Patients suddenly feel black lines in front of their eyes, which may be caused by mosquito, fundus diseases and other reasons. 1. Flying mosquito syndrome: If patients suddenly feel black lines in front of their eyes, it may be caused by vitreous clouding, also known as flying mosquito, which is a common ophthalmic disease, mainly occurring in the middle-aged and elderly as well as highly myopic people. When the patient’s vitreous body appears cloudy material, blocking the light, the patient will feel the field of vision appeared in the black line or black spots, usually do not need special treatment. 2. Fundus diseases: If patients suddenly feel black lines in front of their eyes, it may also be caused by retinal detachment and other diseases, retinal detachment in the early stage of the patient’s eyes will appear black floaters, and in the later stage of the eye can appear black shadows blocking, vision gradually decline. It is recommended that patients who suddenly have black lines in front of their eyes should consult a hospital in time to assess their condition and receive standardized treatment under the guidance of a doctor.