Localized hypertrophic mononeuropathy of the trigeminal nerve.

We report a case of localized hypertrophic mononeuropathy that involved the trigeminal nerve in a 61-year-old woman with a 5-year history of progressive drooping of the left eyelid, double vision, and left-sided temporal and retro-orbital pain. A mass that occupied the patient's left cavernous sinus was found to display localized hypertrophic mononeuropathy characterized by "onion bulbs" that were formed of concentrically proliferating Schwann cell processes around intact axons. This contrasted to the majority of previously reported cases of localized hypertrophic mononeuropathy, predominantly found in peripheral nerves, where the proliferating cell processes have been shown to be of a perineurial cell origin. Differences in architectural arrangement and degree of cellularity between the perineurial and schwannian forms of localized hypertrophic mononeuropathy were noted. These findings suggest important fundamental differences in the pathogenesis of various forms of onion-bulb mononeuropathies.