A sudden or gradual darkening of a toenail, often resulting in a dark brown or black appearance, is medically known as melanonychia. This discoloration refers to the presence of brown or black pigment in the nail plate. The dark color almost always stems from two primary sources: pooled blood underneath the nail or the deposition of the pigment melanin. Although frequently harmless, this change should never be ignored, as it can occasionally signal a serious underlying health concern. Understanding the specific cause requires looking closely at how and when the discoloration first appeared.
Immediate Cause: Blood Under the Nail
The most frequent reason for a black toenail that appears suddenly is a subungual hematoma, which is a bruise forming beneath the nail plate. This occurs when trauma causes blood vessels in the nail bed to rupture, leading to blood accumulation trapped between the nail and the underlying tissue. The pressure from this trapped blood often causes throbbing pain and results in a deep purple or black appearance.
The trauma leading to a subungual hematoma is not always a single, crushing injury, such as dropping a heavy object on the toe. Repetitive microtrauma is a common cause, particularly in athletes like runners or hikers whose toes repeatedly strike the front of a shoe. Shoes that are too tight or too loose can facilitate this repetitive impact.
When the nail bed bleeds, the blood coagulates and darkens over time, creating a fixed black or deep maroon spot beneath the nail. The hematoma must heal by growing out with the nail. Since toenails grow slowly, taking approximately six to nine months for complete replacement, the discoloration will persist for many months as the nail grows.
If the pressure and pain are significant, a healthcare provider may perform trephination to drain the pooled blood. This involves making a small hole through the nail plate, which instantly relieves the pressure and pain. Since the hard nail itself has no nerve endings, anesthesia is not required. For smaller, painless hematomas, the conservative approach is observation, allowing the body to reabsorb the blood as the nail grows out.
Gradual Causes: Fungal Infections and Pigmentation
When discoloration develops slowly over weeks or months without clear trauma, the cause is typically related to a fungal infection or pigmentation changes. Fungal infections of the nail, known as onychomycosis, often begin with white or yellowish discoloration and thickening. Chronic or severe infections can lead to a dark brown or black color due to a substantial buildup of debris under the nail plate.
This dark debris is often composed of keratin, fungal elements, and sometimes secondary bacterial colonization. The warm, moist environment inside shoes and socks creates an ideal breeding ground for the dermatophytes, yeasts, or molds responsible for these infections. Unlike a hematoma, which is fixed and moves forward with nail growth, fungal discoloration may appear more diffuse or irregular.
Increased melanin production is another gradual cause, a condition termed longitudinal melanonychia. This presents as a dark, typically brown or black, vertical band running from the cuticle to the nail tip. It occurs when the melanocytes, the pigment-producing cells in the nail matrix, are activated to deposit melanin into the growing nail plate.
This hyperpigmentation is a benign finding in most cases, particularly in individuals with darker skin tones, where it can affect multiple nails. Trauma or certain systemic medications, such as chemotherapy agents or antibiotics, can also activate these pigment cells, resulting in a new or darker stripe. The slow growth of the nail means this line develops and changes appearance gradually over time.
When to Seek Medical Attention and Treatment Options
While most black toenails are benign, professional medical consultation is necessary to distinguish common causes from the rare but aggressive subungual melanoma, a form of skin cancer. Specific warning signs, often referred to as the ABC rule, require immediate evaluation by a dermatologist or podiatrist. These include a band that is Age-related (new onset in an adult), Broad (wider than 3mm), and Changing (getting darker or wider over time).
A particularly concerning sign is the extension of the dark pigment onto the skin surrounding the nail plate, known as Hutchinson’s sign. Other red flags include a dark spot that appears without known injury, discoloration that affects only a single nail, or a nail that begins to split, bleed, or lift from the nail bed. Unlike a bruise, a cancerous lesion will not resolve or grow out with the nail.
Diagnosis for a suspicious black toenail often involves examination with a specialized magnifying tool called a dermatoscope to analyze the pigment pattern. If melanoma is suspected, a small surgical biopsy of the nail matrix is performed to analyze the tissue for cancerous cells. Early detection is directly linked to better treatment outcomes.
Treatment is entirely dependent on the final diagnosis. For a confirmed fungal infection, topical or oral antifungal medications are prescribed, which can take many months to clear. Minor subungual hematomas are observed, but larger, painful ones can be quickly treated with trephination to decompress the area. If melanoma is confirmed, treatment involves surgical removal of the affected tissue, often requiring a complex procedure to remove the tumor while preserving the toe.