Nails bulging horizontally is mainly caused by diseases, common causes include onychomycosis, nail dystrophy, hemangioma, etc., need to go to the hospital in time for treatment. 1. Ringworm of the nail: ringworm of the nail is a nail lesion caused by the invasion of dermatophytes into the nail plate and other parts of the body, commonly known as “gray nails”. Microbial invasion of the nail layer will destroy the structure of the nail layer, the patient’s clinical symptoms include the appearance of opaque spots on the nail plate, nail plate and nail bed separation, nail half moon and nail root thickening. 2. Nail dystrophy: Nail dystrophy refers to a variety of factors affecting the nail matrix, resulting in the appearance of various shapes and colors of the nail plate and other abnormalities. Patients often show some symptoms due to the lack of vitamin A, calcium, etc. The main clinical symptoms are transverse nail grooves, longitudinal nail grooves, nail splitting, increased brittleness, thickening, and so on. 3. Hemangioma: Hemangioma is a tumor disease originated from the endothelial cells of blood vessels, babies may suffer from hemangioma when they are born, and hemangioma occurs in different parts of the body and shows different symptoms. When hemangioma grows in nails, it can lead to thickening of nails, pain and other symptoms. Patients should go to the hospital immediately when they experience discomfort.