## Epidemiology of Acanthosis Nigricans **Key Point:** Acanthosis nigricans is a cutaneous manifestation of systemic disease, with insulin resistance being the most common cause, accounting for 90% of cases. ## Pathophysiology Insulin resistance leads to: 1. Elevated circulating insulin levels (hyperinsulinemia) 2. Stimulation of insulin-like growth factor (IGF) receptors on keratinocytes and fibroblasts 3. Epidermal hyperplasia and dermal fibrosis 4. Resulting velvety, hyperpigmented appearance ## Clinical Association with Metabolic Syndrome **High-Yield:** Acanthosis nigricans is strongly associated with: - Type 2 diabetes mellitus - Obesity - Polycystic ovary syndrome (PCOS) - Metabolic syndrome ## Malignancy-Associated Acanthosis Nigricans | Feature | Insulin-Resistant AN | Malignancy-Associated AN | |---------|----------------------|-------------------------| | Frequency | 90% of cases | 5–10% of cases | | Age of onset | Usually > 40 years | Often > 50 years, rapid onset | | Associated tumors | — | Gastric, lung, breast, lymphoma | | Prognosis | Chronic, benign | May regress with tumor treatment | **Clinical Pearl:** In a patient with metabolic risk factors (diabetes, obesity), insulin resistance is the default diagnosis. Malignancy should be suspected only if acanthosis nigricans appears suddenly in an older patient without metabolic disease or if other B symptoms are present. **Warning:** Do not automatically assume malignancy in every case of acanthosis nigricans—this is a common trap. The vast majority of cases are metabolic. ## Other Rare Causes - Thyroid dysfunction (hypothyroidism): can worsen insulin resistance but is not a primary cause - Addison's disease: causes hyperpigmentation but not acanthosis nigricans - Medications: corticosteroids, oral contraceptives (via insulin resistance) [cite:Fitzpatrick's Dermatology 9e Ch 73]
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