What is acanthosis nigricans?
Acanthosis nigricans (AN) is a skin condition that can be recognized by the dark, thickened areas of skin it causes, primarily in the neck and underarm folds. This dermatosis is most often quite benign and associated with obesity, but it can also be a sign of an underlying disease such as a malignant tumor.
The appearance of darker, thicker, rougher and drier, but painless areas of skin is characteristic of acanthosis nigricans. Their color results from hyperpigmentation (increased melanin) and thickening from hyperkeratosis (increased keratinization). Wart-like growths may develop.
These spots can appear on all parts of the body, but they preferentially affect the folds of the skin on the neck, armpits, groin and genito-anal areas. They are less frequently observed on the knees, elbows, breasts and navel. An accurate diagnosis must rule out Addison’s disease, which causes similar spots.
The origins of the disease
Researchers suspect that acanthosis nigricans is a resistance reaction of the skin to excessive levels of insulin, the hormone produced by the pancreas that regulates blood glucose. Insulin resistance can be associated with a variety of disorders, including obesity and type 2 diabetes.
In its mild form, the most common and known as pseudoacanthosis nigricans, these are skin manifestations associated with obesity and reversible with weight loss. Medications may also be involved in some cases, such as growth hormones or certain oral contraceptives.
Acanthosis nigricans can also be the outward, visible sign of an underlying, silent disorder. This malignant form is fortunately much rarer because the causative disease often turns out to be an aggressive tumor: it is seen in 1 in 6,000 patients with cancer, most often affecting the gastrointestinal or genitourinary tract. The average life expectancy of a patient with malignant AN is reduced to a few years. (1)
Men and women are equally affected and acanthosis nigricans can appear at any age, but preferably in adulthood. Note that darker-skinned individuals are more frequently affected, with a prevalence of AN of 1-5% among whites and 13% among blacks. (1) This skin manifestation is seen in about half of adult individuals with severe obesity.
The disease is not contagious. There are familial cases of AN, with autosomal dominant inheritance (i.e., a person with AN has a 50% chance of passing the disease on to his or her children, both girls and boys).
Prevention and treatment
Treatment of mild AN is to reduce blood insulin levels with a proper diet, especially since AN can be a warning sign of diabetes. In any case, a dermatologist should be consulted if a darker, thicker area of skin appears. When AN appears in a person who is not overweight, comprehensive examinations should be performed to ensure that it is not related to the underlying presence of a tumor.
American Academy of Dermatology. Acanthosis nigricans: Overview. [Https://www.aad.org/public/diseases/color-problems/acanthosis-nigricans]
National Organization for rare Disorders (NORD). [Http://rarediseases.org/rare-diseases/acanthosis-nigricans/]