Melanonychia is the medical term for brown or black discoloration of the nail plate, caused by the presence of the pigment melanin. This discoloration results from the activation of melanocytes, the pigment-producing cells that normally lie dormant in the nail matrix, the area where the nail grows. Although the physical appearance of a dark streak on a nail often causes anxiety, the condition is common and benign in the vast majority of cases.
Understanding the Appearance of Nail Pigmentation
The appearance of melanonychia can vary widely in color, width, and pattern, which helps medical professionals determine its cause. The most frequent presentation is longitudinal melanonychia, appearing as a band or stripe running vertically from the cuticle to the free edge of the nail. This longitudinal pattern occurs because activated pigment cells in the nail matrix continuously deposit melanin into the growing nail plate.
The color of the stripe can range from light tan to dark black, reflecting the amount of melanin being produced. Less commonly, the pigment may appear as transverse melanonychia, a horizontal band running across the width of the nail plate. This transverse pattern is often associated with temporary systemic events, such as certain medications or acute trauma, that briefly stimulate the pigment cells.
Common Causes of Pigmentation
Increased melanin production in the nail matrix is typically caused by either temporary stimulation of pigment cells or a benign increase in their number. One frequent cause is racial or constitutional activation, a normal physiologic variation in individuals with darker skin tones. Nearly all individuals of African descent, for example, will develop one or more pigmented bands on their nails by age 50.
Physical damage is another common trigger, as trauma can easily activate pigment cells in the nail matrix. This includes a single acute injury and repetitive microtrauma, such as chronic friction from tight-fitting shoes or habits like nail biting. Benign growths like a nail matrix nevus (a mole within the nail-producing tissue) can also cause a persistent longitudinal streak.
Systemic conditions and medications can also lead to pigment changes, often affecting multiple nails simultaneously. Causes include endocrine disorders, such as Addison’s disease, or nutritional issues like Vitamin B12 deficiency. A wide range of medications, including certain chemotherapy agents and antimalarial drugs, are known to stimulate melanocytes, resulting in either a longitudinal or a transverse pattern.
Warning Signs and Differentiation from Melanoma
While most melanonychia is harmless, a dark stripe on a single nail can, in rare instances, be a sign of subungual melanoma, a serious form of skin cancer. Early-stage melanoma is difficult to distinguish from a benign pigment band, requiring a thorough medical evaluation of any new or changing lesion. Dermatologists use specific criteria to identify features that raise suspicion for malignancy.
One primary warning sign is Hutchinson’s sign: the extension of dark pigment from the nail plate onto the surrounding skin of the cuticle or lateral nail folds. This spillage suggests that the melanin-producing cells are proliferating uncontrollably and spreading beyond the nail unit. Any rapid change in the band’s appearance is also a red flag for potential malignancy.
The “ABCDEF” mnemonic is used by clinicians to assess the risk of a pigmented nail lesion.
ABCDEF Mnemonic
- Age: Melanoma is more common in adults between 50 and 70, and in individuals of African, Asian, or Native American heritage.
- Band: Refers to the stripe’s size, specifically if the brown-black stripe is wider than three millimeters or has blurred borders.
- Change: Noting any rapid increase in width or color intensity, or a lack of improvement after treating a presumed cause.
- Digit: Indicates the digit most often involved, typically the thumb, the great toe, or the index finger.
- Extension: Refers to the extension of pigment onto the surrounding skin (Hutchinson’s sign).
- Family: Refers to a family or personal history of melanoma.
Diagnosis and Management
When a patient presents with melanonychia, the first step is a detailed history and a physical examination of the nail unit. A medical professional often uses a specialized magnifying tool called dermatoscopy (or onychoscopy) to closely examine the pigment pattern. This helps differentiate benign causes from concerning features. Dermatoscopy allows visualization of parallel lines in the nail plate; irregular lines, spacing, or color are suspicious indicators.
The definitive way to rule out melanoma is through a nail matrix biopsy, which involves surgically taking a small tissue sample from the area where the nail originates. This procedure is performed under local anesthetic and requires careful technique to minimize the risk of permanent scarring or nail deformity. A pathologist then analyzes the biopsy specimen to determine if the melanocytes are benign or malignant.
For cases diagnosed as benign, management is usually limited to reassurance and monitoring for future changes. If the cause is drug-related, the pigmentation may fade slowly once the medication is stopped, though this can take several months as the nail grows out. If the biopsy confirms subungual melanoma, surgical removal of the cancerous tissue is necessary to prevent the disease from spreading.