Although conjunctivitis is the most common cause of red eyes presented to the family practitioner, a review by Vafidis emphasizes the importance of recognizing alternative diagnoses, some of which are vision-threatening.

Insidious onset of symptoms over approximately 36 hours in a watery red eye is characteristic of viral conjunctivitis. Soreness can last up to 10 days, and the condition is infectious until the excessive watering stops. Daily cleaning to remove debris is adequate for mild conjunctivitis, hay fever–related inflammation, and blepharitis, but broad-spectrum topical antibiotics often are prescribed in the belief that they prevent secondary infection. Bacterial conjunctivitis usually starts suddenly with copious, sticky discharge and puffy eyelids. Treatment requires the use of topical antibiotics. Patients with allergic conjunctivitis have prominent chemosis and stringy secretions. Red hypertrophic papillae can be found under the upper eyelid. Treatment consists of reducing response to the allergen, and use of hygiene and topical anti-allergy medications, such as cromolyn or lodoxamide.

In addition to the various forms of conjunctivitis, episcleritis and subconjunctival hemorrhage can present as a painless red eye with normal vision. A spontaneous subconjunctival hemorrhage could indicate high blood pressure, diabetes, or bleeding disorders but more commonly results from reduced lubrication in the eye. Episcleritis resembles a localized area of conjunctivitis without discharge. It indicates autoimmune activity and sometimes heralds a new diagnosis of sarcoidosis, arthritis, or inflammatory bowel disease. Most cases resolve spontaneously and should not trigger an extensive work-up for potential autoimmune disease.

The four main causes of painful red eyes are corneal lesions, uveitis, acute glaucoma, and scleritis. Corneal problems such as keratitis, ulceration, and arc damage are intensely painful, often requiring local anesthetic before the eye can be examined. Ulcers or tears in the cornea can be identified with dilute fluorescein and blue-light examination.

Abrasions are most painful on waking because of adhesions between the new epithelium and the eyelid. Lubrication can be required for up to six weeks, and the pain can require use of systemic analgesia. Corneal infections are frequently associated with herpes simplex or the use of contact lenses. Serious corneal abscess can develop, and herpetic lesions, which can recur, carry the risk of scarring. In both uveitis and acute glaucoma, patients complain of deep, aching pain with loss of vision. The cornea is hazy with perilimbal redness and irregular pupils. Patients with uveitis are usually younger and have intense photophobia. Patients with acute glaucoma are likely to be older than 50 and to be profoundly ill, with pain and systemic upset, such as vomiting and anorexia. Both conditions should be urgently referred to a specialist. Emergency treatment for glaucoma includes antiemetics, analgesics, and acetazolamide.