Redness of the eye is one of the commonest signs in ophthalmology, being a feature of a wide range of ophthalmological conditions, some of which are severe and sight threatening, whereas others are mild and of little consequence. Occasionally, the red eye can be the first sign of important systemic disease. It is important that every practicing doctor has an understanding of the differential diagnosis of this common sign, and a categorisation of the signs, symptoms and management of the red eye will now be made from the standpoint of the nonspecialist general practitioner.
The simplest way of categorising these patients is in terms of their visual acuity. As a general rule, if the sight, as measured on the Snellen test chart, is impaired, then the cause might be more serious. The presence or absence of pain is also of significance, but as this depends in part on the pain threshold of the patient, it can be a misleading symptom. Disease of the conjunctiva alone is not usually painful, whereas disease of the cornea or iris is generally painful.
The red eye will, therefore, be considered under three headings: the red eye that sees well and is not painful, the red painful eye that can see normally, and the red eye that does not see well and is acutely painful.
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