Schwann cell-onion bulb tumor of the trigeminal nerve: hyperplasia, dysplasia or neoplasia?

Onion bulbs are concentric lamellar structures formed by Schwann or perineurial cells, which may be seen in several generalized or localized diseases of the peripheral nerve. There is debate regarding the pathogenesis of localized tumefactions displaying these microscopic structures. We report the fifth case, to our knowledge, of a Schwann cell-onion bulb tumor, which arose in the trigeminal nerve of a child with an unclassifiable, probably distinct, neurocutaneous syndrome; we also provide evidence for a neoplastic or hamartomatous origin. Molecular studies failed to establish an abnormality in the NF1, NF2, PMP22, or Connexin 32 genes. Similar and previously reported cases are discussed, as well as other onion bulb-forming entities.