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.
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.
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.