A dark or black discoloration beneath a single fingernail can be alarming. While many people worry about a serious underlying condition, the cause is often a common and harmless injury. Understanding the possible origins is the first step in determining if the spot is a benign bruise that will resolve or a sign requiring medical evaluation. The appearance and evolution of the discoloration are the most important clues.
Trauma: The Primary Reason for a Black Nail
The most frequent reason for a sudden black spot under the nail is a subungual hematoma, which is a bruise beneath the nail plate. This occurs when an injury, such as slamming a finger or hitting it with a hammer, causes small blood vessels in the nail bed to rupture. The blood becomes trapped between the nail bed and the hard nail plate. This pooling blood initially appears reddish-purple and may cause throbbing pain due to the intense pressure.
The hematoma typically darkens to deep brown or black as the trapped blood coagulates and breaks down. A key sign of a hematoma is that the discoloration moves outward as the healthy nail grows. Since a fingernail takes about six months to grow fully, the spot will gradually migrate toward the edge and eventually be clipped away. If the injury was severe, the pressure may cause the nail plate to separate and fall off, but a new nail will grow back.
Pigmentation Changes: Benign Melanonychia
When a dark spot appears without known trauma, it may be melanonychia, which is the presence of melanin pigment in the nail plate. This condition presents as a vertical, linear band of brown or black color running from the cuticle to the free edge of the nail. Melanonychia results from increased melanin production by melanocytes located in the nail matrix beneath the cuticle.
In individuals with darker skin tones, this is often a normal variant known as racial melanonychia, frequently appearing on multiple fingers. Other benign causes include a nail matrix nevus, which is a non-malignant mole forming a stable, pigmented stripe. Unlike a hematoma, this discoloration is painless, stable, and remains visible for years. The key distinction from a bruise is that this linear pigmentation originates at the nail matrix and lacks a history of acute injury.
The Critical Concern: Nail Melanoma
The most serious, though rare, cause of a black streak is subungual malignant melanoma, a form of skin cancer arising in the nail unit. Melanoma often presents as longitudinal melanonychia, a dark stripe easily mistaken for a benign pigment change or bruise. It frequently affects the thumb, index finger, or big toe.
To differentiate melanoma from benign pigmentation, a modified ABCDE mnemonic helps with initial screening:
- A is for Age, with peak incidence in adults between the fifth and seventh decades of life.
- B is for Band, referring to a pigmented stripe that is three millimeters or wider with irregular or blurred borders.
- C signifies Change, meaning the band is rapidly changing in width or color, or the nail fails to improve despite treatment.
- D is for Digit, most commonly the thumb or big toe.
- E is for Extension of the pigment onto the surrounding skin of the cuticle or nail folds, known as Hutchinson’s sign.
This spread of color beyond the nail plate indicates that the pigment-producing cells are proliferating and migrating into the adjacent skin.
Knowing When to Seek Professional Diagnosis
While most black spots under a single nail are harmless bruises, professional evaluation is warranted when certain red flags are present. Seek a doctor if the discoloration appeared without preceding trauma, making a hematoma unlikely. This is especially important if the dark spot has been visible for several months without growing out with the nail.
An immediate visit is necessary if any dark band or spot is rapidly changing in size, shape, or color. The presence of Hutchinson’s sign, where pigment extends onto the surrounding skin, is an urgent signal for evaluation. Additionally, if the nail plate begins to crack, split, or lift away, or if there is persistent pain or bleeding, the area should be assessed. Timely diagnosis of serious conditions, like subungual melanoma, greatly improves the outlook.