Birthmarks are common skin markings, typically presenting as a discoloration or growth present at birth or appearing shortly afterward. They form due to abnormalities in the development of blood vessels, pigment-producing cells, or other skin tissue components. While most birthmarks are flat, some develop into raised, dark patches that sprout thick, dark hair. This excessive hair growth, known medically as hypertrichosis, stems from a unique biological interaction between the birthmark cells and the surrounding hair follicles.
Identifying the Birthmarks That Grow Hair
The specific type of birthmark responsible for hair growth is the Congenital Melanocytic Nevus (CMN). This mole is present at birth or appears within the first few weeks of life, forming from an abnormal proliferation of melanocytes, the skin’s pigment-producing cells. CMN can range in color from light brown to black, and their texture may be smooth, bumpy, or warty.
The size of a CMN is classified based on its projected adult diameter, which is important for monitoring and hair growth. CMN larger than 20 centimeters are called giant congenital nevi, and these are most frequently associated with pronounced hypertrichosis, often termed “giant hairy nevus.” These pigmented marks differ from vascular birthmarks, like hemangiomas or port-wine stains, which are composed of blood vessels and typically do not grow hair.
The Biological Reason for Hair Growth
The hair growth is a direct consequence of the physical location and chemical signaling of the nevus cells within the skin. During development, the melanocytes forming the nevus often extend deep into the dermis, clustering around the hair follicles. This proximity allows the nevus cells to directly influence the normal hair growth cycle.
Research shows that nevus cells are in a state of senescence, meaning they have permanently stopped dividing but remain metabolically active. These senescent cells release specific signaling molecules and growth factors, such as osteopontin, into the local tissue environment. Osteopontin acts on nearby hair follicle stem cells, hyperactivating them. This causes the follicles to exit the resting phase (telogen) and rapidly enter the active growth phase (anagen). The follicle spends a longer time in this growth phase, resulting in the production of thicker, longer, and more pigmented terminal hair than is found on the surrounding skin.
When to Monitor the Birthmark
Most CMN are benign, but they carry a small, increased risk of developing into melanoma, a serious form of skin cancer. The risk is significantly higher for larger CMN, particularly those greater than 20 centimeters in projected adult diameter. Therefore, long-term surveillance by a dermatologist is necessary for managing these birthmarks. Parents and patients should monitor the birthmark for any notable changes in appearance.
Signs that warrant immediate medical evaluation include:
- A change in the color, especially a darkening or the appearance of multiple shades of black or blue.
- An increase in the size of the lesion.
- The development of uneven or irregular borders.
- A sudden change in texture.
- Any persistent bleeding or ulceration on the surface.
Though the presence of hair itself is not a sign of malignancy, these birthmarks require regular professional check-ups to detect malignant transformation early.
Management and Removal Options
Management of a hairy birthmark is often driven by cosmetic concerns or the psychological impact of the lesion. Approaches involve managing the hair growth or removing the birthmark entirely. Temporary methods, such as shaving or plucking, only address the hair shaft above the skin surface and do not eliminate the follicle.
Hair Removal Options
More permanent options, like laser treatment, target the pigment in the hair follicle. However, laser treatment on a CMN is complicated because the nevus cells also contain high concentrations of pigment. This pigment can interfere with the laser’s effectiveness and potentially obscure signs of malignant change.
Surgical Excision
The only way to permanently remove the source of the hair growth is through surgical excision of the birthmark. This procedure removes the nevus cells and the hyperactive hair follicles. For larger lesions, complete removal may require multiple surgeries, tissue expansion, or skin grafting.