Thank you for the question.
There are various causes of horizontal ridges of the nails. Trauma e.g. picking and repeated injury to the nail matrix is a common cause of horizontal ridging.
Horizontal ridging can be caused by underlying medical conditions. There includes eczema or psoriasis and paronychia which is an inflammation of the skin around the nail. Disease like thyroid problems, infections, drugs and nutritional deficiencies can also cause transverse nail ridging. Sometime, no underlying cause is found.
It is important to determine the underlying cause through a detailed history and screening blood tests may sometimes be carried out. If an underlying cause is found, this should be addressed as this can lead to improvement of the ridging as the new nail develops.