Distal subungual onychomycosis, or DSO, is the most common type of onychomycosis. The site of the invasion in this form of the disease is the nail bed as well as the underside of the nail plate. As such, the two main consequences of distal subungual onychomycosis are onycholysis and a thick subungual region, the former of which is when the nail plate detaches from the nail bed. DSO can affect both the fingernails and toenails, though infections of the toenails are far more common. This is likely due to the fact that tinea pedis, an infection of the feet, occurs more frequently than tinea manuum, an infection of the hands. Distal subungual onychomycosis is often hereditary in nature.
Proximal subungual onychomycosis, also known as PSO or proximal white subungual onychomycosis, is a rare subtype of onychomycosis that involves all layers of the nail. The infection begins in the proximal nail fold and spreads distally throughout the nail. This then leads to proximal onycholysis, subungual hyperkeratosis, destruction of the proximal nail plate, and leukonychia. PSO is often seen in AIDS patients and is therefore considered to be an early indication of HIV infection. The fingernails and toenails are affected equally and the dermatophytic fungi known as Trichophyton rubrum are usually responsible.
White superficial onychomycosis, or WSO, is a less common type of onychomycosis that is characterized by a fungal invasion of the nail plate. It is more common in toenails than fingernails and is most often caused by Trichophyton interdigitale. Sometimes the infection spreads from the nail plate to the nail bed and hyponychium, the latter of which refers to the skin beneath the free edge of the nail. In approximately 10% of cases, the infection develops to a point where the entire surface of the nail is covered in dull patches with white edges. As a result, the affected nail becomes rough and fragile, increasing the risk that it will crumble.
Total dystrophic onychomycosis, also known as TDO or severe onychomycosis, is the most advanced type of onychomycosis which can take up to 15 years to develop. In this form of end-stage nail disease, the whole nail thickens and becomes dystrophic. Moreover, there is complete destruction of the nail and the structure is completely lost. When this stage is reached and the nail is not responding to treatment, more extreme measures such as nail removal or toe amputation may be necessary.
Candidal onychomycosis, otherwise known as a yeast infection of the nail, is an uncommon type of onychomycosis that affects the nail and nail folds. It has been classified as its own category of onychomycosis because of how it differs from dermatophytic onychomycosis. In contrast to other types of onychomycosis, Candidal onychomycosis more often affects fingernails than toenails and can be very painful. In addition, the infection usually begins in the structures surrounding the nail, instead of in the nail itself.