Acanthosis Nigricans
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Direct answer
Velvety, symmetric, hyperpigmented thickening of the flexures — neck, axillae, groins — is acanthosis nigricans, and in nine of ten Indian clinic patients it externalises insulin resistance, making it the skin's billboard for prediabetes, obesity, polycystic ovarian syndrome and type 2 diabetes. Beyond the common obesity-associated form live the endocrine variants (type A genetic insulin-receptor syndromes such as Rabson-Mendenhall, type B autoantibodies against the receptor in lupus), drug-induced disease (nicotinic acid, systemic steroids, oral contraceptives, growth hormone) and the malignant form — sudden-onset, rapidly spreading, intensely pruritic acanthosis with tripe palms in an older adult, pointing to gastric adenocarcinoma above all. Treatment is the cause: weight, glycaemia and metformin first, retinoids and lasers for cosmesis second.
What you must remember
- Commonest cause: obesity-associated insulin resistance; hyperinsulinaemia drives IGF-1 receptor stimulation on keratinocytes and fibroblasts — the mechanism sentence every viva wants.
- Clinical feel: velvet to touch, dirty-looking hyperpigmentation with skin tags in the flexures; neck involvement brings patients in for "black neck" — the presenting complaint in Indian OPDs.
- Malignant red flags: age above 40, onset over months, rapid spread, severe pruritus, thickened palms (tripe hands), mucosal and mucocutaneous involvement, weight loss — search the upper gastrointestinal tract, where gastric adenocarcinoma dominates.
- Endocrine classifications: type A — young women with genetic insulin-receptor defects (Rabson-Mendenhall, Alstrom, Berardinelli-Seip lipodystrophy syndromes) with ovarian hyperandrogenism; type B — middle-aged women with autoantibodies to the insulin receptor, often with lupus or other autoimmune disease.
- Drug list: nicotinic acid (the classic), glucocorticoids, combined oral contraceptives, growth hormone, insulin itself at injection sites, and stilboestrol historically.
- Workup of the common form: fasting glucose, HbA1c, fasting insulin, lipid profile, and in women with hirsutism an androgen and ovarian ultrasound panel for PCOS.
- Separate entity: acral acanthotic anomaly — benign darkening of the palms and dorsa of hands in older adults with no insulin resistance — commonly mistaken, examinable.
- Therapy reality: no cream erases it; weight loss plus metformin softens it over months; keratolytics, topical retinoids and lasers offer cosmetic fringe benefit.
Two patients, two utterly different meanings
A 19-year-old engineering student presents with her mother, worried about a "black collar" that will not wash off; her body mass index is 31, the velvet plaques cover the nape and both axillae, and skin tags stud the fringes. The conversation pivots from cosmetic to cardiovascular: fasting glucose 112 mg/dL, HbA1c 6.2 per cent — prediabetes — fasting insulin high, lipids borderline. Her prescription is lifestyle plus metformin, with the explicit prediction that the neck will lighten only as insulin falls, over six to twelve months. Contrast a 58-year-old lean man with three months of explosive acanthosis, itching that keeps him awake, palms like tripe leather and 8 kg of weight loss. Here the flexures are a metastatic rumour of the stomach: upper gastrointestinal endoscopy is arranged as the "biopsy of the skin", gastric adenocarcinoma confirmed, and the dermatology team's job becomes tumour-directed therapy plus comfort, because the skin follows the cancer's curve. Same physical sign, opposite prognoses — that contrast is the entire teaching of this disease.
Where candidates slip
The reflex error is treating every acanthosis nigricans as benign obesity stigmata: the malignant form is the exam's deliberate trap, signalled by the words sudden, pruritic, rapidly progressive, acral or mucosal, and lean adult. The second slip is the mechanism: candidates say "pigment deposition" — histologically the darkness is hyperkeratosis and papillomatosis with only mild basal hyperpigmentation, which is why it feels velvet and returns after laser without metabolic correction. Third, the PCOS link: a young woman with acanthosis, hirsutism and irregular cycles should trigger an androgen and ultrasound workup, not just a dermatology prescription. And know acral acanthotic anomaly as the differential for isolated palm darkening, so a benign elderly finding is not escalated into a cancer hunt.
Frequently asked questions
Which malignancy is most associated with malignant acanthosis nigricans?
Gastric adenocarcinoma, with other abdominal malignancies trailing; the skin may precede, accompany or follow tumour detection.
What drives obesity-associated acanthosis nigricans?
Hyperinsulinaemia crossing over to stimulate IGF-1 receptors on keratinocytes and dermal fibroblasts, producing hyperkeratosis and papillomatosis.
Which features separate malignant from benign acanthosis?
Sudden onset after age 40, rapid progression, severe pruritus, tripe palms, mucosal involvement and weight loss in a non-obese adult.
Which drugs can induce acanthosis nigricans?
Nicotinic acid classically, plus systemic corticosteroids, oral contraceptives, growth hormone and high-dose stilboestrol historically.
What is acral acanthotic anomaly?
A benign, acquired velvety darkening of the palms and dorsal hands in older adults, unlinked to insulin resistance or malignancy.