The sudden appearance of a dark spot beneath a toenail can be alarming, but this discoloration, often referred to as subungual pigmentation, is a common occurrence. The color change, ranging from reddish-brown to dark black, indicates a change in the tissue or nail plate itself. While a black toenail might suggest a single cause, the reasons are varied, spanning from simple physical injury to more complex biological processes. Understanding the source of the discoloration is the first step in determining whether observation is sufficient or if medical attention is required.
The Primary Culprit Trauma and Bruising
The most frequent reason for a black toenail is a subungual hematoma, which is essentially a bruise under the nail plate. This condition results from physical injury that causes blood vessels in the nail bed to rupture and bleed. The trauma can be acute, such as stubbing a toe or dropping a heavy object on it, or chronic, caused by repetitive microtrauma. Repetitive impact often occurs in athletes, particularly runners, whose toes repeatedly press against the inside of a shoe.
The bleeding creates a hematoma trapped between the nail bed and the nail plate. Since the blood has no place to escape, it exerts pressure, which causes the throbbing pain often associated with this injury. The characteristic dark color develops as the trapped blood oxidizes, changing from bright red to dark brown, purple, and eventually black. This change is similar to what happens with any bruise on the skin.
Unlike a bruise on the skin, this discoloration does not fade or get reabsorbed quickly; instead, it remains trapped under the nail. The body’s natural process for resolving a subungual hematoma involves the nail simply growing out, slowly carrying the discolored blood with it. Because toenails grow slowly, this process can take several months to a year until the black spot completely disappears. If the hematoma is large and painful, a medical professional may perform a simple procedure called trephination to drain the blood and relieve the pressure.
Infections and Pigmentation Changes
While trauma is a common cause, dark discoloration can also arise from biological and environmental factors. Fungal infections, medically termed onychomycosis, can lead to nail discoloration that may present as dark brown or black. Specific organisms, such as certain molds like Scytalidium or Alternaria, can produce pigments that stain the nail plate a dark color. This discoloration is often accompanied by other signs of infection, including nail thickening, brittleness, and separation of the nail plate from the nail bed (onycholysis).
Bacterial infections can also contribute to dark discoloration, often presenting as a dark green or greenish-black hue. These infections thrive in the moist, warm environment created by a partially separated nail and can cause debris buildup underneath the nail plate. In these cases, the dark color is not blood but rather a byproduct or pigment produced by the microorganisms. External sources can also stain the nail, such as residual dirt, chemicals, or dyes from dark-colored footwear, which can seep into the space between the nail and the nail bed.
A more concerning biological cause of dark pigmentation involves the production of melanin under the nail. This is known as longitudinal melanonychia, which appears as a dark, pigmented streak running from the base to the tip of the nail. The streak occurs when melanocytes, the pigment-producing cells located in the nail matrix, become activated and deposit melanin into the growing nail plate. While often benign—caused by chronic trauma, certain medications, or being more common in individuals with darker skin tones—this mechanism is also how subungual melanoma manifests.
Determining When to See a Doctor
Distinguishing a benign cause from a potentially serious one often lies in observing specific warning signs and the history of the discoloration. If the dark spot is clearly moving outward with the growth of the nail and is not changing its shape or color, it is likely an old hematoma that can be safely monitored. However, several red flags warrant a prompt consultation with a healthcare provider, such as a podiatrist or dermatologist.
One significant warning sign is a dark, persistent longitudinal streak that is changing in width, color, or border irregularity. Another serious indicator is Hutchinson’s sign, which is the spread of dark pigmentation from the nail plate onto the surrounding skin of the cuticle or nail folds. This periungual spread of pigment suggests that the pigment-producing cells in the nail matrix are proliferating and extending beyond the confines of the nail bed.
Other concerning features include the lack of a clear injury history, the discoloration being wider at the base of the nail than at the tip, or the presence of nail dystrophy, such as splitting or thinning. Seeking medical advice is recommended if the discoloration is new, rapidly changing, or if there is uncertainty about its origin. A doctor can perform a physical examination and may use tools like a dermatoscope or recommend a biopsy to accurately diagnose the cause and rule out subungual melanoma.