Thin axons relative to myelin spiral length in hereditary motor and sensory neuropathy, type I.

The relationship of axonal to myelin area in semithin transverse sections of myelinated fibers obtained from sural nerves at the ankle level was morphometrically assessed using computer imaging. Ten patients with hereditary motor and sensory neuropathy, type I and 41 control subjects were examined. In large- and intermediate-diameter myelinated fibers of diseased nerves, axons were significantly attenuated relative to the amount of myelin. Using electron micrographs, a similar finding was obtained when axon area was regressed on myelin spiral length. The altered relationship was found to be greater with more severe fiber loss. These data, plus other evidence, indicate that in this disorder there is a progressive atrophy of axons, usually most severe in distal aspects of lumbosacral neurons and associated with secondary segmental demyelination and remyelination and hypertrophic neuropathy, preceding distal axonal loss. Because the process may begin in utero or in infancy, not only atrophy but also maldevelopment of axons may be involved.