The sudden appearance of dark pigmentation beneath a toenail is often concerning. This discoloration, medically known as melanonychia when the pigment is melanin, prompts many people to search for its cause. While the underlying reason is frequently minor, possibilities range from benign trauma to serious systemic conditions. Understanding the potential origins of this darkness is the first step toward determining the appropriate course of action.
The Most Common Cause Trauma and Bruising
The most frequent explanation for a dark spot under the toenail is a subungual hematoma, essentially a bruise beneath the nail plate. This condition results when blood vessels in the nail bed are damaged, leading to blood trapped between the nail and the underlying tissue. Acute incidents, such as stubbing a toe or dropping a heavy object, are obvious causes.
A subungual hematoma can also develop from repetitive, low-grade trauma, which is common in the toes. Activities like running, hiking, or wearing ill-fitting shoes that cause the toes to press repeatedly against the shoe’s end can lead to micro-bleeding. The spot initially appears red or purple but darkens over several days as the trapped blood clots, turning a deep brown or black.
Since the blood is trapped, the spot must grow out with the nail. Because toenails grow slowly, a hematoma may take six to twelve months to fully disappear as the nail plate advances. If the injury is severe and causes significant blood accumulation, it can create painful pressure. This may require a medical professional to drain the blood through a small hole in the nail, a procedure called trephination.
Infections That Darken the Nail Bed
A second common cause of dark discoloration involves microbial invaders, primarily fungal infections known as onychomycosis. Fungi thrive in the warm, moist environment of shoes and can penetrate the nail plate, causing color changes. While often presenting as yellow or white, certain molds and yeasts can result in brown or black pigmentation.
Specific organisms, such as Scytalidium or pigmented strains of dermatophytes, produce dark pigment that incorporates into the nail structure. This darkness is distinct from a bruise because it does not appear suddenly following trauma and may present as a streak or diffuse discoloration. Fungal infections also cause other changes to the nail, including thickening, crumbling, and separation of the nail plate from the bed.
Fungal infections often lead to a buildup of debris underneath the nail, which can appear dark and contribute to the discoloration. Though less common, bacterial infections, particularly those involving certain Gram-negative bacteria, can also cause pigmentation, especially in people with weakened immune systems. These microbial causes require specific antifungal or antibacterial treatments, making a proper diagnosis necessary.
When Discoloration Signals Serious Illness
Though rare, persistent or changing dark pigmentation under the nail can signal subungual melanoma, a form of skin cancer that develops in the nail matrix. This condition often presents as a brown or black longitudinal stripe, called melanonychia striata, that runs the length of the nail. Physicians often use an adapted system to assess the risk of malignancy.
A helpful tool for identifying suspicious lesions is the ABCDE mnemonic:
- Age and specific populations: Melanoma is more common in older adults and in individuals of African American or Asian heritage.
- Band of pigment: A width of 3 millimeters or more, irregular borders, or varied color within the band raises concern.
- Change or lack of response to treatment: A band that rapidly grows or changes color warrants immediate attention.
- Digit most commonly involved: This is typically the thumb or the big toe.
- Extension of the pigment: Pigment extending onto the skin surrounding the nail, known as Hutchinson’s sign, is a strong indicator of melanoma.
Beyond melanoma, dark nail pigmentation can signal certain systemic diseases or be a side effect of medication. Conditions like Addison’s disease, Cushing’s syndrome, and certain nutritional deficiencies can activate pigment-producing cells in the nail matrix, leading to multiple pigmented bands. Drugs used for chemotherapy or anti-malarial treatments are also known to cause nail discoloration, often appearing as transverse bands.
Home Monitoring and When to See a Doctor
When a dark spot appears without a clear memory of trauma, careful observation is recommended to distinguish between benign and serious causes. Track the spot’s size, shape, and color over several weeks, ideally by taking clear, dated photographs. A spot caused by trauma should move gradually forward and disappear as the nail grows out.
If the discoloration fails to move with the nail growth, spreads rapidly, or is accompanied by unexplained pain, a professional evaluation is necessary. Any sign of the dark pigment extending beyond the nail plate onto the surrounding skin is a red flag requiring immediate medical consultation. A medical professional can accurately diagnose the cause, whether it is a fungal infection, a sign of underlying systemic illness, or subungual melanoma.