A dark or black area appearing beneath a toenail can be a concerning observation. This discoloration signals a change in the tissue underneath the hard nail plate, which can be caused by various factors, ranging from a common, minor injury to a rare but serious medical condition. The opaque nature of the nail often makes it difficult to determine the exact origin of the darkened pigment without professional assessment. Understanding the different potential causes and their typical appearances is important for determining the appropriate course of action. A careful look at the history of the discoloration helps in deciding if the issue can be monitored at home or requires consultation with a healthcare provider.
Discoloration Due to Physical Injury
The most frequent reason a portion of the toenail turns black is a condition called a subungual hematoma. This occurs when trauma causes blood vessels underneath the nail bed to rupture and bleed, trapping the blood in the confined space between the nail plate and the underlying tissue. The resulting dark color is essentially a bruise under the nail, which can appear brown, dark purple, or black as the blood oxidizes.
An acute impact, such as dropping a heavy object directly onto the toe, is a common cause. Alternatively, repetitive microtrauma from activities like long-distance running or hiking can cause the toe to repeatedly strike the inside of a shoe. This repeated pressure can generate enough force to cause bleeding under the nail, often affecting the big toe.
If the bleeding is significant, the accumulated blood can exert considerable pressure, leading to throbbing pain. For minor cases without severe pain, home care involves applying ice and elevating the foot to reduce swelling and discomfort. The dark area will not disappear; instead, it will slowly grow out with the nail, a process that can take up to a year or more for a toenail to fully replace itself.
Non-Traumatic Medical Causes
When a dark spot appears under the nail without any clear history of trauma, the cause may stem from internal conditions related to pigmentation or disease. The most concerning of these is subungual melanoma, a type of skin cancer that develops in the nail matrix. This condition often presents not as a bruise, but as a dark, irregular vertical streak, a pattern known as longitudinal melanonychia.
This streak usually starts at the base of the nail and may be wider than three millimeters, sometimes with blurred or irregular borders. A particularly concerning sign is the spread of the dark pigment from the nail plate onto the surrounding skin of the cuticle or nail fold, which is medically termed Hutchinson’s sign. While rare, this form of cancer requires immediate investigation.
Other causes of dark streaks are benign, such as a nail matrix nevus, which is essentially a mole occurring under the nail. These are also characterized by longitudinal melanonychia, but they typically remain stable in size and color over time. Differentiation between a benign nevus and melanoma requires a professional evaluation.
Fungal infections, while typically causing yellow or white discoloration, can also lead to a black appearance in chronic or severe cases. This happens because the fungus causes a buildup of dark debris underneath the nail plate, or the fungus itself produces a dark pigment. Systemic health issues or medications can also cause nail discoloration due to changes in pigmentation or blood flow. Certain drugs or conditions, such as those affecting adrenal gland function, can cause increased pigment production that manifests in the nails.
Determining When to Seek Medical Attention
While many cases of black toenails resolve on their own, professional medical evaluation is necessary if there is no known injury, or if the discoloration is unexplained, sudden, or persistent. If the dark area begins to change shape, size, or darkness over weeks or months, this suggests an active pathological process instead of a resolving bruise.
A discoloration that extends beyond the edges of the nail plate, coloring the skin of the cuticle or the nail folds, should be seen by a doctor promptly. This finding raises the suspicion of subungual melanoma. Similarly, if the dark streak has a width greater than three millimeters, these features increase the probability of a more serious underlying issue.
Persistent or increasing pain, warmth, redness, or discharge around the nail could indicate an infection or a fracture in the underlying toe bone. If the discoloration fails to move outward toward the tip of the toe as the nail grows over a few months, it means the pigment is being continuously produced, which necessitates a biopsy for diagnosis. Individuals with underlying conditions such as diabetes or circulatory problems should also seek medical advice for any nail changes.