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.
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.
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.
The cause is AUTOIMMUNE. However, the new weakness of PPS
appears to be related to the infection 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.