Localized hypertrophic neuropathy: possible focal perineurial barrier defect.

Localized hypertrophic neuropathy (hypertrophic mononeuropathy) is a rare benign condition that generally occurs in people under 40 years of age. Our immunocytochemical (S-100 protein) study of four new cases confirms previous observations that the cells forming the hypertrophic onion bulb are composed of perineurial cells. These observations and previously published illustrations, reveal a curious hyalinization of the outer perineurium of affected fascicles which suggests the absence of a perineurial barrier. Compartmentation (compartmentalization) of the endoneurium in hypertrophic mononeuropathy closely mimics the transient compartmentation which occurs in the distal nerve stumps of axotomized nerves, particularly in nerves in which re-innervation is prevented. Compartmentalization also can be produced by resection of the perineurial sheath. These findings suggest that hypertrophic mononeuropathy may be a reactive condition due to focal damage to the perineurial barrier.