Acanthosis nigricans
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 | |
|---|---|
| Type | Characteristics |
| Obesity-associated acanthosis nigricans |
|
| Syndromic acanthosis nigricans |
|
| Benign acanthosis nigricans |
|
| Drug-induced acanthosis nigricans |
|
| Hereditary benign acanthosis nigricans |
|
| Malignant acanthosis nigricans |
|
| Mixed-type acanthosis nigricans |
|
What are the features of acanthosis nigricans?
- Thickened brown velvety textured patches of skin that may occur in any location but most commonly appear in the folds of the skin in the armpit, groin and back of the neck.
- Papillomatosis (multiple finger-like growths) is common on cutaneous and mucosal surfaces.
- Skin tags often found in and around affected areas.
- Pruritus (itching) may be present, particularly in acanthosis nigricans associated with malignancy (paraneoplastic pruritus).
- Acanthosis nigricans lesions may also appear on the mucous membranes of the oral cavity, nasal and laryngeal mucosa and oesophagus.
- Lesions involving the mucosa, palms and soles tends to be more extensive and more severe in malignant acanthosis nigricans.
- Patients with malignant acanthosis nigricans tend to be middle-aged, not obese and lesions develop abruptly.
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.
- Correct hyperinsulinaemia through diet and medication
- Lose weight with obesity-associated acanthosis nigricans
- Excise or treat underlying tumour
- Stop offending medicines in drug-induced acanthosis nigricans
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.
