Acanthosis Nigricans (Dark Thick Skin Folds) Causes and Pictures

What is acanthosis nigricans?

Acanthosis nigricans is a type of skin condition characterized by thick dark (hyperpigmented) skin which tends to occur on the skin folds – armpits (axillae), neck, and groin. Typically the hyperpigmented skin has a velvety texture and can therefore be clearly differentiated from hyperpigented areas of normal skin. Acanthosis nigricans can occur with certain diseases and has gained much attention in recent years due to its association with obesity and insulin resistance as is seen in type 2 diabetes mellitus (sugar diabetes).

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There are two types of acanthosis nigricans with several different catgeories. The same process is involved in all types and forms of acanthosis nigricans which involves rapid stimulation of the dermis and epidermis of the skin by most commonly by hormonal factors. This results in overgrowth (hyperplasia) and thickening (hyperkeratosis) of the skin with increased melanin deposition (hyperpigmentation) although the number and size of the melanocytes (pigment-producing skin cells) remains the same. The proliferation of the dermal papillae contributes to the velvet texture of the affected area. Acanthosis nigricans is a benign skin condition. Even when associated with malignancy, it is not due to cancer of the skin or at the site but a manifestation due to a tumor elsewhere in the body (paraneoplastic).

Causes and Types of Acanthosis Nigricans

There are broadly two types of acanthosis nigricans :

  1. Benign – an overgrowth of tissue due to an excessive number of normal cells associated with hormonal factors associated with endocrine disorders, genetic factors, hormone-secreting tumors and congenital syndromes.
  2. Malignant (paraneoplastic) – seen with cancer at other sites in the body but not in the affected area.

The exact reason why acanthosis nigricans develops in certain patients with specific disorders and not others has not as yet been clearly ascertained. Acanthosis nigricans is associated with a number of disorders, particularly endocrine diseases, and most are of the benign type. It should be noted that this type of skin lesion can serve an important marker of benign and malignant processes and should not be ignored despite its innocuous nature.

Due to the various causes, acanthosis nigricans can be subdivided into different categories :

  • Obesity-associated. As the name suggests is associated with obesity and the most common type of acanthosis nigricans. Hyperpigmentation diminishes or even resolves completely with weight control. Insulin resistance is often present.
  • Acral. Not associated with any disease and has a greater tendency to occur in darker skinned individuals with prominent thickening on the knuckles and dorsal aspect of hands and feet (back of hands, top of foot).
  • Syndromic. This is when acanthosis nigricans is associated with various syndromes including diabetes mellitus, PCOS (polycystic ovarian syndrome), SLE (systemic lupus erythematosus), Hashimoto’s thyroiditis and other autoimmune diseases.
  • Unilateral. This is an inherited form of acanthosis nigricans and may come and go in life.
  • Familial. Also due to genetic factors, the lesions tends to begin in childhood and either stops growing or resolves by puberty.
  • Generalized. Primarily occurs in infants and children and affects most of the body. May be associated with underlying systemic disorders and certain types of cancer but it is rare.
  • Drug-induced. Seen with various medications that affect the females hormones, including oral contraceptives and drugs for treating conditions like PCOS or endometriosis. Also seen with mega doses of niacin and usually resolves once the medication is discontinued.
  • Malignant. Often seen with gastric adenocarcinoma (stomach cancer).
  • Mixed-type. This may involve one or more of the types above.

Location and Pictures of Acanthosis Nigricans

Acanthosis nigricans has a tendency to occur on the skin folds, particularly the posterior neck, armpits (axillae), groin and anogenital region. Patients often ignore the lesions until it becomes prominent in visible areas like the neck due to the aesthetic and psychosocial impact. With acral acanthosis nigricans, patients may also seek medical attention sooner as the lesions on the hands can have a pronounced impact of social interaction. The lesions are usually symmetrical and often there are skin tags in the area. Rarely, acanthosis nigricans may occur on the eyelids, areola and even the mucous membranes of the oral or nasal cavity, and  conjunctiva.

Benign acanthosis nigcricans of the armpits (Picture from Dermatology Atlas, courtesy of Samuel Freire da Silva, M.D.)

Benign acanthosis nigcricans of the armpits (Picture from Wikimedia Commons)

Benign acanthosis nigcricans of the neck (Picture from Dermatology Atlas, courtesy of Samuel Freire da Silva, M.D.)

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