PostPolio Syndrome/ Pathophysiology and Clinical Management-1

Post-Polio Syndrome: Pathophysiology and Clinical Management

Anne Carrington Gawne and Lauro S. Halstead
The Post-Polio Program, National Rehabilitation Hospital, 102 Irving St., NW, Washington, DC 20010
Critical Reviews in Physical and Rehabilitation Medicine, 7(2): 147-188 (1995)
© 1995 by Begell House, Inc.
Lincolnshire Post-Polio Library copy by kind permission of Dr. Gawne.
ABSTRACT: Post-polio syndrome (PPS) is a progressive neuromuscular syndrome characterized by symptoms of weakness, fatigue, pain in muscles and joints, and breathing and swallowing difficulties. Survivors of poliomyelitis experience it many years after their initial infection. Although the etiology for these symptoms is unclear, it may be due to motor unit dysfunction manifested by deterioration of the peripheral axons and neuromuscular junction, probably as result of overwork. An estimated 60% of the over 640,000 paralytic polio survivors in the U.S. may suffer from the late effects of polio. Their physical and functional rehabilitation care presents a challenge for practitioners in all disciplines. To evaluate these symptoms, a comprehensive assessment must be done, as frequently PPS is a diagnosis of exclusion. Care of the patient with PPS is best carried out by an interdisciplinary team of rehabilitation specialists. This article reviews the epidemiology, pathophysiology, characteristics, assessment, and rehabilitation care of the patient with PPS.
KEY WORDS: poliomyelitis, post-polio syndrome, weakness, fatigue, exercise, pain therapy, respiratory complications.
This article is a comprehensive review of postpolio syndrome (PPS). Although poliomyelitis epidemics came to a dramatic 
end in most countries with the introduction of the Salk polio vaccination in 1955, acute polio remains a threat in many 
parts of the third world. In addition, many of those earlier survivors are now developing new problems and face a new challenge to restore strength and function.
In the past, even in the field of rehabilitation, residual motor loss from paralytic polio was generally considered a chronic, stable lesion. Following the acute illness and a period of rehabilitation, patients eventually reached a plateau 
of neurological and functional recovery that was believed to remain essentially static.[1] However, more recently, research shows that over one half of the survivors of paralytic polio experience new health problems related to their original 
illness.[2] These problems occur about 30 to 50 years after their initial polio and include new weakness, fatigue, pain, and functional loss. The cause of these new symptoms remains uncertain; however, it appears to be related to motor unit dysfunction manifested by a deterioration of the peripheral axons and the neuromuscular junction.
This review begins with a discussion of the historical and epidemiological aspects of poliomyelitis, as well as the pathophysiology of polio. We examine possible etiologies for the development of PPS. We then review the characteristics 
of PPS, including physical, electrodiagnostic, and morphologic features. Finally, we discuss the assessment, differential diagnosis, and rehabilitation management of some of the problems that patients experiencing the late effects of polio may face.
For more than 100 years, it has been recognized that new muscle weakness occurs in polio survivors many years after their initial illness. The first descriptions appeared in 1875 when four separate case histories were reported in the French 
literature by Carriere,[3] Raymond,[4] and Cornil and Lepine.[5] All of these patients were young men who had paralytic polio in infancy. They developed new weakness not only in previously affected muscles but also in muscles believed to be uninvolved. They all had physically demanding jobs and performed repetitive activities. In a commentary on one of the 
cases, Jean Martin Charcot suggested that a previous disease of the spinal cord may leave an individual more susceptible
 to a subsequent spinal disorder and that the new weakness was secondary to overuse of the involved limbs.[4] In a presentation to the Royal Society of Medicine in 1962, Zilkha reviewed 11 patients with motor neuron disease who developed progressive weakness 20 to 40 years later.[6] He stated "It could be suggested that the subsequent development 
of disease of the motor neuron in those patients with a previous history of poliomyelitis, usually 25 years before, is related 
to the occurrence of that earlier disease".
Since these initial reports, there have been other sporadic reports of similar phenomenon. In 35 reports of almost 250 cases, authors have described new problems, including weakness and fatigue occurring up to 71 years after the acute polio episode.[7]

These neurological changes were most commonly diagnosed as a form of progressive muscular atrophy, chronic anterior poliomyelitis, late motor denervation, and forme fruste amyotrophic lateral sclerosis.[8,9] However, it was not until the 
early 1980s, approximately 40 years after the major epidemics of the 1940s and 1950s, that PPS became widely recognized.
As the numbers of persons experiencing these new symptoms increase, this subject has been studied in depth. This review concentrates on some of the more recent research. Although the incidence of acute poliomyelitis has decreased since the introduction of the Salk vaccine in 1955 and the Sabin trivalent oral polio vaccine (TOPV) in 1961, some of the principles learned through the study of PPS can be applied to other similar neurological diseases and aging with a disability.
During the epidemics in the U.S. from 1952 to 1954, the incidence of new cases was approximately 15/100,000, as Figure 1 demonstrates.[10] The incidence peaked in 1952 with over 57,879 new cases reported in the U.S. After the Salk vaccine 
was introduced in 1955 and the Sabine vaccine in 1961 the incidence dropped to 0.04/100,000 by 1963. The last 
confirmed case of paralytic polio from domestic wild virus in the U.S. occurred in 1979.[10] Paralytic polio is now a rare complication of the current Sabin (oral) vaccination. In the U.S. between 1961 and 1964, there was an incidence of 
paralysis in 4.9/10 million doses compared with an incidence of 0.23/10 million in 1989, as Figure 2 shows.[10]
The greatest risk is from the initial immunization (1/700,000).[11]

FIGURE 1. Reported rates per 100,000 persons of poliomyelitis and of death from poliomyelitis, United States, 1932 to 1989. (IPV = inactivated polio vaccine; OPV = oral polio vaccine.)
A survey by the National Center for Health Statistics in 1987 found there were more than 640,000 survivors of paralytic polio.[12] Thus, despite the virtual elimination of new cases, paralytic polio remains one of the most prevalent
 neuromuscular diseases in this country. While PPS is more common in those who have experienced moderate to severe paralysis,13 population-based surveys show that as many as 64% of polio survivors develop new symptoms.[2] If these samples are representative of a typical distribution, then it can be estimated that over 409,000 people may experience 
the late effects of polio. While some of this population has died since the original survey, there has also been an unknown
 and largely unexpected increase in the number of polio survivors in the U.S. resulting from the influx of affected 
immigrants, refugees, and illegal aliens from Southeast Asia and Latin America. In 1991, the World Health Organization (WHO) estimated that 85% of children world-wide received three doses of the TOPV vaccine, a significant increase over the percentage in 1971 (15%), largely due to the effort WHO put into their vaccination campaign.[14] While no cases of wild polio infection occurred in the Americas in 1993, WHO estimates that 96,500 new cases occurred in the developing
 world; in particular Africa, Asia, and India, due to inadequate immunization.[15]
 FIGURE 2. Reported cases of paralytic poliomyelitis (total and vaccine-associated per 10 million doses of Sabin vaccine), U.S., 1960 to 1989.

Knowledge of the pathophysiology of acute polio is necessary to understand the possible causes for PPS and to provide a rational basis for its management. The poliovirus is a positive single-stranded RNA enterovirus belonging to the Picornavirus group. The virus is relatively small (30 nm in diameter) and the poliovirus genome is about 7400 nucleotides long.[16] It lacks a lipid coat or capsule but has a protein coat shaped like a polyhedron with 20 faces.[17] There are three polio viruses, numbers 1, 2, and 3, defined by the configuration of the capsid proteins. Therefore, theoretically, a person could be infected more than once.
