Beau's lines are horizontal grooves that run across the nail plate, caused by a temporary stop in growth at the matrix after illness, injury, chemotherapy or heavy use of gel and acrylic nails.
Also known as: Beau lines, horizontal nail ridges, transverse nail grooves, dents across the nail
Beau's lines are grooves that run across the nail from side to side. They form when the matrix, the tissue that makes the nail plate, stops or slows production for a time. When growth resumes, the thinner section moves forward with the plate as a dent. DermNet defines them as a transverse depression on all nails caused by an acute systemic illness that halts nail growth (DermNet).
Causes include a severe illness with high fever, chemotherapy, zinc deficiency, injury to the base of the nail, and prolonged use of artificial nails, acrylics or gel manicures (Cleveland Clinic). A line on one nail points to a local cause such as a knock or a tight cuticle tool. Lines at the same height on every nail point to something that affected the whole body.
The distance from the cuticle shows when the event happened. Fingernails grow about 3 to 4 mm a month, so a groove 6 mm from the base formed roughly two months earlier (Medscape). The line grows out with the nail and is gone in about 6 months on fingers, longer on toes.
The groove grows out on its own. Care is for the cause and for the nail while it grows. Cleveland Clinic advises addressing the cause, moisturising daily, taking breaks from gel and acrylic, gentle buffing and gloves for wet work. Vertical ridges, which run from base to tip, are a different and usually harmless sign of ageing or dry skin, and a ridge filler hides them under polish. New Beau's lines on all nails without an obvious cause are a reason to see a GP (Mayo Clinic).