Noninflammatory nail changes starting at the free edge of the nail are usually due to trauma, psoriasis, or a fungus infection. By contrast the inflammatory processes are usually due to either bacterial or monilial (Candida albicans) infection and have their origin in the living tissue of the lateral walls of the nail groove.
Thickening of the nail starting at the free edge is usually thought to be a sure sign of a fungus infection (onychomycosis). Unfortunately the same appearance of the nail occurs in psoriasis (Fig 1) when it affects the epidermis underlying the free edge of the nail. Although in psoriasis the thickening is not due to true nail plate formation but rather to a heaping up of keratin under the free edge of the nail, the clinical distinction is often impossible to determine. Another diagnostic sign of onychomycosis is the crumbling and destruction of the thickened nail, but
Halprin KM. Afflictions of a Vestigial Appendage: III. Disorders of Free Edge and Lateral Margins of the Human Nail (Psoriasis, Onychomycosis, Monilial, Bacterial Infections). JAMA. 1968;203(7):513. doi:10.1001/jama.1968.03140070069015
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