Acanthosis Nigricans

Acanthosis nigricans (AN) is a cutaneous marker of insulin resistance and, less commonly, malignancy, characterised by velvety hyperpigmented plaques in flexural areas.

Prevalence is up to 20 to 40% in obese adults, more in people of African, Hispanic, and Native American descent.

Types: obesity and insulin resistance (commonest), endocrine (PCOS, Cushing syndrome, acromegaly, hypothyroidism, lipodystrophy), syndromic (Type A and B insulin resistance), drug induced (nicotinic acid, oral contraceptives, glucocorticoids, insulin, growth hormone), hereditary benign, and malignant.

Malignant AN is associated with gastric adenocarcinoma (about 60%) and other adenocarcinomas, appearing abruptly, with extensive involvement and mucosal and palmar changes.

• Symmetric, hyperpigmented, velvety, papillomatous plaques with accentuated skin markings in the neck, axillae, groin, antecubital and popliteal fossae, and under the breasts.
• Skin tags, keratosis pilaris, and acrochordons often coexist.
• Malignant AN: rapidly progressive, pruritic, with oral papillomatosis, "tripe palms" (thickened rugose palms), and weight loss.
• Cosmetic disfigurement and odour in severe disease.

• Clinical diagnosis; biopsy if atypical or rapidly evolving (papillomatosis, hyperkeratosis, mild acanthosis, hyperpigmentation).
• Metabolic workup: fasting glucose, HbA1c, OGTT, fasting insulin, HOMA-IR, lipid profile, liver tests, blood pressure, and BMI.
• Endocrine evaluation if indicated: testosterone, DHEAS, 17-OH progesterone, TSH, cortisol, and IGF-1.
• If malignant AN is suspected: upper GI endoscopy, CT chest/abdomen/pelvis, tumour markers, mammography, and colonoscopy.
• Medication review.

• Treat underlying cause: weight loss (5 to 10% reduces severity), exercise, metformin 500 to 2000 mg daily, GLP-1 receptor agonists (e.g., semaglutide), treatment of PCOS, thyroid replacement, and stopping of causative drugs.
• Topical: tretinoin 0.025 to 0.1% cream nightly, urea 10 to 20%, salicylic acid 2 to 6%, lactic acid 12%, calcipotriol 0.005%, and mild steroid creams for irritation.
• Systemic or procedural: oral isotretinoin 20 to 40 mg daily, acitretin, laser (Q-switched, fractional CO2), and dermabrasion for refractory plaques.
• Antibacterial washes (benzoyl peroxide 5%) for odour.
• Malignant AN: treat the underlying cancer, with regression after tumour resection.
• Educate on cardiometabolic risk and screen children with obesity for diabetes.