A darkened toenail, medically termed melanonychia, signifies a change in pigment beneath the nail plate. This discoloration often presents as shades of brown, red, or black. While many instances are benign and caused by common injuries, any unexpected or persistent change in nail color warrants careful attention. Understanding the causes helps determine if a black toenail is a simple physical reaction or a sign of a deeper health concern.
Physical Trauma and Subungual Hematoma
The most frequent cause of a black toenail is a physical injury resulting in bleeding underneath the nail plate, known as a subungual hematoma. This injury occurs from a single, acute event, such as stubbing a toe or dropping an object onto the foot. The trauma ruptures small blood vessels, causing blood to pool in the confined space beneath the nail.
This pooling blood initially appears red or purple, but it quickly darkens to deep brown or black as the hemoglobin oxidizes. The pressure from the accumulated blood is often the source of significant throbbing pain, which peaks within the first few hours after the injury. The discoloration will not move until the damaged portion of the nail plate grows out, which can take many months.
Repetitive, low-level trauma, often called microtrauma, is common among athletes like runners or hikers. This occurs when ill-fitting shoes allow the toes to repeatedly strike the front of the shoe box, sometimes called “Runner’s Toe.” The constant friction causes repeated damage to the nail bed’s capillaries, leading to small, layered hematomas that collectively turn the nail dark.
Fungal Infections and Discoloration
Another prevalent cause of toenail darkening is a fungal infection, medically referred to as onychomycosis. While these infections typically cause the nail to turn yellow or white, certain species can lead to a distinct black or dark brown appearance. This discoloration can stem from the accumulation of dark debris underneath the nail plate as the infection progresses.
Less common fungal organisms, such as those from the Scytalidium or Exophiala genera, produce dark pigment directly. These fungi synthesize melanin, which is deposited into the nail plate, causing a true black coloration. A fungal infection is often accompanied by other signs, including a thickened, brittle, or crumbling nail texture.
The infection may also cause the nail to separate from the nail bed, a process called onycholysis, creating an open space for dark debris to collect. Onychomycosis requires specific antifungal treatment to clear the infection. Since trauma can create a pathway for fungi to enter the nail bed, a black toenail may sometimes involve both a hematoma and a subsequent fungal infection.
Underlying Medical Conditions
While less common, black toenails can signal serious or systemic medical issues. The most concerning cause is subungual melanoma, a rare form of skin cancer that develops in the nail matrix. This condition typically presents as a dark, vertical band or streak that is often irregular in color and width.
A warning sign for melanoma is the spread of pigment to the skin surrounding the nail, known as Hutchinson’s sign. Unlike a bruise, a melanotic streak will not grow out with the nail and may widen or change shape over time. Other systemic causes that activate pigment-producing cells include certain medications, such as chemotherapy drugs and antimalarials.
Specific health conditions may also cause diffuse or streaked darkening across multiple nails. Examples include Addison’s disease, which affects the adrenal glands, or a deficiency in Vitamin B12. These systemic issues are often accompanied by other body-wide symptoms.
Management and When to Consult a Doctor
For a black toenail caused by acute trauma and a minor subungual hematoma, management focuses on reducing swelling and pain. Immediate steps include rest, applying ice to the affected toe, and elevating the foot. Over-the-counter pain relievers can help manage discomfort caused by the pressure of the blood.
If the hematoma is large and causes intense, persistent throbbing pain, a doctor may need to perform a procedure called trephination. This involves creating a small hole in the nail plate to release the trapped blood and relieve the pressure quickly. This drainage is generally only possible within a few days of the injury before the blood fully clots.
Professional medical evaluation is necessary for any black toenail presenting with “red flag” symptoms. These include a dark spot or streak that appears without known injury, or one that is not receding with normal nail growth. Consultation with a dermatologist or podiatrist is advised if the discoloration involves the cuticle or surrounding skin, or if the nail shows signs of severe infection, such as pus, warmth, or redness.