Bloodshot eyes appear red because the vessels in the surface of the white portion of the eye (sclera) become swollen. This may result from dry air, too much sun, dust, something in the eye, allergies, infection, or injury.
One common cause of a red eye is straining or coughing. This can lead to a bright red, dense bloody area on the white part of the eye. This is called a subconjunctival hemorrhage. Although this bloody area may appear alarming, it is a fairly common occurrence and of little significance. If you notice a bloody spot in one eye that doesn't hurt, but just looks bad, don't worry. It should clear up on its own within a week or two.
Eye infections or inflammation can occur, causing redness as well as possible itching, discharge, pain, or vision problems:
- Blepharitis -- Swelling of the eyelash along the edge of the eyelid.
- Conjunctivitis -- Swelling or infection of the tissue that lines the eyelids and coats the surface of the eye (the conjunctiva). This is often referred to as "pink eye."
- Corneal ulcers -- Ulcers on the outer covering of the eye, usually because of a bacterial or viral infection.
- Uveitis -- Swelling of the uvea, which includes the iris, ciliary body, and choroid. This is often related to an autoimmune disorder, infection, or exposure to toxins. Often, only the iris is inflamed, which is called iritis.
Other potential causes include:
- Cold or allergies.
Acute glaucoma -- a sudden increase in eye pressure that is extremely painful and causes serious visual disturbances. This is a medical emergency. Most times, glaucoma is chronic and gradual.
- Corneal scratches caused by sand, dust, or overuse of contacts.