When a toenail suddenly turns dark, it can be quite startling. This discoloration typically involves the nail plate or the tissue directly beneath it, known as the nail bed. While alarming, a black toenail is frequently caused by common and generally benign factors. This article explores the most frequent reasons behind this change in toenail color.
Physical Trauma and Injury
The most common reason a toenail turns black is physical trauma or injury. This occurs when small blood vessels beneath the nail rupture, leading to bleeding into the confined space between the nail plate and the nail bed. The accumulation of blood forms a subungual hematoma, which appears as a dark red, purple, or black discoloration. The color darkens as the blood clots and breaks down.
This type of injury can result from a single incident, such as dropping a heavy object onto the toe. It also frequently develops from repetitive, minor impacts, often seen in activities like running, hiking, or walking long distances. Ill-fitting footwear is a major contributor; shoes that are too tight, too narrow, or too loose can cause toes to repeatedly jam against the front or top of the shoe. This constant pressure and friction can lead to microtrauma and bleeding.
A subungual hematoma causes throbbing pain and pressure under the nail. The discoloration will not grow out with the nail; instead, the discolored area gradually moves forward as the nail grows, eventually reaching the tip and being clipped away. If bleeding is extensive, the pressure can be significant enough to cause the nail to lift or even detach entirely from the nail bed over several weeks or months.
Fungal and Other Infections
Beyond physical injury, fungi can also cause toenail discoloration. Onychomycosis, a common fungal infection of the nail, can lead to changes in nail appearance, including thickening, crumbling, and lifting of the nail plate. As fungal organisms grow and accumulate debris under the nail, they can produce dark pigments, resulting in black, brown, or dark green discoloration.
These infections thrive in warm, moist environments, making communal areas like public showers, swimming pools, and shared locker rooms common places for contraction. Discoloration from a fungal infection progresses slowly and can affect multiple nails over time. Unlike a bruise, it does not involve pain unless the nail becomes severely thickened and presses against footwear.
While less common, certain bacterial infections can also contribute to nail discoloration. Some bacteria produce pigments that can stain the nail, or they can accumulate debris that appears dark. Bacterial infections causing black toenails are considerably rarer than fungal infections and often present with additional signs such as redness, swelling, or pus around the nail.
Serious Underlying Conditions
In rare instances, a black toenail can signal a more serious underlying health condition that warrants medical attention. One such concern is subungual melanoma, a form of skin cancer that develops under the nail. This condition often appears as a dark brown or black streak that runs vertically through the nail. Unlike a bruise, this discoloration does not grow out with the nail or fade over time.
Melanoma under the nail may also exhibit irregular borders, vary in color intensity, or spread from the nail plate onto the surrounding skin, a phenomenon known as Hutchinson’s sign. This spreading indicates a higher likelihood of malignancy. Subungual melanoma can be aggressive, making early detection necessary. While much less common than traumatic injuries or fungal infections, medical professionals consider it, especially when there is no clear history of injury or the discoloration behaves unusually.
Rarer systemic conditions or certain medications can lead to nail discoloration. For example, some chronic illnesses or specific drug therapies can affect nail pigmentation or blood flow, leading to changes in nail color. These instances are identified by a medical professional who considers a patient’s full health history and other symptoms.
When to Consult a Doctor
While many black toenails resolve on their own, seeking medical advice is advisable in specific circumstances. Consult a healthcare provider if the discoloration is accompanied by persistent or worsening pain that does not subside after a few days. Any signs of infection, such as increasing redness, swelling, warmth, or pus around the nail, warrant immediate medical evaluation.
Also, see a doctor if the black discoloration does not grow out with the nail over several months, or if you notice a dark streak that widens or spreads into the surrounding skin, particularly without clear injury. If the cause of the black toenail is unknown and there was no history of trauma, a medical professional can accurately diagnose the issue. They can differentiate between benign causes and more serious conditions, ensuring appropriate management and treatment.