Discovering a black spot under a toenail can be surprising and concerning. This discoloration, medically known as melanonychia, has several causes, ranging from common, harmless injuries to rare but serious conditions. Understanding the spot’s characteristics is the first step in determining its origin and whether it requires professional attention.
Blood Under the Nail
The most frequent cause of black spots on a toenail is a subungual hematoma, which is essentially a bruise beneath the nail plate. This occurs when physical trauma causes blood vessels in the nail bed to rupture and bleed, pooling blood between the nail and the underlying tissue.
The trauma may be a single, sudden event, such as stubbing a toe or dropping a heavy object on the foot. More commonly on the toenails, a hematoma results from repeated, minor impacts, such as wearing ill-fitting shoes or the constant jarring from running.
A subungual hematoma typically appears as a solid black, purple, or reddish-brown spot, often uniform in color, and may be accompanied by throbbing pain and pressure. The spot is fixed to the nail bed and will grow out with the nail plate, migrating slowly toward the tip over several months.
Natural Pigmentation and Moles
Another cause for black spots involves the natural production of melanin, the pigment that gives color to skin and hair. This is referred to as melanonychia striata when it presents as a vertical band of color. It results from melanocytes, the pigment-producing cells in the nail matrix, depositing melanin into the nail plate as it grows.
In many individuals, particularly those with darker skin tones, this pigmentation is a normal variant and may appear in multiple nails. The resulting stripe is usually a uniform brown or black color, and it is a benign finding. A benign nevus, or mole, in the nail matrix can also cause a dark spot or stripe.
A mole-related spot is typically stable, meaning its width and color remain consistent over many years. The key distinguishing feature of these benign pigmented lesions is their stability and slow change.
Identifying Subungual Melanoma
While most dark spots are benign, a rare and serious cause is subungual melanoma, a form of skin cancer that originates in the nail matrix. This cancer has a poorer prognosis due to delayed diagnosis. Unlike a bruise, this discoloration often appears without trauma and does not grow out with the nail.
Warning signs that differentiate melanoma from harmless pigmentation are summarized by the “ABCDEF” rule. “B” stands for a brown-to-black band greater than 3 millimeters wide. “C” refers to a change in width, color, or shape; a spot that rapidly changes or remains fixed while the nail grows is concerning.
“D” signifies a high-risk digit, with the thumb and the great toe being the most common sites. “E” stands for extension of the pigment onto the surrounding skin, known as Hutchinson’s sign. This means the pigment has spread to the skin of the cuticle or the nail folds.
The borders of a melanoma spot, represented by “F” for form, are often irregular, blurred, or notched. The color may be varied, containing streaks of different shades of brown, black, or blue. A non-traumatic spot showing irregular growth or color change warrants prompt medical evaluation.
When to Seek Medical Attention
While many black spots are harmless, certain characteristics require immediate evaluation by a healthcare professional, such as a podiatrist or dermatologist. Any dark spot or stripe that appeared without a known injury and fails to move forward as the nail grows should be examined. This lack of distal migration suggests the pigment source is continuously active in the nail matrix.
A medical consultation is necessary if the discoloration is rapidly changing in size, shape, or color, or if its borders become irregular. The presence of Hutchinson’s sign, where the pigment spreads onto the surrounding skin of the nail fold or cuticle, is a concerning feature. Bleeding, ulceration, or the development of a bump or nodule beneath the nail also require an urgent biopsy.
If the black spot is accompanied by signs of infection, such as increasing pain, swelling, warmth, or pus, a medical visit is also necessary. Seeking a professional opinion provides definitive diagnosis and peace of mind.