When a dark spot appears beneath a toenail, it is often caused by an injury related to footwear. Tight or ill-fitting shoes can cause a black toenail, but the discoloration is always a symptom of physical trauma. This common issue is frequently seen in athletes, especially runners, but it can affect anyone wearing shoes that repeatedly put pressure on the toes. Understanding the mechanism behind this injury and distinguishing it from other causes of nail discoloration is important for proper care.
The Direct Cause: How Footwear Pressure Creates a Black Toenail
The black appearance under the nail plate is caused by bleeding, medically termed a subungual hematoma. This condition results from direct trauma to the blood vessels in the nail bed, the tissue directly beneath the nail. Tight shoes trigger this injury through repeated microtrauma, where the toe is constantly jammed against the inside of the shoe.
This repetitive pressure, particularly during activities involving stopping, starting, or running downhill, causes friction and impact. The blood vessels under the nail bed rupture and leak blood, which then pools in the confined space between the nail plate and the underlying tissue. Since the blood has nowhere to drain, it coagulates and appears dark blue, purple, or black through the translucent nail.
The pooling of blood creates pressure, leading to the throbbing sensation and pain associated with a newly formed hematoma. The body treats the trapped blood like a bruise that must grow out with the nail. Depending on the size of the hematoma, the pressure can cause the nail plate to eventually lift and detach from the nail bed.
Distinguishing Other Causes of Nail Discoloration
While trauma from tight shoes is a frequent cause of a black toenail, other conditions can produce similar discoloration, making differentiation important. One common alternative is a fungal infection, or onychomycosis, which typically causes a yellow-brown or white discoloration first. The nail may become thickened and brittle, and the dark color can sometimes result from debris buildup or pigmentation changes.
A more concerning, though rare, cause is subungual melanoma, a form of skin cancer that develops in the nail bed. Unlike a hematoma, melanoma usually appears as a dark brown or black streak that runs vertically from the cuticle to the tip of the nail. This discoloration often has irregular borders and may extend into the surrounding skin, known as the Hutchinson sign.
A key way to distinguish a traumatic hematoma from a melanoma is by observing its progression over time. A hematoma will move forward as the nail grows out, eventually disappearing at the tip. Conversely, a melanoma streak will remain fixed in position and may widen or darken over time, independent of nail growth. Trauma-induced discoloration is typically a smudge or a patch, while melanoma is more often a distinct, persistent linear band.
Prevention Strategies and When to Seek Medical Care
Preventing black toenails involves ensuring your footwear is properly fitted, especially if you engage in regular physical activity. A shoe should allow approximately a thumb’s width of space between the tip of your longest toe and the end of the shoe. This space accommodates the natural movement and swelling of the foot during exercise, preventing the toes from repeatedly hitting the shoe box.
Choosing shoes with a deep and wide toe box can reduce pressure on the nails. Keeping toenails trimmed straight across and relatively short minimizes the chance of them pressing against the shoe. Wearing moisture-wicking socks can also help reduce friction.
When to Seek Medical Care
Medical advice should be sought if the black discoloration appears without any memory of trauma, or if the dark area does not begin to grow out with the nail. Consultation is also necessary if the discoloration involves a dark streak that extends onto the skin around the nail. A medical professional should be consulted if the toe is intensely painful, shows signs of infection like pus or excessive swelling, or if the pain is severe enough to require draining the pooled blood to relieve pressure.