Brown toenails, medically known as chromonychia, are a change from the nail’s typical clear appearance. This common discoloration can arise from various sources. While often benign, a brown toenail can also signal underlying health issues. Understanding these factors helps identify potential causes and next steps.
External Factors and Injuries
One frequent cause of brown toenails stems from external factors, particularly trauma or injury to the toe. When an impact, such as dropping an object on the foot or wearing ill-fitting shoes, damages the nail bed, it can lead to a subungual hematoma. This condition involves bleeding underneath the toenail, and the trapped blood appears dark red, brown, or even black as it dries and oxidizes. The discoloration typically grows out with the nail over several weeks to months.
Beyond direct injury, external substances can also stain toenails brown. Dark nail polish, left on for prolonged periods, can penetrate the nail plate, resulting in a yellowish-brown or brown tint. Dyes from shoes or socks, or certain topical medications, might also transfer to the nail. These temporary stains generally resolve as the nail grows out or with cleaning.
Fungal and Bacterial Infections
Microbial invaders are a common reason for toenail discoloration, including shades of brown. Fungal infections, known as onychomycosis, are a prevalent cause, accounting for about 50% of nail diseases. These infections occur when fungi, often dermatophytes, yeasts, or molds, enter the nail, typically through small cracks or breaks. As the fungus grows, it consumes keratin, leading to changes in color, which can range from white or yellow to reddish-brown or dark brown. Infected nails may also become thickened, brittle, or misshapen.
Bacterial infections can also contribute to toenail discoloration. Pseudomonas aeruginosa is a bacterium that commonly causes a greenish-brown or greenish-black discoloration, often referred to as “green nail syndrome” or chloronychia. This bacterium thrives in moist environments, such as those found under artificial nails or within a compromised nail structure. The green hue is due to the production of a pigment called pyocyanin, which diffuses into the nail plate.
Systemic Health Conditions and Medications
Toenail discoloration can sometimes be a subtle indicator of underlying systemic health conditions. For instance, chronic kidney disease can cause various nail changes, including a yellowish or brownish discoloration. In some cases, individuals with kidney disease may develop “Lindsay’s nails,” characterized by the proximal half of the nail appearing white and the distal half showing a reddish-brown or darker hue. This phenomenon is thought to be related to the accumulation of nitrogenous waste products in the bloodstream.
Diabetes can also affect toenail health, making individuals more susceptible to fungal infections that lead to brown or yellow discoloration due to impaired circulation and elevated blood sugar levels. Additionally, certain medications can cause nail discoloration as a side effect. Some chemotherapy drugs, for example, can lead to nails turning black, brown, or blue, and these changes are usually temporary. Other medications, like certain antimalarials or tetracycline antibiotics, have also been reported to cause nail color changes, including shades of brown or blue.
Rare but Serious Causes
While less common, some serious conditions can cause toenails to turn brown, necessitating prompt medical attention. Subungual melanoma, a rare form of skin cancer, develops beneath the nail. It typically appears as a brown or black streak running vertically down the nail. Unlike a bruise, which grows out with the nail, a melanoma streak may widen, change in intensity, or extend to the skin surrounding the nail (Hutchinson’s sign).
Early detection of subungual melanoma is important for successful treatment, as it can spread to other parts of the body if left untreated. Any new, persistent, or changing brown or black streak under a toenail, especially if it affects a single nail or extends to the surrounding skin, warrants immediate evaluation by a healthcare professional.