Keys To Diagnosing Metastatic Melanoma In The Foot And Ankle

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Christopher Witt, DPM, and Steven Geary, DPM

Given that melanoma in the foot is commonly misdiagnosed, these authors present the case of a 45-year-old with a history of multiple melanotic lesions and emphasize the importance of early diagnosis.

A review of the literature reveals that melanoma is the sixth most common type of cancer in the United States and we commonly misdiagnose it in the foot and ankle.1 Melanoma can arise from a preexisting nevus or develop de novo on the skin. The lifetime risk for men developing the disease is 1:120 and the risk is 1:95 for women.2

   Foot and ankle lesions represent approximately 3 to 15 percent of all cutaneous melanomas.2 In a review of 63 cases, 29 percent of lesions were on the dorsum of the foot, 19 percent on the heel, 17 percent on the plantar aspect of the foot, 17 percent on the ankle and 13 percent at the digits.3 Forty-seven of these patients were women and 16 were men.

   There are four major types of melanoma. Superficial spreading melanoma is the most common and most frequently occurs on the dorsum of the foot.2 Acral lentiginous melanoma is the only type that arises equally across all skin types, representing about half of the melanoma occurring on the hands and feet.2 Nodular melanoma is the most frequently occurring melanoma in the older population. Lentigo maligna melanoma occurs almost exclusively on the face and neck due to sun damage.2

   There are also two types of nail unit melanoma, longitudinal melanonychia and amelanotic. Nail unit melanoma has the worst prognosis of all melanomas and represents 1.4 percent of melanomas.2 The five-year survival rate is 88 percent with Breslow thickness less than 2.5 mm and this decreases to 44 percent when the Breslow thickness is greater than 2.5 mm.2

Pertinent Guidelines For Diagnosing Melanoma

It can be difficult for one to distinguish between a nail unit melanoma and a dystrophic nail. One of the principles of nail unit anatomy is that melanocytes only occur on the matrix and nail folds, not the nail bed. This means that if there is a pigment change in only the nail bed, it is not a melanoma. A source of pigment that is clear proximally is usually a hematoma. A pigmented area that is present only within the nail bed and has a normal nail matrix and nail fold is also not likely to be melanoma.

   The shape of the change in pigment can also be helpful in distinguishing a melanoma from a hematoma. Pools of irregular shaped pigment are most likely a hematoma whereas a well-organized linear band with consistent width that begins in the matrix is likely to be a melanonychia. In dark-skinned patients, hyperpigmentation is more likely to occur in the nail unit with an increase in age. The more common causes for this are trauma, fungus and lichen planus.

   One of the classic clinical findings for nail unit melanoma is Hutchinson’s sign. A true Hutchinson's sign is when pigment is visible on the nail fold or distal pulp of the digit. This usually results in a poorer prognosis for the individual. We often misdiagnose this as a pseudo Hutchinson’s sign, which is pigment visible in the clear edges of the nail fold as it becomes the cuticle. Another rule of thumb is that if the pigment becomes wider more proximally, this is also most likely melanotic.2

   The common misdiagnoses for melanoma of the foot and ankle include paronychia, subungual hematoma, pyogenic granuloma, neuropathic ulceration and tinea pedis. There are multiple mnemonics that we can use as tools to help determine the probability of melanoma.

   One mnemonic is ABCDE, which has been in use for years for melanoma detection for the entire body. It stands for:2

• Asymmetry
• Borders of irregularity
• Colors
• Diameter greater than 6 mm
• Evolution

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