Bloodshot eyes, a condition in which the white part of the eyes become red, occur because blood vessels in the eyes dilate or swell. As there are a variety of causes of this dilation and swelling, the condition itself is a pretty common one. Allergies, infections and trauma can cause bloodshot eyes, as well as certain other medical conditions. Any of these causes can be mild or severe, but the amount of redness in the eyes does not necessarily determine the severity of a person’s overall condition. More important symptoms to look at are accompanying pain, or vision difficulties.
Allergies can definitely be the cause of bloodshot eyes. Drugs, environmental factors and animals are a few things that can cause a person to have an allergic reaction. For example, a penicillin shot can cause a person to exhibit eye redness, in addition to other allergy symptoms. Pollen is an example of an environmental factor that can also lead to redness in the eyes. Also, certain animals can trigger an allergic reaction as well, such as a bite from a particular insect.
The list of infections and inflammations that can cause bloodshot eyes is varied and wide. While this makes it impossible to discuss them all, a few examples include blepharitis, conjunctivitis and uveitis. With blepharitis, an infection occurs in an eyelash follicle because of skin bacteria. Conjunctivitis, also known as pink eye, is an inflammation due to bacterial or viral causes. Uveitis is the inflammation of the choroid, ciliary body, and iris. In addition to eye redness, an infection can also cause the eyes to be itchy or painful and might even lead to vision problems.
Trauma to the eye can also be responsible for bloodshot eyes. This can be anything from coughing or straining, to eye surgery, to foreign objects in the eye. Coughing and straining can actually cause a condition known as subconjuctival hemorrhage, which is when a blood vessel breaks and causes one spot of redness in the white part of the eye; this minor bleeding usually goes away on its own within a couple weeks time. Bleeding in the eye can also cause bloodshot eyes and can explain the redness if a person just had eye surgery. It is understandable that foreign objects, like contact lenses, can cause red eyes because they can scratch the cornea.
Bloodshot eyes can also occur as a result of other medical conditions. A few examples are dehydration, dry eyes or eye fatigue. In addition, other conditions that can result in bloodshot eyes include acute glaucoma, keratoconus and pterygium.