Sudden congestion of the eye can occur in two ways: a dilated conjunctival blood vessel and a subconjunctival hemorrhage. In the former case, a single dilated conjunctival blood vessel can be seen, while the latter case presents as a patchy red conjunctival hemorrhage, where a single conjunctival blood vessel is not visible in the hemorrhagic area. Conjunctival congestion is usually due to irritation of the conjunctiva from various causes, resulting in dilatation of the conjunctival vessels. Irritation can be caused by many factors, such as inflammation of the conjunctiva caused by pathogens such as bacteria, viruses, chlamydia, allergic conjunctivitis, overuse of the eyes, staying up late, dry eyes, lid gland dysfunction, blepharitis, foreign bodies, chemical, ionizing radiation or sand irritation, etc. Subconjunctival hemorrhage is caused by the rupture of small blood vessels under the bulbar conjunctiva. Because of the laxity of the subconjunctival tissue, hemorrhage tends to become flaky and often occurs in only one eye, more often in the elderly, but can occur in any age group. The exact cause can rarely be found, and may be related to hypertension, hyperlipidemia, diabetes, trauma and other factors, and occasionally a history of increased intra-abdominal pressure due to violent coughing and vomiting. Therefore, if there is a sudden congestion of the eyes, you should not handle the medication by yourself to avoid delaying the condition, but you need to visit a hospital ophthalmologist for diagnosis and identification of the specific cause by a specialist.