Discussion
SOCs are a rare, usually asymptomatic, nail abnormalities which are diagnosed incidentally in the progress of investigating other diseases like melanomas. Previous studies have described the possibility of an association between SOC and trauma, clubbing, and subsequently cyst formation. Clubbing can affect the dermis of the nail by fibroblast proliferation which causes cyst formation. Age, sex, lesion site, differential diagnosis (macroscopic and microscopic), underlying diseases, and treatments are listed in Table 1 of previous cases in this field.
SOCs may have different clinical presentations, including onycholysis, onychodystrophy, pigmentation of the nail bed, ridging, and thickening. It most commonly affects single digits, mainly thumbs and great toenails, and pain is not uncommon (3).
The subungual epidermoid inclusions that Lewin first referred to as the follicular microcysts of the nail bed are bulbous proliferations of the extremities of rete ridges, occasionally with the development of microcysts (4). These microcysts seldom lose their attachment to the nail bed epithelium and appear superficially within the dermis. The production of homogeneous keratin without a granular layer characterizes the keratinization of this superficial epithelial inclusion. The cyst’s epithelium also mimics the follicular isthmus (4).
Onycholemmal is the new term that is being used in literature. It depicts a specific type of subungual tumor with a pattern of onycholemmal microcysts and trichilemmal keratinization (5). All of these results point to the presence of vestiges of follicular units in the nail bed epithelium. In contrast to palms and soles, the nail epithelium is an invagination of the dorsal epidermis overlaying the digit that includes a few hair germs, consistent with embryology. The clinical presentation in our index case included onychodystrophy and onycholysis on a nail of the second right finger with no history of recent trauma, pain, or bleeding. Subungual melanomas and onycholemmal carcinomas can mimic SOC presentations and nail bed biopsy is required for appropriate diagnosis (2).
On histopathology, onycholemmal cysts originate from the nail bed epithelium and are restored with eosinophilic keratin in the absence of a granular layer. The follicular isthmus outer root sheath is homologous to SOC and it is keratinized with no granular layer (6). The nail bed biopsy with partial or total nail avulsion is required for appropriate diagnosis.
The differential diagnosis of onycholemmal cyst include subungual keratoacanthoma. squamous cell carcinoma (SCC), verrucous carcinoma (VC), glumus tumor, subungual metastasis and onycholemmal carcinoma. The characteristic histological findings can help the exact diagnosis of these lesions.
There are no specific treatment recommendations (3). The biopsy from the affected area revealed multiple free-lying cysts within the dermis of the nail bed, and in the near region to the epithelium of the nail bed. The cysts were lined by the stratified squamous epithelium with no granular layer or any cellular atypia. The cysts included luminal onycholemmal keratin (3).
This report highlighted the variable clinical presentations of subungual onycholemmal cysts (SOC), which can mimic different nail malignancies, including subungual melanomas and onycholemmal carcinomas. Early diagnosis of SOC by nail biopsy can improve the treatment outcome.