|Classification and external resources|
Acanthosis nigricans on axilla
Acanthosis nigricans is a brown to black, poorly defined, velvety hyperpigmentation of the skin. It is usually found in body folds, such as the posterior and lateral folds of the neck, the axilla, groin, umbilicus, forehead, and other areas.
It typically occurs in individuals younger than age 40, may be genetically inherited, and is associated with obesity or endocrinopathies, such as hypothyroidism, acromegaly, polycystic ovary disease, insulin-resistant diabetes, or Cushing's disease. Not associated with hyperthyroidism.
The most common cause of acanthosis nigricans is insulin resistance, which leads to increased circulating insulin levels. Insulin spillover into the skin results in its abnormal increase in growth (hyperplasia of the skin). The high insulin concentrations probably activate growth factor receptors (perhaps the IGF1 receptor) that drive the proliferation of keratinocytes and melanocytes. The condition most commonly associated with insulin resistance is type 2 diabetes mellitus, but is also a prominent feature of obesity, polycystic ovary syndrome, Donohue syndrome, and Rabson-Mendenhall syndrome. Acanthosis nigricans may also be seen with certain medications that lead to elevated insulin levels (e.g., glucocorticoids, niacin, insulin, oral contraceptives, and protease inhibitors).
In the context of a malignant disease, acanthosis nigricans is a paraneoplastic syndrome and is then commonly referred to as acanthosis nigricans maligna. Involvement of mucous membranes is rare and suggests a coexisting malignant condition.
When seen in individuals older than age 40, this disorder is commonly associated with an internal malignancy, usually adenocarcinoma, and most commonly of the GI tract or uterus; less commonly of the lung, prostate, breast, or ovary. The stomach is the most common site. Acanthosis nigricans of the oral mucosa or tongue is highly suggestive of a neoplasm, especially of the GI tract.
- Benign acanthosis nigricans
- Acanthosis nigricans associated with obesity
- Syndromic acanthosis nigricans
- Malignant acanthosis nigricans
- Acral acanthosis nigricans
- Unilateral acanthosis nigricans
- Drug-induced acanthosis nigricans
- Mixed acanthosis nigricans
Other Classifications of Acanthosis nigricans 
Acanthosis nigricans may also be divided into the following types:506:
- Acral Acanthotic Anomaly (Acral Acanthosis Nigricans)
- Acanthosis nigricans type I
- Acanthosis nigricans type II
- Acanthosis nigricans type III
Signs, tests, and misdiagnoses 
Physicians can usually diagnose acanthosis nigricans by simply looking at a patient's skin. A skin biopsy may be needed in unusual cases. If no clear cause of acanthosis nigricans is obvious, it may be necessary to search for one. Blood tests, an endoscopy, or x-rays may be required to eliminate the possibility of diabetes or cancer as the cause.
Additionally acanthosis nigricans has similar visual characteristics (neck discoloration) with Casal collar, which is a symptom of pellegra (a nutrient deficiency disease, easily remedied with supplementation). In early stages of discoloration, it is hard for a non trained eye to distinguish one from the other.
People with acanthosis nigricans should be screened for diabetes and, although rare, cancer. Controlling blood glucose levels through exercise and diet often improves symptoms. Acanthosis nigricans maligna may resolve if the causative tumor is successfully removed.
Acanthosis nigricans often fades if the underlying cause can be determined and treated appropriately.
- "acanthosis nigricans" at Dorland's Medical Dictionary
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- James, William; Berger, Timothy; Elston, Dirk (2005). Andrews' Diseases of the Skin: Clinical Dermatology. (10th ed.). Saunders. ISBN 0-7216-2921-0.
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