Wild poliovirus enters the body by oral ingestion, then replicates in the lymphoid tissue of the pharynx and ileum and spreads regionally to lymphoid tissue. It is extremely infectious and usually benign. The vast majority of infected individuals (95 to 99%) remain asymptomatic or experience a self-limited illness characterized by fever, myalgia, and gastrointestinal symptoms.[18] However, in 1 to 5% of persons, viremia may follow, with invasion of the anterior horn cells of the central nervous system (CNS). These patients usually develop a headache, stiff neck, and back pain, similar to viral meningitis. Only 1 to 2% of all those infected develop paralysis. Finally, the rate of paralysis varies with the strain of the virus and the patient's age. In children, paralysis occurs in 1/1000 cases, while in adults 1/75 develop paralysis. Asymmetric, flaccid paralysis occurs, with legs more commonly involved than arms. Severe bulbar weakness occurs in 10 to 15% of all paralytic cases. Less frequently, there is ophthalmoplegia and bladder involvement.[18] The pathological findings of acute polio consist of inflammation of meninges and anterior horn cells, with loss of spinal and bulbar motor neurons.[19] Less prominent findings include abnormalities in the cerebellar nuclei, reticular formation, thalamus, hypothalamus, cortical neurons, and dorsal horn.[19]
Once the virus has invaded the CNS, neurological and functional loss occurs as anterior horn cells are lost, and thus the muscle fibers innervated by them are "orphaned". Recovery begins in weeks and reaches a plateau in 6 to 8 months. The extent of neurological and functional recovery is determined by three major factors: 
(1) the number of motor neurons that recover and resume their normal function, 
(2) the number of motor neurons that develop terminal axon sprouts to reinnervate muscle fibers left orphaned by the death of their original motor neurons, and 
(3) muscle hypertrophy. 
The phenomenon of terminal axon sprouting makes it possible for an uninvolved or recovered motor neuron to "adopt" these orphaned muscle fibers. Stalberg has shown that a motor neuron cell can adopt five to seven additional muscle fibers commonly and occasionally, as many as 20 for every muscle cell innervated originally.[20] A single motor neuron that originally innervated 100 muscle fibers iriight eventually innervate 700 to 2000 fibers. As a result, the survivors of acute polio may be left with a few, significantly enlarged motor units doing the work previously performed by many units.[21] Figures 3, 4, 5, and 6 provide a schematic illustration of this phenomenon. Both electrophysiological evidence, including single fiber and macro-EMG and morphological data support this concept.[22-27]

In addition to this reinnervation, the remaining muscle fibers hypertrophy to increase the strength of the muscle group.[25,28] Because this mechanism of neurophysiological compensation is so effective, a muscle can retain normal strength even after 50% of the original motor neurons have been lost. Therefore, in some patients, manual muscle testing (MMT) may be normal when more than half the original anterior horn cells are destroyed.[29]
 Figure 4


Figure 6

The pathological changes that cause the symptoms of PPS are not well understood; however, a number of possible theories have emerged in recent years.[30] These are summarized in Table 1. We explore each of these separately.
TABLE 1 Proposed Etiologies of Post-Polio Syndrome.
Motor unit dysfunction due to overuse or premature aging of large motor units.
Musculoskeletal overuse.
Musculoskeletal disuse.

Loss of motor units with normal aging.
Predisposition to motor neuron degeneration because of glial, vascular, and lymphatic changes caused by acute polio.
Chronic polio virus infection or virus reactivation.
An immune mediated syndrome.
The effect of growth hormone.
The combined effects of disuse, overuse, pain, weight gain, or other illnesses.
A. Motor Unit Dysfunction Due to Overwork or Premature Aging of Polio Affected Motor Units.
In 1902, Gowers proposed that progressive weakness in many degenerative neuronal diseases is due to abiotrophy; the neurons are exhausted and they simply wear out.[31] The original viral attack of the anterior horn cells may have left some motor neurons functional but impaired, making them more vulnerable to dysfunction as time passes. Tomlinson observed that many neurons were smaller than normal in the spinal cords of persons who survived long after the acute polio episode.[32,33] Consistent with Bodian’s findings,[34] his observation led him to conclude that the protein synthetic mechanisms of any cell invaded by polio are likely to be permanently damaged.
At this time, we can only speculate that premature exhaustion may be due to abnormalities in DNA and RNA repair or protein synthesis.[30] If this is a major factor, it is most likely that this is a combination of damage to both neurons affected by polio and those that have increased metabolic demands because of increased motor unit territory.[35] According to this theory, neurological dysfunction results from this increased metabolic load after a critical number of years. This has been demonstrated in electrophysiological studies. Weichers and Hubbel[26,27] and Dalakas et al.[36] found neuromuscular transmission abnormalities suggesting that the giant motor neurons may not be able to sustain indefinitely the metabolic demands of all their sprouts. As a result, individual terminals slowly deteriorate and reinnervated muscle fibers drop off, as Figure 7 shows.[21] 

Possibly then, it is prolonged overwork with increased metabolic demand of the greatly enlarged motor units that compounds injury to the motor unit sustained during acute infection. The overworked anterior horn cells' control over a greater than normal percentage of muscle function may cause them to succumb "prematurely" to the aging process, resulting in pronounced weakness beginning as early as the fourth decade and steadily worsening with advancing age. Thirty to 40 years after recovery, the giant motor units appear to have lost their ability to sustain all of the terminal sprouts supplying so many muscle fibers. Consequently, the number of muscle fibers driven by each motor neuron declines, and the polio survivor experiences new weakness and other symptoms of neurological dysfunction.
While there is no direct experimental data demonstrating that this increased metabolic demand results in premature aging of the neuron soma, pathologically it does appear that collateral reinnervation is greater, not surprisingly, in weak muscles.[30,37] The more muscle fibers lost, the more apparent is the slowly progressive weakness. This hypothesis is intuitively attractive and conceivably explains new weakness in some polio patients but remains unsubstantiated by muscle biopsy changes like group atrophy that would reflect the new loss of whole motor units. Also, no studies have demonstrated permanent biochemical or physiological damage to the surviving motor neurons.[9]
B. Muscle Overuse.
Muscle overuse is less well understood, although studies suggest relationships between the number of motor units, muscle damage, exercise intensity, and duration.[38-42] However, the extent to which a primary muscle defect is weakening some polio survivors remains unknown. Overuse has a cumulative effect over time. Chronic mechanical strains on joints, ligaments, and soft tissues that have not been supported well for 30 or more years produce a self-perpetuating cycle of further complications. Recognizing overuse complications early and implementing effective interventions may avert severe post-polio disablement in middle or old age.
Both Windebank et al. at the Mayo Clinic and Maynard found that in persons with similar neurological involvement, new weakness occurred more often in the weight-bearing muscles of the legs than in the non-weight-bearing muscles of the arms.[38,39] And those limbs affected the most by the original disease were the most susceptible to new weakness. Perry et al. observed that patients with new lower-extremity weakness had a less efficient gait, with an increase in both the duration and intensity of the extensor muscle contraction.[40] Halstead and Gawne showed more patients had mild involvement or subclinical polio in their arms, while clinically unstable polio or atrophic polio was more common in the lower extremities.[41] Agre and co-workers found that symptomatic post-polio subjects had evidence of more severe original polio involvement by history, were weaker and capable of performing less work than asymptomatic subjects, and recovered strength less readily than controls.[42] Finally, evidence of anatomical damage to muscle fibers is indirectly shown by elevation of creatinine kinase levels found in patients with unstable polio.[43,44]
C. Muscle Disuse.
It is well known that disuse leads to both deconditioning and muscle weakness in healthy individuals.[45] Similarly, polio individuals have been noted to have similar short-term increased weakness when forced to remain sedentary with illness or injury.[46] What part this plays in the development of long-term weakness, however, is less clear.
