What is the diagnosis for a diabetic patient with a history of Deep Vein Thrombosis (DVT) and malignant melanoma, presenting with painful leg swelling and crustations?

Diagnosis: Necrobiosis Lipoidica Diabeticorum

The clinical presentation of a diabetic patient with painful leg swelling, "ugly limb," and crustations is most consistent with necrobiosis lipoidica diabeticorum (NLD), particularly when the lesions have progressed to ulceration. 1, 2

Clinical Reasoning

Key Diagnostic Features of NLD

• NLD characteristically presents on the lower extremities as erythematous papules and plaques that can progress to confluent ulcers with prominent granulation tissue and an orange-yellow hue 1
• The condition affects approximately 0.3% of diabetic patients and is a chronic granulomatous skin disorder, with the legs being the most common site 2
• The "ugly limb" appearance with crustations strongly suggests ulcerative NLD, which develops when the characteristic plaques break down 1, 2

Why Not Hemosiderin Deposition?

• Hemosiderin deposition (from chronic venous insufficiency or prior DVT) typically presents as brownish discoloration without the painful ulcerative crustations described 3
• While this patient has a history of DVT, hemosiderin staining alone would not explain the acute painful swelling and crusted ulcerations 3
• The clinical picture described goes beyond simple pigmentary changes to include active tissue breakdown 1

Critical Differential Consideration

Malignant Melanoma Must Be Excluded

• Given this patient's history of malignant melanoma excision, a biopsy is mandatory to exclude melanoma recurrence or new primary melanoma masquerading as a diabetic foot complication 4, 5, 6
• Acral lentiginous melanoma and amelanotic melanoma are frequently misdiagnosed as diabetic foot ulcers due to atypical clinical morphologies 5, 6
• Any non-healing ulcer in a diabetic patient with a history of melanoma should undergo biopsy, even when clinical suspicion is low 5, 6

Diagnostic Approach

Immediate Steps Required

• Perform incisional biopsy of the lesion to confirm NLD and definitively exclude malignant melanoma 1, 5, 6
• The biopsy should show granulomatous inflammation with necrobiosis and collagen degeneration characteristic of NLD 1, 2
• Assess for signs of infection, as diabetic foot infections can complicate any wound and require evaluation for erythema, warmth, tenderness, pain, induration, or purulent secretions 3

Additional Evaluation

• Evaluate the affected limb for arterial ischemia and venous insufficiency, as these can complicate wound healing 3
• Assess glycemic control, as improved glucose management may help with lesion resolution 2
• Consider imaging to exclude recurrent DVT if there is significant unilateral swelling, though the crusted ulcerative appearance points more toward NLD 3

Common Pitfalls to Avoid

• Failing to biopsy atypical or non-healing ulcers in diabetic patients with melanoma history can lead to catastrophic delays in cancer diagnosis 4, 5, 6
• Assuming all leg lesions in diabetics are simply diabetic complications without considering other serious pathology 6
• Delaying biopsy while attempting empiric wound care, which can allow malignancy to progress if present 5