Red eye is a common ophthalmologic condition and accounts for 2% to 3% of office visits in the primary care setting.1 Although most cases of red eye are benign, detailed history-taking (onset and duration of eye complaint) and physical examination are essential to recognizing vision-threatening eye disorders requiring prompt ophthalmologic referral and emergent management. Commonly encountered diagnoses include infectious, inflammatory, and systemic conditions. 

Self-limiting to vision-threatening disorders are discerned through the compilation of subjective and objective distinguishing factors. Equipment such as a slit lamp and tonometer can assist in evaluating the structure and intraocular pressure, respectively. Symptoms of eye pain, vision change, trauma, and foreign body must be explored; the depth of examination will depend on the practice setting.


Conjunctivitis is a common condition primary care providers encounter.2,3 Conjunctivitis is the general term for inflammation that occurs to the thin mucus membrane covering the eyeball and the inner aspects of the tarsal plates.2 The inflammatory process causes dilatation of the vessels and cellular infiltration resulting in conjunctival injection, discharge, and/or chemosis.4 The annual incidence of bacterial conjunctivitis in the United States is estimated to be 135 per 10,000 people.3 The corresponding cost burden is of concern when considering the costs of diagnostic tests, medications, and employee absenteeism.3 Acute conjunctivitis, defined as occurring for less than 4 weeks, can be categorized as infectious or noninfectious and is often diagnostically challenging to differentiate between the 2 conditions.2,3 

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Viral Conjunctivitis 

The most common cause of infectious acute conjunctivitis is viral and comprises 80% of cases.2,3 Of those, adenovirus (90%) is the most common causative agent.2,3 Conjunctivitis occurs in 12% of patients diagnosed with COVID-19, usually early in the illness.5 Viral conjunctivitis is highly contagious and patients typically present with symptoms that may begin in 1 eye but quickly becomes bilateral.3,6 Discharge is watery and has a mild mucoid component.4,6 Given the viral cause, patients often present with concomitant upper respiratory infection symptoms that assist with differentiating between other causes of conjunctivitis.3 Patients may also experience eye irritation or a gritty feeling.3  

Herpes zoster involving the trigeminal nerve and herpes simplex infection are 2 viral causes of conjunctivitis that may progress to keratitis and often require urgent referral to an ophthalmologist.3,7 Typical herpetiform or vesicular skin lesions may be visualized.7 Photophobia, visual changes, and preauricular lymphadenopathy may be present.7,8 Dendritic lesions may be noted with fluorescein staining in 30% to 50% of patients with herpetic eye infection.8 

Bacterial Conjunctivitis 

Bacterial conjunctivitis is the second most common infectious cause of acute conjunctivitis and is most commonly caused by Staphylococcus aureus in adults.3 Hemophilus influenza and Streptococcus pneumoniae are also frequently encountered.3 The typical presentation is diffuse unilateral conjunctival injection with profuse, thick, purulent discharge that may spread bilaterally.3,6 Matted crusting of the eyes in the morning and purulent discharge on physical examination have been identified as bedside findings that increase the probability of positive bacterial cultures.9   

Pseudomonas aeruginosa eye infection is common in individuals wearing contact lenses secondary to unwashed lens containers.3,4 Infection with Neisseria gonorrhoeae is possible and presents with an abrupt onset of extreme amounts of purulent discharge. If either organism is suspected, patients should be emergently evaluated by ophthalmology secondary to potential complications such as severe keratitis, corneal ulceration (Figure 1), and perforation.3,10

Figure 1. Bacterial conjunctivitis caused by Pseudomonas aeruginosa. Credit: Eguchi et al. BMC Research Notes.10

In neonates, ophthalmia neonatorumis is most commonly caused by Chlamydia trachomatis and can be sight-threatening.11 Herpes simplex should also be considered. Transmission of organisms occurs during delivery and symptoms usually occur within the first 28 days of life.11 

