Nail horizontal depression may be caused by nail dystrophy, lichen planus and so on, need to consult a doctor in time to clarify the cause of the disease. 1. Nail dystrophy: this is a multifactorial nail damage, can be caused by heredity, local stimulation, drugs, physical injury, etc., can appear on the surface of the nails to lose luster or horizontal lines, or even thinning deformation caused by the nails transverse depression. 2. Lichen planus: the etiology is not clear, it may be related to immunity, heredity, viral infection, neuropsychiatric factors, drugs and so on. It may also lead to changes in the nails, such as loss of gloss on the surface of the nails, the appearance of horizontal lines and lateral depression of the nails. There are many causes of lateral nail indentation, and it is recommended to consult a doctor promptly to clarify the cause of the disease and to treat it under the doctor’s guidance.