A dark spot on a toenail is a common observation that frequently prompts concern regarding its origin. This discoloration, which can appear brown, black, or even green, is a visual sign of an underlying change within the nail unit or the tissue beneath it. The cause ranges widely from a minor physical impact to an infection or, in rare instances, a serious medical condition. Observing the characteristics of the spot is the initial step in determining the source. Understanding the various causes is important for proper monitoring.
Subungual Hematoma: Discoloration from Injury
The most frequent cause of a dark spot on the toenail is a subungual hematoma, which is the accumulation of blood beneath the nail plate. This occurs when blood vessels in the nail bed are damaged by trauma, such as stubbing a toe, dropping a heavy object, or repetitive micro-trauma from ill-fitting footwear. The blood pools in the confined space between the nail bed and the nail plate, leading to immediate pressure and often throbbing pain.
The appearance of the spot is typically a solid patch of red, purple, or dark brown that turns black as the blood clots and dries. Because the blood is trapped, the discoloration remains fixed to the nail plate. The only way for the dark spot to resolve is for the nail to grow out completely, slowly carrying the trapped blood toward the tip.
Given the slow growth rate of toenails, this process can take a year or longer for full resolution. If the injury was severe, the nail may become thicker, brittle, or even separate from the nail bed entirely, a condition known as onycholysis. If the spot appeared suddenly after a known impact, it likely represents this benign type of bruising.
Infectious Causes: Fungal Growth and Debris
Dark discoloration can also result from the activity of microorganisms, most commonly fungal infections known as onychomycosis. While these infections frequently cause a yellow or white appearance, they can also lead to brown or black spots due to the buildup of keratinous debris underneath the nail plate. This debris, a byproduct of the fungal growth, often appears crumbly and can lift the nail away from the bed.
Specific organisms can produce distinct colors. For instance, the bacterium Pseudomonas aeruginosa is notorious for creating a greenish-black discoloration, termed chloronychia. This bacterium thrives in moist environments and often colonizes a nail that has already separated from the nail bed, a common result of fungal infection or trauma. Dark molds, such as certain species of Aspergillus, are another type of non-dermatophyte fungus that can invade the nail unit, contributing to a dark color and significant thickening of the nail.
The presence of a fungal infection is often accompanied by other signs, including a foul odor, thickening of the entire nail plate, and brittleness. In these cases, the dark appearance is caused by the pigments created by the microorganisms themselves or by the dark accumulation of subungual material.
Pigmented Lesions: Moles and Melanoma
True dark spots caused by pigment production involve melanocytes, the cells responsible for melanin in skin and hair, which are also located in the nail matrix. When these cells are activated, they produce a streak of pigment in the nail plate called longitudinal melanonychia. This can be a benign mole or nevus, which typically presents as a stable, narrow, uniform band of color.
The more serious concern is subungual melanoma, a rare but aggressive form of skin cancer that begins in the nail matrix. Distinguishing between a benign mole and malignancy is determined by observing specific clinical features, often summarized by the ABCDEF rule. The “B” in the mnemonic refers to a brown-black band with a breadth of three millimeters or more or blurred borders.
The “C” stands for a change in the band, such as rapid widening or darkening. The “E” represents the extension of pigment onto the skin surrounding the nail, known as Hutchinson’s sign. This spread of pigment onto the cuticle or lateral nail fold is a significant warning sign that the melanocytes are behaving abnormally. Unlike a hematoma, a true pigmented lesion is a streak of color within the nail that grows with the nail plate, and malignancy is signaled by changes in its width, color, and border characteristics.
Critical Indicators for Medical Evaluation
A dark spot on a toenail warrants professional evaluation if certain changes or characteristics are present, as early diagnosis of serious conditions significantly improves outcomes. A spot that appeared without any history of trauma or injury should be assessed by a dermatologist or podiatrist. Any dark streak that rapidly changes in width, color, or shape over weeks or months is a significant red flag.
The appearance of pigment bleeding onto the surrounding skin, known as Hutchinson’s sign, is a concerning feature that necessitates immediate attention to rule out melanoma. Other signs that indicate a need for professional diagnosis include severe pain not relieved by home care, signs of infection such as redness, swelling, or pus discharge, or if the nail is splitting or bleeding over the darkened area. If a dark spot, which is not a bruise, fails to grow out with the nail over several months, it indicates an ongoing process that requires definitive diagnosis, often through a biopsy.