Onychomycosis is a common infection that typically results in the nail becoming thick, brittle, and discolored. While most people associate the condition with white or yellow discoloration, a change to a black color can occur. This dark presentation can be caused by the fungus itself or by other conditions, making a proper diagnosis important. Understanding the potential reasons behind this color change is the first step toward effective management.
Understanding the Color Change: Pigmented Fungi
The black appearance of a fungal infection is often due to the presence of pigmented fungi. These organisms naturally produce melanin, a dark brown to black pigment, which becomes incorporated into the fungal cell walls and the nail structure. Specific culprits include non-dermatophyte molds such as Neoscytalidium dimidiatum and certain Acremonium species.
This black pigmentation is distinct from the more common white or yellow appearance caused by dermatophyte fungi, which are the most frequent cause of onychomycosis. Fungal melanonychia, the term for this dark presentation, is a variety of onychomycosis that presents a specific diagnostic and therapeutic challenge. The accumulation of fungal debris, dirt, and byproducts of the infection under the nail can also contribute to a dark, black appearance.
Differential Diagnosis: Other Causes of Black Toenails
When a toenail turns black, it is important to consider common non-fungal causes, as some require urgent medical attention. The most frequent cause is a subungual hematoma, resulting from trauma like stubbing a toe or from repetitive impact, such as in runners. This discoloration typically appears immediately after an injury, may be painful initially, and will grow out gradually as the nail plate advances. The blackness in a hematoma often has a purple or reddish-black hue and a clearly defined boundary.
A more serious, though rare, cause is subungual melanoma, a form of skin cancer that develops in the nail unit. This condition usually presents as a dark brown or black longitudinal streak that may widen over time and often only affects a single nail, most commonly the big toe or thumb. Hutchinson’s sign is a specific concerning sign where the dark pigment extends from the nail plate onto the surrounding skin of the nail fold or cuticle. Unlike a hematoma, which resolves as the nail grows, melanoma discoloration does not improve and may worsen over time, often beginning without any history of injury.
Certain systemic medications can also cause black or dark pigmentation in the nail plate. Chemotherapy drugs and some antibiotics like minocycline are known to activate pigment-producing cells in the nail matrix. Minocycline, for instance, can cause a slate-gray or blue-black discoloration of the nail bed, while chemotherapy agents often cause longitudinal or diffuse brown-black bands.
Assessing Severity and When to Seek Medical Care
Consulting a healthcare provider is necessary to rule out serious conditions. Seek an evaluation if the black discoloration appears suddenly without any known trauma or if it is spreading rapidly. Pain, swelling, or the presence of pus or discharge around the nail are signs of a possible bacterial co-infection that requires immediate medical treatment.
The appearance of a dark streak or band that extends into the skin around the nail (Hutchinson’s sign) requires evaluation for potential melanoma. If the black area does not move forward as the nail grows over several months, or if the nail structure begins to crack, split, or lift from the nail bed without a clear cause, a medical consultation is necessary. Since the visual characteristics of fungal infections, trauma, and cancer can overlap, laboratory testing is the only way to establish a definitive diagnosis.
Treatment Approaches for Pigmented Nail Infections
If a pigmented fungal infection is suspected, a medical professional will first take a sample of the nail and debris for diagnostic testing. Common procedures include a potassium hydroxide (KOH) preparation for immediate microscopic examination and a fungal culture to identify the specific species, which can take several weeks. Nail clippings may also undergo histopathology with special stains like Periodic Acid-Schiff (PAS) to confirm the presence of fungal elements within the nail structure.
Treatment for pigmented onychomycosis, particularly those caused by dematiaceous molds, is often more challenging than for standard fungal infections. These non-dermatophyte molds are frequently resistant to common oral antifungals like terbinafine, the preferred treatment for typical onychomycosis. Due to this resistance, systemic oral antifungals such as itraconazole or voriconazole are often required, as they demonstrate better activity against these specific types of fungi.
Topical treatments like medicated nail lacquers are generally ineffective as a sole therapy for severe or deeply pigmented infections because they struggle to penetrate the thickened nail plate. Treatment success relies on a combination of professional nail debridement, which involves thinning or removing the infected nail material, along with a prolonged course of systemic medication. The entire treatment process can take many months, as the medication works by allowing a new, healthy nail to grow out slowly.