Onychomadesis is the spontaneous separation of the nail plate from the matrix area; the distal part of the nail plate being still firmly attached to the nail bed. The remaining nail is eventually shed. Profound nail matrix arrest of a minimum of 1-2 weeks halts nail production with generation of a partial or full thickness nail defect. It is thought to represent the severe form of a Beau line. Due to the subsequent shedding of the nail plate, it is considered a severe form of onycholysis.

Nail matrix arrest may be due to either local or systemic disease. Known associations with onychomadesis include infections, medications (particularly chemotherapies), radiation, autoimmune diseases (pemphigus vulgaris, alopecia areata, bullous pemphigoid), finger or wrist fracture with cast immobilization, high altitude, deep-sea dives, systemic disease (reflex sympathetic dystrophy, diabetes mellitus, dialysis, Raynaud phenomenon), yellow nail syndrome, keratosis punctata, acute paronychia, and repeated local trauma from footwear. Cases may be idiopathic. A rare familial form with autosomal dominant inheritance has been reported.

When acute, the condition may be associated with swelling of the proximal nail fold due to an elevation of the proximal part of the nail plate. The course is typically self-limited.

Pathophysiologically, retronychia is similar to onychomadesis, but the new nail plate grows vertically into the proximal nail fold.

In children, hand-foot-and-mouth disease is a common cause of onychomadesis.