The neurophysiological processes involved in recovery following acute paralytic poliomyelitis are quite effective; muscles are able to retain normal strength even if half the original motor neurons have been lost. Paralysis remaining after one year is likely to be permanent, although some recovery of muscle strength is possible up to 18 months after infection.One mechanism involved in recovery is nerve terminal sprouting, in which remaining brainstem and spinal cord motor neurons develop new branches, or axonal sprouts. These sprouts can reinnervate orphaned muscle fibers that have been denervated by acute polio infection, restoring the fibers capacity to contract and improving strength. Terminal sprouting may generate a few significantly enlarged motor neurons doing work previously performed by as many as four or five units: a single motor neuron that once controlled 200 muscle cells might control 800 to 1000 cells. Other mechanisms that occur during the rehabilitation phase, and contribute to muscle strength restoration, include myofiber hypertrophy – enlargement of muscle fibers through exercise and activity – and transformation of type II muscle fibers to type I muscle fibers.In addition to these physiological processes, the body can compensate for residual paralysis in other ways. Weaker muscles can be used at a higher than usual intensity relative to the muscles maximal capacity, little-used muscles can be developed, and ligaments can enable stability and mobility. Complications
Residual complications of paralytic polio often occur following the initial recovery process. Muscle paresis and paralysis can sometimes result in skeletal deformities, tightening of the joints, and movement disability.
Following the introduction of polio vaccines in the 1950s polio incidence declined rapidly. Once infected, there is no specific treatment. The disease can be prevented by the polio vaccine, with multiple doses required for lifelong protection. There are two broad types of polio vaccine; an injected vaccine using inactivated poliovirus and an oral vaccine containing attenuated (weakened) live virus. Through the use of both types of vaccine, incidence of wild polio has decreased from an estimated 350,000 cases in 1988 to 6 confirmed cases in 2021, confined to just three countries. There are rare incidents of disease transmission and/or of paralytic polio associated with the attenuated oral vaccine and for this reason the injected vaccine is preferred. Signs and symptoms
The term "poliomyelitis" is used to identify the disease caused by any of the three serotypes of poliovirus. Two basic patterns of polio infection are described: a minor illness which does not involve the central nervous system (CNS), sometimes called abortive poliomyelitis, and a major illness involving the CNS, which may be paralytic or nonparalytic. Adults are more likely to develop symptoms, including severe symptoms, than children.In most people with a normal immune system, a poliovirus infection is asymptomatic. In about 25% of cases, the infection produces minor symptoms which may include sore throat and low fever. These symptoms are temporary and full recovery occurs within one or two weeks.In about 1 percent of infections the virus can migrate from the gastrointestinal tract into the central nervous system (CNS).