A sudden red patch at the corner of the eye is usually a subconjunctival hemorrhage caused by a ruptured blood vessel on the surface of the eye, which is a very common eye disease. There are many causes of this condition, such as trauma to the eye, local inflammation, staying up late, violent coughing, or having high blood pressure or blood disorders, all of which may cause the rupture of the subconjunctival blood vessels, and the exact cause is usually difficult to trace. However, the effects of this condition on the eye are usually minimal and have no effect other than on appearance. In the early stages of bleeding it is usually recommended to apply cold compresses locally, and after 48 hours it is replaced by hot compresses, which usually return to normal in about two weeks.