Acanthosis nigricans: causes, treatment and when to see a GP

The NHS says that once a GP knows what is causing acanthosis nigricans, they can recommend the best treatment, and the patches should fade over time once the cause is treated. There is no specific treatment for the patches themselves. A skin specialist (dermatologist) may be able to suggest treatments to improve their appearance, but finding and treating the cause is usually recommended first.

The Primary Care Dermatology Society (PCDS) says there is no reliably effective treatment and that the primary aim is to correct the underlying cause, which, if possible, can lead to an improvement in symptoms. It lists weight loss where appropriate, correcting hyperinsulinaemia (high insulin levels) through diet and medication including metformin, and stopping the causal medication if possible in drug-induced acanthosis nigricans.

For the skin itself, PCDS says topical calcipotriol or retinoids might be helpful in some cases, and that laser can be used for papillomatous areas (those with small, wart-like bumps). Suspected malignant acanthosis nigricans needs urgent referral.

The NHS says the most common cause is obesity, and lists type 2 diabetes, conditions that affect hormone levels (such as Cushing's syndrome, polycystic ovary syndrome or an underactive thyroid), and taking certain medicines, including steroids or hormone treatments like the contraceptive pill. Rarely it is caused by cancer, usually stomach cancer, or by a faulty gene inherited from your parents. Sometimes healthy people with no other conditions get it.

PCDS describes numerous causes that may occur alone or in combination. It says insulin resistance secondary to obesity is the most common cause, other causes of insulin resistance include PCOS, hereditary acanthosis nigricans can be inherited as an autosomal dominant trait, and drugs appear to be an uncommon cause, with systemic corticosteroids, nicotinic acid and stilboestrol among the associated medications.

PCDS adds that the likely common mechanism is stimulation of growth factor signalling pathways in the skin, and that in insulin resistance syndromes high levels of circulating insulin activate the insulin-like growth factor 1 receptor.

The NHS says the main symptom is patches of skin that are darker and thicker than usual. They can appear anywhere on the body, are dry and feel similar to velvet, and are most common in skin folds such as the armpits, neck or groin. Some people also have tiny growths (skin tags) on the patches. The patches often appear gradually without any other symptoms, and sometimes the skin may be itchy.

PCDS says it predominantly affects the neck and flexures. The most common sites are the intertriginous areas of the armpit, groin, anogenital region, and the back and sides of the neck, and the neck is the most commonly affected site in children. The dorsal surfaces of the hands, feet, elbows and knees are involved in acral acanthosis nigricans. Lesions begin as hyperpigmented patches which thicken with a velvety texture, and many small papillomatous lesions develop within affected areas.

• Darker, thicker, velvety patches of skin

• Most often in the armpits, back and sides of the neck, and groin

• Sometimes skin tags on the patches

• Usually no other symptoms; sometimes itchy

The NHS says to see a GP if you have new dark patches on your skin or any skin changes you are unsure about. It says a GP can usually tell if it is acanthosis nigricans by looking at your skin, and that you may need some tests to find out what is causing the patches.

PCDS says malignant acanthosis nigricans is associated with internal malignancy, usually a gastric adenocarcinoma, and that it tends to develop in middle-aged and elderly patients and be more abrupt in presentation. The vast majority of cases are secondary to stomach adenocarcinoma, but other malignancies sometimes found include other gastrointestinal tumours, lung, uterus, ovaries and urinary tract. Additional features that should increase suspicion include tripe palms (velvety, rugose thickening of the palms), the Leser-Trélat sign (abrupt appearance of multiple, sometimes itchy, seborrhoeic keratoses), and florid cutaneous papillomatosis.

The NHS says a GP can usually tell if it is acanthosis nigricans by looking at your skin. PCDS says diagnosis is usually clinical, and that if a diagnostic biopsy is required, histology shows hyperkeratosis and papillomatosis with finger-like upward projections of dermal papillae.

PCDS says that in the majority of patients it is important to test for diabetes mellitus, and that in women hyperandrogenism should also be considered. Where malignant acanthosis nigricans is suspected, a thorough work-up for underlying malignancy is needed, although PCDS notes that occasionally malignant acanthosis nigricans can precede internal manifestations.

PCDS says that in hereditary forms, acanthosis nigricans tends to arise in childhood, and that the neck is the most commonly affected site in children.

PCDS says acanthosis nigricans is more common in patients with skin of colour. In one study it was observed in 1% of white people, 5.5% of people living in a rural, tropical area of Brazil or another country in Latin America, and 13.3% in African American populations. It also notes that acral acanthosis nigricans, with lesions most prominent over the backs of the hands and feet, is much more common in skin of colour.

On the NHS, a GP is the usual first step, and the NHS says a GP can usually tell if it is acanthosis nigricans by looking at your skin and may arrange tests to find the cause. A referral to a dermatologist may be made for cosmetic treatment ideas, or urgently if cancer is suspected.

If you see a private dermatologist, ask how the underlying cause will be investigated, who will coordinate any blood tests for diabetes or hormone problems, and how the plan will be shared with your GP, because the NHS says finding and treating the cause is usually recommended first.