Beau’s lines — first described by French physician Joseph Honoré Simon Beau in 1846 — are horizontal, grooved lines on a fingernail or toenail that run parallel to the lunula (the half-moon shaped whitish area that appears at the bed or proximal area of a nail). The deep ridges tend to eventually grow out as the nail lengthens, which can sometimes create pain if the misshapen nail presses deep into the flesh.
Beau’s lines are thought to be caused by a temporary cell division at the proximal base of the nail (in other words, the body stops producing nail cells for a specific period of time). This can be caused by a variety of systemic diseases and other conditions, including:
Because Beau’s lines grow out as the nail lengthens, the timing of when the nail injury or underlying disease can be determined by measuring the distance from the line to the nail’s proximal base.
Beau’s lines will eventually grow out on their own, meaning there is no need for treatment of the lines. However, it is strongly recommended that you visit a doctor if you have Beau’s lines, as they may signify a serious underlying medical condition like the illnesses listed above.