Post-polio syndrome (PPS, poliomyelitis sequelae) is a group of latent symptoms of poliomyelitis (polio), occurring in more than 80% of polio infections. The symptoms are caused by the damaging effects of the viral infection on the nervous system and typically occur 15 to 30 years after an initial acute paralytic attack. Symptoms include decreasing muscular function or acute weakness with pain and fatigue. The same may also occur years after a nonparalytic polio infection. The precise mechanism that causes PPS is unknown. It shares many features with myalgic encephalomyelitis/chronic fatigue syndrome, but unlike that disorder, it tends to be progressive and can cause 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.
Signs and symptoms After a period of prolonged stability, some people who recover from polio infections begin to experience new signs and symptoms, characterised by muscular atrophy, weakness, pain, and limb 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 and 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. Problems breathing or swallowing, sleep-related breathing disorders, such as sleep apnea, and decreased tolerance for cold temperatures are other notable symptoms. Increased activity during healthy years between the original infection and the onset of PPS can amplify the symptoms. Thus, contracting polio at a young age can result in particularly disabling PPS symptoms.
Mechanism Numerous theories have been proposed to explain post-polio syndrome. Despite this, no absolutely defined causes of PPS are known. 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 enlargement of the motor neuron fibers places added metabolic stress on the nerve cell body to nourish the additional fibers. After years of use, this stress may be more than the neuron can handle, leading to the gradual deterioration of the sprouted fibers and eventually, the neuron itself. 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. The normal aging process also may play a role. Denervation and reinnervation are going on, but the reinnervation process has an upper limit where the reinnervation cannot compensate for the ongoing denervation, and motor unit loss occurs. What disturbs the denervation-reinnervation equilibrium and causes peripheral denervation, though, is still unclear. With age, most people experience a decrease in the number of spinal motor neurons. Because polio survivors have already lost a considerable number of motor neurons, further age-related neuron loss may contribute substantially to new muscle weakness. The overuse and underuse of muscles also may contribute to muscle weakness. Another theory is that people who have recovered from polio lose remaining healthy neurons at a faster rate than normal. Little evidence exists to support this idea. Finally, the initial polio infection is thought to cause an autoimmune reaction, in which the body's immune system attacks normal cells as if they were foreign substances. Again, compared to neural fatigue, the evidence supporting this theory is quite limited.
Diagnosis Diagnosis of PPS can be difficult, since the symptoms are hard to separate from complications due to the original polio infection, and from the normal infirmities of aging. No laboratory test for post-polio syndrome is known, nor are any other specific diagnostic criteria. Three important criteria are recognized, including a previous diagnosis of polio, a long interval after recovery, and a gradual onset of weakness. In general, PPS is 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 that quantify strength, such as the Medical Research Council (MRC) scale. Magnetic resonance imaging, neuroimaging, and electrophysiological studies, muscle biopsies, or spinal fluid analysis may also be useful in establishing a PPS diagnosis.
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