A 34-year-old man presented with a 2-year history of a slowly enlarging painless papule on the left side of his face. Apart from gentle shaving, there was no history of trauma to the area. He was otherwise in excellent health and had no history of other skin disease. There was no family history of similar lesions, neurofibromatosis, or multiple endocrine neoplasia (MEN) type 2B.
Physical examination revealed a firm, flesh-colored, smooth, dome-shaped papule superior to the mucocutaneous junction of the patient’s left lateral upper lip. The lesion measured 4 mm in diameter. The rest of the physical examination findings were normal. In particular, there were no clinically apparent features of neurofibromatosis or MEN 2B.
A biopsy of the lesion revealed a partially encapsulated, well-circumscribed papule composed of fascicles of small spindle cells with wavy slender nuclei interlaced with prominent clefts. Immunohistochemically, the majority of the tumor cells were positive for S100 protein, and staining for neurofilaments revealed numerous axons. Some cells in the periphery were positive for epithelial membrane antigen.
Palisaded encapsulated neuroma, also known as solitary circumscribed neuroma, is a distinctive variant of cutaneous neuroma characterized by a benign hamartomatous proliferation of the Schwann cells and axons of the cutaneous peripheral nerve fibers, in roughly equal ratios.1 The term palisaded encapsulated neuroma is derived from the histologic appearance of the lesion, which shows palisading Schwann cells surrounded by an incomplete capsule of perineural tissue.2 The condition was first described by Reed and colleagues in 1972.3
EPIDEMIOLOGY AND ETIOPATHOGENESIS
The vast majority of cases occur in middle-aged adults. The condition may, however, arise at any age, including during childhood.4 There is no sex predilection.5 The stimulus that triggers the hamartomatous proliferation of the Schwann cells and axons has not yet been identified. The occurrence of a palisaded encapsulated neuroma is usually sporadic.
Histologically, a palisaded encapsulated neuroma is a well-circumscribed, partially encapsulated, intradermal, spindle cell tumor.6 The capsule is characteristically absent near the superficial aspect where the nerve merges with the dermis.7 The tumor is composed of interdigitating Schwann cells with slender, tapered nuclei and axons grouped in distinct fascicles separated from one another by artifactual clefts.1 Schwann cell nuclei often have a palisaded appearance, especially those near the periphery of the lesion.6 Nuclear pleomorphism and mitosis are characteristically absent.8
Immunohistochemically, Schwann cells stain positively for S100 protein, type IV collagen, and vimentin, while perineural cells stain positively for epithelial membrane antigen.1,6,9 Axons can be identified with neurofilament staining.1,6,9
Typically, a palisaded encapsulated neuroma presents as an asymptomatic, solitary, pink or skin-colored, slow-growing, firm or rubbery, sessile papule or nodule. The size is usually 2 to 6 mm in diameter with 1 to 3 mm elevation.6,8,10 Characteristically, the lesion has no overlying hair, and there is loss of overlying skin markings.8,10 Telangiectasia is minimal or absent.
A palisaded encapsulated neuroma is most frequently found on the face near the margin of a mucocutaneous junction, as in the present case (Figure 1).1,5,10 Less than 10% of lesions occur elsewhere, such as on the trunk, shoulders, and acral sites (Figure 2).1,5 Rarely, mucosal sites such as the oral cavity, nasal mucosa, and glans penis may also be affected.9 While palisaded encapsulated neuromas are traditionally solitary, the occurrence of multiple lesions within the same patient also has been described.11
The diagnosis is usually suspected clinically based on typical physical features and is confirmed histologically based on the characteristic histologic findings.
The main differential diagnoses are traumatic neuroma, neurofibroma, and schwannoma. Other differential diagnoses include basal cell carcinoma, verruca vulgaris, dermatofibroma, intradermal melanocytic nevus, epidermal cyst, leiomyoma, and skin appendage tumor.1,10 A traumatic neuroma, also known as an amputation neuroma, is characterized by regenerative proliferations of nerve fibers secondary to extrinsic damage to the nerve fibers and can occur at any body site.1 Histologically, the nerve fascicles are arranged haphazardly and are separated by a fibrotic, sometimes inflamed, stroma.7
Typically, a cutaneous neurofibroma presents as a painless, soft, flesh-colored, nodular or pedunculated lesion that often invaginates into the skin and causes a “button-holing” when gentle digital pressure is applied to the surface.12 The presence of 2 or more neurofibromas forms a major diagnostic criterion for neurofibromatosis type 1. Histologically, a neurofibroma lacks a capsule and well-formed fascicles separated by clefts, contains a mucopolysaccharide substance, and has fewer axons and myelin sheaths.5,7,8,10
A schwannoma, also known as neurilemmoma, is a benign nerve-sheath tumor composed of Schwann cells. It is typically found subcutaneously, lacks axons and a thick collagenous capsule, and contains Antoni A and B tissue types and Verocay bodies.5,8,10 The axons are either absent or sparse and are confined to the periphery of the lesion.7
PROGNOSIS AND MANAGEMENT
A palisaded encapsulated neuroma is cosmetically unsightly and may have an adverse impact on quality of life. There is no known association between a palisaded encapsulated neuroma and neurofibromatosis or MEN 2B.1
The treatment of choice is surgical excision, which is considered curative.1,9 Untreated, the lesion tends to persist and may slowly enlarge. Recurrence is rare after surgical excision.