The sudden appearance of a dark spot or black discoloration beneath a big toenail is medically known as melanonychia. While a darkened nail often raises immediate concern, the vast majority of cases are not related to a serious health issue. Understanding the difference between a common injury and a more complex condition is the first step toward appropriate care. Identifying the origin of this physical change depends largely on its specific appearance and the circumstances surrounding its onset.
Trauma: The Most Frequent Culprit
The most common reason for a black big toenail is physical injury, resulting in a condition called a subungual hematoma. This occurs when blood vessels rupture beneath the nail plate, causing blood to pool between the nail and the nail bed. A single, acute event, such as stubbing a toe or dropping a heavy object on the foot, can easily cause this type of bruising.
More frequently, the hematoma results from repetitive microtrauma, often seen in runners or athletes. This happens when the toe repeatedly slides forward and impacts the inside of a shoe, particularly if the footwear is ill-fitting. The constant pressure damages the tissue, leading to a slow accumulation of dried blood that appears dark red, purple, or black. This discoloration is uniform and will migrate outward as the nail grows, eventually disappearing at the tip. For a large hematoma, pressure buildup can cause throbbing pain, which is relieved if a medical professional drains the trapped blood.
Distinguishing Other Benign Causes
Not all dark discoloration is caused by trauma; other non-serious conditions can also change the nail’s appearance. Fungal infections (onychomycosis) can sometimes lead to a blackish hue, though they more commonly cause yellow or white changes. The fungus grows beneath the nail, leading to a buildup of keratinous debris that can appear dark brown or black. This discoloration is often accompanied by a thickened or crumbly nail texture, distinguishing it from the solid, bruise-like appearance of a hematoma.
External factors, such as specific medications or dyes, can also stain the nail plate. Systemic health issues, including diabetes, kidney disease, or anemia, may alter nail health and color, leading to various forms of darkening. Unlike a subungual hematoma, these changes develop gradually over time and are not associated with a single traumatic event. The discoloration from staining or underlying systemic conditions often lacks the distinct, localized color change seen with blood under the nail.
Recognizing Serious or Urgent Concerns
The most serious, though rare, cause of a black toenail is subungual melanoma, a form of skin cancer that develops in the nail-forming tissue. This malignancy arises from the pigment-producing cells (melanocytes) and presents as a dark, irregular streak or spot that does not grow out normally with the nail. The discoloration is not related to any recent injury and persists or worsens over time.
A particularly concerning sign is the Hutchinson sign, which involves dark pigmentation spreading from the nail bed onto the surrounding skin of the cuticle or nail fold. Other features that raise suspicion include rapid changes in the size or color of the dark area, irregular borders, or a band of pigment wider than three millimeters. While subungual melanoma is uncommon, recognizing these specific warning signs warrants immediate medical evaluation.
What Happens Next: Diagnosis and Care
When a black toenail appears without a clear cause, or if it exhibits any signs of malignancy, a professional medical evaluation is necessary. The diagnostic process begins with a thorough examination of the nail and a detailed history of any recent injuries or changes. If a fungal infection is suspected, the doctor may perform a potassium hydroxide (KOH) test, which involves scraping a small sample of the nail for microscopic analysis.
If the discoloration is concerning for melanoma, a nail matrix biopsy is the definitive diagnostic procedure to rule out cancer. For a painful subungual hematoma, a doctor may perform a drainage procedure, called trephination, by creating a small hole in the nail plate to relieve pressure. Otherwise, most trauma-induced black toenails require patience, as the darkened area will slowly grow out with the nail over several months. Treatment for a fungal cause involves targeted antifungal medications, which may be topical or oral depending on the infection’s severity.