Dark streaks running lengthwise down a nail are medically termed Longitudinal Melanonychia. This appearance represents an accumulation of the pigment melanin within the nail plate. The pigmentation starts in the nail matrix, the tissue beneath the cuticle where the nail is formed, and is carried forward as the nail grows. These lines can be caused by factors ranging from harmless, common occurrences to rare, serious underlying health conditions. Understanding the characteristics of these streaks helps determine if medical attention is necessary.
Common and Benign Causes
Most black lines on the nails are benign and result from increased melanin production. The most frequent cause is physiologic melanonychia, a normal variation in pigmentation common in people with darker skin tones. Nearly all individuals of African descent develop some nail pigmentation by age 50, and it is also common in people of Asian descent. This pigmentation often affects multiple nails and is considered the equivalent of a mole or freckle on the skin.
Another frequent non-cancerous cause is a subungual hemorrhage, which is a bruise under the nail resulting from trauma. This can occur from a single acute injury, such as crushing a finger, or from chronic repetitive microtrauma. The color is caused by trapped blood, appearing red, purple, or black, and may present as a streak or a diffuse blotch. The discolored area migrates outward with nail growth, eventually clearing completely, which distinguishes it from stable pigment production.
Benign growths within the nail matrix, such as a nevus or a lentigo, also cause longitudinal melanonychia. A nevus is a mole formed by pigment-producing cells and is the most common cause in children. A lentigo is similar to a freckle, involving an increase in pigment-producing cells, and is more frequently seen in adults. Both are stable bands of pigment that do not typically change in width or color over time.
Nail Pigmentation Related to Systemic Health
The lines may also signal an underlying systemic condition or be a side effect of medication. Certain drugs activate melanocytes in the nail matrix, causing pigment production that results in a streak. This is noted with some chemotherapy agents, antimalarial drugs, and certain antivirals used to treat HIV. The pigmentation often resolves once the causative medication is discontinued.
A variety of internal diseases can also manifest as nail pigmentation, though this is less common. Hormonal disorders, such as Addison’s disease, can stimulate melanin production, sometimes leading to streaks in multiple nails. Inflammatory skin conditions like Lichen Planus can involve the nail matrix and cause a pigmented band. Certain infections, including advanced HIV/AIDS, have also been associated with nail discoloration.
Identifying Potential Malignancy
While most black lines are harmless, the most concerning cause is subungual melanoma, a rare but aggressive form of skin cancer developing in the nail matrix. Identifying potential malignancy relies on recognizing specific warning signs. Dermatologists use an adapted version of the “ABCDE” rule to screen for signs of subungual melanoma.
The first letter, A, stands for Age, as melanoma is most commonly diagnosed in adults between 50 and 70 years old. It is disproportionately found in individuals of African, Asian, or Native American descent. B addresses the Band characteristics: a streak that is brown-black, wider than three millimeters, and has irregular or blurred borders is suspicious. Benign streaks typically have uniform color and sharp, parallel edges.
C signifies Change, meaning the streak has rapidly increased in width, darkened, or changed color over time. This also applies if an existing band fails to improve despite treatment. D is for Digit involved, as melanoma frequently appears on the thumb, big toe, or index finger.
E stands for Extension of pigment onto the skin surrounding the nail, known as Hutchinson’s sign. This is the spread of pigment from the nail plate onto the cuticle or the lateral nail fold. This indicates that pigment-producing cells are moving beyond the confines of the nail matrix. Benign pigmentation usually stops abruptly at the edge of the nail plate.
Medical Evaluation and Treatment Options
Any dark line that is new, rapidly changing, or exhibits suspicious ABCDE features warrants prompt evaluation by a dermatologist. The initial diagnostic process involves a physical examination, a detailed history, and often the use of a specialized magnification tool called a dermatoscope. This device allows the dermatologist to closely examine the pattern, color, and structure of the pigment within the nail plate and matrix.
Biopsy Procedures
If the lesion is suspicious, a biopsy of the nail matrix is required to establish a definitive diagnosis. There are two primary types of biopsies: excisional and incisional. An excisional biopsy involves the surgical removal of the entire lesion. This is the preferred method when melanoma is strongly suspected, as it allows for complete staging and assessment of the tumor’s depth.
Treatment
An incisional biopsy removes only a small portion of the lesion, typically a narrow strip of the nail matrix. This method is generally used for very large lesions or when clinical suspicion is low. Treatment for confirmed subungual melanoma is surgical, often involving the removal of the entire nail unit and potentially the amputation of the fingertip. Benign lesions usually only require regular monitoring.