Post-polio syndrome (PPS)

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. 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.

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

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

Diagnosis

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.

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 using various muscle scales which quantify strength, such as the Medical Research Council (MRC) scale. Magnetic resonance imaging (MRI), neuroimaging, and electrophysiological studies, muscle biopsies, or spinal fluid analysis may also be used in establishing a PPS diagnosis.

In general, PPS is not life-threatening. The only exception is in patients left with severe residual respiratory difficulties, who may experience new severe respiratory impairment. Studies have proven 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. PPS can be confused with Amyotrophic lateral sclerosis (ALS), which progressively weakens muscles. PPS patients do not have an elevated risk of ALS.

Symptoms

Symptoms include new or increased muscular weakness, pain in the muscles, and fatigue. Fatigue is often the most disabling symptom, as often even slight exertion can produce disabling fatigue and also increase other symptoms.

Some post-polio patients report having memory problems, or various other cognitive difficulties that are difficult to distinguish from normal aging. Some physicians have suspected that post-polio patients have an increased sensitivity to anesthetics, but rigorous work on the subject remains to be done. Weight gain is also a frequently noted symptom, though it is hard to tell if this is due to the disorder directly or due to the decreased level of physical activity that usually accompanies the disorder.

Causes

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.

Neural fatigue

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.

Mitochondrial disruption

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.

Reticular activating system damage

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.

Mechanical overwork

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.