Red eye, or hyperaemia, is one of the most common presentations to primary care  and the emergency department . However, despite this the underlying cause is often misdiagnosed which can lead to severe, sight-threatening conditions being missed .
Thorough evaluation with an appropriate history and examination can help to distinguish the sight-threatening from the benign red eye. In this article, we aim to discuss common causes of red eye and provide a comprehensive approach to decoding the red eye.
What are the causes?
Redness of the eye can be unilateral or bilateral. It is the primary sign of ocular inflammation usually occurring due to engorgement of superficial blood vessels or can occur when a blood vessel ruptures . Most of the cases tend to be benign as they are self-limiting or require minimal intervention. Only a handful of cases go on to be serious and in need of urgent referral.
There are a myriad of causes that lead to a red eye. The following is a red eye sieve to help consider the possible differential diagnoses :
History and examination
As with all presentations, good history and examination is key to an accurate diagnosis and initiation of appropriate management. A thorough history is essential as the presence of a red eye may be associated with an underlying systemic disease. Therefore, a complete history including a systems review, past medical, drug, family and social history is necessary.
In order to determine the underlying cause for the red eye, there are two key questions to ask in the history of the presenting complaint: 1) Is there any pain? 2) Is there any loss of vision? Other important features to enquire about include whether the symptoms are unilateral or bilateral, the duration of symptoms, presence of photophobia, recent eye trauma or surgery, contact with anyone unwell, whether there is a history of contact lens use and, if so, contact lens hygiene.
A stepwise examination approach incorporates a thorough inspection of the structures of the eye as well as eye function [5,6].
Red flags 
NICE recommends considering a same day ophthalmology referral if the following features are present, as they may be indicative of serious underlying pathology::
- Reduced vision or visual acuity
- Deep pain in the eye or tenderness on palpation of the globe
- Unequal pupil or abnormal pupil reactions
- High velocity or chemical injury
- Contact lens use
- Fluorescein staining
- Neonatal conjunctivitis – conjunctivitis in the infant in the first 28 days of life
Diagnosing the red eye [8,9]
Please click the PDFs to see two further tables connected to this article:
The acute non-painful red eye [10,11]
The acute painful red eye [10,11]
TAKE HOME MESSAGE
- Presentations of red eye are usually benign and can often be managed in the community, however, it is important not to miss a sight-threatening diagnosis!
- Golden questions: Is there any pain? Is there any loss of vision? Presence of these symptoms can be associated with more serious pathology.
- The sight threatening causes of a red eye warranting immediate ophthalmology referral include:
o Acute angle closure glaucoma
o Corneal ulcer / abscess
o Ocular trauma
o Chemical injuries
- All chemical eye injuries are potentially blinding injuries.
- Abrasions may be missed if fluorescein is not used.
- If in doubt of the aetiology of a red eye, refer to ophthalmology!
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2. Gilani C, Yang A, Yonkers M, Boysen-Osborn M. Differentiating Urgent and Emergent Causes of Acute Red Eye for the Emergency Physician. Western Journal of Emergency Medicine 2017;18(3):509-17.
3. Frings A, Geerling G, Schargus M. Red Eye: A Guide for Non-specialists. Deutsches Aerzteblatt Online 2017.
4. Olver J, Cassidy L, Jutley G, Crawley L: Ophthalmology at a Glance. Wiley Blackwell; 2005.
5. Sibal N, Tyagi A. Red eye: what's the diagnosis? Guidelines in practice 2018:
6. Allen R, Harper R: Basic Ophthalmology. San Francisco, Calif: American Academy of Ophthalmology; 2016.
7. Scenario: Management of red eye. NICE 2020:
8. Cronau H, Kankanala R, Mauger T. Diagnosis and management of red eye in primary care. American Family Physician 2010;81(2):137-44.
9. Worthing J, Tse A. The Red Eye. Core EM 2019:
10. Dennison A, Murray P: Oxford Handbook of Ophthalmology. 4th ed. Oxford University Press; 2018.
11. Khaw P, Shah P, Elkington A: ABC of Eyes. Wiley; 2013.
(All links last accessed February 2021)