Allergic and Noninfectious Conjunctivitis 

Allergic conjunctivitis affects approximately 20% of the population annually.3 It often presents with bilateral conjunctival injections, lid swelling, pruritis, and watery eye discharge.2,6 Key differentiating findings include the presence of pruritus and cobblestoning on physical examination and the appearance of closely packed squares on the palpebral conjunctive caused by enlarged papillae of the palpebral conjunctiva.2,6 Symptoms occur more often in the spring and summer, similar to seasonal allergies.6 Patients may also have a history of atopy or allergies (nonseasonal or seasonal).6 

A lack of lids being matted together in the morning and the inability to visualize a red eye from 20 feet away has been identified as features that increase the probability of a nonbacterial cause of red eye.9 Dry eyes, allergens, irritants such as dust and smoke, and uncorrected refectory errors can cause noninfectious conjunctivitis.4


Episcleritis is a condition affecting the superficial vasculature of the eye and is a self-limiting disorder with typical resolution occurring within 3 weeks of initial presentation (Figure 2).3,12  Approximately 120,000 new cases of episcleritis occur per year in the United States with 70% to 74% affecting middle-aged women.3 The etiology is most often idiopathic but approximately one-third of cases are connected to autoimmune, systemic collagen vascular, and viral illnesses.3  

Figure 2. Diffuse right episcleritis with injection in the superficial episcleral vessels. Credit: Parentin et al. J Med Case Reports.12

Patients may be asymptomatic or present with unilateral or bilateral inflammation and report mild discomfort in the affected region.3 Vision is unaffected, and it is rare for patients to report photophobia or discharge.3 The presence of additional symptoms should lead to suspicion of alternative diagnoses and consideration of scleritis in patients with pain, vision changes, or photophobia.4 


Differentiating scleritis from episcleritis is important because scleritis usually necessitates more aggressive interventions. Scleritis is an inflammation of the scleral tissues and deep vessels that tends to be more destructive than episcleritis.3 Middle-aged individuals and women are most commonly affected.3 Scleritis can be either anterior or posterior. An alarming symptom of anterior scleritis that helps to differentiate it from more benign causes of red eye is severe pain that is worse at night and may even wake the patient.3 Anterior scleritis presents with striking symptoms and includes a deep red and purple colored eye accompanied by vision loss and severe eye pain that radiates to the face or head.3,7 A concerning symptom of posterior scleritis is vision loss with or without pain.3

To help differentiate between episcleritis and scleritis, phenylephrine can be instilled. Redness will improve with phenylephrine use in episcleritis but not in scleritis because affected vessels are more superficial in episcleritis and deeper in scleritis.2,7 Gradual onset, photophobia, vision changes, and pain are common in patients with scleritis and facilitate the differentiation of scleritis from episcleritis.4 

Scleritis is associated with multiple complications such as corneal ulcer, uveitis, glaucoma, and keratitis and, thus, should be evaluated by an ophthalmologist within a couple of days.3 Similar to episcleritis, systemic autoimmune diseases such as rheumatoid arthritis or vasculitis are often associated with scleritis; therefore, appropriate evaluation for underlying disorders is paramount in the management of patients with scleritis.3,4  


Keratitis is inflammation of the cornea that is commonly caused by bacteria but can also be due to viruses, parasites, or fungi.3 Infections can permeate the corneal layers from the epithelium through the stroma and down to the endothelium.3 The most significant risk factor is inappropriate use of contact lenses, notably overnight use. Other causes include dry eyes, corneal surgery, trauma, and immunosuppressive conditions such as diabetes and topical steroid use.3 Patients present with red eye, foreign body sensation, photophobia, and discharge (purulent or nonpurulent).3 Corneal opacity, infiltrates, or ulcers may be identified on physical examination.3 Hypopyon (white blood cells in the anterior chamber) may be present in fulminant cases.3 

