A dark mark on a toe, whether on the skin or under the nail, often causes concern. The appearance of a black spot is a common reason for seeking medical advice, as causes range from a simple injury to a serious, though rare, form of skin cancer. Understanding the potential origins of this discoloration is the first step toward determining if the spot is harmless or requires professional attention. The spot may result from trauma, changes in natural pigmentation, or external factors.
The Most Common Cause: Trauma and Blood
The most frequent explanation for a black spot under the toenail is a subungual hematoma, which is a blood blister beneath the nail plate. This condition results from an injury that causes blood vessels in the nail bed to rupture, leading to a collection of blood under the nail. Blunt force trauma, such as stubbing a toe or dropping a heavy object, is a common culprit. Repetitive microtrauma from tight-fitting shoes or activities like running can also cause this injury.
The spot appears dark purple, brown, or black because the trapped blood clots and dries. This pooling blood creates pressure on the nail bed, often causing throbbing pain immediately following the injury. If the hematoma is not drained, the discoloration will grow out slowly with the nail.
Because toenails grow slowly, it can take six to twelve months for the discolored section to move completely from the base to the tip. Another minor cause is splinter hemorrhages, which are thin, vertical lines of blood under the nail. These occur when small capillaries in the nail bed are damaged, typically from minor trauma, and they also migrate outward as the nail grows.
Pigmentation Issues and Benign Lesions
Some dark spots are caused by melanin, the natural pigment that colors skin and hair. When melanocytes create excess melanin in the nail matrix, the result is a vertical brown or black band called longitudinal melanonychia. This condition is often benign, representing a mole (nevus) or a freckle (lentigo) in the nail bed, or simply an activation of pigment cells due to trauma.
In individuals with darker skin tones, multiple bands of longitudinal melanonychia are common and almost always benign. Unlike a hematoma, which is a round spot that moves out with the nail, a benign pigmented streak usually maintains a regular width, color, and border as it grows.
Pigmentation on the skin can also be caused by external factors, such as the fungal infection Tinea Nigra. This superficial infection, caused by Hortaea werneckii, typically presents as a flat, painless dark brown or black patch on the sole or toes. The discoloration is caused by the pigmented fungus itself. Tinea Nigra is usually treated successfully with topical antifungal medication and must be ruled out, as it can be confused with more concerning melanocytic lesions.
The Critical Concern: Recognizing Melanoma
The most serious, though uncommon, cause of a black spot on the toe is acral lentiginous melanoma (ALM), a subtype of melanoma that appears on the palms, soles, or under the nails. This cancer arises from the pigment-producing cells in the nail matrix and often presents as a widening, darkening longitudinal melanonychia. To aid in early detection, the ABCDEF rule is used to evaluate suspicious pigmented lesions under the nail.
The ABCDEF rule includes:
- Age and Ancestry: Noting a higher risk in people between 50 and 70, particularly those of African, Asian, or Native American descent.
- Brown-black band: With a width of three millimeters or more and irregular borders.
- Change: In the size or growth rate of the band, or a lack of improvement after treatment for a presumed benign condition.
- Digit: Most commonly involved, which is the big toe or the thumb.
- Extension: Of the pigment onto the skin surrounding the nail plate, known as Hutchinson’s sign. This sign, where the pigment bleeds onto the cuticle or lateral nail folds, is a strong indicator of subungual melanoma.
- Family: Or personal history of melanoma.
While Hutchinson’s sign is a key indicator, a similar-looking “pseudo-Hutchinson’s sign” can occur in benign conditions when nail bed pigment shows through a translucent nail fold. Any pigmented band that is new, changing, or involves the surrounding skin requires immediate medical evaluation to distinguish between a benign growth and a malignancy.
When to Seek Professional Evaluation
Any dark spot on the toe or under the nail that cannot be linked to a recent injury or that shows concerning features requires a professional assessment. A visit to a dermatologist is necessary if the spot is new, rapidly changing, or if the discoloration extends onto the adjacent skin (the classic Hutchinson’s sign). Other red flags include a pigmented band that is wider than three millimeters, has irregular or blurred borders, or is accompanied by nail destruction, bleeding, or ulceration.
The diagnostic process typically begins with a physical examination using a dermatoscope, a specialized magnifying tool that allows the doctor to see the pigment’s pattern within the nail plate and nail bed. If the clinical findings are inconclusive, the only way to establish a definitive diagnosis is through a nail matrix biopsy. This procedure involves taking a small tissue sample for laboratory analysis to determine if the cells are benign or malignant. Prompt evaluation of any suspicious or changing spot is important for early detection of subungual melanoma.