The sudden appearance of a dark spot on a toe, especially under the nail, can be a source of concern. This discoloration, medically termed melanonychia when related to pigment, has a broad range of causes, with most being benign and related to common injuries. In rare cases, it may signal a more serious underlying issue. Careful observation of the spot’s appearance, location, and behavior over time provides important clues to understand its nature.
Common and Harmless Explanations
The most frequent cause of a dark spot is a subungual hematoma, which is a bruise beneath the nail plate. This occurs when trauma, such as dropping an object on the toe or repetitive impact from tight-fitting shoes, causes small blood vessels in the nail bed to rupture. The trapped blood appears as a black, deep purple, or reddish-brown spot that is sharply demarcated. Unlike pigmentation, this spot will migrate outward and eventually disappear as the nail grows.
Discoloration can also arise from benign pigment changes within the nail matrix, similar to a mole or freckle on the skin. These are called nevi or, when presenting as a linear streak, longitudinal melanonychia. Such lines are significantly more common in individuals with darker skin tones, often affecting multiple nails without worry. Fungal infections (onychomycosis) are another common explanation, causing the nail to turn yellow, brown, or black, often accompanied by thickening or crumbling. External factors, like residual dye from dark socks, shoes, or nail polish, can also temporarily stain the nail plate, which will grow out over time.
Underlying Circulation Issues
Systemic health problems that affect blood flow can manifest as dark spots on the toes. Peripheral vascular disease, where blood vessels narrow, compromises circulation to the extremities, potentially leading to discoloration. This reduced blood flow (ischemia) can cause the skin and nail unit to appear darker due to poor oxygenation.
Tiny streaks of blood under the nail, called splinter hemorrhages, are often signs of micro-trauma. However, these linear marks, which run in the direction of nail growth, can point to systemic issues if they appear under multiple nails without a clear cause of injury. Conditions such as bacterial endocarditis, vasculitis, or diabetes can damage the small capillaries in the nail bed, making them prone to rupture.
Certain oral medications can also induce hyperpigmentation in the nail unit, known as drug-induced melanonychia. Chemotherapy agents, specific antibiotics like minocycline, and antimalarial drugs can activate pigment-producing cells in the nail matrix. The resulting pigment appears as diffuse areas or longitudinal bands, typically brown, black, or blue-gray, and generally fades once the medication is stopped.
Identifying Potential Serious Conditions
The most concerning, though rare, cause of a dark spot is subungual melanoma, a form of skin cancer originating in the nail matrix. This condition is often initially mistaken for a bruise or a benign mole, leading to delayed diagnosis. The malignancy typically presents as a dark, irregular, longitudinal band on a single nail, most commonly affecting the big toe or the thumb.
To help distinguish this serious condition from benign causes, dermatologists use the adapted ABCDEF rule for nail lesions:
- A: Age, noting a peak incidence between 50 and 70 years, and also pointing to African-American, Asian, or Native American heritage, where this melanoma is disproportionately common.
- B: Band, which is typically brown to black, three millimeters or wider, and has irregular or blurred borders.
- C: Change in the band’s size, color, or shape over time, or a lack of improvement despite treatment for other presumed causes.
- D: Digit involved, with the big toe being one of the most frequent sites.
- E: Extension of the pigment onto the skin surrounding the nail plate, known as Hutchinson’s sign. This spread into the cuticle or lateral skin fold is a strong indicator of a cancerous process.
- F: Family or personal history of melanoma or atypical moles, which increases the overall risk.
Next Steps and Medical Consultation
Observing the characteristics of the dark spot is the first step toward understanding its cause. If the spot resulted from a known injury, monitoring its progress as the nail grows will confirm if it is a bruise. A subungual hematoma moves distally toward the free edge of the nail, but if the spot remains stationary or spreads, a medical evaluation is warranted.
There are specific changes that warrant an immediate visit to a healthcare professional, such as a dermatologist or a podiatrist. Seek consultation if:
- The spot is rapidly growing, intensely black or blue-black, or involves bleeding or ulceration.
- The appearance of Hutchinson’s sign, where the pigmentation extends to the skin around the nail.
- Any dark streak involves only one nail.
- The spot does not resolve or grow out with the nail plate over a period of months.