Acanthosis nigricans is the medical term for darkened, thickened patches of skin in the armpit and around the groin and neck.
It's not a condition in itself, but may be a sign of an underlying or internal condition. The underlying condition is usually not serious, although the skin changes can sometimes be a sign of early diabetes and, rarely, cancer.
What are the features?
Dark, velvety patches
If you have acanthosis nigricans, you'll have thickened, brown velvety patches of skin.
These patches may occur anywhere but are usually seen around the neck, in the armpit, around the groin and sometimes in other skin folds. The skin may be itchy.
Sometimes, the skin over the joints of the fingers and toes may be affected, as well as the skin on the lips, palms of the hands and soles of the feet. However, this is more common in elderly people whose acanthosis nigricans is a sign of cancer. This is called acanthosis nigricans maligna, and is very rare.
In rare cases of acanthosis nigricans that signal cancer, the patches are very aggressive and spread quickly. Patches tend to be more extensive and severe. In these cases the mouth, tongue, throat, nose and windpipe may 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.
What are the possible underlying causes?
The cause is uncertain, but acanthosis nigricans has been associated with a wide range of underlying conditions.
There are different types of acanthosis nigricans.
Obesity-associated acanthosis nigricans
This is the most common type. Obesity can cause 'insulin resistance'. This is when some of the cells in the body no longer respond to insulin.
Acanthosis nigricans can be a sign of insulin resistance. Because of the link between obesity and insulin resistance, acanthosis nigricans often occurs in people who are overweight and have diabetes or are at risk of developing diabetes.
Syndromic acanthosis nigricans (sign of an underlying syndrome)
Acanthosis nigricans may be associated with an internal underlying syndrome, such as Cushing's syndrome or polycystic ovary syndrome.
Cushing's syndrome is a group of symptoms (weight gain, bruising, stretch marks) caused by very high levels of the hormone cortisol in the body. Polycystic ovary syndrome is a condition that affects how a woman's ovaries work. It can lead to excessive body hair, irregular periods, infertility, acne and weight gain.
Ethical (racial) acanthosis nigricans
This type is commonly seen in dark-skinned people, especially those of African descent. Thick dark patches develop over the tops of hands and feet in people who are otherwise in good health.
This is a harmless (benign) form of acanthosis nigricans.
Drug-induced acanthosis nigricans
Acanthosis nigricans is sometimes triggered by medicines including insulin, corticosteroids and hormone treatments (such as human growth hormone or the contraceptive pill).
Malignant acanthosis nigricans (sign of cancer)
When 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.
In such rare cases the patches are more severe and the mouth, tongue and lips may also be affected.
This tends to affect elderly people who are not overweight and (in contrast to the benign form), malignant acanthosis nigricans has no link with ethnicity.
How is it diagnosed?
Your GP 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 level (to see if it's a sign of insulin resistance), and you may need an endoscopy or an X-ray.
How is it treated?
There is no specific treatment to get rid of the patches, but they should not cause you any trouble.
Treatment aims to correct the underlying cause of the skin condition. Usually, correcting this will cure the acanthosis nigricans.
Depending on the cause, you may be advised to lose weight or you may need to switch to a different medication, if this is triggering the acanthosis nigricans.
If it's caused by an underlying syndrome, you will need to have this treated. Read about the treatment of Cushing's syndrome and thetreatment of polycystic ovary syndrome.
In the hereditary form, the patches tend to gradually get bigger before staying the same or fading on their own.
What's the outcome?
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.
Only in cases of malignant acanthosis nigricans, where there is underlying cancer, is the situation serious – but if the tumour is successfully treated, the condition may disappear.
Acanthosis nigricans on the neck of a 13-year-old Mexican girl with obesity