Post-polio syndrome (PPS, or post-poliomyelitis syndrome) is a condition that affects approximately 50% of people who have previously contracted poliomyelitis—a viral infection of the nervous system—after recovery from the initial paralytic attack. Typically the symptoms appear 15-30 years after the original infection, at an age of 35 to 60. Symptoms include acute or increased muscular weakness, pain in the muscles, and fatigue. The precise mechanism that causes PPS is unknown. It shares many features with the post-viral chronic fatigue syndrome, but unlike that disorder it tends to be progressive, and as such can cause a tangible loss of muscle strength. Treatment is primarily limited to adequate rest, conservation of available energy, and supportive measures, such as leg braces and energy-saving devices such as powered wheelchairs, analgesia (pain relief) and sleep aids.
After a period of prolonged stability individuals who had been infected and recovered from polio begin to experience new signs and symptoms, characterised by muscular atrophy (decreased muscle mass), weakness, pain and fatigue. PPS is a very slowly progressing condition marked by periods of stability followed by new declines in the ability to carry out usual daily activities. Most patients become aware of their decreased capacity to carry out daily routines due to significant changes in mobility, decreasing upper limb function and lung capability. Fatigue is often the most disabling symptom; even slight exertion often produces disabling fatigue and can also intensify other symptoms. Increased activity during intervening healthy years between the original infection and onset of PPS can amplify the symptoms. Thus, contracting poliomyelitis at a young age can result in particularly disabling PPS symptoms.
Numerous theories have been proposed to explain post-polio syndrome. Despite this, there are currently no absolutely defined causes of PPS. The most widely accepted theory of the mechanism behind the disorder is "neural fatigue". A motor unit is a nerve cell (or neuron) and the muscle fibers it activates. Poliovirus attacks specific neurons in the brainstem and the anterior horn cells of the spinal cord, generally resulting in the death of a substantial fraction of the motor neurons controlling skeletal muscles. In an effort to compensate for the loss of these neurons, surviving motor neurons sprout new nerve terminals to the orphaned muscle fibers. The result is some recovery of movement and the development of enlarged motor units. The neural fatigue theory proposes that the remaining motor neuron fibres, that were originally damaged by the initial poliomyelitis infection become overworked and eventually wear out. After years of use, these enlarged motor units become unstable, and may not be able to maintain the metabolic demands of all the new sprouts, resulting in the slow deterioration of motor units. This causes muscle weakness and paralysis. Restoration of nerve function may occur in some fibers a second time, but eventually nerve terminals malfunction and permanent weakness occurs. When these neurons no longer carry on sprouting, fatigue occurs due to the increasing metabolic demand of the nervous system.
Diagnosis of post-polio syndrome can be difficult, since the symptoms are hard to separate from complications due to the original poliomyelitis infection, and from the normal infirmities of aging. There is no laboratory test for post-polio syndrome, nor are there any other specific diagnostic criteria. PPS is usually a diagnosis of exclusion whereby other possible causes of the symptoms are eliminated. Neurological examination aided by other laboratory studies can help to determine what component of a neuromuscular deficit occurred with polio and what components are new and 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 useful in establishing a PPS diagnosis.
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. 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. PPS has be confused with amyotrophic lateral sclerosis (ALS), which progressively weakens muscles. PPS patients do not have an elevated risk of ALS. 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.