Fingernails and toenails marked by horizontal waves or ridges are typically the result of the medical condition known as Beau's lines, according to Skinsight. An abrupt interruption of nail keratin synthesis causes the waves. As the nail grows, the lines tend to vanish.
While Beau's lines are common among chemotherapy patients, they can strike people of any age or ethnic background, reports Skinsights. Others with the condition may suffer from negative reactions to other types of medication as well as various systemic causes, including heart attacks, malnutrition, metabolic disturbances and severe infections.
Nail Pro says that extreme cold and local injury are the most common causes of Beau's lines. People who engage in frequent running or kicking may develop Beau's lines on their toenails, for example, and people who have inflammation around a nail due to conditions such as eczema can develop Beau's lines on their fingernails.
Nail Pro reports that French medical doctor Joseph Honoré Simon Beau was the first physician to describe the condition. Beau noted that in determining the cause of the nail condition, it was helpful to measure the distance of the ridge from the nail bed and recall what the patient was experiencing physically about the time it first developed.