A black toenail, medically known as melanonychia, is a darkening or discoloration beneath the nail plate. This change in color, which can range from red-brown to dark black, is a visible symptom of an underlying issue in the nail bed or nail matrix. While often the result of a common, minor injury causing a localized bruise, the discoloration can also signal a more serious condition. Therefore, any black toenail warrants attention to determine the source of the pigment.
Causes Related to Physical Trauma
The most frequent cause of a black toenail is a subungual hematoma, which is a bruise forming beneath the nail plate. This occurs when trauma causes small blood vessels in the nail bed to rupture and bleed. Since the blood is trapped between the hard nail plate and the underlying nail bed, it pools and darkens as it clots, resulting in a black or dark purple appearance.
Acute, single-event injuries, such as stubbing a toe or dropping an object on the foot, commonly trigger this hematoma. The pooling blood creates pressure, which is often the source of intense, throbbing pain accompanying the discoloration. The color is caused by deoxygenated and dried blood, which may evolve from reddish-purple to dark brown or black over several days.
Repetitive micro-trauma often affects athletes like runners or hikers. This involves continuous, low-level impact where the toes repeatedly strike the inside of a shoe, particularly during downhill running or sudden stops. Wearing ill-fitting footwear encourages this repeated striking of the nail against the shoe’s toe box. This sustained pressure similarly damages the capillaries, leading to a gradual buildup of blood that causes the nail to darken over time.
Non-Traumatic Medical Causes
Black discoloration can arise from conditions unrelated to physical injury. One common non-traumatic source is onychomycosis, a fungal infection of the nail. While fungal infections often present as white or yellow discoloration, they can sometimes cause a dark appearance due to the accumulation of dark debris beneath the nail plate.
Systemic health issues, such as diabetes, anemia, and chronic kidney or heart disease, can also affect nail health and pigmentation. These conditions may alter circulation or nutrient delivery, leading to various types of discoloration that occasionally appear dark. Some medications or rare benign conditions can also cause a dark line or patch to appear on the nail.
The most serious non-traumatic cause is subungual melanoma, a rare but aggressive form of skin cancer that develops in the nail matrix. This cancer typically presents as a dark, irregular, vertical band that grows in width and intensity over time. A specific warning sign is Hutchinson’s sign, where the dark pigmentation spreads from the nail plate onto the surrounding skin of the cuticle and nail fold. Unlike traumatic causes, this discoloration often appears without injury and does not resolve as the nail grows out.
Recognizing When to Consult a Doctor
While most traumatic black toenails resolve on their own, certain signs indicate the need for professional medical evaluation. Immediate attention is warranted if a black toenail develops without any preceding injury or if the discoloration does not gradually clear as the nail grows out. This persistence or unexplained onset raises the possibility of a non-traumatic cause like melanoma or a fungal infection that requires specific treatment.
You should seek care if the black toenail is accompanied by signs of infection, such as increasing pain, significant swelling, warmth around the toe, or the presence of pus or foul odor. Severe throbbing pain caused by a large subungual hematoma may necessitate a minor procedure to drain the trapped blood and relieve pressure. This procedure is generally most effective if performed within the first day or two of the injury.
Any dark streak that changes in size, shape, or color, or involves the skin surrounding the nail plate, should be evaluated immediately. The presence of Hutchinson’s sign, where pigment extends onto the cuticle, is a significant marker that requires a biopsy to rule out subungual melanoma. If the discoloration covers more than half of the nail area or if the injury was severe enough to potentially fracture the underlying bone, a medical assessment is necessary.
Prevention and At-Home Management
Preventing traumatically induced black toenails centers on reducing the friction and impact on the toes. For athletes, selecting properly fitted footwear is paramount, ensuring there is a thumb’s width of space between the longest toe and the shoe’s end. Keeping toenails trimmed short and straight across minimizes the chance of the nail catching on the shoe’s interior or adjacent toes. Wearing moisture-wicking socks can also help keep the feet dry, reducing the risk of fungal infections that can lead to discoloration.
For mild subungual hematomas that are not significantly painful, home management involves protecting the toe and waiting for the nail to grow out. Applying a cold compress immediately after a minor injury can help limit swelling and bleeding. Once the nail is discolored, the dark area will slowly move toward the nail tip over several months as the new, clear nail grows in from the base. If the nail loosens or separates from the nail bed, it is important to keep the area clean and protected to prevent secondary infection.