Sudden darkness of the eyes is a clinical symptom of blackout. The clinical symptom of blackout is most commonly caused by cardiovascular diseases, such as sudden onset of tachyarrhythmias or severe slow arrhythmias. Sudden onset of tachyarrhythmia or severe slow arrhythmia may lead to insufficient blood supply to the brain, thus causing the clinical manifestation of black clouding. In addition, patients with severe anemia often experience blackouts when they change position. For blackouts caused by tachyarrhythmias, the onset of tachyarrhythmias should be actively controlled; for severe slow arrhythmias, pacing therapy should be given; for blackouts caused by severe anemia, the cause of the anemia should be actively sought and treated, and the anemia should be corrected.