palisaded encapsulated neuroma
Palisaded encapsulated neuroma of the skin, also referred to as solitary circumscribed neuroma, is a benign cutaneous tumor of the skin that clinically presents as a firm, flesh-colored, dome-shaped papule with loss of overlying skin markings.
The lesions typically measure between 2 and 6 mm and have slowly enlarged over several years.
Although hairs, ulcerations, and telangiectasias are typically absent or minimal, our patient presented with a single telangiectasia. Such dilated capillaries and venules have been reported to occur in PENs with recurrent trauma, such as in shaving.
While PENs overwhelmingly occur on the face particularly at mucocutaneous junctions, reports have described these lesions occurring on the eyelid, oral mucosa, nasal fossa, shoulder, arm, hand, foot, and glans penis.
The clinical differential diagnosis of PEN includes neurofibroma, basal cell carcinoma (BCC), melanocytic nevus, epidermal cyst, and skin appendage tumor.
Features that distinguish PEN from BCC are the longer presence and relative lack of telangiectasias and ulceration of the PEN.
Histopathologic examination of the PEN reveals a solitary, well circumscribed, partially encapsulated, intradermal nodule comprised of interdigitating spindle cells grouped in distinct fascicles. Additionally, there is a lack of nuclear pleomorphism and mitoses.
Although the nodular growth pattern occurs most frequently, others include epithelioid, plexiform, multinodular and fungating. Immunohistochemical analyses demonstrate positive tumor cell staining for S100 protein, collagen type IV and vimentin characteristic of Schwann cell differentiation.
The capsule is comprised of elongated perineural cells that stain positively for epithelial membrane antigen (EMA). Based on histologic similarities, the differential diagnosis includes neurofibroma, schwannoma, traumatic neuroma and leiomyoma.
Dissimilar to PEN however, neurofibromas that are associated with Neurofibromatosis 1 lack a capsule, contain mucopolysaccharide ground substance and possess fewer ensheathed axons.
Schwannomas are usually subcutaneous, contain Antoni A and B type tissue with Verocay bodies and lack axons. Although traumatic neuromas display abundant Schwann cells and axons, inflammatory cells and scaring are also visualized. Leiomyomas are distinguished by the presence of muscle cells in the tumor and positive staining for vimentin.
The proposed pathophysiology of the PEN focuses on a hamartomatous growth of Schwann cells predominating over the number of axons.
However, the stimulus that triggers these Schwann cells to proliferate has not been identified. Most important is the recognition and biopsy of these lesions such that a definitive diagnosis can be made.
In the case of the PEN, patients can be reassured that excision is curative with a low probability of recurrence, which alleviates unnecessary further testing or concern for systemic disease or malignancy.
While neurofibromas have been associated with Neurofibromatosis 1 and underlying neurofibrosarcoma, mucosal neuromas are linked with the tumors of multiple endocrine neoplasia II.
To date, there are no reports of PENs in patients with Fragile X, and thus we do not believe that the PEN is part of the Fragile X syndrome.
However, this case highlights that well circumscribed solitary papules, particularly near mucocutaneous junctions, necessitate definitive histologic diagnosis because they can mimic tumors with underlying systemic disease or malignant potential.
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