Onychomycosis types

DLSO is the most common subtype. Infection starts at the free edge of the nail or one of the side margins, typically on the toenails, and spreads beneath the nail plate. As the fungus advances, it leads to thickening under the nail (subungual hyperkeratosis), separation of the plate from the bed (onycholysis), and a yellow–brown discolouration. Dermatophytes, especially Trichophyton rubrum, are frequent causes. Because of its high prevalence, DLSO is often the main focus of treatment guidelines and clinical management. 

In superficial onychomycosis, the infection is limited to the surface layers of the nail plate. It usually appears as chalky white patches, although darker streaks or spots may occur in some cases. Since the infection remains on the outer surface, deeper structures such as the nail bed are often unaffected. Early treatment of onychomycosis tends to be more successful because the fungus has not penetrated the nail’s deeper layers. 

PSO occurs when fungi enter through the proximal nail fold or cuticle region, affecting the newly formed part of the nail and gradually moving outward. A white patch near the cuticle is characteristic, and separation of the nail plate may begin proximally rather than distally. This form is less common in healthy individuals and may indicate an underlying immune disorder, so its presence sometimes prompts further medical evaluation. 

In mixed-pattern infections, a single nail displays features of more than one subtype. For example, distal subungual changes combined with superficial white patches, or a combination of PSO and WSSO. Mixed patterns may occur in long-standing infections or when more than one fungal species is involved. These cases can be more challenging to diagnose and may require a broader therapeutic approach. 

TDO represents the advanced stage of fungal nail disease. The entire nail becomes thickened, brittle, discoloured, and often severely deformed. Large amounts of debris accumulate beneath the plate, and in some cases the nail structure is almost completely destroyed. TDO may develop after years of untreated DLSO, PSO, or mixed-pattern infection. Treatment is more difficult at this stage, and regrowth may be limited. 

Secondary onychomycosis occurs when a fungal infection develops on top of an existing nail disorder. Conditions such as nail psoriasis, chronic paronychia, lichen planus, or previous trauma may weaken the nail’s structure, making it more susceptible to fungal invasion. These underlying issues can obscure the clinical appearance, so both conditions usually need to be addressed to achieve effective treatment.