Fungal keratitis is often caused by filamentous fungi and yeast, particularly in tropical and temperate regions.3 Adenovirus often causes conjunctivitis that may invade the cornea resulting in keratoconjunctivitis.3 Photokeratitis is caused by UV rays.4 Risk factors include recent history of welding or a trip to a sunny place such as the mountains.4 Symptoms are typically unbearable and occur within 6 to 8 hours after exposure.4 Pronounced blepharospasm results in a functional inability to see.4 Damage to corneal epithelium typically begins to resolve within 48 hours.4 


The uvea, which includes the iris, choroid, and ciliary body, is inflamed in uveitis.13 Anterior uveitis refers to iris (iritis) and/or ciliary body (iridocyclitis) involvement. Posterior uveitis involves the choroid (choroiditis) and retina (retinitis) or a combination of both (chorioretinitis).13 Panuveitis includes all components of the uveal tract.1 The cause is idiopathic in 30% of cases.3 However, a multitude of infectious and systemic inflammatory diseases have been identified as causes.3 Thus, extraocular or systemic involvement should be investigated.3 Although not an exhaustive list, infectious causes are often viral and include herpes simplex, herpes zoster (Figure 3), cytomegalovirus, toxoplasmosis, tuberculosis, West Nile virus, and Zika virus.3,14 Some examples of systemic inflammatory diseases that cause uveitis include sarcoidosis, psoriatic arthritis, lupus erythematosus, Kawasaki disease, and inflammatory bowel disease.3 Ophthalmology follow-up within 24 hours is appropriate in patients with suspected uveitis.3  

Figure 3. Sectorial iris atrophy is typical of herpetic uveitis. Credit: Muñoz-Negrete, et al. Biomed Res Int.14

Patients typically present with unilateral intraocular pain with prominent photophobia to the extent the patient prefers a darkened room. Patients also experience blurry vision.7 The conjunctival injection is often more prominent near the limbus (ciliary flush) and direct light in the affected eye may cause pain and pupillary restriction in both eyes.7 Clouding to the anterior chamber and cell (individual cells) and flare (cloudy appearance secondary to protein release) on slit-lamp examination can be visualized with uveitis.2 Although rare, hypopyon may occur with uveitis.15 However, according to 1 study, individuals with hypopyon secondary to uveitis did not experience worse visual outcomes compared with those without hypopyon.15 

Acute Angle-Closure Glaucoma

In the US, acute angle-closure glaucoma (AACG) affects more than 2.7 million people aged 40 years and older.16 Glaucoma is sight-threatening and, thus, is considered a medical emergency.2 Glaucoma is the second leading cause of blindness among Black individuals and the third leading cause of blindness among White individuals.16 Increased intraocular pressure (IOP) is caused by the obstruction of the aqueous humor flow through the trabecular meshwork.3 Anatomical variants, such as a shallow anterior chamber, anterior placed lens, or thicker iris may lead to a decreased outflow of aqueous humor and increased IOP.3 An increase in IOP may lead to compression of the optic nerve and subsequent optic neuropathy that may affect vision.3 

Patients present with sudden onset of unilateral painful red eye without discharge.7 The presence of vision loss, headache, nausea, and vomiting may also occur and help to differentiate glaucoma from many other benign ocular illnesses.7 On physical examination, conjunctival injection, cloudy cornea, and mid-dilatation nonreactive pupil may be present.2 The lack of pupillary response can be secondary to ischemia and paralysis of the pupillary muscles may be present.4 Using tangential lighting, a crescent-shaped shadow may be noted in the medial aspect of the iris indicating a shallow anterior chamber.4 Intraocular pressure readings are normally found to be ≤21 mm Hg but may be in the range of 70 mm Hg in acute angle-closure glaucoma.7 Some individuals may normally have an increase in IOP, thus comparison of readings with the unaffected eye may help identify false-positive results.3 