D. Loss of Normal Motor Units with Aging.
While anatomical and electrophysiological studies have demonstrated that there is a loss of motor neurons with advancing age, this becomes prominent only after 60 years of age.[33,35] While some studies of post-polio survivors have failed to show a positive relationship between the onset of new weakness and chronological age,[38,47] a recent study by Trojan et al. demonstrates that older individuals are more likely to develop PPS.[48] Nevertheless, the most consistent variable is the length of the interval between onset of polio and the appearance of new symptoms, so most patients develop new weakness 30 to 40 years after their initial infection, and the age of onset of symptoms is variable. Therefore, while chronological age may contribute to the development of new weakness, it is probably not the primary causative factor. Also, it is unlikely that polio patients retain "normal" motor neurons. So, the first hypothesis (overuse of large motor units) is more likely.
E. Predisposition to Motor Neuron Degeneration Due to Glial, Vascular, and Lymphatic Damage.
Some investigators have suggested that damage to the glial cells and vascular supply at the time of infection can lead to secondary dysfunction of anterior horn cells.[49] While vascular damage is sometimes seen, it is believed that this is secondary to the severe inflammatory responses that can occur. Most studies show that polio affects only the neural cells and not the glial or vascular endothelial cells.[50,51] For these reasons, it is unlikely that these changes play a large part in the clinical deterioration seen with post-polio syndrome.
F. Virus Reactivation or Persistent Infection.
Animal studies have shown that poliovirus and other picornaviruses may persist in the CNS and produce late or chronic disease.[52,53] When looking for evidence of persistent infection, investigators either study the immune response, by examining oligoclonal bands in the cerebral spinal fluid (CSF), isolating the virus through histochemical or hybridization studies, or looking for evidence of viral genetic material using the polymerase chain reaction (PCR) technique and probe detection. Recently, much work has been done in this area.
Dalakas and co-workers first found evidence of oligoclonal IgG bands in the CSF in seven of 13 symptomatic post-polio patients.[36] However, total IgG levels, the IgG index, and IgG synthesis in the CSF were normal, and there were no antibodies to poliovirus. No oligoclonal bands were found in the CSF of six asymptomatic patients.
Evidence for possible reactivation of polio virus was demonstrated by Sharief et al. in 1991 in a study that examined the intrathecal immune response in polio survivors.[54] They assessed the antibody response to poliovirus and the production of interleukin-2 and soluble interleukin-2 receptors in 36 patients with PPS and 67 controls, including 13 who had a history of polio but no new weakness and 18 patients with amyotrophic lateral sclerosis. Oligoclonal IgM bands specific to poliovirus were .

Detected in the CSF of 21 of 36 patients with PPS but in none of the controls. In quantitative studies, there was evidence of increased intrathecal synthesis of IgM antibodies to poliovirus only in those with PPS and no increased IgM to other viruses. The patients with PPS had significantly higher mean CSF levels of interleukin-2 and soluble interleukin-2 receptors, corresponding to increased levels of IgM. The presence of an intrathecal immune response to poliovirus in patients with PPS is suggestive that recrudescence of weakness may be caused by persistent or recurrent infection with some part of the poliovirus.
Following publication of Sharief’s article in the New England Journal of Medicine, letters to the editor by Drs. Salazar-Grueso, Roos, Dalakas, Jubelt, and Cashman pointed out that there was IgM detected in 21 patients, with poliovirus specific IgG in only seven patients.[55] They felt it was inconsistent to have IgM but no IgG in a chronic viral infection. More studies were then done to duplicate the findings.
In 1994, Muir and Sharief examined CSF in 24 patients with PPS, 36 with stable polio, and 36 controls.[56] Three of 24 patients with PPS had evidence of enterovirus RNA compared with no patients in the other groups. All three of these patients had high intrathecal levels of poliovirus specific oligoclonal IgM bands, which they suggest is evidence that patients with a history of polio are susceptible to persistent enterovirus infection.
In another study, Melchers et al. examined skeletal muscle biopsies in six patients and CSF specimens in an additional 10 patients who met the criteria for PPS, examining the CSF for IgM antibodies to poliovirus and the muscle biopsy for presence of poliovirus RNA by PCR.[57] In none of these specimens was any evidence of poliovirus detected. Control CSF specimens in patients with acute poliovirus were positive, while controls in patients with aseptic meningitis (not polio) were negative.
On the other hand, Jubelt et al. examined sera and CSF from 19 post-polio patients and found increased anti-poliovirus (anti-PV) antibodies to type 1 and 2 in seven.[58] However, in another study, no anti-PV antibodies were found.[59] 
They have concluded that there was little evidence of intrathecal production of anti-PV antibody.
Leparc and colleagues examined CSF from eight patients with PPS and 10 controls.[60] Although no viruses were cultured, using enzymatic amplification of viral DNA, genome sequences were found in several PPS patients and in no controls. They suggest that these results are in favor of the persistence of poliovirus for several decades in PPS patients.
Finally, in 1994, Monzone and Dalakas examined serum and CSF of patients with PPS, comparing them to patients with acute polio, other neurological diseases, and normal controls.[61] While both IgM and IgG were highest with acute polio, moderate levels of both were seen in PPS patients compared with the controls. Poliovirus was seen in one of 18 PPS patients by PCR, and amplified genetic product was seen in four of 12 PPS patients. They concluded that high titers of IgM antiPV antibodies imply an ongoing antibody response to antigen, and the presence of viral RNA suggested possible viral persistence.
In summary, regarding the hypothesis of persistent or reactivated infection, an active controversy still exists. However, the lack of consistent findings from study to study and the failure to find conclusive findings in all patients with PPS suggests that this is not the single cause of new weakness. Most researchers feel this is an area that needs to be studied further.
G. An Immune-Mediated Syndrome.
Another hypothesis proposes immunologic involvement. A study by Pezeshkpour and Dalakas described what appeared to be evidence of an ongoing inflammatory or immune response -- active inflammatory gliosis, neuronal chromatolysis, and axonal spheroids in the spinal cords of polio patients who died many years later of other causes.[62] Steegman also found inflammatory infiltrates, including lymphocytes, plasma cells, and macrophages in the parenchyma and perivascular spaces in five of seven post-polio patients.[49] Whether these changes represent a primary lesion in the cord or a response to a lesion in the distal axon is unknown.
Ginsberg et al. described activated T cells, including significant alternations in CD4+ subsets in both symptomatic and asymptomatic post-polio subjects when compared with normal controls supporting the possibility that immunologic factors may contribute to late disease progression.[63] Dalakas et al. have reported preliminary evidence of a lymphocytic response in the form of anti GM1 neuronal antibodies and IgG oligoclonal bands in the CSF of some patients with new weakness, whereas patients with no new weakness had no oligoclonal bands in their CSF.[21,36] He later examined muscle biopsies and found perivascular or interstitial inflammatory cells consisting of CD8+ cells, CD4+ cells, and macrophages. These findings suggest that there is a slow hut ongoing inflammatory process not only in the spinal cord but also in the muscle specimens.[64] More recently, immunopathological studies of a patient with PPS showed evidence of focal perivascular interparenchymal inflammatory infiltrates in the CNS. Immunoperoxide staining demonstrated that these were virtually all B lymphocytes, with rare macrophages and no T cells.[65] These antibodies could be directed toward neurons, nerve terminals, or postsynaptic antigens. This suggests that PPS could be an autoimmune disorder mediated by antibodies produced in situ, and not a cell-mediated process. It is possible that autoantigens from neurons, axons, and muscle membranes might be released during the acute phase of polio. Antibodies against neuronal elements or antiidiopathic antibodies could play a role in the pathogenesis of PPS.
