When a black spot appears beneath a toenail, the sudden discoloration can be startling, prompting concern about serious health conditions. This type of dark spot, known medically as subungual discoloration, is a common complaint that has several possible causes ranging from the completely harmless to conditions that warrant swift medical attention. Understanding the characteristics of the spot is the first step in determining the likely cause. This information is intended solely for educational purposes and is not a replacement for professional medical advice or diagnosis.
Bruising Under the Nail Plate
The most frequent cause of a black spot under the toenail is a subungual hematoma, which is blood trapped beneath the nail plate. This occurs when blood vessels in the nail bed rupture, causing blood to pool in the confined space between the nail and the underlying tissue. The deep red or purple color of the blood oxidizes over time, often turning dark brown or black.
Acute trauma, such as dropping a heavy object on the toe or stubbing it forcefully, is the most common trigger for a hematoma. This injury is typically accompanied by immediate, throbbing pain due to the pressure exerted by the accumulating blood. The discoloration usually appears immediately or within hours of the incident and may cover a small area or the entire nail.
Repetitive microtrauma is another form of injury that causes subungual hematomas, often without a single memorable incident. Activities like long-distance running, hiking, or wearing poorly fitting shoes cause constant, minor impacts that damage the nail bed over time. This repeated friction and pressure results in small, recurring bleeds that gradually form a dark spot.
Unlike a stain, the hematoma moves forward with the natural growth of the nail plate. Since toenails grow slowly, the spot can take 12 to 18 months to grow out completely. If the pressure is severe, a healthcare provider may perform trephination, which involves carefully draining the blood to relieve pain and prevent nail damage.
Fungal Discoloration and Benign Pigment Changes
Causes other than trauma can result in black discoloration, including certain infections and natural pigmentation processes. Fungal infections, medically termed onychomycosis, can cause the nail to change color, often starting as white or yellow and progressing to shades of brown, green, or black. The dark color is frequently due to a buildup of keratinous debris underneath the nail plate, or from the pigments produced by the specific mold or yeast organism.
Fungal discoloration is usually distinguished by accompanying changes in the nail’s texture and structure. The nail often becomes thickened, brittle, and may crumble around the edges, sometimes lifting away from the nail bed. Since fungi thrive in warm, moist environments, this condition is more common in individuals who frequently wear tight, closed-toe shoes or have underlying conditions like diabetes.
Another benign cause of black or brown discoloration is the natural production of pigment, known as melanonychia. This condition is essentially a mole or nevus occurring in the nail matrix. It typically presents as a thin, linear band of pigment running the entire length of the nail plate. This benign pigmentation occurs when melanocytes in the nail matrix produce melanin, the pigment that colors skin and hair.
This linear pigmentation is often stable, meaning the width and color of the band remain consistent over many years. It is much more common in individuals with darker skin tones, where it can affect multiple nails and is often a normal finding. The pigment is deposited into the nail plate as it grows, creating a distinct stripe.
Identifying Urgent Symptoms
While most black spots are harmless, the discoloration can, in rare instances, signal a serious condition, most notably subungual melanoma, a form of skin cancer. Subungual melanoma often appears as a dark brown or black vertical streak, which can be mistaken for a mole or an old bruise. Unlike a hematoma, which grows out with the nail, the discoloration from melanoma persists and often changes.
Certain features of the spot suggest a need for immediate medical evaluation by a dermatologist or podiatrist. One significant warning sign is a dark streak that begins to widen or change in color intensity over a short period. The borders of a cancerous band are often blurry, irregular, or may show varying shades of color within the stripe.
A particularly concerning feature is the spread of the pigment from the nail plate onto the surrounding skin, especially the cuticle or the nail fold, known as Hutchinson’s sign. Other symptoms include the development of a nodule or ulcer under the nail, persistent bleeding without a clear injury, or the nail plate splitting or separating from the nail bed. These changes indicate a need for professional assessment to rule out malignancy.
Other less common but serious causes of nail discoloration include certain systemic diseases, such as Addison’s disease, or side effects from specific medications. If a black spot appears suddenly without trauma, fails to grow out completely, or is accompanied by pain, swelling, or drainage, consultation with a healthcare provider is prudent.