Beau's lines are horizontal or transverse depressions in the nail (they can be seen in the fingernails, thumbs, toenails, or all nails) that may be confused with nail ridging. This phenomenon was originally described in 1846 by the French physician Joseph Honoré Simon Beau. These lines are caused by diseases that affect the entire body, including malnutrition, heart attack (myocardial infarction), liver diseases, severe infections (such as mumps, measles, pneumonia, and others), and metabolic disturbances, including poorly controlled diabetes. However, Beau's lines can result from any disease process or illness that is severe enough to affect the growth plate of the nail. Malnutrition and chemotherapy drugs for cancer can also cause Beau's lines. Trauma or injury may also be a cause of Beau's lines or nail ridges. The timing of the disease can be estimated by measuring the distance from the Beau's line to the nail bed.
Brady, Mark P. "Fingernail and Toenail Abnormalities: Nail the Diagnosis." Mar. 9, 2018. Medscape.com. <https://reference.medscape.com/features/slideshow/fingernail-abnormalities>.
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