H. The Effect of Growth Hormone.
There is an intriguing suggestion by Shetty, Matsson, and Rudman that the aging of the hypothalamus growth hormone (GH) axis may be a precipitating factor in the development of PPS.[66,67] It has been shown that GH secretion drops off dramatically in approximately one third of normal adults over the age of 40. This results in a fall in somatomedin C (SmC) or insulin growth factor (IGF-I), which plays an important role in accelerating the synthesis of DNA and skeletal muscle protein, aids in the proliferation of muscle satellite cells and the regeneration of peripheral nerve sprouting.[68]
In a survey of 10 men with PPS and 94 healthy men ages 35 to 63, 100% of those with PPS had SmC less than or equal to 0.40 µ/ml and 90% had values of 0.35 µ/ml or less, while in the healthy population the numbers were 40 and 27%, respectively. In the PPS group, the values did not correlate significantly with either age, functional level, body weight, or years since acute polio.[66] In a study of 12 stable polio patients and 10 patients with PPS, Rudman and Shetty found that SmC was markedly depressed in those with PPS and was normal in those without PPS.[67]
In a subsequent study with 124 polio survivors and 261 age-matched healthy controls, Rao et al. measured IGF-I in 124 polio survivors and found that the level was significantly lower in those with a history of polio. The IGF-I levels in that group significantly correlated with age, gender, body mass index, dependency, pain, and difficulty with activities of daily living (ADLs). However, it did not correlate with subjective report of recent decline in functional status.[69]
Recently, this hypothesis was tested in six PPS patients with low IGF-I levels.[70] These patients were given low-dose human GH treatment for 3 months. Although two of five patients demonstrated improvements in strength and endurance, this was not consistent, and the overall impression was that there was no significant improvement. It was suggested that a longer trial would be necessary. Further investigation into the role of GH therapy for treatment of PPS presents a challenge for the future.
I. The Combined Effects of Overuse, Disuse, Pain, Weight Gain, or Other Illnesses.
Finally, it is hypothesized that a combination of musculoskeletal disuse, musculoskeletal overuse, or motor unit dysfunction may play a significant role in the development of progressive weakness. Furthermore, they may interact with each other in such a way to multiply the effects of any single factor, as illustrated in Figure 8.[71] With overuse, weakness may develop. This may lead to disuse, weight gain, and further weakness. Musculoskeletal disuse leads to atrophy, weakness, contractures, and diminished endurance, which are complications that have been studied in other groups with sedentary lifestyles or neuromuscular lesions. If there is overuse, musculoskeletal pain may occur, causing the patient to either rest, developing deconditioning, or compensate with improper body mechanics, leading to further overuse, and possibly pain elsewhere. In our experience, most patients present with some combination of these. Treatment can then be centered on minimizing the effects of one or more of these in order to allow the remaining muscles to function at a more optimal level.
There is disagreement about the most appropriate names or diagnostic labels to describe post-polio patients with new health problems. A number of terms have been proposed, including postpolio syndrome (PPS), post-polio muscular atrophy (PPMA), late effects of polio, and postpolio sequelae. One reason for a lack of consensus is that previously these terms lacked specific diagnostic criteria. This is due to the absence of any pathognomonic tests and our incomplete understanding of the underlying pathophysiology of the presenting complaints. Another reason no single term is suitable for all individuals is that there may be one, two, or more pathologic processes present at any one time producing similar, overlapping symptoms. Separating out the origin of each symptom may not only be impractical but impossible, which gives rise to the need for a more general and less precise diagnostic term. Recently, an attempt was made to be more specific, and these terms have been defined more clearly.
PPMA refers to the clinical and pathological features seen in muscles of post-polio patients who are developing new weakness and atrophy in muscles both affected and apparently unaffected by the polio virus previously. Specifically, there is documented evidence of neuromuscular deterioration with muscle biopsy showing evidence of active denervation in the form of scattered angulated fibers.[36] Mulder first proposed the following criteria for the late progression of weakness in polio survivors in 1972: (1) a credible history of polio, (2) partial recovery of function, (3) a minimum of 10 years of stabilization, and (4) the subsequent development of progressive muscle weakness.[9]
In contrast to PPMA, PPS is a more heterogeneous term and therefore more practical in the typical clinical setting. However, it should not be used indiscriminately for every person with a history of paralytic polio with a new complaint. Criteria for making this diagnosis are outlined in Table 2.[72]
TABLE 2. Criteria for the Diagnosis of Post-Polio Syndrome.
A prior episode of paralytic polio confirmed by history, physical exam, and typical findings on EMG.
Standard EMG evaluation demonstrates changes consistent with prior AHCD: increased amplitude and duration of motor unit action potentials, an increased percentage of polyphasic potentials and, in weak muscles, a decrease in the number of motor units on maximum recruitment; fibrillations and sharp waves may or may not be present.
A period of neurologic recovery followed by an extended interval of neurological and functional stability preceding the onset of new problems; the interval of neurologic and functional stability usually lasts 20 or more years.
The gradual or abrupt onset of new neurogenic, nondisuse weakness in previously affected and/or unaffected muscles; this may or may not be accompanied by other new health problems such as excessive fatigue, muscle pain, joint pain, decreased endurance, decreased function, and atrophy.
Exclusion of medical, orthopedic, and neurologic conditions that might cause the health problems listed above.
The first criterion is a documented history of polio. The diagnosis of paralytic polio usually can be confirmed by examining, whenever possible, the original medical records; eliciting a credible history of an acute, febrile illness producing motor but no sensory loss; noting whether other members of the patient’s family or neighbors had a similar illness; and by observing certain features during physical examination. One very characteristic feature is the presence of focal, asymmetric weakness, and/or atrophy on examination.
The second criterion is a characteristic pattern on EMG. The changes on routine EMG compatible with prior polio include large polyphasic motor unit action potentials (MUAPs) and a decrease in the number of motor units on maximum recruitment in weak muscles. Occasionally, fibrillations are present. These are discussed in detail later in this article.
The third criterion is a characteristic pattern of recovery. In patients with late complications of polio, the pattern of events from onset of polio to onset of new problems is so characteristic that when it is absent the diagnosis should be seriously questioned. The pattern generally consists of three stages, as shown in Figure 9: (1) paralytic polio in childhood or later in life, (2) partial to fairly complete neurologic and functional recovery, (3) a period of functional and neurologic stability lasting many years, and (4) the onset of new health problems.[72]
FIGURE 9. Natural history data from post-polio clinic in Houston, Texas. A = birth; B = onset of polio; C = maximum recovery; D = onset of new health problems; E = time of evaluation; F = death. (Ref. 72.)

The fourth criterion is the onset of new neurogenic, non-disuse weakness that may come on either gradually or abruptly. New neurogenic weakness is essential for making the diagnosis of PPS and presumably reflects new or continuing dysfunction of previously injured motor units. Often this new weakness is accompanied by one or more of the other new health problems listed in Table 2. Although the distinction is not always readily apparent, new neurogenic weakness, in contrast to disuse weakness, can frequently be inferred by the onset of diminished function despite maintaining the usual level and intensity of activity.
And, finally, the fifth criterion is the exclusion of other conditions that might cause the weakness and other health problems listed in Table 2. In addition to distinguishing between disuse and neurogenic weakness, there are several other dilemmas in making the diagnosis of PPS. First, the symptoms are frequently so general that ruling out all possible causes is not practical and can be prohibitively expensive, and, second, co-existing medical, orthopedic, and/or neurologic conditions may be present that can produce a similar set of overlapping signs and symptoms. As indicated in Figure 8, once a problem such as weakness occurs -- regardless of the underlying etiology -- it may initiate a chain reaction of other complications that makes the original problem impossible to identify.
This fifth criterion is often the most difficult to establish. A history of paralytic poliomyelitis does not exempt anyone from getting the chronic illnesses, diseases, or psychiatric disturbances that afflict the general population. When medical, orthopedic, or neurologic conditions coexist with post-polio problems, a similar set of overlapping signs and symptoms may occur. Compression neuropathies, radiculopathies, degenerative arthritis, disc disease, obesity, anemia, diabetes, thyroid disease, and depression are some common examples. Nevertheless, because non-disuse weakness is such an important indicator of PPS, and different etiologies dictate very different management strategies, every attempt should be made to differentiate post-polio weakness from other possible causes.
