It is imperative to recognize the presence of nail unit tumors early because of the risk for permanent nail plate dystrophy and the possibility of a malignant tumor.4,5 Subungual onycholemmal cysts may present with a wide spectrum of clinical findings including marked subungual hyperkeratosis, onychodystrophy, ridging, nail bed pigmentation, clubbing, thickening, or less often a normal-appearing nail. Based on reported cases, several trends are evident. Although nail dystrophy is most often asymptomatic, pain is not uncommon.5,6 It most commonly involves single digits, predominantly thumbs and great toenails.7,8 This predilection suggests that trauma or other local factors may be involved in its pathogenesis. Of note, trauma to the nail may occur years before the development of the lesions or it may not be recalled at all.
Diagnosis requires a degree of clinical suspicion and a nail bed biopsy with partial or total nail plate avulsion to visualize the pathologic portion of the nail bed. Because surgical intervention may lead to the implantation of epithelium, recurrences after nail biopsy or excision may occur.
In contrast to epidermal inclusion cysts arising in the skin, most SOCs do not have a granular layer.9 Hair and nails represent analogous differentiation products of the ectoderm. The nail matrix is homologous to portions of the hair matrix, while the nail bed epithelium is comparable to the outer root sheath of the hair follicle.7 Subungual onycholemmal cysts originate from the nail bed epithelium, which keratinizes in the absence of a granular layer, similar to the follicular isthmus outer root sheath. Thus, SOCs are comparable to the outer root sheath–derived isthmus-catagen cysts because of their abrupt central keratinization.8
Subungual onycholemmal cysts also must be distinguished from slowly growing malignant tumors of the nail bed epithelium, referred to as onycholemmal carcinomas by Alessi et al.10 This entity characteristically presents in elderly patients as a slowly growing, circumscribed, subungual discoloration that may ulcerate, destroying the nail apparatus and penetrating the phalangeal bone. On histopathology, it is characterized by small cysts filled with eosinophilic keratin devoid of a granular layer and lined by atypical squamous epithelium accompanied by solid nests and strands of atypical keratinocytes within the dermis.11 When a cystic component and clear cells predominate, the designation of malignant proliferating onycholemmal cyst has been applied. Its infiltrative growth pattern with destruction of the underlying bone makes it an important entity to exclude when considering the differential diagnosis of tumors of the nail bed.
Subungual melanomas comprise only 1% to 3% of malignant melanomas and 85% are initially misdiagnosed due to their rarity and nonspecific variable presentation. Aside from clinical evidence of Hutchinson sign in the early stages in almost all cases, accurate diagnosis of subungual melanoma and differentiation from SOCs relies on histopathology. A biopsy is necessary to make the diagnosis, but even microscopic findings may be nonspecific during the early stages.
We report a case of a 23-year-old woman with horizontal ridging and tenderness of the right great toenail associated with pigmentation of 5 years’ duration due to an SOC. The etiology of these subungual cysts, with or without nail abnormalities, still remains unclear. Its predilection for the thumbs and great toenails suggests that trauma or other local factors may be involved in its pathogenesis. Because of the rarity of this entity, there are no guidelines for surgical treatment. Subungual onycholemmal cysts may be an underrecognized and more common entity that must be considered when discussing tumors of the nail unit.