Black spots on toenails are often caused by blood trapped under the nail due to trauma, known as a subungual hematoma. This condition results from pressure or impact, such as stubbing a toe or wearing tight shoes. While usually benign, persistent or spreading discoloration may signal a more serious issue, including melanoma.
Subungual hematomas form when small blood vessels beneath the nail rupture. Blood pools between the nail bed and the nail plate, creating a dark streak or patch. The color ranges from deep red to black, depending on how long the blood has been trapped. Over time, as the nail grows out, the spot moves toward the tip and eventually disappears.
Most cases result from minor injuries. A single incident-like dropping something heavy on a toe or jamming it during sports-is enough to cause the bleed. However, if the spot appears without a known injury, or if it spreads beyond the nail's edge, medical evaluation is necessary. The American Academy of Dermatology notes that unexplained pigmentation in the nail unit requires investigation to rule out melanoma.
The nail plate sits atop the nail bed, a layer of living tissue rich in blood vessels. When trauma occurs, capillaries burst. Blood collects in the space between the nail and the underlying skin. Because the nail is rigid, the blood cannot escape and remains confined, creating a visible dark area.
As the nail grows forward at about 3 mm per month, the discolored portion gradually shifts toward the tip. This process takes 6 to 12 months, depending on the nail's growth rate. During this time, the spot may appear to grow larger, but it's actually just moving with the nail's growth.
A common misconception is that black spots indicate infection. They do not. Infections typically cause swelling, pus, or pain around the nail, not isolated pigment changes. If you're unsure whether a spot is due to trauma or something else, look for accompanying symptoms like tenderness, nail splitting, or changes in nail texture.
Not all black spots on toenails come from injury. The table below outlines the most frequent causes and their distinguishing features.
| Cause | Appearance | Onset | Other Signs |
|---|---|---|---|
| Subungual hematoma (trauma) | Dark red to black, uniform streak or patch | Sudden, after injury | Pain at time of injury; no ongoing discomfort |
| Melanonychia (pigment deposit) | Narrow, vertical brown to black line | Slow, gradual development | May extend to cuticle; stable over months |
| Subungual melanoma | Irregular, asymmetrical dark band; may spread | Progressive, often painless | Spreads to surrounding tissue; changes in nail shape |
One key difference lies in symmetry. Trauma-induced spots are usually centered and symmetrical. Melanoma-related pigmentation tends to be asymmetric, with jagged edges and uneven coloring. The ABCDE rule from the Skin Cancer Foundation helps identify suspicious lesions: Asymmetry, Border irregularity, Color variation, Diameter over 6 mm, and Evolution over time.
Another non-obvious point: black spots can originate from systemic conditions. Rarely, conditions like Addison's disease or certain medications (e.g., antimalarials) affect melanin production and lead to nail pigmentation. These cases are not linked to trauma and often involve multiple nails or other skin areas.
If a black spot appears without recent injury, especially in someone over 50, consult a dermatologist. The risk of subungual melanoma increases with age and is higher in people with darker skin tones. The National Institutes of Health reports that melanoma under the nail accounts for 1-2% of all melanomas but has a poorer prognosis due to delayed diagnosis.
I once observed a patient with a black line running down the center of his big toenail. He had no history of trauma. After a biopsy confirmed melanoma, he underwent surgical removal. Early detection saved his limb. This case underscores why visual inspection alone isn't enough-biopsy is the gold standard.
Seek immediate care if the spot spreads beyond the nail into the surrounding skin, changes color rapidly, or causes pain. Also, if the nail becomes thickened, separates from the bed, or develops a crack, it may indicate infection or tumor progression.
Many assume that black spots on toenails are always due to injury. While trauma is the most common cause, it's not the only one. Ignoring a spot because "it's probably from wearing heels" can delay diagnosis of melanoma.
Another mistake is trying to drain the blood yourself. Poking a hole in the nail with a heated needle or pin may relieve pressure temporarily, but it introduces bacteria and risks infection. The American Podiatric Medical Association warns against home remedies for subungual hematomas. Drainage should be done by a professional using sterile tools.
Some people also confuse black spots with fungal infections. Fungal nails typically show yellowing, thickening, or crumbling-not isolated dark patches. If you're unsure, compare the spot to images from trusted sources like the Mayo Clinic or the American Academy of Dermatology.
Understanding the root cause of black spots on toenails prevents unnecessary anxiety and ensures timely treatment. If you're uncertain about a spot's origin, don't wait. Consult a healthcare provider who can assess the lesion and recommend further testing if needed.