Dark discoloration of a toenail, ranging from deep brown to black, is a common observation that often prompts concern. While the appearance can be alarming, the underlying causes vary widely, from mechanical injury to serious medical conditions. Understanding the source of the darkening is important for determining whether the symptom is a harmless bruise or requires professional evaluation. Causes of dark toenails involve physical trauma, biological invaders, or changes in the body’s natural pigment production.
Blood Under the Nail (Trauma)
The most frequent cause of a dark toenail is a subungual hematoma, which is a bruise beneath the nail plate. This occurs when blood vessels in the nail bed rupture, trapping blood between the nail and the tissue below. The resulting color is initially reddish-purple, but it quickly oxidizes to a dark brown or black hue.
Trauma can stem from an acute, forceful injury, such as dropping a heavy object on the foot or severely stubbing a toe. More commonly, it results from chronic, repetitive micro-trauma, often seen in runners or athletes wearing ill-fitting footwear. Repeated impact against the shoe’s toe box causes minor, cumulative damage to the nail bed. As the nail grows, the discolored area will slowly migrate toward the tip and eventually grow out completely, confirming its benign origin.
Infections and Discoloration
Biological organisms, primarily fungi and bacteria, can lead to toenail discoloration. Fungal infections, known as onychomycosis, are widespread and typically cause the nail to become thickened, brittle, and discolored. Although often presenting as yellow or white, the infection can turn dark brown or black due to the accumulation of pigmented debris or specific fungal strains that produce dark pigments.
Bacterial invaders can produce distinct colors. For example, the bacterium Pseudomonas aeruginosa causes chloronychia, which manifests as a blue-green or dark green-black discoloration. These infections often thrive in moist environments and may be accompanied by the separation of the nail plate from the nail bed. Unlike a bruise, infections usually involve changes in the nail’s texture, leading to crumbling, lifting, or distortion of the nail structure.
Pigment Changes and Serious Conditions
Darkening can be caused by the production and deposition of melanin, the pigment responsible for skin and hair color. Benign longitudinal melanonychia appears as a brown or black streak running the length of the nail. This results from a localized increase in pigment production by melanocytes in the nail matrix. This is common and often considered a normal variant in individuals with darker skin tones, functioning much like a mole or birthmark.
A more concerning cause of pigment change is subungual melanoma, a rare but aggressive form of skin cancer originating in the nail matrix. This condition presents as a dark, often irregular, longitudinal streak that may widen or change color over time. A warning sign is Hutchinson’s sign, which describes the pigment spreading from the nail plate onto the surrounding skin of the cuticle or nail folds. Certain systemic treatments, such as chemotherapy drugs, can also induce benign nail hyperpigmentation that appears as dark bands.
When to Seek Professional Advice
While many dark toenails are simply bruises that will resolve on their own, certain features warrant consultation with a healthcare professional, such as a podiatrist or dermatologist. Any dark discoloration that appears without a clear history of trauma should be evaluated, especially if it affects only one digit. Prompt advice is necessary if the dark area is rapidly changing in size, shape, or color, or if the pigmentation is irregular.
A consultation is also recommended if the dark streak is wider than three millimeters, or if the pigment extends beyond the nail plate onto the adjacent skin (Hutchinson’s sign). Chronic pain, swelling, pus, or any signs suggesting a spreading infection should be immediately checked. Individuals with underlying health conditions like diabetes should have any dark nail change assessed to prevent complications.