Beau's lines are multiple horizontal ridges on the nails, often indicating malnutrition or severe illnesses that temporarily affect nail growth, such as coronary artery disease leading to coronary artery thrombosis. Slow blood flow affects nail growth, resulting in Beau's lines.
Brittle nails refer to nails that are fragile and easily cracked, with the nail plate becoming thin, brittle, and prone to splitting. The occurrence of brittle nails is often associated with chronic diseases of the nail matrix, frequent exposure to chemicals, chronic inflammatory or keratinization disorders such as psoriasis and chronic eczema, as well as peripheral circulation disorders, hypochromic anemia, myxedema, and hypoparathyroidism.