Schwannoma: Another dermoscopic pitfall?

Sir,

A 34-year-old man presented with 6 months history of a painful lesion over the abdomen. There was no history of trauma, paresthesia, or loss of weight. Cutaneous examination revealed a solitary 5 × 5 mm light brown nodule over the left side of the abdomen [Figure 1]. On palpation, the nodule was firm, tender, and not fixed to the skin of the anterior abdominal wall. Clinically differential diagnoses of dermatofibroma, neurofibroma, and leiomyoma were made.

Figure 1:: Solitary nodule of size 5 mm×5 mm over the left side of abdomen

Dermoscopic examination of the nodule revealed a gray-brown structure-less area surrounded by the pigment network [Figure 2]. White dots corresponding to normal eccrine duct openings were also seen. Histopathological examination showed epidermal hyperplasia, increased basal layer hyperpigmentation, and a well-circumscribed capsulated dermal nodule [Figure 3]. Areas of different cellular densities and arrangement, forming Antoni A, Antoni B, and Verocay body, were evident [Figure 4A and B]. Immunohistochemistry (IHC) showed positivity for S100 [Figure 4C]. A diagnosis of schwannoma was made.

Figure 2:: Dermoscopy (HEINE DELTA 20^® non-polarized contact dermoscope, 10× magnification) showing gray-brown structure-less area surrounded by pigment network

Figure 3:: Histology showing epidermal acanthosis, increased basal pigmentation, and well-circumscribed encapsulated dermal tumor (H&E, 50×)

Figure 4:: A. Dermal nodule showing spindle-shaped cells forming Antoni A, Antoni B, and Verocay body (H&E, 10×). B. Basal layer hyperpigmentation (H & E, 5×). C. IHC analysis showing positivity for S100 (IHC, 10×)

Cutaneous schwannomas, also known as neurilemmomas, are benign, slow-growing tumors seen along peripheral or cranial nerves. These encapsulated tumors originate from Schwann cells, which are responsible for myelination of peripheral nerves, and have a predilection for developing along sensory nerves.^[1] They are commonly found in the head and neck region and flexor surface of extremities owing to the higher concentration of nerve fibers over these areas.^[2] Schwannomas are usually detected in the fourth to fifth decade of life and have no sexual predilection. The majority of tumors are solitary and sporadic. However, multiple schwannomas may be present in patients with neurofibromatosis type I, schwannomatosis, and Carney complex.^[3] Cutaneous schwannomas are usually situated at the level of the deep dermis or subcutaneous tissue, although rarely they can be present in the upper dermis also. They are usually asymptomatic, but large tumors may compress the nerve of origin or adjacent structures, causing pain or dysesthesia. The patient may experience shooting paresthesia along the course of the nerve upon percussion of the tumor (Tinel’s sign) if it is present along a sensory nerve.^[2]

Histopathology of schwannoma is characterized by a thin fibrous capsule and the tumoral body is constituted by two histological patterns: Antoni A areas consisting of closely packed spindle cells arranged in compact fascicles with their nuclei arranged in palisades and Antoni B areas which are less cellular areas constituted by randomly arranged fusiform cells in a network of reticulated fibers and mucinous matrix. Verocay bodies are another characteristic feature seen in Antoni A areas, which are free bands of amorphous substance between the rows of nuclei. Based on the predominant features in histopathology, schwannomas can be classified into five variants: classic, cellular, plexiform, ancient (degenerated), and melanotic.^[3]

The gray-brown structure-less area in dermoscopy corresponds to increased epidermal pigmentation, and the pigment network corresponds to increased pigmentation of rete ridges. After extensive search, we could find four previous reports of dermoscopy of schwannoma in literature, and the common feature in all of them was the presence of brownish pigmentation. Non-specific erythema and normal pigmentary network were noticed in dermoscopy of an infraorbital schwannoma by Reyna-Rodríguez et al.^[4] The second report was that of a hemorrhagic schwannoma which demonstrated homogeneous brown areas and reddish-blue areas at the periphery with blue-white veil covering the lesion on dermoscopy.^[5] The remaining two reports state arborizing vessels as a prominent feature in dermoscopy of schwannoma in addition to the brown pigmentation.^[6,7] The presence of arborizing vessels should prompt us to consider basal cell carcinoma (BCC) and palisaded encapsulated neuroma (PEN) also in the dermoscopic differential diagnosis of schwannoma.^[8] Although differentiating the lesion from BCC may not be difficult due to the absence of other characteristic features such as blue-gray ovoid nests, globules, “wheel-spoke” areas, etc., it is tougher with PEN as arborizing vessels are the main consistent feature in its dermoscopy.

The absence of arborizing vessels in our patient may be because of the deeper location of the tumor. There are conflicting reports regarding the presence of vasculature in schwannoma. While, according to some authors, vasculature is usually not prominent in schwannomas, few others have described dilated and irregular vessels in the histopathology.^[1] Also, in long-standing lesions such as ancient schwannomas, vascular sclerosis is known to occur as part of degenerative changes.

IHC can confirm the diagnosis as Schwann cells stain positive for S100, Leu 7(CD 57), collagen type IV, and SOX10, whereas the capsule will stain positive for epithelial membrane antigen. The treatment of choice in cutaneous schwannoma is local excision, and recurrence following complete excision is rare. Malignant transformation of schwannoma can occur very rarely in the form of malignant peripheral nerve sheath tumor or angiosarcoma.

Schwannoma is making to the list of lesions (seborrheic keratosis, dermatofibroma, leiomyoma, solar lentigo, and supernumerary accessory nipple) that defy the two-step algorithm in differentiating melanocytic and non-melanocytic lesions, by presenting with pigment network.^[9] The dermoscopic features described for different painful skin tumors that can mimic schwannoma are dermatofibroma (central white scar-like patch and peripheral pigment network, linear/irregular crypts, blue-gray homogeneous pigmentation, and erythematous homogeneous area),^[10] leiomyoma (pigment network and central scar-like area, oval and/or elongated hyperpigmented structures within the central area, white cloud-like area),^[11] eccrine spiradenoma (serpentine-branched vessels and blue clods),^[12] and glomus tumor (multicolored structure-less pattern with central purple area and peripheral whitish homogeneous area).^[13]

To sum up, more case reports/series are required to conclude about consistent dermoscopic features in schwannoma. Although dermoscopy gives clues in the diagnosis of cutaneous schwannoma, biopsy and IHC are mandatory to rule out the dermoscopic mimics like dermatofibroma and leiomyoma.

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Conflicts of interest

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