Darkened toenails can have numerous underlying origins. The resulting discoloration, which may appear as black, brown, or deep purple, is typically caused by alterations occurring within the nail plate or the tissue of the nail bed beneath it. Understanding the specific reason for the change is important, as the causes range from common, harmless physical events to rare, more serious medical conditions. While many cases resolve without intervention, others require attention from a medical professional.
Physical Injury and Trauma
The most frequent cause of a dark toenail is a subungual hematoma, which is simply a bruise beneath the nail plate. This occurs when trauma, such as stubbing a toe or dropping a heavy object on the foot, causes blood vessels in the nail bed to rupture. The resulting collection of blood becomes trapped beneath the hard nail, leading to immediate discoloration that can range from reddish-purple to deep black.
Repetitive, minor impacts can also cause this condition, a phenomenon often seen in runners or hikers whose toes repeatedly strike the front of tight footwear. The pressure from the pooled blood often causes throbbing pain that can be relieved by a medical professional draining the blood, a procedure known as trephination. If left untreated, the hematoma will grow out slowly with the nail over several months, a process that can take up to a year for toenails.
Fungal Infections and Environmental Factors
A fungal infection of the nail, known as onychomycosis, is a very common cause of toenail discoloration. While often associated with yellow or white changes, some fungal species produce pigments that result in brown or black discoloration. The infection can also cause a buildup of dark debris under the nail, a mixture of fungal matter and dead skin cells, which contributes to the overall darkening.
The nail plate may become thickened, brittle, and distorted as the fungal organism invades the keratin structure. Toenails are particularly susceptible because they are frequently exposed to the warm, moist, and dark environments created by shoes and socks. Poor foot hygiene and conditions like athlete’s foot or peripheral vascular disease increase the risk for these infections. Distinguishing this cause from a hematoma involves noting that the discoloration usually begins at the edge of the nail and is accompanied by a change in nail texture.
Systemic Conditions and Natural Pigmentation
Darkening of the toenail can occur due to internal physiological processes or systemic conditions unrelated to trauma or infection. One such phenomenon is longitudinal melanonychia, which presents as a brown or black vertical band running the length of the nail. This banding is caused by active melanin production from melanocyte cells located in the nail matrix.
This pigmentation is a normal variant in many individuals, particularly those with darker skin tones, where it can affect multiple nails and is considered benign. Melanonychia can also be triggered by certain medications, such as chemotherapy drugs or minocycline, which stimulate melanin production. Underlying medical issues like Addison’s disease or chronic kidney disease can also cause nail discoloration, often affecting multiple nails simultaneously.
Identifying Serious Symptoms and Seeking Treatment
While most dark toenails are due to benign causes like bruising or fungus, a rare but serious concern is subungual melanoma, a form of skin cancer. Differentiating a bruise or benign pigmentation from this malignancy requires attention to specific visual cues. A dark spot or streak that appears without any history of trauma and rapidly changes in size or shape should prompt immediate medical evaluation.
One important sign to look for is Hutchinson’s sign, which is the spread of the dark pigment from the nail bed onto the skin surrounding the nail, specifically the cuticle or nail folds. Unlike a hematoma, which grows out with the nail over time, a malignant dark streak often has irregular borders, varies in color, and widens proximally. If a dark streak is wider than three millimeters or affects only a single nail, especially the big toe, consultation with a dermatologist or podiatrist is warranted for an accurate diagnosis.