Subungual dermal nerve sheath myxoma of the left thumb: Case report and literature review

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We report the case of a 59-year-old woman referred for evaluation of a progressively enlarging, painless subungual mass on the left thumb, which had been present for two years. She denied any prior trauma, infection or systemic symptoms. Clinical examination revealed longitudinal splitting affecting over half of the nail plate, caused by a tender, well-defined subungual nodule measuring approximately 1.5 cm at the distal nail margin [Figure 1a]. Onychoscopic evaluation showed linear vessels and whitish streaks on an erythematous-orange structureless background [Figures 1b and c]. Radiographic imaging excluded bone involvement or calcification. Surgical excision was performed to establish a definitive histopathological diagnosis. After application of a tourniquet and distal digital nerve block, the lesion was carefully dissected away from the ventral surface of the nail plate. A longitudinal elliptical incision extending from the distal nail matrix to the tip of the nail bed/hyponychium was made. Gross examination revealed a gelatinous, well-circumscribed mass. Histopathological evaluation demonstrated a multilobulated tumour composed of spindle-shaped and stellate cells embedded within abundant myxoid stroma. Immunohistochemical staining showed strong S100 positivity, confirming a neural origin [Figures 2a-d]. These findings supported the diagnosis of dermal nerve sheath myxoma (DNSM).

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Figure 1a:

Subungual mass covered by a nail plate with longitudinal splitting extending along its entire length (using non polarised HEINE DELTA 20 T dermatoscope (HEINE Optotechnik GmbH & Co. KG, Gilching, Germany) at 10× magnification).

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Figure 1b:

Anterior view showing streaks on a pinkish background (using non polarised HEINE DELTA 20 T dermatoscope (HEINE Optotechnik GmbH & Co. KG, Gilching, Germany) at 10× magnification).

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Figure 1c:

Onychoscopic examination reveals linear vessels and whitish streaks on an erythematous-orange structureless background (using non polarised HEINE DELTA 20 T dermatoscope (HEINE Optotechnik GmbH & Co. KG, Gilching, Germany) at 10× magnification).

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Figure 2a:

Multilobulated tumour composed of spindle-shaped and stellate cells embedded within abundant myxoid stroma (black arrow). (Haematoxylin and eosin, 10x)

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Figure 2b:

Dermis composed mainly of spindle cells (black arrow). (Haematoxylin and eosin, 40x)

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Figure 2c:

On immunohistochemistry, spindle cells showing S100 positivity (black arrow). (S-100, 100x).

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Figure 2d:

Alcian blue stain highlighting myxoid stroma (black arrow). (100x).

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DNSM is a rare, benign soft-tissue tumour of peripheral nerve origin, most commonly arising in the dermis or subcutaneous tissue.¹ Previously known as neurothekeoma, DNSM is now recognised as a distinct entity, characterised by a proliferation of schwannian cells embedded in a prominent myxoid matrix.² According to the latest classification by World Health Organisation (WHO), DNSM is differentiated from cellular neurothekeoma based on its histological and immunohistochemical features, particularly its strong S100 protein positivity.^3,4 While DNSM typically occurs on the head, neck and extremities, subungual presentation is exceptionally rare, posing significant diagnostic and therapeutic challenges.^2,5

Including this case, only five prior reports of subungual DNSM have been described in the literature [Table 1].^2,5-7 Notably, these earlier cases were diagnosed before the WHO reclassification and were labelled as myxoid neurothekeomas.^{2,4 7} Given their S100 positivity, they now fall under the DNSM category.^3,4 Across all reported cases, patients typically presented for dermatologic evaluation after long delays, with the shortest reported symptom duration being six months.^5-7 This delay is likely attributable to the slow, asymptomatic growth of the tumour. Pain was documented in only one case, possibly due to nerve compression or secondary inflammation.⁶ While trauma was suggested as a contributing factor in one report, its role remains uncertain.⁷

Preoperative imaging plays a key role in assessing bony involvement. Among the documented cases, radiographs were performed in three, with one showing a scalloped depression of the distal phalanx.^5,7 Magnetic resonance imaging used in one case, revealed a well-circumscribed, lobulated soft-tissue mass without bone infiltration.⁶ Based on clinical and radiographic features, differential diagnoses included myxoid cyst, neurofibroma, glomus tumour and subungual papilloma.^2,5,6 Histologically, all reported cases shared a multinodular architecture with spindle and rounded cells in a myxoid background, separated by fibrous septa. Immunohistochemical analysis consistently showed strong S100 positivity, supporting the schwannian origin.^1,3 Surgical excision has been the treatment of choice across all reported cases, with techniques adapted according to lesion size and location.^2,5,6 Despite marginal excision in some instances, no recurrences have been reported during follow-up periods ranging from 12 to 18 months.^1,5,7 Importantly, no cases of malignant transformation have been documented.^1,3,4

This case underscores the importance of considering DNSM in the differential diagnosis of painless subungual nodules. Surgical excision remains the mainstay of management, offering excellent long-term prognosis.^1,3,5 Early recognition, meticulous surgical planning and thorough histopathological evaluation are essential to achieving optimal outcomes and minimising unnecessary morbidity ^2,4,6