Onycholysis is defined as the detachment of the nail plate from its bed at its distal end and/or from lateral attachments. Detachment of the nail plate creates a subungual space that may gather dirt and keratin debris. As moisture accumulates beneath the nail, secondary infection by bacteria and yeasts can also occur.

Fingernail onycholysis is more common in women than men and is common in patients with hand dermatitis. Fingernail and toenail onycholysis is most often caused by trauma. Other causes of onycholysis include hyper- and hypothyroidism, pregnancy, porphyria, pellagra, syphilis, atopic dermatitis, psoriasis, irritant or allergic contact dermatitis, lichen planus, congenital abnormalities of the nails, onychomycosis and other infections, and underlying malignancies. Tetracycline, quinolones, oral contraceptives, and certain chemotherapeutic agents are the most frequently associated medications. Systemic retinoids may also be culprits.

Blistering diseases, such as porphyria cutanea tarda or pseudoporphyria, will give rise to pain followed by onycholysis when a subungual vesicle or bulla appears. This is induced by sunlight and is known as photoonycholysis.

In children, onycholysis is most often due to trauma particularly when one or a few nails are affected. Psoriasis, lichen planus, and congenital abnormalities are also possibilities. If these etiologies have been ruled out, a systemic cause should be explored.