Diseases of the Nail Unit

The human nail is a complex unit that includes five major modified cutaneous structures: the nail matrix, nail bed, nail plate, nail folds, and cuticle (eponychium). This chapter discusses the function and structure of the five nail components and the pathophysiology affecting each. Also reviewed are nail findings associated with underlying systemic and dermatologic conditions: splinter hemorrhages, koilonychia, transverse nail-plate depressions (Beau’s lines), onycholysis, leukonychia, clubbing, nail-plate pitting, and longitudinal pigmented bands. Infections of the nail are discussed, which include bacterial paronychia, chronic paronychia, and onychomycosis. Figures illustrate the longitudinal section of the fingernail, multiple pigmented longitudinal bands, psoriasis involving the fingernail, late-stage lichen planus of the fingernail, transverse linear grooves, Pseudomonas aeruginosa causing a green nail, psoriasis of the nail, melanonychia striata, and a nail specimen for potassium hydroxide preparation. Tables describe antifungal treatment for toenail onychomycosis as well as selected dermatologic disorders that affect the nail unit. This chapter contains 50 references.