(See also Overview of Nail Disorders.)
About 10% of people have onychomycosis, which most often affects the toenails rather than the fingernails. It is most prevalent among older people, particularly men, and people with poor circulation in the feet (peripheral arterial disease [see Foot care]), diabetes (see Foot problems in diabetes), a weakened immune system (caused by a disorder or drug), athlete's foot, or nail dystrophies.
Onychomycosis returns often, even after prolonged treatment.
Most cases are caused by dermatophytes. Dermatophytes are molds (a type of fungi). The fungus can be acquired through contact with an infected person or through contact with a surface where the fungus is present, such as a bathroom floor.
Infected nails have an abnormal appearance but are not itchy or painful. In mild infections, the nails have patches of white or yellow discoloration. A chalky, white scale may slowly spread beneath the nail’s surface. In more severe infections, the nails thicken and appear deformed and discolored. They may detach from the nail bed (see Tumors of the Nails). Usually, debris from the infected nail collects under its free edge.
A doctor usually makes the diagnosis based on the appearance of the nails. To confirm the diagnosis of onychomycosis, the doctor may need to examine a sample of the nail debris under a microscope and sometimes culture it to determine which fungus is causing the infection or clip off part of the nail and do a test called polymerase chain reaction (PCR). The PCR test is used to produce many copies of a gene from the fungus, making the fungus much easier to identify.
These fungal infections are difficult to cure but do not usually cause complications, so treatment is recommended only if symptoms are particularly severe or bothersome or if the person is at risk of complications. For example, people who have diabetes or peripheral vascular disease and onychomycosis are at risk of developing potentially serious infection of the skin and soft tissues in the feet and legs (called cellulitis).
If treatment is desired, the doctor usually needs to prescribe a drug taken by mouth (orally), such as terbinafine, fluconazole, or itraconazole. These antifungal drugs are taken for a long time, typically at least several months. However, even then the nail will not appear normal until new, healthy nail finishes growing out, which can take 12 to 18 months. The existing deformed or discolored nail does not improve, but newly growing nail should appear normal.
Ciclopirox is an antifungal drug that is mixed in a nail lacquer but is not very effective when used without oral drugs. Efinaconazole and tavaborole are new antifungal drugs that can be applied directly on the nail (topically). They are not as effective as oral drugs but can increase the chance of curing the infection when taken with an oral drug. They can also be used by people who cannot take one of the oral drugs.
To reduce the possibility of a recurrence, the nails should be kept trimmed short, the feet should be dried after bathing (including between the toes), absorbent socks should be worn, and antifungal foot powder or cream should be used. Old shoes may contain a high concentration of fungal spores and, if possible, should not be worn.