Post polio syndrome
Post-polio syndrome (PPS) is a condition that affects polio survivors years after recovery from an initial acute attack of the poliomyelitis virus. PPS is mainly characterized by new weakening in muscles that were previously affected by the polio infection and in muscles that seemingly were unaffected. Symptoms include slowly progressive muscle weakness, unaccustomed fatigue (both generalized and muscular), and, at times, muscle atrophy. Pain from joint degeneration and increasing skeletal deformities such as scoliosis are common. Some patients experience only minor symptoms. While less common, others may develop visible muscle atrophy, or wasting.
PPS is rarely life-threatening. However, untreated respiratory muscle weakness can result in underventilation, and weakness in swallowing muscles can result in aspiration pneumonia.
The severity of residual weakness and disability after acute poliomyelitis tends to predict the development of PPS. Patients who had minimal symptoms from the original illness will most likely experience only mild PPS symptoms. People originally hit hard by the poliovirus and who attained a greater recovery may develop a more severe case of PPS with a greater loss of muscle function and more severe fatigue. It should be noted that many polio survivors were too young to remember the severity of their original illness and that accurate memory fades over time.
According to estimates by the National Center for Health Statistics, more than 440,000 polio survivors in the United States may be at risk for PPS. Researchers are unable to establish a firm prevalence rate, but they estimate that the condition affects 25 percent to 50 percent of these survivors, or possibly as many as 60 percent, depending on how the disorder is defined and which study is quoted.
Patients diagnosed with PPS sometimes are concerned that they are having polio again and are contagious to others. Studies have shown that this does not happen.
Causes
The cause is unknown. However, the new weakness of PPS appears to be related to the degeneration of individual nerve terminals in the motor units that remain after the initial illness. A motor unit is a nerve cell (or neuron) and the muscle fibers it activates. The poliovirus attacks specific neurons in the brainstem and the anterior horn cells of the spinal cord. In an effort to compensate for the loss of these neurons, ones that survive sprout new nerve terminals to the orphaned muscle fibers. The result is some recovery of movement and enlarged motor units.
Years of high use of these enlarged motor units adds stress to the neuronal cell body, which then may not be able to maintain the metabolic demands of all the new sprouts, resulting in the slow deterioration of motor units. Restoration of nerve function may occur in some fibers a second time, but eventually nerve terminals malfunction and permanent weakness occurs. This hypothesis is consistent with PPS's slow, stepwise, unpredictable course.
Through years of studies, scientists at the National Institute of Neurological Disorders and Stroke and at other institutions have shown that the weakness of 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.
Diagnosis
Physicians arrive at a diagnosis of PPS by completing a comprehensive medical history and neuromuscular examination and by excluding other disorders that could explain the symptoms. Researchers and physicians typically use the following criteria to establish a diagnosis: