Size-dependent myelinated fiber loss in the corticospinal tract in Shy-Drager syndrome and amyotrophic lateral sclerosis.

Morphometric evaluation was performed on myelinated fibers of the corticospinal tract at the seventh thoracic spinal cord segment from three patients with Shy-Drager syndrome (SDS), six patients with amyotrophic lateral sclerosis (ALS), and five patients with nonneurologic symptoms. In SDS, small-sized myelinated fibers were nearly completely depleted, while large-sized myelinated fibers were considerably well preserved. In ALS, on the contrary, large myelinated fibers were predominantly decreased. These results suggested that selective vulnerability of axonal loss depends on fiber size and should be considered in interpretation of pathology of corticospinal tracts.