On Pseudo-Onion Bulb Intraneural Proliferations of the Non-Major Nerves of the Oral Mucosa

Abstract

Perineurial cells (PCs) participate in reactive and neoplastic processes, of the latter pure perineurial being intraneural (IP) and soft tissue perineuriomas with oral examples being reported in both. In our review of over 500 peripheral nerve sheath tumors including granular cell tumor, we identified a single ostensible case of IP occurring on the tongue of a 45-year-old African-American male that was characterized by classic perineurial pseudo-onion bulbs (PsOb), proliferating PCs among these PsOb, sclerosis apparently due to long term duration and a plexiform pattern. We have also encountered 37 examples of apparently reactive, hyperplastic or traumatic, PsOb intraneural pseudoperineuriomatous proliferation (IPP) simulating microscopically some of the properties of IP. The majority of the lesions occurred in women and close to 80 % affected the tongue. Three microscopic patterns were appreciated. Type I lesions were those where IPP was seen only focally, type II where it was seen in roughly half of the lesion, and type III where the majority of the lesional tissue or the lesion itself was characterized by IPP. Immunohistochemically, IPP featured PsOb with generally a single layer of PCs decorated by epithelial membrane antigen, glut-1 or claudin-1, and decreased numbers of S-100 positive Schwann cells. The number of axons was not apparently altered. A prominent collagenous intraneural component was occasionally evident among PsOb and the affected nerve featured discontinuous or absent perineurial envelop. While type I and II IPP can be distinguished from IP, the distinction from type III lesions can be problematic. However, the discontinuity of the perineurium of the affected nerve, the spacing and collagenization among PsOb, the limited perineurial cell layer defining the pseudo-onion bulbs, the absence of proliferating PCs between PsObs and the decreasing number of Schwann cells may be of help in the distinction from IP.

Intraoral Pseudo-Onion Bulb Intraneural Proliferations in a Patient with Hemimandibular Hyperplasia: A Case Report and Review of the Literature

“…Clinically, these lesions appear nodular or polypoidsimilar to fibrous hyperplasia [11]. The lesions in our case were fibrotic and had a polypoid, warty appearance.…”

“…In their case series, Koutlas and Scheithauer [11] found 37 examples of intraneural pseudoperineuriomatous proliferations, and all of the lesions reported were solitary lesions. Clinically, these lesions appear nodular or polypoidsimilar to fibrous hyperplasia [11].…”

“…The lesions in our case were fibrotic and had a polypoid, warty appearance. The histologic criteria that Koutlas and Scheithauer [11] used to make the diagnosis included pseudo-onion bulbs that were characterized by a concentric arrangement of one or rarely two spindle cells with indistinct cytoplasmic borders around a residual nerve fiber, varying degrees of intrafascicular collagen deposition, usually a discontinuity or absence of the perineurial sheath, and varying degrees of stromal fibrous hyperplasia. Immunohistochemically, the lesions stain the same as true intraneural perineuriomas with EMA highlighting the perineurial cells of the pseudoonion bulb-one to two layers for this entity-and S-100 highlighting the residual Schwann cells.…”

“…Koutlas and Scheithauer [11] had proposed the term ''intraneural pseudoperineuriomatous proliferation'' to describe a reactive process mimicking a perineurioma. It is seen most commonly in women, and up to 80 % of cases involve the tongue [11]. The lower lip is the second most frequent site.…”

“…The lower lip is the second most frequent site. The preference for frequently traumatized sites such as the tongue and lower lip supports a traumatic rather than neoplastic etiology [11]. To date, chromosome 22 deletion status in these lesions has not been assessed.…”