A black spot or streak on a fingernail or toenail can be alarming. This discoloration, medically termed melanonychia when involving melanin pigment, has a wide range of potential origins. The cause can be as benign as a minor physical injury or, in rare instances, a sign of a serious underlying medical condition. Understanding the source is the first step toward appropriate action, as treatment varies from simply waiting for the nail to grow out to requiring immediate medical evaluation.
The Most Common Culprit: Bruising Under the Nail
The most frequent cause of a black spot beneath the nail plate is a subungual hematoma, which is essentially a deep bruise. This occurs when trauma, such as crushing a finger or dropping a heavy object, causes blood vessels in the nail bed to rupture. The resulting collection of blood becomes trapped between the nail plate and the underlying tissue.
The spot is often a solid, dark purple, red, or black color, and it frequently causes throbbing pain due to pressure buildup beneath the rigid nail. A key feature is that the spot moves outward as the nail grows, confirming it is old, clotted blood that will eventually clear as the nail plate is replaced. Fingernails typically take four to six months to fully grow out, while toenails require nine to twelve months or more for resolution. If the hematoma is large—covering more than 25% of the nail—or intensely painful, a medical professional may drain the pooled blood through trephination, which involves creating a small hole in the nail to relieve pressure.
Pigmentation Related to Moles and Medications
Beyond injury, black or brown coloring can result from the deposition of melanin, the pigment that gives skin and hair its color. This condition is often called longitudinal melanonychia. This appearance is typically a dark line or band running the length of the nail, but it can present as a spot if the pigment is concentrated. The cause is often benign, stemming from activated melanocytes, the pigment-producing cells located in the nail matrix beneath the cuticle.
One common benign cause is a nail matrix nevus, which is essentially a mole located in the nail matrix, causing a streak of pigment to be incorporated into the growing nail plate. These benign streaks are particularly common in people with darker skin tones and may involve multiple nails. Certain systemic medications can also trigger melanin production, resulting in dark spots or bands, a phenomenon known as drug-induced melanonychia. Chemotherapy drugs and some antimalarial medications are well-known examples that activate melanocytes, leading to discoloration that typically resolves after the medication is discontinued.
Discoloration Caused by Infections or Illnesses
Not all dark spots are related to trauma or melanin; some are caused by infections or systemic health issues. While most fungal nail infections (onychomycosis) present with yellow or white discoloration, certain types of fungi can produce darker pigments. For instance, non-dermatophyte molds can cause the nail to appear dark green, brown, or black due to debris accumulating beneath the nail plate.
Another type of dark marking that can appear as small black lines or spots is a splinter hemorrhage, which looks like a tiny splinter embedded under the nail. These are small clots of blood caused by damage to the capillaries in the nail bed. While often caused by minor trauma, multiple splinter hemorrhages appearing without injury can be a sign of underlying systemic conditions. These rarer causes include specific infections like bacterial endocarditis, an infection of the heart’s inner lining, or systemic diseases that involve blood vessel inflammation, such as vasculitis or lupus.
Identifying the Serious Warning Sign: Subungual Melanoma
The most serious, though rare, cause of a black spot or streak is subungual melanoma, a form of skin cancer that originates in the nail matrix. Early detection is a concern because this condition can be easily mistaken for a simple bruise or a benign pigment streak. This cancer usually presents as a dark, irregular band that changes over time, most commonly affecting the thumb or the big toe.
Physicians use the ABCDEF mnemonic to help identify suspicious pigmented lesions within the nail unit:
- A stands for Age, as it is most common in adults between 50 and 70, and in certain ancestries like African American or Asian.
- B refers to a Brown-black band that is wide—typically 3 millimeters or more—and has irregular borders.
- C is for Change in the size, color, or shape of the band, or a lack of change in nail morphology despite treatment.
- D highlights the Digit most commonly involved, particularly the thumb or big toe.
- E represents the Extension of the pigment onto the skin surrounding the nail, known as Hutchinson’s sign.
- F stands for a Family or personal history of melanoma or atypical moles.
The presence of any of these signs, particularly a dark spot or streak that is rapidly changing or involves the surrounding skin, warrants an immediate consultation with a dermatologist for a definitive diagnosis, which may require a biopsy.