Acanthosis nigricans (AN) is a skin condition associated with dark, gray-brown to black velvety plaques. These occur especially in the folds of the skin, such as the back or side of the neck, armpits, abdomen, breasts and groin. The plaques can also occur elsewhere on the body, including on the knuckles, elbows, knees, scalp, eyelids, face and palms. The skin changes usually appear gradually over months to years. "Acanthosis” means thickening of the skin. “Nigricans" refers to the dark discoloration.

There are two major types of AN:

• Benign acanthosis nigricans. This type can be divided into three categories:
  
  
• Endocrine. Most commonly affiliated with type 2 diabetes (a condition characterized by the body’s inability to use the hormone insulin effectively) or insulin resistance, a condition that can lead to diabetes.
  
  
• Drug induced. Often results from the use of oral steroids, birth control pills or nicotinic acid (a cholesterol-lowering agent).
  
  
• Idiopathic. The cause is unknown.
  
  
• Malignant acanthosis nigricans. This type is rare and associated with cancerous internal tumors (abnormal growths). It is usually fast growing and has a sudden onset. It is often secondary to gastrointestinal or genitourinary cancers. It may also be associated with lymphomas.

Benign AN is harmless and is primarily a cosmetic concern that affects many people with diabetes. Almost 75 percent of children with type 2 diabetes have benign acanthosis nigricans, according to the American Diabetes Association (ADA). The prevalence of AN is expected to rise along with the soaring rate of childhood obesity and people with type 2 diabetes. Some people with AN inherit the condition, which tends to appear during childhood before plaques stabilize or improve over time.

Reports of the association between AN and insulin resistance and diabetes in children have prompted calls by some parents and diabetes groups to screen schoolchildren for AN. The U.S. Centers for Disease Control and Prevention (CDC) responded to the request, but denied the applicability of widespread school-based screening for AN as a reliable method of identifying children at risk for type 2 diabetes.

People who have symptoms of AN are urged to consult a physician. Although there is no particular treatment for AN, the skin discoloration often fades when the underlying condition that causes it is successfully treated.