The terms "the late effects of polio", or "post-polio sequelae" are less specific, and refer to the myriad of new symptoms that patients with a history of polio may experience, regardless if there is evidence of actual new motor unit dysfunction. These symptoms: (1) can be attributed directly to damage caused by the poliovirus, such as cold intolerance, or musculoskeletal imbalance; (2) are thought to be related to the body’s failure to maintain the level of recovery that was achieved following the infection such as new weakness; or (3) result from a secondary trauma, such as the development of carpal tunnel syndrome (CTS) after years of crutch walking.[73] As new information becomes available about the underlying mechanism(s) that produce late onset complications, these criteria will undoubtedly change and new terminology be developed to fit our improved understanding.
Because diagnostic criteria remain nonspecific and pathognomonic tests unavailable, a consistent diagnostic name has not yet been established for new health problems associated with former polio. Indeed, several pathologic processes may interact at any given time to produce similar, overlapping symptoms. The use of a general rather than a precise diagnostic term takes into account the impossibility of determining a distinct origin for each new symptom. The diagnosis "PPS" should be reserved for those patients whose symptomatology indicates motor unit dysfunction with variable musculoskeletal overuse.
A variety of studies over the past 10 years demonstrate that the majority of post-polio patients develop similar symptoms, the frequency and relative rank of these are taken from a number of studies and shown in Table 3.[13,72,73,76,77] Of these reports, two are population based (Codd and Ramlow), one is based on a questionnaire (Halstead), and the remainder are clinically based, which accounts for the relative differences in the prevalence of symptoms. In addition to the more frequent symptoms, other problems that have been reported include increased sleep requirements, dizziness, syncope, and headaches. Major new functional problems are listed in Table 4.[72,74,75,77]
Comparison of Most Common New Symptoms in Subjects with a History of Paralytic Polio Reported in Six Studies
Symptom Codd[77]
N = 28 Halstead[72]
N = 132 Chetwynd[76]
N = 694 Agre[75]
N = 79 Ramlow[13]
N = 474 Halstead[74]
N = 539
Fatigue 59% 89%[a]
48% 86% 34% 87%[a]
Joint Pain 74%[a]
71% 60%[a]
77% 42%[a]
Muscle Pain 48% 71% 52% 86% 38% 79%
New Weakness 71% N/A 47% 69% 38% N/A
  Affected muscle 66% 69% N/A 80% N/A 87%
  Unaffected muscle 15% 50% N/A 53% N/A 77%
Cold Sensitivity 46% 29% N/A N/A 26% N/A
a = Most frequent symptom.
Note: N = Number of subjects reported.
Most Common New Functional Problems in Persons with a History of Paralytic Polio Reported in Four Studies
Problem Codd[77]
N = 28 Halstead[72]
N = 132 Agre[75]
N = 79 Halstead[74]
N = 539
Difficulty walking 25%[a]
N/A 85%[a]
Difficulty climbing Stairs N/A 61% 67%[a]
Difficulty with ADLS 14% 17% 16% 62%
a = Most common new problem.
Note: N = Number of subjects reported.
In general, the patients most at risk for developing new problems are those who experienced more severe polio at onset, although some patients with typical post-polio symptoms had seemingly very mild polio with excellent clinical recovery. Table 5 summarizes the factors associated with progressive weakness.[47,48] Inasmuch as the age at onset of polio is a factor, persons who were older when they contracted polio appear to be at an increased risk for new neurologic symptoms. The onset of these new problems is most commonly insidious, but in many persons they are precipitated by specific events such as a minor accident, period of bed rest, or weight gain. Patients characteristically say that a similar event several years earlier would not have caused the same decline in health and function. Likewise, new problems may begin when coexisting medical problems such as diabetes develop or worsen. More specific details regarding these complaints are covered later in this article.
The time course for the development of late symptoms is fairly characteristic, as demonstrated in Figure 9. While the range between the initial episode and onset of symptoms is 8 to 71 years, the mean interval is around 35 years.[78]
TABLE 5 Factors Associated with the Development of Progressive Weakness.[47,48]
Age of initial infection more than 10 years old. [a]
History of hospitalization.
Ventilator use.
Paralytic involvement in all four limbs.
More weakness at time of acute polio.
A longer time since acute infection.
Recent weight gain.
Muscle pain associated with exercise.
Greater age at time of presentation to clinic.
a. Reports differ from study to study.
The EMG in acute poliomyelitis is characterized by poor MUAP recruitment at the onset of weakness, followed by the development of fibrillation potentials in widespread muscle groups after 3 to 4 weeks.[79] The fibrillation potentials subside as reinnervation by surviving motor units proceeds during recovery. In muscles with too few motor units to have adequate reinnervation, fibrillation potentials can persist indefinitely; however, in both partially and completely denervated muscle the fibrillation potential amplitude decays over time. Kraft found that in subjects with partial traumatic peripheral nerve lesions, initially the amplitude of the fibrillation potentials were as high as 1600 µV, with most greater than 700 µV.[80] One year later, all potentials were less than 100 µV.
The same is true in chronic polio. Some investigators report that there is an increase in muscle membrane instability between those patients that are clinically stable (no new weakness) and those who are clinically unstable (new weakness). Nelson found that in those patients who were seen for unrelated medical problems, three of 27 had fibrillation potentials, while in those with delayed weakness, 14 of 29 had fibrillations (p <0.01).[43] Martinez studied 34 patients, 17 with stable polio and 17 with new weakness. He found that 41% of those with unstable polio had fibrillations, while none with stable strength did.[37] However, other investigators, including Cashman, Weichers, Hayward, and Seaton, examined groups of patients with and without new weakness and found evidence of ongoing enervation in the form of fibrillations in both groups.[23,26,27,81,82] Some electromyographers have found that muscle membrane instability is more pronounced in the more severely affected muscles.[83,84] The muscles that become extremely weak and atrophic have few to no MUAPs, but virtually all, including clinically normal muscles, will have large, polyphasic MUAPs and abnormal, decreased recruitment.[79] In a study comparing the size of MUAPs in control subjects and polio patients with and without new weakness, Agre found that the size of MUAPs was significantly larger in all polio patients, and that those with new weakness had larger units than those without. He found no significant correlation between strength and MUAP size.[42]
Single fiber studies have shown abnormalities, including increased jitter and blocking and increased fiber density 
and motor unit territory.[85] When comparing electrophysiological and biopsy findings, a significant correlation 
between the percentage of fibers exhibiting jitter and increased fiber density has been found, suggesting that muscles 
with the most enlarged motor units as a result of sprouting are more likely to exhibit instability later in life.[85]
Defects at the neuromuscular junction have been studied using repetitive stimulation techniques by Trojan et al.
 Single fiber EMG was performed, and jitter was measured during both low- and high-frequency stimulation in 17 
PPS patients and nine normal controls. In five of 17 PPS patients and one control, jitter was significantly higher at high-frequency stimulation. The remaining patients were not significantly different from controls. Those PPS patients with abnormal jitter had a significantly longer time since their acute polio. The authors concluded that the neuromuscular
 junction defect in PPS patients is probably due to ineffective conduction along immature nerve sprouts and exhaustion of acetylcholine stores, and that this may be dependent on time after acute poliomyelitis.[86]
Findings on macro-EMG include increased macro-EMG amplitude, up to 2000% above the mean. Lange et al. found that polio-affected muscles with normal strength had large-amplitude macro-EMG signals (1000 to 4110 µV), weak but stable muscles also had increased large macro-EMG amplitude, while muscles with new weakness had smaller than expected 
macro-EMG amplitude (130 to 450 µV). Fiber density and jitter were increased in all three groups, but percent blocking doubled in weak muscles.[87]
In summary, EMG abnormalities seen in chronic polio include large polyphasic MUAPs in both conventional and macro-EMG. Although reports differ, it is a fair consensus that there can be evidence of muscle membrane instability in the form of fibrillations in both stable and unstable muscles, although generally these fibrillation potentials are small and sparse.
 Findings on single fiber EMG include increased jitter, blocking, and fiber density. High-frequency stimulation accentuates this increased jitter, especially in those muscles with new weakness, reflective of defects in the neuromuscular junction. 
