Excerpt from the
Handbook on the Late Effects of Poliomyelitis for Physicians and Survivors©
Electromyography records the changes in electric potential of muscles by means of surface or needle electrodes. An electromyogram (EMG), the record of electromyography, characterizes the electrical activity of motor units and, in polio survivors, will typically show evidence of chronic neuropathy (a disease of the nerves) which reflects the paralysis of many years ago. During recovery from the acute disease, the terminal ends of the motor nerves sent out "sprouts" (Wohlfart, 1958) to the orphaned muscles resulting in larger than normal motor units, detectable by EMG (see Pathology).
EMG findings from polio survivors appropriately diagnosed with post-polio syndrome, and those not experiencing symptoms, are not significantly different (Cashman et al., 1987). Consequently, an EMG is not a test to diagnose post-polio syndrome. The diagnosis is a clinical one (see Evaluation) and a diagnosis of exclusion. Clinicians do use an EMG to confirm a history of prior polio if there is doubt, and to eliminate other diseases that may be causing the symptoms and/or co-existing with post-polio syndrome (Peach, 1997).
Research using EMG suggests that the enlarged motor units are not stable and that the disintegration of the reinnervated motor unit occurs over time (Wiechers & Hubbell, 1981). One study demonstrated an increased motor unit size with time after acute paralytic poliomyelitis (Stålberg & Grimby, 1995). Using motor-unit number estimation (MUNE), a study reported that 87% of previously affected limbs exhibited denervation as did 65% of supposedly unaffected limbs, and that the progression cannot be attributed to normal aging (McComas et al., 1997).