Perineurial cell hypertrophic mononeuropathy manifesting as carpal tunnel syndrome.

A 42-year-old man showed signs and symptoms suggestive of carpal-tunnel syndrome, but EMG showed an isolated motor axon-loss lesion affecting the right median nerve distally. After the MRI revealed a mass in the median nerve, surgical exploration showed a diffusely swollen median motor branch. Biopsy showed a lesion with marked onion-bulb formation composed of perineurial cells as identified by immunohistochemical analyses and electron microscopic examination. Although we previously coined the term "perineurioma" for this condition, re-reviews of our cases do not support the idea that the onion-bulb lesion is a benign tumor; instead, it appears to be reactive hyperplasia. Although rare, electromyographers and neurologists need to be aware of this problem because it is self-limited and does not require surgical resection.