Koilonychia: an update on pathophysiology, differential diagnosis and clinical relevance.

Koilonychia, a concave nail dystrophy, has multiple aetiologies and may be hereditary, acquired or idiopathic. Within dermatology, koilonychia is often a manifestation of an inflammatory dermatosis such as psoriasis or lichen planus, or a sign of onychomycosis. Other disease associations include iron store abnormalities, Plummer–Vinson Syndrome, nutritional deficiencies and occupational or traumatic aetiologies. In young children, koilonychia of the toenails is commonly transient and idiopathic, although familial and syndromic cases are reported. The dermatologist must be aware of the potential cutaneous and systemic associations with koilonychia in order to guide appropriate workup, treatment and/or referral. An algorithm for evaluation of koilonychia is presented along with discussion of common causes of koilonychia and a comprehensive list of all known associations.