Acanthosis nigricans

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.

Causes
It typically occurs in individuals younger than age 40, may be genetically inherited, and is associated with obesity or endocrinopathies, such as hypothyroidism or hyperthyroidism, acromegaly, polycystic ovary disease, insulin-resistant diabetes, or Cushing's disease.

Endocrine
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 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).

Malignant
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.

Other
Other causes of acanthosis nigricans are familial, drug-induced and idiopathic.

Types
=== Classification of Acanthosis nigricans according to Schwartz, 1994 ===
 * Benign acanthosis nigricans
 * Acanthosis nigricans associated to 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 :
 * 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.

Treatment
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.

Prognosis
Acanthosis nigricans often fades if the underlying cause can be determined and treated appropriately.