What is Acanthosis Nigricans?

Acanthosis Nigricans (AN) is a skin condition, characterised by the formation of dry, dark patches. These patches appear most often under the armpits and at the back of the neck and feel velvet-like to the touch. They can also develop in the groin, behind the knees and elbows and under the breasts; or anywhere that the skin forms folds or creases. Sometimes the patches also develop small growths or become itchy, but often they are asymptomatic. They are not contagious and not harmful. However, if they appear suddenly, or are accompanied by other symptoms, such as unexplained weight loss, you should see your doctor.

What causes Acanthosis Nigricans?

Obese people seem more likely to develop AN. This is most likely because insulin signaling pathways are defective in both conditions. Further evidence for this comes from the fact that those with type 2 diabetes mellitus (T2DM) and polycystic ovary syndrome (PCOS) are at increased risk of developing AN patches. Both T2DM and PCOS are strongly associated with insulin resistance. In fact, it is thought that AN could potentially be a warning sign of pre-diabetes. Obesity driven AN is also known as pseudoacanthosis nigricans; perspiration and friction can exacerbate the condition, particularly in areas where the skin is in folds). Other endocrine conditions, such as Cushing’s syndrome and thyroid conditions are also associated with the development of AN. 

Some medications can cause AN, including corticosteroids and the oral contraceptive pill.

In rare cases, AN can be a sign of cancer. This type of AN is known as the malignant form; it has a rapid onset, usually affecting people over the age of 40 (although in rare cases children have been affected too), and is often accompanied by other symptoms, such as sudden weight loss. Stomach cancer is the malignancy most commonly associated with AN (although it is still very rare); other cancer types that might manifest with AN patches are those of the colon and the liver, and the lymphomas.

Another rare cause of AN is familial inheritance. The condition is passed on in an autosomal dominant fashion, meaning that only one of your parents needs to have it for you to also inherit it. This type of AN develops gradually, worsening during puberty and in adolescence, before stabilising and improving in adulthood.

How common is Acanthosis Nigricans?

AN is common and, with obesity rates rising globally, it is likely to affect more people. There are racial differences, with Native Americans and African Americans much more likely to develop AN patches than those of European descent. The prevalence of AN in healthy caucasians with a normal BMI is thought to be less than 1%; however, the rate in Native Americans is between 30 and 70%, with higher rates seen in those with co-existing conditions and a high BMI.

More than 60% of children who are considered to be morbidly obese (BMI > 98th percentile) have AN, regardless of their race. As global obesity rates continue to increase, this clearly demonstrates why AN prevalence is going to rise, even amongst those populations/races not previously considered to be high risk. 

How is Acanthosis Nigricans diagnosed and treated?

Diagnosis of AN is usually straight-forward; your doctor should be able to diagnose on sight. It may take longer to identify the underlying cause of your AN, particularly if there is no genetic component.

There are limited options for specific treatment of the patches; most doctors will attempt to first of all establish and treat the cause of your AN. Over time this will result in skin improvements and the patches should start to fade.

If obesity is thought to be the trigger, lifestyle changes should be implemented with a view to losing weight. If medications are causing the patches, your doctor will probably try different options, to find an approach that you are better able to tolerate. If your AN is found to have a malignant basis, you will be treated by an oncologist.

One option that has generated some interest is the use of retinoids. These are vitamin A-like compounds, found to improve the appearance of skin affected by AN. Retinoids are used to treat a number of skin complaints, including acne and psoriasis, as they regulate cell growth, including the growth and differentiation of the cells that form the outer layer of the skin (the stratum corneum). Topical retinoids are likely to have fewer adverse effects than those taken orally.

Other possible treatment options include:

• Vitamin D
• Laser treatment
• Melatonin
• Metformin.

For a comprehensive review of treatment options used in the management of AN, click here. 

The outlook

In many cases, AN patches fade and disappear with time. Whilst, AN may cause cosmetic distress; in isolation, the patches are rarely harmful and are unlikely to manifest with any other symptoms. The condition can, however, serve a very valuable purpose, as it can be a warning sign or early indicator of a more severe underlying disease.

Children with AN are more likely to become obese and/or experience insulin resistance. Identifying that you might have a predisposition to unhealthy weight gain means you can take steps to alleviate it before it becomes a medical issue. Furthermore, AN that appears rapidly and without warning can be a sign of cancer. It goes without saying that the earlier cancer is diagnosed and treatment initiated, the better the overall prognosis.  

If you notice any unusual skin changes it is advisable to speak to your doctor as soon as possible.  

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Sources:

• “Acanthosis Nigricans.” NHS Choices, NHS, www.nhs.uk/conditions/acanthosis-nigricans/.
• “Acanthosis Nigricans.” NORD (National Organization for Rare Disorders), rarediseases.org/rare-diseases/acanthosis-nigricans/.
• “Acanthosis Nigricans: Overview.” American Academy of Dermatology, www.aad.org/public/diseases/a-z/acanthosis-nigricans-overview.
• Brickman, Wendy J., et al. “Acanthosis Nigricans: A Common Finding in Overweight Youth.” Pediatric Dermatology, vol. 24, no. 6, 2007, pp. 601–606., doi:10.1111/j.1525-1470.2007.00547.x. 
• Higgins, Steven P et al. “Acanthosis nigricans: a practical approach to evaluation and management.” Dermatology online journal vol. 14,9 2. 15 Sep. 2008.
• Phiske, Meghanamadhukar. “An Approach to Acanthosis Nigricans.” Indian Dermatology Online Journal, vol. 5, no. 3, July-Sept 2014, pp. 239–249., doi:10.4103/2229-5178.137765.
• Yosipovitch, Gil, et al. “Obesity and the Skin: Skin Physiology and Skin Manifestations of Obesity.” Journal of the American Academy of Dermatology, vol. 56, no. 6, June 2007, pp. 901–916., doi:10.1016/j.jaad.2006.12.004.