At this time, however, there is no single EMG finding that can distinguish muscles that are developing new weakness.
Dalakas et al. evaluated a population of 27 post-polio patients and documented new weakness over an 8-year period.[24] Biopsy findings at follow-up included changes in cell morphology, fiber type grouping, small angulated fibers, and hypertrophy of muscles that were less affected. He then separated morphologic findings according to the severity of involvement (or presumed involvement) of polio. These muscles were placed into one of four subgroups. We briefly summarize the characteristics of each of these groups.
Subgroup I included muscles originally affected, but partially recovered, with subsequent new weakness. These muscles showed a combination of myopathic features, with new and old neurogenic changes. The myopathic features included increased connective tissue, occasional necrotic/phagocytosed fibers, variations of fiber size with big and small but
rounded fibers, fiber splitting and an abundance of internal nuclei. The old neurogenic changes included fiber type 
grouping, groups
 of small fibers, and small angulated fibers.
New neurogenic changes seen included small scattered esterase-positive angulated fibers.
Subgroup II were those muscles originally affected but fully recovered, with new weakness. These showed evidence of extensive reinnervation with fiber type grouping consisting of very large groups of up to 170 normal size fibers and 
variable small scattered angulated fibers.
Subgroup III were those muscles originally clinically spared with new weakness. These muscles like those in group 
II showed evidence of chronic enervation/reinnervation with large fiber type grouping and occasional angulated atrophic fibers. Contrary to group II, hypertrophic fibers, internal nuclei, and fiber splitting were rare. Only minimal perivascular inflammation was noted.
Subgroup IV were muscles that were asymptomatic: either apparently unaffected or unaffected with subsequent
 improvement. These muscles showed fiber type grouping but no small angulated fibers or evidence of inflammation.
Grimby and Einnarsson have demonstrated that morphologic changes seen in muscles of patients with PPS weakness 
include an increase in fiber area with hypertrophy and an increase percentage of type I fibers. These researchers documented in some polio survivors the fiber area of type I and type IIA myofibers on the average are twice the area seen in control
They also demonstrated there is a fiber type transformation from type II (fast twitch, glycolytic) fibers to type I (slow twitch, oxidative) fibers based on these findings. They suggest that excessive use of remaining muscle fibers leads to hypertrophy, and that this is most prominent in the weakest muscles.[25,28,88] Borg also documented an increased percentage in type I fibers. In addition, internal nuclei and fiber splitting were prominent findings.[89]
In summary, in severely involved muscle myopathic morphologic features include evidence of inflammation and increased connective tissue. Muscles with new enervation may show small angulated fibers, internal nuclei, and fiber splitting. In muscles with reinnervation, large fibertype grouping, fiber-type transformation, and hypertrophy may occur. All of these studies demonstrate the unpredictable nature and wide variability with which polio affects the muscles. No two persons, or even two muscles within the same person, may be affected equally. Because of this, no two people should be treated quite 
the same. Thus lies the challenge in treating the person with PPS.
Proper assessment of post-polio patients presents both a challenge and a dilemma: a challenge because of the diversity and nonspecific nature of the problems they may present with, and a dilemma because of the absence of specific diagnostic
 tests, the continuing uncertainty of the underlying cause or causes, and the lack of any curative therapeutic intervention.
 Our approach to the assessment of post-polio patients is based on a number of assumptions concerning their past health
 experience and present needs. These assumptions guide the format and content of our evaluation. They are based on the experience gained in assessing and managing over 1000 patients over nearly a decade and the lessons learned in organizing and running two major polio programs in two different institutional settings. Clearly, these assumptions represent a 
particular bias and we recognize that other professionals, with a different perspective or with different resources available to them, may want to use a modification of the approach outlined here. We believe it is essential that the evaluation includes 
the following elements:[90]
A. Comprehensive and Interdisciplinary Evaluation.
Because of the number, diversity, and complexity of the problems presented by these patients, a comprehensive, 
coordinated assessment is required. For many of these patients the evaluation in the post-polio clinic may be one of the first examinations done by a group of specialists that are familiar with the features of PPS. The best way to provide a 
comprehensive, coordinated evaluation that looks at the medical, functional, and psychosocial and vocational issues
 of this population is to use an interdisciplinary rehabilitation team including the physician and nurse and physical and 
occupational therapists. A social worker or psychologist, orthotist, and respiratory therapist may complete this team.
B. Diagnosis by Exclusion.
Because post-polio syndrome is a diagnosis by exclusion, it is essential that every patient receive a careful history and physical examination, along with appropriate laboratory, radiological, and diagnostic studies to rule out other medical, orthopedic, or neurologic conditions that might be causing or aggravating the symptoms. A psychosocial evaluation is 
often helpful, along with an assessment of function, gait, and orthotic needs. In addition, a baseline measure of strength and endurance in key muscle groups is essential to observe for the appearance of new weakness. Because manual muscle testing alone is not a reliable and valid measure of strength over time in post-polio patients in comparison to more quantitative measurements, we recommend objective testing of key muscle groups in the good to normal range with a myometer or isokinetic test.[91,92,93] For virtually every patient, we feel a standard electromyogram/nerve conduction
 study (EMG/NCS) of all four extremities and appropriate paraspinals is essential. We have found that this test is
 invaluable in confirming the presence of an old anterior horn cell disease (AHCD), identifying major muscle groups with subclinical involvement, establishing a baseline and helping exclude certain other neurologic and myopathic
 conditions, and in detecting additional diagnoses, including carpal tunnel syndrome (CTS), ulnar neuropathy, and radiculopathy. A summary of our experience with 100 consecutive patients is shown in Table 6.[94] We do not believe 
more sophisticated studies with single fiber EMU or macro-EMG are indicated in the routine clinical setting because they have not helped separate the symptomatic from the asymptomatic patient or proven useful in guiding clinical management.
Additional Electrodiagnostic Findings in 100 Consecutive Post-Polio Patients
(Ref. 94.)
Carpal Tunnel Syndrome (CTS)
Ulnar Neuropathy at the Wrist
CTS and Ulnar Neuropathy
Peripheral Neuropathy
Brachial Plexopathy
Tibial Neuropathy
Subclinical Polio
A standard battery of screening tests such as an electrolyte panel, fasting glucose, etc. used on a routine basis are generally
 not helpful; however, when indicated we may obtain a hematocrit, blood glucose, creatine phosphokinase 
(CPK), or thyroid function tests. Whether it is useful to monitor CPK levels on a regular basis to assist in determining long-term prognosis or as an aid in clinical management is still not clear.
Patients who had respiratory involvement initially and have a history of pulmonary disease or scoliosis should have a screening vital capacity (VC) and functional expiratory volume (FEV1) measured along with their other vital signs.
 If the vital capacity is less than 50% of predicted or the history and clinical situation warrant, further pulmonary
 function tests (PETs) are obtained. If the patient has significant spinal curvature, a 36-in gravity loaded scoliosis film is obtained to provide a baseline for follow up exams. If degenerative joint disease (DJD) or other skeletal abnormalities are suspected, appropriate radiographs are obtained.
C. Expectation of Improvement.
We believe that everyone who comes to the clinic can be helped, regardless of the etiology or severity of disability. As a result, our goal is that everyone, even if they can implement only some of the recommendations and 
interventions, will feel better physically and emotionally and achieve an improved level of function.
D. Convenience and Efficiency.
Because of the comprehensive, interdisciplinary nature of our evaluation, and frequently the decreased stamina these 
patients may have, we attempt to complete the evaluation in 1 or 2 days, having team members come to the patient in a
 central location rather than having the patient come for a series of single service outpatient evaluations. All laboratory, radiological, and electrodiagnostic procedures are performed on location the day of the evaluations.
A typical evaluation in the NRH Post-polio Clinic extends over 2 days, with the first day reserved for evaluations by team members, including a rehabilitation nurse, physician, physical therapist, occupational therapist, social worker, and, if
 needed, an orthotist.
