Conjunctivitis has many causes, but in most cases it results from infection with a virus or bacterium or from a reaction to an allergen. No matter the cause, conjunctivitis always involves inflammation (swelling) of the thin layer that lines the inside of the eyelid and covers the white part of the eye (the conjunctiva). But conjunctivitis also has other signs and symptoms, which may vary, depending on the cause. These signs and symptoms can be used to diagnose the type of conjunctivitis.
Viral conjunctivitis can often be diagnosed from signs and symptoms, and patient history. For example, if conjunctivitis accompanies a common cold or respiratory tract infection and if discharge from the eye is watery rather than thick, the cause is likely a virus. The history the patient gives (for example, having contact with someone with conjunctivitis or having allergies) and examination of the eye can also help a doctor make a firm diagnosis.
Laboratory tests are not usually needed to diagnose viral conjunctivitis. However, testing may be done if a more severe form of viral conjunctivitis is suspected, such as conjunctivitis caused by herpes simplex virus or varicella-zoster virus. This testing is done using a sample of the discharge from an infected eye.
Depending on the cause of viral conjunctivitis, some patients may have additional symptoms or conditions, such as the following:
Bacterial conjunctivitis can usually be diagnosed by a doctor, nurse, or other healthcare provider from signs and symptoms, and patient history. For example, if conjunctivitis accompanies an ear infection and if discharge from the eye is thick rather than watery, the cause may be a bacterium. Although not routinely done, your healthcare provider may obtain a sample of eye discharge from the conjunctiva for laboratory analysis to determine which form of infection you have and how best to treat it.
Depending on the cause of bacterial conjunctivitis, some patients may have additional symptoms or conditions, such as the following:
Allergic conjunctivitis can be diagnosed from signs and symptoms, and patient history; for example, allergic conjunctivitis may occur seasonally when pollen counts are high, and it can cause the patient's eyes to itch intensely. This type of conjunctivitis is a common occurrence in people who have other signs of allergic disease, such as hay fever, asthma, or eczema. Allergic conjunctivitis results from a person's reaction to substances they are allergic to, such as pollen, dust mites, animal dander, medications, cosmetics, and other allergy-provoking substances.
Signs & Symptoms
It can be hard to determine the exact cause of every case of conjunctivitis. This is because some signs and symptoms of the condition can differ depending on the cause, and other signs and symptoms are similar no matter what caused the conjunctivitis.
Symptoms of conjunctivitis can include