Dark lines on fingernails can be a cause for concern. While these lines can sometimes signal serious underlying health conditions, many causes are benign and not a reason for immediate alarm. Understanding the various reasons behind these nail changes, from minor physical impacts to systemic health issues, can provide clarity and guide appropriate action.
Common Non-Pigmented Causes
Splinter hemorrhages are a common non-pigmented cause of dark lines. These appear as tiny blood clots underneath the nail, resembling thin, vertical red or reddish-brown lines running in the direction of nail growth. They get their name because they can look like a wood splinter lodged under the nail. They form when small blood vessels (capillaries) in the nail bed rupture due to damage.
Trauma is the most frequent cause, resulting from direct injury to the nail, such as stubbing a toe or hitting a finger. They appear closer to the end of the nail and can be 1 to 3 millimeters long. Initially reddish, they often darken to brown or black within a few days. These lines usually resolve as the nail grows out and do not require specific treatment unless accompanied by other symptoms.
Understanding Pigmented Lines (Melanonychia)
Dark lines caused by melanin, the pigment responsible for skin color, are called melanonychia. These appear as brown or black streaks running lengthwise along the nail plate, from the cuticle to the nail tip. Melanonychia results from activated melanocytes, the cells that produce melanin, within the nail matrix, the area where the nail originates. This increased melanin can be due to increased production (melanocytic activation) or an increased number of melanocytes (melanocytic hyperplasia).
Benign melanonychia is common, especially in individuals with darker skin tones, sometimes called racial or physiological melanonychia. It is prevalent in Afro-Caribbean, Japanese, and Hispanic populations. These lines are usually harmless, often affect multiple nails, and may increase in width with age. A benign mole (melanocytic nevus) within the nail matrix can also produce pigment, leading to a persistent dark line. Fungal infections can cause nail discoloration, sometimes appearing as dark lines, and may include nail thickening or brittleness.
A more serious, though rare, cause is subungual melanoma, a type of skin cancer under the nail. This manifests as a brown or black streak that often starts at the bottom of the nail and may gradually widen. Warning signs include a streak that changes in size, shape, or color, has irregular borders, or shows pigment spreading to the skin surrounding the nail (Hutchinson’s sign). Unlike a bruise, a melanoma streak will not heal or move as the nail grows.
Systemic Illnesses and Medications
Dark lines on fingernails can also indicate systemic illnesses or be a side effect of certain medications. Conditions like Addison’s disease, which affects the adrenal glands, can lead to increased pigmentation in the nails. Laugier-Hunziker syndrome, a benign condition, also causes brown or black longitudinal streaks on nails, often accompanied by similar pigmentation in the oral mucosa.
Various medications induce nail discoloration, including dark lines. Chemotherapy agents are a common cause, leading to dark lines, darkening of the nail plate, or even lifting of the nail. Antimalarial drugs, such as hydroxychloroquine and chloroquine, can cause brownish discoloration. Tetracyclines, particularly minocycline, may cause bluish-gray pigmentation. These drug-induced changes are often reversible once medication stops, though resolution can take time.
When to Seek Professional Guidance
Observing a dark line on a fingernail warrants professional evaluation, especially if certain characteristics are present. Consult a doctor if the line is new, appears on a single digit, or if there is no known history of trauma. Any changes in an existing line, such as an increase in width, darkening of color, or irregular borders, should prompt a medical visit. Also concerning is pigment spreading from the nail plate onto the surrounding skin or cuticle.
Further concerns include a nail that bleeds without injury, separates from the nail bed, or shows signs of thinning, cracking, or distortion. If there is any uncertainty about the cause of a dark line, seeking medical advice is a reasonable step. A dermatologist or general practitioner can diagnose the cause through examination, and if necessary, perform a biopsy to rule out serious conditions.