Acanthosis nigricans is the medical term for darkened, thickened patches of skin that usually develop in the armpit and around the groin and neck.
It's not a condition in itself, but can be a sign of an underlying health problem. These underlying conditions aren't usually serious, although occasionally acanthosis nigricans can be sign of cancer.
Signs and symptoms
Dark, velvety patches
If you have acanthosis nigricans, you'll have thickened, brownish-grey or black patches of skin.
The patches will be dry and rough, feeling similar to velvet. They may also be itchy.
These patches may occur anywhere, but are usually seen around the neck, in the armpit, around the groin and sometimes in other skin folds. Occasionally, the skin over the joints of the fingers and toes may be affected, as well as the lips, palms of the hands and soles of the feet.
The patches usually develop slowly over time. Patches that grow and spread quickly are more likely to be associated with cancer. In these cases, the mouth, tongue, throat, nose and windpipe may also be affected.
Tiny growths on the skin
You may also have lots of tiny finger-like growths from the patches. This is known as papillomatosis.
There may also be skin tags around the affected area. These are small flesh-coloured or pigmented growths that hang off the skin and look a bit like warts.
When to see your GP
Acanthosis nigricans is usually harmless, but as it can be a sign of something serious it's a good idea to see your GP if you think you have it. They may suspect acanthosis nigricans just by looking at your skin.
If the cause isn't clear, you may have blood tests to check your blood sugar or hormone levels, and you might need additional tests such as an endoscopy or X-ray.
What causes acanthosis nigricans?
Acanthosis nigricans sometimes occurs in people who are otherwise in good health, particularly dark-skinned people of African descent.
However, in most cases it's a sign of an underlying problem or condition such as obesity, diabetes or abnormal hormone levels. Some of the main causes of acanthosis nigricans are outlined below.
Acanthosis nigricans is usually the result of obesity. This is known as obesity-associated acanthosis nigricans.
It occurs because obesity can cause insulin resistance (when the body is unable to properly use the hormone insulin), which may lead to high levels of insulin in the blood, affecting the skin cells.
Insulin resistance can also cause type 2 diabetes, so acanthosis nigricans can be an early sign that you have diabetes or are at risk of developing the condition.
Syndromes and hormone problems
Acanthosis nigricans is sometimes associated with an underlying syndrome or hormone problem, such as:
This type of acanthosis nigricans is known as syndromic acanthosis nigricans.
Acanthosis nigricans is sometimes triggered by medicines, including insulin, corticosteroids, and hormone treatments such as human growth hormone or the contraceptive pill.
This is known as drug-induced or medication-associated acanthosis nigricans.
In rare cases, acanthosis nigricans can be caused by a faulty gene inherited directly from your parents. This is known as familial or benign genetic acanthosis nigricans.
This type is usually passed on in an autosomal dominant pattern, which means it can be passed on if only one of your parents carries the faulty gene.
If the dark skin patches come on suddenly and spread quickly, it may be a sign you have cancer (usually stomach cancer). This is known as malignant acanthosis nigricans.
This is a rare condition that tends to affect middle-aged or elderly people, regardless of their weight or ethnic background.
The patches are more severe and the mouth, tongue and lips may also be affected. The skin may also become irritated and itchy.
How acanthosis nigricans is treated
Treatment for acanthosis nigricans aims to correct the underlying cause of your symptoms. This will usually cure the acanthosis nigricans or significantly improve the appearance of your skin.
Treatments that may be recommended include:
There's no specific treatment to get rid of the patches, but a dermatologist (skin specialist) may be able to recommend treatments such as creams or tablets that may help improve the look of your skin.
Most cases of acanthosis nigricans are harmless and not a sign of anything serious. The skin patches often fade with time as the underlying condition is treated.
If you have inherited acanthosis nigricans from your parents, your patches may gradually get bigger before staying the same or eventually fading on their own.
Only in cases where there is underlying cancer is the situation very serious. If the tumour is successfully treated, the condition may disappear, but unfortunately the types of cancer that cause acanthosis nigricans tend to spread quickly and a cure is often not possible.