Post-polio syndrome

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Post-polio syndrome (PPS) is a condition that affects survivors of poliomyelitis, a viral infection of the nervous system, after recovery from an initial paralytic attack of the virus.

The precise mechanism that causes post-polio syndrome is unknown.

PPS has been confused with amyotrophic lateral sclerosis (ALS), which progressively weakens muscles. PPS patients do not have an elevated risk of ALS.

PPS shares many features in common with myalgic encephalomyelitis, a form of chronic fatigue syndrome that is apparently caused by viral infections, but unlike those disorders it tends to be progressive, and can cause tangible loss of muscle strength.

In general, PPS is not life-threatening. The major exception are patients left with severe residual respiratory difficulties, who may experience new severe respiratory impairment.There have been no sufficient longitudinal studies on the prognosis of post-polio syndrome; however, speculations have been made by several physicians based on experience. Fatigue and mobility usually return to normal over a long period of time. The prognosis also differs depending upon different causes and factors affecting the individual. An overall mortality rate of 25% exists due to possible respiratory paralysis of persons with post-polio syndrome; otherwise, post-polio syndrome is usually non-lethal.^[1]

Typically the symptoms appear 20-40 years after the original infection, at an age of 35 to 60. Symptoms include new or increased muscular weakness, pain in the muscles, and fatigue.^[2]

Diagnosis of post-polio syndrome can be difficult, since the symptoms are hard to separate from the original symptoms of polio and from the normal infirmities of aging. There is no laboratory test for post-polio syndrome, nor is there any other specific diagnostic, and diagnosis is usually a “diagnosis of exclusion” whereby other possible causes of the symptoms are eliminated.^[3]

PPS may be difficult to diagnose in some because it is hard to determine what component of a neuromuscular deficit is old and what is new: Neurological examination aided by other laboratory studies can help to exclude all other possible diagnoses. Objective assessment of muscle strength in PPS patients may not be easy. Changes in muscle strength are determined in specific muscle groups sing various muscle scales which quantify strength, such as the Medical Research Council (MRC) scale.

Muscle biopsies and spinal fluid analysis may also be used in establishing a PPS diagnosis.

Magnetic resonance imaging (MRI) and neuroimaging may also be used in establishing a PPS diagnosis.

Electrophysiological studies may also be used in establishing a PPS diagnosis.

Treatment generally is limited to supportive measures, primarily leg braces and energy-saving devices such as powered wheelchairs, plus pain relievers, sleep aids, etc.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

The precise mechanism that causes post-polio syndrome is unknown.

Studies have shown that, compared to control populations, PPS patients lack any elevation of antibodies against the poliovirus, and because no poliovirus is excreted in the feces, it is not considered a recurrence of the original polio. Further, there is no evidence that the poliovirus can cause a persistent infection in humans.

Several theories have been proposed to explain post-polio symptoms. One theory of the mechanism behind the disorder is that it is due to “neural fatigue” from overworked neurons. The original polio infection generally results in the death of a substantial fraction of the motor neurons controlling skeletal muscles, and the theory is that the remaining neurons are thus overworked and eventually wear out.

Another theory holds that the original viral infection damages portions of the lower brain, possibly including the thalamus and hypothalamus. This somehow upsets the hormones that control muscle metabolism, and the result is a metabolic disorder similar to mitochondrial disorder that causes muscle pain and injury (via rhabdomyolysis) and also causes the fatigue. Some also believe that the original polio caused the atrophying of some muscles and as the person ages the weakness caused by loss of muscle mass due to aging is accelerated due to the person starting off with less muscle. Another possibility is that post-polio symptoms are due to some combination of mechanisms.

The most widely accepted theory is the “neural fatigue” one. Motor neuron fibers were originally damaged by the polio virus and were subsequently over-stressed because too few surviving neurons activated too many muscles. Eventually these neurons become fatigued and die, leading to the slowly advancing loss of muscle function that is typical of post-polio. This scenario may be accelerated by the fall-off in production of nerve growth factor (NGF) that occurs with menopause/andropause.

This theory assumes that the major symptoms of PPS are a result of some interference with the action of mitochondria in the muscles and possibly the nerves. Failure of the mitochondria to produce sufficient energy would result in the muscle pain typical of PPS, and would, over time, cause muscle death (rhabdomyolysis) due to exerting the muscle beyond its ability to recover. The cause of this interference with mitochondrial action is presumably a change in the body’s hormone balance, as mediated by the hypothalamus and other lower brain areas that control hormones (and which were, presumably, damaged by the original polio virus infection). As with the neural fatigue theory, menopause/andropause accelerates the process, though this time by most likely disrupting the NOTCH pathway that controls cell differentiation and damage repair.

One significant argument in favor of the mitochondrial disruption theory is that it explains the fatigue and cognitive difficulties (“brain fog”) symptoms that usually accompany post-polio better than the neural fatigue theory does.

Damage to the reticular activating system and related areas such as the thalamus can also produce most of the fatigue, “brain fog”, and dysautonomia symptoms of post-polio, and may be able to cause hormonal changes that result in progressive muscle weakness. Post-mortem examinations of polio patients have shown damage to these areas, and some PPS patients show lesions in these areas when examined by MRI. Many authorities believe that these areas are damaged by the initial polio infection, either as a direct result of the polio virus, or due to an autoimmune reaction following the polio infection.

One problem with this theory, though, is that it doesn’t easily explain the delayed onset of PPS. It may be that this theory needs to be combined with one of the others to explain delayed onset.

The stresses placed on nerves, muscles, and joints in a polio survivor are in many cases several times those experienced by other people. Problems with gait, in particular, can greatly over-stress joints and the surviving muscles, and the polio survivor is also likely to compensate for weakened arms by jerking more when lifting/pulling something. Over time (and again with menopause/andropause), this results in fatigue and damage.

An early theory stated that PPS is caused by reactivation of latent polio virus in the body, similar to the way that shingles is a reactivation of the chicken pox virus. This theory has been discredited by laboratory studies that show no active polio virus in the body.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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PPS has been confused with amyotrophic lateral sclerosis (ALS), which progressively weakens muscles. PPS patients do not have an elevated risk of ALS.

PPS shares many features in common with myalgic encephalomyelitis, a form of chronic fatigue syndrome that is apparently caused by viral infections, but unlike those disorders it tends to be progressive, and can cause tangible loss of muscle strength.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Post-polio syndrome occurs in approximately 25–50% of people who survive a poliomyelitis infection.^[1] On average, it occurs 30–35 years afterwards; however, delays of between 8–71 years have been recorded.^[2][3] The disease occurs sooner in persons with more severe initial infection.^[3] Other factors that increase the risk of postpolio syndrome include increasing length of time since acute poliovirus infection, presence of permanent residual impairment after recovery from the acute illness,^[2][3] and female sex.^[4]

Post-polio syndrome is documented to occur in cases of nonparalytic polio (NPP). One review states late-onset weakness and fatigue occurs in 14% to 42% of NPP patients.^[5]

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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In general, PPS is not life-threatening. The major exception are patients left with severe residual respiratory difficulties, who may experience new severe respiratory impairment.There have been no sufficient longitudinal studies on the prognosis of post-polio syndrome; however, speculations have been made by several physicians based on experience. Fatigue and mobility usually return to normal over a long period of time. The prognosis also differs depending upon different causes and factors affecting the individual. An overall mortality rate of 25% exists due to possible respiratory paralysis of persons with post-polio syndrome; otherwise, post-polio syndrome is usually non-lethal.^[1]

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