Acanthosis Nigricans

1. Classification and Causes
   1. Type 1: Hereditary Benign Acanthosis Nigricans
      1. Idiopathic onset during childhood or Puberty
   2. Type 2: Benign Acanthosis due to Insulin Resistance
      1. Diabetes Mellitus
      2. Androgen Excess (Hyperandrogenism)
         1. Acromegaly (Gigantism)
         2. Cushing's Syndrome
         3. HAIR-AN Syndrome
      3. Hypogonadism
         1. Addison's Disease
         2. Hypothyroidism
   3. Type 3: Pseudo-Acanthosis Nigricans
      1. Obesity resulting in Insulin Resistance
      2. Seen in patients with darker pigmentation
      3. Specific findings
         1. Numerous Skin Tags in axilla and groin
         2. Velvety patch at inner upper thigh areas of chafing
   4. Type 4: Drug-induced Acanthosis Nigricans
      1. Nicotinic Acid (high dose)
      2. Glucocorticoids
      3. Diethylstilbestrol (DES)
      4. Oral Contraceptives (androgenic Progesterone)
      5. Growth Hormone therapy
   5. Type 5: Malignant Acanthosis Nigricans
      1. Causes
         1. Paraneoplastic tumors (Adenocarcinoma)
         2. Lymphoma
      2. Specific findings
         1. Palm and sole hyperkeratosis
         2. Lip vermilion border and Oral Mucosa involved
2. Signs
   1. Characteristics
      1. Hyperpigmentation patches (dirty-appearing)
      2. Velvety feel and appearance develops over time
   2. Distribution
      1. Nape of neck
      2. Axillae
      3. Groin
      4. Vulva and Perineum
      5. Antecubital fossa
      6. Knuckles
      7. Submammary region
      8. Umbilicus
3. Evaluation
   1. Consider malignancy
      1. Onset over age 35 years
      2. Non-obese patient
   2. Evaluate for cause of Insulin Resistance
   3. Evaluate for Hyperandrogenism
4. References
   1. Elmer (2001) Am Fam Physician 63(12):2385-90
   2. Fitzpatrick (1992) Atlas Dermatology, McGraw, p.732-3
   3. Hacker (1996) Postgrad Med 99(6):177-86