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Why might post-polio syndrome cause new weakness decades after recovery?

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Polio vaccine — polio A health care worker giving a polio vaccine to a child in Katsina state, Nigeria, 2014.

Post-polio syndrome (PPS) is a condition that can emerge years or decades after an initial recovery from polio. It is important to clarify that PPS is not caused by a reactivation of the poliovirus, nor is it a contagious condition.1

The syndrome manifests as a gradual return of symptoms, including increased muscle weakness, atrophy, fatigue, and pain. While the exact cause remains uncertain, it is believed to result from the long-term physiological strain placed on the nerve cells that survived the original infection.21

During the initial illness, the virus destroys motor neurons, which cannot regenerate. To compensate, surviving nerve fibers often grow new branches to maintain muscle function. Researchers hypothesize that as a former patient reaches middle age, these overextended nerve branches may begin to deteriorate due to the decades of extra work, leading to the decline in muscle function associated with the syndrome.21