The patient is seen initially by the nurse, who makes a brief assessment of the past and current health status, clarifies the patient's goals for the clinic visit, coordinates the evaluations by the team members, schedules diagnostic tests, and assists with patient and family education. The medical evaluation consists of a comprehensive history and physical exam, with special attention to the history of the details of the initial illness with acute polio and its management and a special focus during the physical exam on neurologic and musculoskeletal findings. There is also an analysis of station and gait to determine the need for orthoses and other durable medical equipment. In addition, the physician determines the
 need for X-ray, laboratory and electrodiagnostic studies, and initiates referrals to other rehabilitation disciplines, such as a nutritionist, vocational counselor, speech language pathologist, or psychologist, as well as other medical and/or surgical specialists if needed.
The physical therapist's evaluation is based on a protocol outlined by Smith and includes a baseline manual test of major muscle groups, measurement of major joint range of motion and leg length, and an evaluation of habitual postures during standing, sitting, lying down, and walking.[95] It also includes an analysis of activities and positions that provoke or
 relieve muscle and joint pains. The occupational therapist's assessment is based on a format described by 
Young and includes an evaluation of activities that produce pain, weakness, or fatigue; when they occur; and how these problems interfere with the person's activities of daily living (ADL).[96] Special attention is paid to the frequency and intensity of activities in the
 home, at work, in the community and during travel, and to the use or need for adaptive aids.
The social work evaluation focuses on how new health problems and functional loss impact on the patient, the family, significant others, and colleagues at work, school, or elsewhere outside the home. There is also an effort to identify coping strategies used by, and available to, the individual and assess the emotional impact of the original polio experience and 
relate it to current feelings of having a second disability.[97,98] In addition, the social worker facilitates referrals and 
access to community resources and services, including the local post-polio support group. In the afternoon, we obtain any necessary diagnostic tests to help rule out other medical, orthopedic, or neurologic conditions that might be causing or aggravating the patient’s presenting symptoms.
Finally, the morning of the second day is used to complete any unfinished evaluations and to hold a team conference with
 the patient and his/her family. This conference is used to review the results of diagnostic tests and discuss our impressions
 and recommendations for interventions. Patients are given a written copy of recommendations and then seen in follow-up
 6 to 8 weeks later to evaluate the effectiveness of the interventions and make any modifications or additional suggestions
 for management. Thereafter, patients are seen as needed and at annual intervals for a repeat functional evaluation and
 manual muscle test as well as an interim history and a physical.
In summary, the assessments provided by special diagnostic testing are generally more fruitful in excluding certain 
conditions than in assisting either with the diagnosis or management of post-polio syndrome. Despite the growing body of evidence
 that suggests that the major pathologic process is motor unit dysfunction, there is still no objective method to predict who might develop new weakness in the future or to monitor the progress of the underlying pathology in the subject who has already becoming weaker. Specifically, no serologic, enzymatic, electrodiagnostic, or muscle biopsy test can
 diagnose PPS. In general, we rely on a careful patient history to distinguish between those patients who have no new weakness (clinically stable) and those who are experiencing new weakness (clinically unstable).
In general, we have found that patients most at risk for developing new problems are those who experienced more severe 
polio at onset, although it is not unusual to see patients with typical post-polio symptoms who had seemingly very mild 
polio with excellent clinical recovery. Most commonly, the onset of these new problems is insidious, but in many persons they may be precipitated by specific events such as a minor accident, fall, period of bed rest, or weight gain.
 Characteristically, patients state that a similar event experienced several years earlier would not have caused the same 
decline in health and function. Likewise, new problems may begin when coexisting medical problems, such as diabetes, develop or worsen.
The symptoms experienced by polio survivors, unfortunately, are fairly common and nonspecific. The lack of specificity
 and a characteristic cluster of symptoms have led some observers to question both the validity of the symptoms and the existence of a diagnosis of post-polio syndrome. Until a pathognomic test is found, this dilemma will undoubtedly persist. However, health professionals who have become experienced in this field in recent years generally agree there are definite qualitative features of these symptoms that are reasonably characteristic. The differential diagnosis of symptoms experienced by persons with a history of polio is complex, and they must be evaluated in a symptom by symptom manner. 
We review these symptoms, their differential diagnosis, and outline an approach to the evaluation and rehabilitation management that we have found to be successful.
A. Weakness and Functional Loss.
1. Differential Diagnosis.
Because of the importance of weakness as a cardinal symptom of motor neuron dysfunction and post-polio changes in 
general, it should be addressed early and one should make the differentiation between post-polio syndrome as a cause 
of new weakness or other causes of new weakness. New weakness may appear in muscles previously affected and/or
 believed to be previously spared. The weakness is usually most prominent, however, in the muscles most severely involved
 in the initial illness. Diminished functional capacity tends to parallel the muscle weakness and can be quite dramatic if functional reserve was marginal. One of the characteristics of many polios was their ability to appear "normal" or function 
at an extraordinarily high level of performance on relatively few good muscle groups. This was possible because of the random, scattered nature of the motor deficits and the body's uncanny ability to compensate with unconventional muscle
 and joint function. In this situation then, late onset weakness of a critical muscle often leads to disruption of a delicate balance that has been maintained for years, leading to a disproportionate amount of functional loss. Persons with
 involvement of one or both legs may have increased difficulty in walking, standing, climbing stairs, or other endurance activities. individuals with presumably normal upper extremities who have been "walking" on their arms with crutches for years may find that
 ambulating, transfers, driving a car, or even dressing are more taxing and the time to recover takes longer than it used to.
When patients present with complaints of "new weakness", the first task the practitioner needs to do is differentiate between true weakness (a loss in muscle strength) and other synonymous terms the patient may be referring to, such as lassitude, fatigue, lack of energy, and languor. Whereas the polio patient may also experience these symptoms, they are complaints of fatigue, not progressive muscle weakness. If possible, objective muscle testing with a myometer or isokinetic system
 should be done to establish a reliable baseline.[91-93] This baseline is essential for monitoring changes in strength in the future. If there is a history of new weakness (often revealed as decreased endurance with diminished function) combined with objective changes, the major differential diagnoses include focal neurological disease such as a radiculopathy, focal compressive neuropathy, or spinal cord lesion and medical causes of neuropathy such as diabetes, thyroid disease, uremia, alcohol, toxins, and, uncommonly, hereditary neuromuscular disease. In most cases, history and a physical alone can make this distinction; however, when necessary, laboratory or electrodiagnostic data may be needed to clarify the appropriate diagnosis. Once other causes are ruled out, an attempt should be made to distinguish the etiology of this weakness (disuse vs. overuse), so that appropriate recommendations regarding exercise and activity can be made.
For patients with new weakness (with or without atrophy), the major differential confronting the clinician is to distinguish between neurogenic weakness due to polio and disuse weakness caused by diminished activity. Although the distinction is not always readily apparent in the clinical setting, when new neurogenic weakness is present a careful history can usually elicit a pattern of decreased strength, endurance, and function despite attempts to maintain the usual level and intensity of activity. Routine, daily activities that require repetition or sustained contractions, such as walking, climbing stairs, standing, or pushing a wheelchair can sometimes provide a semi-quantitative picture of new weakness when current performance is compared with similar activities in the past, for example, number of stairs climbed without difficulty 1, 3, or 5 years ago vs. now. When the presence of new neurogenic weakness is in doubt, a trial of carefully monitored exercise is indicated to exclude the possibility of disuse weakness.
As indicated earlier, we feel the workup for new neurogenic weakness should include an EMG/NCS. Although electrodiagnostic studies even in the most skilled hands have their limitations, they provide a necessary screening tool to
 help exclude some of the more common causes of neurogenic weakness in this population. Specifically, EMG studies can indicate the presence of radiculopathies from disc disease or other causes and help differentiate old polio from other neuromuscular disorders such as adult onset spinal muscular atrophy and myopathies. Nerve conduction studies can
 identify the presence of localized compression neuropathies as well as generalized peripheral neuropathies. Follow-up laboratory and imaging studies can help clarify the underlying etiology suggested by the electrodiagnostic exam and
 reveal other causes for weakness such as occult tumors, toxic metal exposure, and endocrine disorders. 
