What are Beau's Lines?
Beau's Lines are deep, horizontal grooves or indentations that run side-to-side across the nail plate. Unlike vertical ridges, which are a normal sign of aging, these horizontal lines indicate a period where the nail matrix (the root) completely stopped producing keratin for a period of time.
The Nail as a Timeline
Because nails grow at a predictable rate, a specialist can use a Beau's line to calculate exactly when you were sick.
Growth Math
- Fingernails grow about 0.1 mm per day (3mm/month).
- Toenails grow about 1mm per month.
- If a groove is 3mm from the cuticle on a fingernail, the shock occurred about 1 month ago.
Width of the Line
- The depth of the groove indicates how severe the illness was.
- The width of the groove indicates how long the illness lasted.
What Causes Nail Growth to Arrest?
When the body undergoes extreme physical stress, it diverts all energy away from non-essential functions (like growing nails) to focus on survival and healing internal organs.
- Systemic Infections & High Fever: Illnesses like COVID-19, pneumonia, measles, or mumps frequently cause Beau's lines across all 20 nails.
- Myocardial Infarction: A heart attack causes massive systemic shock.
- Chemotherapy: Cytotoxic drugs intentionally halt rapidly dividing cells, causing pronounced, recurring lines with each treatment cycle.
- Local Trauma: If the line only appears on one toe, it was likely caused by dropping something on it or a tight shoe, not a systemic disease.
Treatment & Resolution
There is no specific treatment for Beau's lines themselves. They are a historical record of a past event. As long as the underlying illness or trauma has resolved, normal nail growth will have already resumed behind the line. You simply have to wait for the groove to grow out entirely and be clipped off (6 months for fingers, 12-18 months for toes).
Patient Frequently Asked Questions
Q Can stress cause Beau's lines?
Q What happens if the line goes all the way through the nail?
Related Clinical Topics
Onychomadesis
The complete shedding of the nail plate, essentially a severe form of Beau's lines where the groove goes all the way through.
Muehrcke's Lines
White bands that are often confused with Beau's lines, but are smooth and not physically indented.