A dark discoloration under a toenail can be a source of concern. While often due to minor, treatable issues, it can sometimes indicate a more serious condition requiring medical attention. Understanding the potential reasons helps determine the appropriate course of action.
Blood Under the Nail
A common cause for a dark toenail is a subungual hematoma, or blood trapped beneath the nail. This typically results from trauma, such as stubbing a toe, dropping a heavy object, or repetitive pressure from ill-fitting shoes. The impact damages small blood vessels, leading to bleeding.
A subungual hematoma initially appears red or purple, then turns dark brown or black as blood clots. This discoloration grows out with the nail over several months, typically 6-9 months for toenails. Pain and throbbing are common symptoms due to the pooled blood’s pressure. For severe pain, a healthcare provider may perform trephination, making a small hole in the nail to drain blood and relieve pressure.
Fungal and Other Infections
Toenail discoloration can also stem from infections, with fungal infections (onychomycosis) being particularly prevalent. Fungus often causes the nail to become discolored (white, yellow, brown, or black), thicken, or become brittle, crumbly, or misshapen.
These infections are typically acquired in moist environments, like public showers or locker rooms, where fungi thrive. Fungi can enter through small cracks in the nail or surrounding skin. While usually not painful in their early stages, severe fungal infections can cause discomfort, difficulty walking, or nail separation. Less commonly, bacterial infections can cause green or black discoloration, often with redness, swelling, and pus around the nail.
Pigmentation Changes and Melanoma
Changes in nail pigmentation can also lead to dark marks. Benign pigmentation often appears as a dark streak running the length of the nail, known as longitudinal melanonychia. This occurs when pigment-producing cells in the nail matrix create melanin, which incorporates into the growing nail plate. Such streaks are more common in individuals with darker skin tones and can affect multiple nails.
Conversely, a dark streak or spot under the nail can indicate subungual melanoma, a rare but aggressive skin cancer. It often presents as a brown or black vertical band, typically wider than 3 millimeters, with irregular borders or varying color. Signs differentiating it from benign causes include rapid growth, changes in size or shape, and discoloration extending to the surrounding skin (Hutchinson’s sign). While Hutchinson’s sign is a strong indicator, it is not always definitive for melanoma, as some benign conditions can also cause periungual pigmentation.
When to Seek Professional Medical Advice
It is important to seek professional medical advice for a dark toenail if the discoloration appears without any known trauma, or if it spreads, changes in size or shape, or affects the surrounding skin. Persistent pain, swelling, or signs of infection such as pus, warmth, or redness also warrant medical evaluation.
If melanoma is suspected, such as an irregular dark streak, rapidly changing mark, or Hutchinson’s sign, consult a healthcare provider. Individuals with diabetes or weakened immune systems should also consult a doctor for any toenail changes, as they are at higher risk for complications. A doctor can accurately diagnose the cause and recommend appropriate treatment, which might involve examination, imaging, or a biopsy.