New weakness may appear in muscles previously affected and muscles believed to be previously spared. The weakness is usually most prominent, however, in the muscles most severely involved initially. Diminished functional capacity tends to parallel muscle weakness and can be quite dramatic if functional reserve was marginal. This occurs because of the functional compensations that are made early within the recovery period.
2. Management.
There has been a controversy about the management of new weakness with exercise in the post-polio patient because the pathophysiology of PPS remains unclear. Traditional therapy such as exercise may cause further weakness, so it must be used cautiously. As there is now a considerable body of literature on this subject -- some of it contradictory -- we will provide a quick review of what is known before proposing specific recommendations.
One of the first formal isotonic strengthening programs for patients with a history of polio was described in 1948 by Delorme and Watkins, who applied the principle of progressive resistive exercises (PREs) in 19 post-polio subjects.[92] In the end, 17 of 27 muscle groups demonstrated gross gains in strength measured with MMT. Muscle power in 15 of 27 quadriceps 
muscles doubled or more than doubled as measured by a spring scale. All except three muscles showed an increase in work capacity. In 1950, Gureswitsch evaluated 13 subjects who were in the initial phases of recovery from their polio.[99] 
They exercised with a modification of Delorme's protocol. After training, both muscle strength and endurance increased 50%.
In the early 1980s, specific exercise studies were carried out for survivors with PPS. In 1984, Feldman and Soskolne 
developed an exercise protocol they described as "non-fatiguing strengthening exercises" for a population of six subjects
 with post-polio symptoms.[100] This was performed three times weekly for a period of 3 to 6 months. They found that by using this routine for at least 24 weeks, 14 muscles (46%) got stronger, 17 (53%) showed no change, and 1 muscle got
 weaker, as measured by myometry. There was no relationship between the initial weakness and improvement in strength.
Grimby and Einarsson reported an isokinetic exercise program for 12 post-polio subjects, including nine with symptoms of PPS, in 1987.[101] These exercises were performed for a total of 6 weeks. Only one leg was trained and the other served as a control. There were significant strength gains in the trained leg. The investigators concluded that increases in strength might be explained through both muscular as well as neural adaptations. In 1991, Einarsson used a similar protocol for muscle conditioning of knee extensors in 30 post-polio subjects.[102] This was performed three times a week for 6 weeks. 
There was a significant improvement in knee extension strength, with no change in the strength of the knee flexors.
 These changes were also associated with subjective improvements in functional tasks and general well-being.
Also in 1991, Fillyaw and colleagues used Delorme's training program with 17 PPS subjects.[103] The quadriceps or biceps were exercised, with the contralateral extremity used as a control. Three sets of 10 repetitions with a 5-min rest between sets were performed every other day. Sixteen of 17 subjects demonstrated significant strength gains, but there was no evidence of increased endurance. They cautioned patients should undergo periodic quantitative muscle testing under the 
supervision of a physical therapist to avoid overwork weakness.
Agre and Rodriguez have described an exercise technique known as "pacing", mixing periods of exercise with 
periods of rest, in post-polio subjects.[104-106] They found when subjects paced themselves they had less evidence 
of muscle fatigue, increased capacity to perform work and increased ability to recover strength after activity.
In addition to strengthening programs, studies were also done to determine the cardiovascular fitness of post-polio
 survivors and to assess their response to a cardiovascular training program. In 1985, Owen and Jones evaluated the cardiovascular endurance of 21 subjects in an EKG-monitored, symptom-limited graded exercise test.[107] 
They found that the subjects had an average maximum fitness level of 5.6 metabolic equivalents (METS), 
indicative of severe deconditioning. They proposed an exercise program as follows: intensity of 65 to 80% of reserve HR (maximum HR-resting HR), a duration of 15 to 30 min, and a frequency of three to five times per week on alternate days.
In 1987, Alba and co-workers evaluated the work capacity of 35 subjects, including 33 that were complaining of new symptoms.[108] Parameters evaluated included muscle strength using MMT, body weight, maximum METS, maximum 
heart rate (Hrmax), maximum oxygen uptake (VO2 max), and vital capacity (VC). Significant findings included a decreased VC, especially in those who smoked or had a history of respiratory involvement.
 There was also a decreased Hrmax, decreased maximal cardiac output, and decreased work capacity. She recommended that post-polio survivors partake in any repetitive activity that appealed to them, if it was considered "safe" and recommended stopping the activity if it caused "undue pain, muscle fatigue or a sense of weakness" that required more than the usual
 time to recover.
Dean and Ross examined the effects of a modified exercise program in three post-polio subjects in 1988.[109]
Each subject met the diagnostic criteria for PPS and was ambulatory without assistive devices. Using one subject as an untrained control, the other two walked on a treadmill three times a week at a submaximal rate for a total of 8 weeks, advancing their walking duration from 22 to 31 min. The rating of perceived exertion (RPE) was monitored using a 1 to 10 scale,[110] and pain was monitored using a 1 to 4 scale. An attempt was made to keep the RPE below 2 (light). 
After training, on submaximal testing, both the trained subjects demonstrated reductions in VO2, HR, blood pressure (BP), RPE, and energy cost at similar work loads compared with the pretest, while the untrained subject showed no change.
 There was no apparent effect of training on pulmonary function. The mechanism for the change was felt to be both cardiovascular conditioning as well as muscle adaptation.
In 1989, Jones and colleagues evaluated the cardiorespiratory responses to aerobic training in 16 post-polio subjects who participated in a 16-week exercise program.[111] Baseline tests, including resting HR and BP, HRmax, BPmax, VO2, and expiratory volume (Ve), were performed on a bicycle ergometer. Subjects were divided into exercise and control groups.
 The subjects exercised three times a week for a period of 15 to 30 minutes using a predetermined target HR. Significant 
results on post-test included an improvement in the total exercise time, total work per time, VO2, and Vemax within the exercise group. In 1992, in a similar study Kriz and colleagues examined the effects of an upper-extremity arm ergometry program in 
29 post-polio subjects.[112] The exercise group participated three times a week in a 16-week training program with a heart rate of 70 to 75% HR reserve, with 1 d of rest between exercise sessions. After the 16-week program, the exercise group 
showed significant improvement in VO2 max, Vemax, power, and exercise time. None of the subjects in the exercise group 
developed problems with pain or overuse.
In summary, although it is true that individuals with a history of polio may develop new weakness many years after the original episode, it has been shown that many can improve both muscle strength and cardiovascular endurance from a well-planned training program. There appears to be positive benefits regardless of whether the muscle group is with or without new weakness. The type of exercise program needs to be selected according to both the needs of the individual and the resources available. An isometric program is of benefit in those who have less than antigravity strength, a painful joint, or a joint immobilized in a cast due to surgery or a fracture. An isotonic program is more appropriate for a home exercise program for a non-painful joint with greater than antigravity strength. An isokinetic program can be used when the equipment is available and the muscles have greater than antigravity strength. While the long-term safety of a strenuous strengthening program on muscle that has been severely affected is not known, the positive effects of exercise on other systems, including cardiovascular and respiratory systems are clear. Therefore, it can be concluded that a carefully monitored program is beneficial for most individuals with a history of polio. It is also possible that many of the secondary symptoms such as generalized fatigue can be reduced as patients become conditioned and are able to perform similar amounts of work with less expenditure of energy. An ideal cardiovascular program should exercise the muscles least affected by polio in order to get maximum cardiovascular benefits, while avoiding overuse or secondary degenerative effects on the more affected extremities. For instance, if the legs are the more involved limbs, then the arms can be used in a more strenuous program. The exercise program should be initially supervised by a physical therapist in order to teach.
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