A large, red blood spot may appear on the white of the eye as a result of a broken blood vessel below the conjunctiva, the clear surface of the eyeball, explains Mayo Clinic. It can happen from coughing or sneezing and is called a subconjunctival hemorrhage in the medical community.
Other reasons a blood vessel beneath the conjunctiva may rupture include trauma to the eye, constipation, blood-thinning medications, heavy lifting and, in rare cases, a vitamin K deficiency or blood-clotting disorder, notes All About Vision. In many cases, there is no obvious explanation for why a patient develops a subconjunctival hemorrhage, but it is typically harmless and clears up on its own without any treatment within one to two weeks, notes Mayo Clinic. The redness on the white part of the eye is seen because blood from the broken blood vessel becomes trapped underneath the conjunctiva, which is transparent and unable to absorb the blood more quickly.
A subconjunctival hemorrhage does not impact vision or cause eye discomfort, according to All About Vision. While the hemorrhage is in the process of healing, the white of the eye may undergo color changes and look more like the area is bruised. Try not to rub the affected eye, as doing so shortly after the hemorrhage occurs can result in rebleeding.