Diagnosis: Subungual melanoma
Due to the aggressive nature of the wound and the large soft-tissue defect, a plastic surgeon was consulted. Biopsy showed malignant melanoma, and the thumb was amputated at the base of the proximal phalanx. Pathology revealed a 3×3.5 cm ulceration at the distal portion of the specimen with malignant melanoma involving the skin, subcutaneous tissue, and bone marrow. Two of 3 axillary lymph nodes were also positive for metastatic malignant melanoma.
History and epidemiology
While public awareness of cutaneous melanoma has been increasing, subungual melanoma remains obscure. First described in 1834 by Alexis Boyer, surgeon to Napoleon, it was later characterized in 1886 by Sir Jonathan Hutchinson, who reported 6 cases of “melanotic whitlow.”1 Hutchinson reported that the lesion was usually first attributed to an injury, and because of this the diagnosis was nearly always missed in the early stages.
Today, subungual melanoma is often neglected by patients and frequently misdiagnosed by physicians. The estimated mean delay in diagnosis ranges from 3 to 24 months—nearly double the diagnostic delay observed with cutaneous melanoma.1,2
One study found 52% of subungual melanomas were mistaken for benign or traumatic lesions of the nail bed such as pyogenic granuloma, paronychia, onychomycosis, chronic infection, subungual hematoma, or pigmented nevus. This mistake is not surprising as these lesions are all in the differential diagnosis of a single pigmented nail streak, and all are far more common than subungual melanoma.
Another study found that two thirds of patients underwent some inappropriate surgical procedure before the correct diagnosis was considered.1 Because of its poor prognosis, often related to delay in diagnosis, maintain a high index of suspicion for subungual melanoma in the proper setting and a sound understanding of which lesions need to be biopsied.
Subungual melanoma disproportionately affects nonwhites. While the total number of cases of subungual melanoma accounts for only 0.3% to 3% of all new cases of malignant melanoma, one study found subungual melanoma accounted for up to 23% of all malignant melanomas in Japanese persons, 17% of malignant melanomas in Hong Kong Chinese persons, and 25 % of malignant melanomas in African Americans.1
The thumb and big toe are the most frequently involved regions, perhaps due to the larger proportion of nail matrix on these digits, with 55% of lesions arising on the hands, and more than half of those involving the thumb.2
Cause is unclear
The cause of subungual melanoma is thought to be different from that of cutaneous melanoma, but it remains unclear. Because the nail filters out UVB light and subungual melanoma most frequently arises from a portion of the nail matrix that is not sun-exposed, UV exposure is not thought to play the same role in pathogenesis of subungual melanoma as it clearly does in cutaneous melanoma.1
Since the time of Hutchinson’s original report there have been numerous case reports and several series that describe antecedent trauma in subungual melanoma; however, there has never been conclusive evidence that trauma is a causative factor.3,4