By the way, doctor: What causes Beau's lines?
Q. You wrote about weak, brittle fingernails with longitudinal ridges. I have strong nails with horizontal ridges. What causes this, and what can I do about it?
A. You may be referring to Beau's lines, which are grooves that run horizontally across the nail plate. They usually develop when nail plate growth, which begins in the nail matrix (located under the cuticle), is temporarily disrupted. This can occur with direct injury to the nail matrix; an inflammatory condition such as psoriasis; infection around the nail plate; repetitive picking at the nails or cuticles; or even a manicure. Systemic causes include nutritional deficiencies, illnesses accompanied by high fever, metabolic conditions, certain drugs (especially chemotherapy agents), and diminished blood flow to the fingers (from Raynaud's phenomenon, for example).
Multiple Beau's lines in one nail are a sign of repetitive local injury to the matrix, while multiple nails with multiple lines probably indicate something systemic, such as several cycles of chemotherapy. Since nails grow slowly — about three millimeters per month for fingernails — a condition affecting the nail matrix won't register in the nail plate for weeks or months. You can estimate when injury to the nail matrix took place by measuring the distance from the cuticle to the groove. Unless the matrix is re-injured, Beau's lines grow out with the nail and eventually disappear. Complete replacement of a fingernail takes about six months. You can't do much about Beau's lines, but you may want to ask a clinician familiar with nail problems (usually a dermatologist) to see whether some other condition is involved.