The sudden appearance of a black or dark discoloration beneath a fingernail or toenail, medically termed melanonychia, is common. While many causes of this pigmentation are benign, a dark spot on the nail requires consideration of all potential origins. Understanding the difference between a simple bruise and a more serious condition is important for knowing when to seek professional medical advice. This article explores the most frequent reasons for nail darkening and highlights signs that signal a need for urgent consultation.
Trauma and Bruising Under the Nail
The most frequent cause of a black nail is bleeding beneath the nail plate, known as a subungual hematoma. This condition results from direct, acute trauma, such as crushing a finger in a door or dropping a heavy object on a toe, which causes the small blood vessels in the nail bed to rupture and leak blood. The resulting blood collection is trapped between the nail plate and the nail bed, appearing as a distinct dark red, purple, or black patch that is often uniform in color.
A subungual hematoma initially causes throbbing pain due to the pressure of the accumulating blood, though the pain typically subsides within a few days. If the hematoma is large or significantly painful, a medical professional may perform a simple procedure called trephination. This involves creating a small hole in the nail plate to drain the trapped blood and relieve pressure. For smaller hematomas, no intervention is required; the discoloration will grow out with the nail over several months.
A similar mechanism, though less acute, involves repetitive microtrauma, such as that experienced by runners wearing tight shoes or individuals whose feet slide forward during intense activity. This chronic stress can also cause bleeding beneath the nail, leading to dark discoloration. The key sign confirming a bruise is that the dark area remains fixed beneath the nail and slowly moves forward as the nail grows.
Pigmentation, Fungi, and Medication Side Effects
Dark nail discoloration can arise from external staining, fungal infection, or internal systemic changes. Exogenous stains involve the superficial layering of color onto the nail plate from outside sources, and they can often be scraped off or filed away. Common culprits include handling hair dyes, exposure to chemicals like silver nitrate or potassium permanganate, and dirt or tar.
Certain types of fungal infections (onychomycosis) can also cause the nail to turn dark brown or black. While most fungal infections result in yellowing and thickening, some molds and dermatophytes produce dark pigment that stains the nail plate. This discoloration is often accompanied by a crumbly texture or a lifting of the nail from the nail bed.
Furthermore, various internal factors can induce pigmentation in the nail matrix, leading to linear or diffuse darkening. Several medications are known to cause drug-induced melanonychia, including certain chemotherapy agents, antimalarial drugs, and the antibiotic minocycline. In rare cases, a systemic condition like Addison’s disease, which causes an overproduction of ACTH, can stimulate melanocytes in the nail bed to produce excess melanin.
Recognizing Nail Cancer
Subungual melanoma, a rare form of skin cancer originating in the nail matrix, is the most serious cause of black nail discoloration. This condition often presents as longitudinal melanonychia, appearing as a dark brown or black streak running vertically down the length of the nail. Unlike a bruise, this streak will not move forward with nail growth or disappear over time.
Dermatologists use a mnemonic device, often referred to as the ABCDEF criteria, to help identify the warning signs that differentiate a benign streak from potential melanoma.
ABCDEF Criteria
- Age range of presentation (typically 50 to 70 years).
- Brown-black Band that is wide (3 millimeters or more) with blurred or irregular Borders.
- Change or lack of Change in the band over time despite treatment.
- Digit most commonly affected (the thumb or big toe).
- Extension of the pigment onto the surrounding skin of the cuticle or nail fold (Hutchinson’s sign).
The presence of Hutchinson’s sign, where dark pigment spreads onto the skin adjacent to the nail, is a strong indicator of potential malignancy and warrants immediate evaluation. While other benign conditions can mimic this appearance, any rapidly changing, wide, or unexplained dark streak should be considered suspicious. Early detection of subungual melanoma significantly improves the prognosis.
Next Steps and Medical Consultation
Knowing when to seek medical attention is crucial for maintaining nail health. If the discoloration results from trauma and the pain is severe or the hematoma covers more than 25% of the nail, consultation is recommended for pain relief and assessment for underlying bone fracture. Any dark streak unexplained by a clear injury, or one that does not resolve or grow out within a few months, should be evaluated by a professional.
Urgent consultation with a dermatologist or podiatrist is necessary if the discoloration is rapidly changing in size, shape, or color. The appearance of Hutchinson’s sign is an immediate red flag that requires biopsy to rule out melanoma. Similarly, immediate medical advice is required if the dark area is accompanied by signs of infection, such as pus, swelling, warmth, or persistent pain. Only a medical professional can provide a definitive diagnosis, which may involve a detailed examination using a dermatoscope or a biopsy of the nail matrix.