Nail disorders in children — branching viva

Opening question

Examiner: Take me through this child. What is the likely diagnosis, and what is your frame for assessing a disordered nail? ^[1]

Candidate: The likely diagnosis is onychomycosis — a thickened, discoloured, crumbly toenail with subungual hyperkeratosis and distal onycholysis, in a child with a household contact, which is a recognised transmission route. My frame for any disordered nail rests on one principle: localise the lesion to the part of the nail unit it arises from. The matrix makes the plate, so matrix disease shows as surface and shape change; the nail bed sits under the plate, so bed disease shows as colour and attachment change; the nail fold surrounds the nail, so fold disease shows as swelling and pus. This child's signs are bed and plate changes — thickening, hyperkeratosis and onycholysis — consistent with fungal invasion of the bed and the underside of the plate. ^{[1] [4]}

Examiner: Walk me through the nail unit anatomy and the growth rate, and why it matters. ^[1]

Candidate: The nail matrix is the germinative epithelium tucked under the proximal nail fold that generates the nail plate; its distal edge shows as the pale lunula. The nail plate is the hard keratin nail, riding forward over the tightly adherent nail bed. The proximal and lateral nail folds frame the nail, and the cuticle or eponychium seals the gap and protects the matrix. The hyponychium is where the plate finally separates from the bed at the tip. Fingernails grow about 3 mm a month and take roughly 6 months to regrow, while toenails take 12 to 18 months — which matters because a brief halt in matrix activity after an illness leaves a Beau line that crawls distally over months, and because a treated toenail takes a year or more to look normal. ^[1]