Longitudinal Melanonychia as the Only Sign of in Situ Squamous Cell Carcinoma

Case Presentation

A 39-year-old man was consulted because of a 2-year history of a pigmented lesion in his right thumbnail (Figure 1A). His personal history was remarkable for previous genital human papillomavirus (HPV) infection. Onychoscopy revealed a pigmented longitudinal melanonychia that affected the cuticle to the distal edge of the nail plate with no other signs or symptoms (Figure 1B). Complete excision of the lesion with narrow margins was performed because of a suspicion of malignancy. Histological examination revealed in situ squamous cell carcinoma (SCC).

Figure 1.

(A) Clinical image of the pigmented lesion in the right thumb nail. (B) Onychoscopy of the lesion demonstrates a browndark, homogeneous, pigmented, longitudinal melanonychia. Hutchison and micro-Hutchinson signs absent.

Teaching Point

SCC and in situ SCC are the most common malignant tumors of the nail [1]. This disease is often misdiagnosed with other common infectious or inflammatory nail diseases due to its multiple clinical presentations. Melanonychia has always been a challenging sign for dermatologists. Longitudinal melanonychia is often associated with benign or malignant melanocytic proliferation, and nail unit melanoma (NUM) should always be ruled out [2]; therefore, dermoscopy of the nail apparatus (onychoscopy) is an indispensable tool in the evaluation of pigmented nail lesions. Any unexplained melanonychia of a single digit in white-skinned individuals should be biopsied to rule out NUM; however, this case demonstrates that nail unit in situ SCC can also present exclusively as longitudinal melanonychia with no other deformity or periungual lesion and should always be included in the differential diagnosis when longitudinal melanonychia is present.