Red eye is a symptom of many different conditions, ranging from insignificant to sight-threatening. A review article from the Cleveland Clinic describes these in three categories: one, conditions that can be managed by a general practitioner, two, conditions that should be referred to an ophthalmologist within 48 hours, and three, conditions needing immediate referral to an ophthalmologist.
The least serious possibilities for red eye are as follows:
1. Conjunctivitis. The conjunctiva is a thin membrane lining the whites of the eyes and insides of both eyelids. Conjunctivitis is inflammation of this membrane, and may be caused by allergy, viral infection, or bacterial infection. Treatment depends on the cause.
2. Subconjunctival hemorrhage. I had this myself. Red eye is the only symptom, and it requires no treatment. It is caused by rupture of some of the small blood vessels in the white of the eye, generally because of a spike in blood pressure. If it happens more than once or if you're not sure what's going on, see your eye doctor or primary care doctor.
3. Blepharitis. This is inflammation of the eyelid margins, at the base of the eyelashes, generally caused by infection. Treatment includes eyelid hygiene and possibly antibiotics.
4. Corneal abrasion. Minor scratches to the cornea produce pain and redness, but are not an eye emergency. No treatment is necessary in many cases. Antibiotic ointment may be prescribed for deeper scratches.
5. Keratoconjunctivitis sicca (dry eye). Artificial tears are the initial treatment. If these are not sufficient, an ophthalmologist can recommend further options.
6. Thyroid eye disease. Eyedrops are sufficient for mild cases.
7. Eyelid malposition. There are several varieties. Lagophthalmos is an inability to fully close the eyes. Treatment includes eye drops, taping at night, and possibly surgery. Entropion is an inward-turning of the eyelids, and ectropion is an outward-turning of the eyelids. Surgery has high success rates. Floppy eyelid syndrome produces symptoms mainly at night. Taping or protective eye shields may be sufficient. Surgery is also an option.
8. Pingueculae, pterygia, and episcleritis.