Contributor: Roger E. McLendon and David S. Enterline
The neurofibroma is an infiltrative tumor of the peripheral nerves consisting of a mixture of cell types including Schwann cells, perineural-like cells, fibroblasts, and cells with intermediate features. The plexiform neurofibroma exhibits all of the above cells intermixed in a mucoid background resulting in a net like, or loosely woven, tumor of elongated spindle cells. The plexiform neurofibroma is largely confined to patients with neurofibromatosis type I and these individuals have a mutation of the NF1 gene, resulting in an abnormal neurofibromin protein. Plexiform neurofibromas may arise in any location along the peripheral nervous system. They most commonly present as a dermal tumor, but may also be found as a circumscribed mass in a peripheral nerve. They occasionally involve spinal roots but are virtually absent on cranial nerves. When found in a peripheral nerve, the tumor diffusely infiltrates the involved segment of nerve, precluding preservative surgery on the nerve. Because schwannomas grow as an expansile mass excluding axons, the presence of axons within a spindle cell tumor may be a useful clue to the diagnosis of neurofibroma. The plexiform neurofibroma is typically found in a subcutaneous location where it arises from small nerve twigs and diffusely spreads into the surrounding dermis and soft tissues entrapping dermal appendages. Cytologically the tumor cells exhibit abundant elongated eosinophilic cytoplasm containing nuclei that are oval to elongated with tapered ends. The cells are engulfed in a mucopolysaccharide ground substance that has an affinity for alcian blue. The hyalinized strands of collagen commonly encountered in this tumor have been likened to the appearance of "shredded carrots." Nerve twigs about the orbit may develop this tumor and give rise to tumors in such locations as the extraocular muscles, skin, and lacrimal glands. The optic nerve may rarely host one of these tumors; the tumor presumably arises from a dural nerve twig, given the lack of peripheral nerve elements in the cerebral vesicle derived optic nerve.