The space beneath the toenail is highly susceptible to injury, often leading to painful fluid collection following trauma or friction. While a fluid-filled lesion can occur, the most frequent presentation is a collection of blood, which people often incorrectly label as a blister. Understanding the nature of the fluid helps determine the best course of action.
Subungual Hematoma Versus Fluid Blister
The most frequent lesion beneath the toenail is a subungual hematoma, a collection of blood, not a true blister. This occurs when injury breaks small blood vessels in the nail bed, trapping blood between the nail plate and underlying tissue. A hematoma presents with distinct dark red, purple, or black discoloration. The blood accumulation in this non-flexible space often leads to intense, throbbing pain due to significant pressure buildup.
A true subungual blister is less common, involving the accumulation of serous fluid (clear or yellowish liquid). This lesion is usually caused by shear stress, where repetitive friction separates skin layers, creating a fluid-filled pocket. A serous blister typically appears lighter in color. Regardless of the fluid type, the rigid nail plate prevents the fluid from escaping, making these under-nail injuries painful.
Common Causes of Under-Nail Injury
Under-nail injuries are triggered by two mechanisms: acute trauma or repeated microtrauma. Acute trauma involves a sudden, forceful impact that directly damages the nail unit. Examples include dropping a heavy object onto the toe or stubbing it against a hard surface. This crushing force ruptures blood vessels, resulting in the rapid formation of a subungual hematoma.
The second, more common cause is repetitive friction, or microtrauma, which frequently affects runners or hikers. This involves the toe repeatedly striking the front of the shoe’s toe box during activity, especially when running downhill or engaging in sports with sudden stops. Poorly fitting footwear, whether too tight or too loose, contributes significantly by allowing the nail to repeatedly press against the inside of the shoe.
Improper nail maintenance, specifically leaving toenails too long, is a contributing factor. An overgrown nail extends past the toe tip, increasing the surface area available to collide with the shoe during movement. This constant pressure can lead to the separation of the nail plate or the formation of a hematoma or blister. This repetitive impact is sometimes colloquially referred to as “runner’s toe” or “black toenail.”
At-Home Management and Pain Relief
Managing a mild subungual injury at home focuses on reducing pain and swelling without risking infection. Immediately following the injury, applying the R.I.C.E. protocol—Rest, Ice, Compression, and Elevation—can help limit the initial swelling and throbbing. Resting the affected foot and applying a cold compress helps constrict blood vessels and numb the area.
Over-the-counter non-steroidal anti-inflammatory drugs (NSAIDs) can manage pain and inflammatory swelling. Keep the injured area clean and dry to prevent bacteria introduction. Due to pressure, the nail often separates from the nail bed (onycholysis). The old, discolored nail will eventually shed as a new, healthy nail grows in.
A toenail takes several months to grow out fully, and discoloration remains until the damaged section is replaced. Crucially, do not attempt to drain the fluid or blood using unsterile objects. This practice carries a high risk of introducing infection to the nail bed, potentially leading to serious complications and long-term nail deformity.
Warning Signs Requiring a Doctor Visit
While many subungual injuries can be managed at home, certain symptoms indicate the need for professional medical attention. Severe, unrelenting throbbing pain that does not improve with rest or medication is an urgent warning sign. This intense pain suggests a significant pressure buildup, which a doctor can relieve through a sterile procedure called trephination.
Signs of a developing infection prompt an immediate visit to a healthcare provider.
Signs of Infection
- Increased warmth, redness, and swelling extending beyond the nail area.
- Pus draining from under the nail.
- Red streaks moving up the toe.
- A fever accompanying the injury.
If the injury was caused by a severe crush or impact, an X-ray may be necessary to rule out an underlying fracture of the distal phalanx (the bone at the tip of the toe). A professional evaluation is also necessary if the discoloration covers more than 50% of the nail plate, or if the nail is significantly loose or appears to have a deep laceration underneath. Medical professionals can perform drainage safely and assess for damage to the nail bed, which helps prevent permanent nail deformity.