Red Flags

Red flags are useful warnings clinicians can employ in the decision-making process and should prompt referral to ophthalmology to rule out severe disorders (Table 1).17     

Table 1. Red Flags for Severe Eye Disorders17

Red Flag Possible Disorders 
Change in vision AACG 
Moderate to severe pain AACG 
Moderate to severe photophobia Uveitis 
Loss of pupillary reactivity AACG 
AACG, acute angle-closure glaucoma

Compared with bilateral erythema, unilateral eye redness has been associated with more serious diagnoses.17 Pain, acute vision loss, and decreased pupillary response often suggest AACG, an urgent ophthalmic condition. Moderate to severe eye pain can manifest with a variety of eye conditions including acute glaucoma, corneal ulceration, scleritis, and uveitis, and should be of concern to clinicians. Clinicians should use the differentiating features of each illness to assist with appropriately determining the cause of red eye in patients and subsequent need for referral (Table 2).2,3,4,7,13,15 

Table 2. Differential Diagnosis of Red Eye2,3,4,7,13,15 

IOP, intraocular pressure
Click Table for larger version

Blunt trauma, injury, or persistent pain are beyond the scope of this article and warrant urgent referral. However, red flags associated with traumatic injuries may include vision change, acute and sudden vision loss, hyphema, and change in pupillary shape (ie, tear-drop pupil) and require immediate ophthalmologic evaluation.

Gina Pontrelli, DHSc, PA-C, is assistant professor of Physician Assistant Studies at Hofstra University in Hempstead, NY; Carina Loscalzo, EdD, MS, PA-C, is assistant professor of Physician Assistant Studies at Hofstra University.


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9. Narayana S, McGee S. Bedside diagnosis of the ‘red eye’: a systematic review. Am J Med. 2015;128(11):1220-1224.e1. doi:10.1016/j.amjmed.2015.06.026.

10. Eguchi H, Miyamoto T, Kuwahara T, Mitamura S, Mitamura Y. Infectious conjunctivitis caused by Pseudomonas aeruginosa isolated from a bathroom. BMC Res Notes. 2013;6:245. doi:10.1186/1756-0500-6-245

11. Maqsood N, Mahmood U. Herpes simplex ophthalmia neonatorum: a sight-threatening diagnosis. Br J Gen Pract. 2020;70(699):513-514. doi:10.3399/bjgp20X712973.

12. Parentin F, Lepore L, Rabach I, Pensiero S. Paediatric Behçet’s disease presenting with recurrent papillitis and episcleritis: a case report. J Med Case Rep. 2011;5:81. doi:10.1186/1752-1947-5-81.

13. Mutawa AM, Alzuwawi MA. Multilayered rule-based expert system for diagnosing uveitis. Artif Intell Med. 2019;99:101691. doi:10.1016/j.artmed.2019.06.007

14. Muñoz-Negrete FJ, Moreno-Montañés J, Hernández-Martínez P, Rebolleda G. Current approach in the diagnosis and management of uveitic glaucoma. Biomed Res Int. 2015;2015:742792. doi:10.1155/2015/742792

15. Zaidi AA, Ying GS, Daniel E, et al; Systemic Immunosuppressive Therapy for Eye Diseases Cohort Study. Hypopyon in patients with uveitis. Ophthalmology. 2010;117(2):366-372. doi:10.1016/j.ophtha.2009.07.025

16. Malihi M, Moura Filho ER, Hodge DO, Sit AJ. Long-term trends in glaucoma-related blindness in Olmsted County, Minnesota. Ophthalmology. 2014;121(1):134-141. doi:10.1016/j.ophtha.2013.09.003

17. Ramanayake RPJC, Basnayake BMTK. Evaluation of red flags minimizes missing serious diseases in primary care. J Family Med Prim Care. 2018;7(2):315-318. doi:10.4103/jfmpc.jfmpc_510_1

This article originally appeared on Clinical Advisor