Acanthosis nigricans

Author: Vanessa Ngan, Staff Writer, 2004.

What is acanthosis nigricans?

Acanthosis nigricans is a skin disorder characterised by darkening (hyperpigmentation) and thickening (hyperkeratosis) of the skin, occurring mainly in the folds of the skin in the armpit (axilla), groin and back of the neck.

Acanthosis nigricans is not a skin disease per se but a cutaneous sign of an underlying condition or disease.

There are two important types of acanthosis: benign and malignant. Although classically described as a sign of internal malignancy, this is very rare. Benign types, sometimes described as ‘pseudoacanthosis nigricans’ are much more common.

What causes acanthosis nigricans?

The cause for acanthosis nigricans is still not clearly defined but it appears to be related to insulin resistance. It has been associated with various benign and malignant conditions. Based on the pre-disposing conditions, acanthosis nigricans has been divided into 7 types.

Types of acanthosis nigricans
Obesity-associated acanthosis nigricans
  • Most common type of acanthosis nigricans
  • May occur at any age but more common in adulthood
  • Obesity often caused by insulin resistance
Syndromic acanthosis nigricans
Benign acanthosis nigricans
  • Also referred to as acral acanthotic anomaly
  • Thick velvety lesion most prominent over the upper surface of hands and feet in patients who are in otherwise good health
  • Most common in dark-skinned people, especially those of African American descent
Drug-induced acanthosis nigricans
  • Uncommon, but acanthosis nigricans may be induced by several medications, including nicotinic acid, insulin, systemic corticosteroids and hormone treatments
Hereditary benign acanthosis nigricans
  • Acanthosis nigricans inherited as an autosomal dominant trait
  • Lesions may manifest at any age, infancy, childhood or adulthood
Malignant acanthosis nigricans
  • Acanthosis nigricans associated with internal malignancy
  • Most common underlying cancer is tumour of the gut (90%) especially stomach cancer
  • In 25-50% of cases, lesions are present in the mouth on the tongue and lips
Mixed-type acanthosis nigricans
  • Patients with one type of acanthosis nigricans whom also develop new lesions of a different cause, e.g. overweight patient with obesity-associated acanthosis nigricans who then develops malignant AN

What are the features of acanthosis nigricans?

What is the workup for acanthosis nigricans?

It is very important to differentiate acanthosis nigricans related to malignancy from that related to benign conditions. Tumours in malignant acanthosis nigricans are usually very aggressive and spread quickly. Death often occurs soon after. If malignant acanthosis nigricans is suspected in a patient without known cancer, it is extremely important to perform a thorough workup for underlying malignancy and identify a hidden tumour. If the tumour can be successfully treated, the condition may resolve.

Other causes of acanthosis nigricans may be identified by screening for insulin resistance and diabetes mellitus.

What is the treatment for acanthosis nigricans?

The primary aim of treatment is to correct the underlying disease process. Often correcting the underlying cause results in resolution of the lesions.

In hereditary acanthosis nigricans, lesions tend to enlarge gradually before stabilising and/or regressing on their own.

There is no specific treatment for acanthosis nigricans. Treatments considered are used primarily to improve cosmetic appearance and include topical retinoids, dermabrasion and laser therapy.

Final outcome of acanthosis nigricans varies depending on the cause of acanthosis nigricans. Benign conditions either on their own or through lifestyle changes and/or treatment have good outcomes. However, the prognosis for patients with malignant acanthosis nigricans is often poor. The associated cancer is often advanced and the average survival of these patients is approximately 2 years.

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