Subungual (Nail Bed) Melanoma
Subungual melanoma is a rare clinical entity, representing up to 3% of cases of melanoma in Caucasians but a higher proportion of melanomas (15–35%) in dark-skinned ethnic groups. Over 75% of Subungual melanomas involve either the great toe or the thumb. Early signs of this lesion include a darkening of the nail bed. Dark pigmentation of the posterior (rear) nail fold, Hutchinson’s sign, is a classic sign of Subungual melanoma. Early diagnosis of subungual melanoma is rare, with the prognosis of this lesion reflecting the most significantly advanced stage at diagnosis. Many patients with subungual melanoma report a recent history of trauma to the digit and attribute the lesion to a poorly healing wound. The differential diagnosis of subungual melanoma includes benign pigmented lesions of the nail bed mechanism (melanonychia striata), in addition to chronic bacterial fungal infection, and subungual hematoma.
A major challenge in the diagnosis of subungual melanoma is obtaining an adequate biopsy. Although nodular as well as amelanotic lesions do occur, the majority of subungual hematomas appear as a sharply demarcated blue-black to brown discoloration of the nail which does not involve the adjacent cuticle. A diagnosis of subungual hematoma may be confirmed by releasing the clotted blood through a large bore puncture or partial removal of the nail plate. Close visual inspection of the nail over a short period of time will reveal that the pigmentation advances distally with growth of the nail plate. Formal biopsy of the nail bed is performed under digital or regional anesthesia block in the office or outpatient setting. The nail plate is then carefully elevated from the nail bed and removed so that the proximal aspect of the lesion in question is visualized clearly. An elliptical incision in the nail bed down to the underlying periosteum (connective tissue surrounding the bone) is then performed, allowing for complete excisional biopsy of the lesion as well as primary closure of the defect with fine absorbable sutures. Larger defects may be repaired with nail bed flaps or skin grafting. Generous incisional biopsy through the central portion of pigmentation is performed for larger lesions not amenable to simple excision.
Melanoma in situ of the nail bed is treated with wide local excision. Negative surgical margins of at least 5 mm are optimal. The surgical defect may repaired with a local flap of skin or may require skin grafting.
Invasive subungual melanomas of the lower extremity are most easily treated with amputation of the toe. The appropriate surgical resection margin width of 1 cm for lesions with thicknesses less than 1.0 mm or 2 cm for lesions with thicknesses greater than or equal to 1.0 mm is achieved through complete amputation of the affected toe. Ray amputation (resection of a toe and the adjacent bone in the foot) may be performed for lesions extending into the web space. In the majority of patients, the resulting surgical defect is easily closed, heals well, and allows for normal ambulation without a specialized prosthesis or orthotics device, even when amputation of the great toe is required. For upper extremity (finger) subungual invasive melanomas, surgical treatment is more individualized. Amputation is performed through the joint closest to the lesion, which represents a more conservative and functionally superior approach to the more radical amputations performed in the recent past . Wound closure is achieved with a flap of volar (from the palm of the hand) tissue while ideally maintaining a margin of at least 1 cm of normal tissue. For subungual melanomas of the thumb, a reconstruction is performed by web space deepening using a Z-plasty (a surgical procedure for scar tissue repair), reducing the length of digit loss by approximately 50%.
Sentinel lymph node mapping and excision is appropriately performed for those patients with melanomas approximately >0.8 mm in Breslow thickness in the absence of clinically palpable regional nodes. This procedure is most accurate and best performed prior to amputation of the affected digit. Patients presenting with palpable nodal metastases in the absence of significant distant metastases undergo complete regional lymphadenectomy.