A Becker naevus (nevus in American spelling) is a late-onset epidermal naevus or birthmark occurring mostly in males. It is also known as Becker melanosis. It is due to an overgrowth of the epidermis (upper layers of the skin), pigment cells (melanocytes) and hair follicles. It develops during childhood or adolescence on the shoulders or upper trunk, occasionally elsewhere.
It is thought that it is due to a gene defect, which has not yet been identified. It may be triggered to develop by circulating androgens (male hormones such as testosterone), which is why it appears in males at puberty.
Features of Becker naevus
A Becker naevus is a large one-sided brown patch, sometimes over half the upper back or chest. After puberty it often becomes darker and quite hairy, a feature also called hypertrichosis. Occasionally acne may develop in the naevus.
Rarely, like other birthmarks, there may be some abnormality of underlying tissues derived from the same embryonic cell type, the ectoderm. This is known as the Becker naevus syndrome, a type of epidermal naevus syndrome. These abnormalities may include:
- Smooth muscle hamartoma (overgrowth of smooth muscle tissue like a deep birthmark)
- Under-development of underlying structures such as breast, pectoral muscle, fat, limb, chest wall, spine
- Over-development of a tissue such as adrenal gland, limb, fingers or toes, scrotum.
Treatment of Becker naevus
There is no effective treatment for the majority of Becker naevi. However, the dark brown colour is less obvious if the affected area is kept out of the sun so that it doesn't tan.
The excessive hairs can be reduced by repeated treatments with a hair removal laser or by electrolysis. The pigmentation can be reduced by a pigment laser, but this is not always effective and it may also be made worse by laser treatment.