Function and ultrastructure of the neuromuscular junction in post-polio syndrome

We performed a detailed morphological and electrophysiological analysis of the neuromuscular junction in muscle biopsies from 10 patients with post-polio syndrome (PPS). This was done to clarify the basis for the apparent neuromuscular transmission impairment in PPS. In six patients, intracellular microelectrode recordings demonstrated either reduction of amplitudes of miniature end-plate potentials (MEPPs) or decreased quantal content or both. In one patient, reduction of quantal content was only present with prolonged or high-frequency nerve stimulation. In three patients no significant abnormalities were found by the intracellular microelectrode studies. Histologically, atrophy of individual muscle fibers were present in 6 out of the 10 biopsies, but grouped atrophy was not seen. Fiber type grouping suggesting reinnervation was seen in 8 out of the 10 muscle biopsies. Fragmentation and dispersion of the end plate was present in three patients. In two of these patients dispersion of the end plate was associated with an apparent increase of the quantal content. Electron microscopy revealed either normal neuromuscular junctions or small axon termini apposed to normal postsynaptic folds. In summary, variable degrees and different types of failure of neuromuscular transmission were seen in association with histological signs of reinnervation in the muscle biopsies of affected patients. Functional and structural abnormalities of the neuromuscular junction, although very common, were not invariably present and, therefore, they do not appear to be a necessary condition to define the post-poliomyelitic syndrome.