Red Eye — Full Text

When a patient has a red eye (or red eyes), our main job is to decide who needs to see an ophthalmologist & who doesnโ€™t.  And that can be summed up in one wordโ€ฆ

PAIN !!!  Painful red eyes are caused by a variety of potentially-blinding conditions, which require truly emergent attention.  But we’d never send anyone to an ER without phoning first, because if thereโ€™s no ophthalmologist on-call, the patient will have traveled from Square 1 to Square 0.

Actually, visual loss would be another reason to refer to ophthalmology, not necessarily total loss, but any decrease found by formal measurement. We’ll elaborate in a moment. The following Table lists all the possible causes.ย  See Anatomy of the Eye to distinguish among the different parts.

Causes of Red Eye(s)

xxxxxxxxNo Eye Pain  xxxxxxxxPainful  !!!  
โ€ข Viral Conjunctivitis (“pink-eye”)
โ€ข Bacterial Conjunctivitis
โ€ข Herpes Conjunctivitis
โ€ข AllergiesAirborne & Contact allergies
โ€ข STD CausesGonorrhea, Chlamydia
โ€ข Foreign Body on Conjunctiva
โ€ข Episcleritis
โ€ข Pterygium (inflamed)
โ€ข Subconjunctival Hemorrhage
โ€ข Corneal Abrasion / Foreign Body
Corneal Ulcer / Keratitis
โ€ข Scleritis
โ€ข Iritis / Uveitis
โ€ข Optic Neuritis (eye often not red)
โ€ข Acute Closed-Angle Glaucoma
โ€ข Corneal Abrasion (recurrent)
โ€ข Contact Lens Wearer

xxxxxxSee also Diagrams — Eye Anatomy

A word about Pain.  Thereโ€™s a big difference between real eye pain on the one hand, and an uncomfortable, scratchy, gritty sensation (like something stuck in the eye) on the other.  Itโ€™s important to distinguish — everything in the second column above causes true unquestionable pain, and (except for the uncomplicated corneal abrasion) requires an immediate trip to an ophthalmologist.  If the patientโ€™s description seems at all ambiguous, it’s not true pain, & such a referral is unlikely to be necessary.

Blurry vision occurring together with new eye pain is really concerning.  However, a lot of my patients say they have “blurry vision on & off.”  That doesn’t count; it’s probably due to tearing (pronounced “TEER-ing”, not “TAIR-ing”).  If blurry vision is caused by eye disease, it’s there all the time.

Some people [myself included] have blurry vision in just one eye all the time, because we’re near-sighted in only that eye.  But since we see fine with both eyes, we never realize it until for some reason we decide to cover an eye to see what happens (like if we have a red eye, happen to be reading this right now, & decide to see if our vision is blurry!). How can we tell if the blurriness is due to bad eye disease, or if that eye had always been near-sighted & we’d never noticed?

Pinhole !!!  Put thumb & index fingers of both hands together to form a tiny hole, and squint through it (see link).  If blurry vision improves with pinholing, the eye is simply near-sighted.  If there’s no change, something else (maybe serious) is wrong with the eye.  If you’re just reading this for fun, & happen to be near-sighted, take off your glasses / out your contacts & try it. See the link for an explanation.

Since most conditions that cause red eyes don’t really hurt, let’s begin with first columnโ€ฆ

Non-Painful Red Eyes

Once again, if there’s any discomfort with the following conditions, it’s described as scratchy, gritty, itchy, โ€œsomething in my eyeโ€ and the like.  But NOT true PAIN.  For all these diagnoses except the extremely rare Gonorrheal Conjunctivitis, vision is normal.  If not, and the abnormality persists despite pinholing, and we donโ€™t find anything like a chronic cataract to explain it, the patient needs a full exam by Ophthalmology.

**  Viral Conjunctivitis  —  Simple common viruses are far and away the most common cause of conjunctivitis (see Differences Among Germs).  People call it “pink eye,” though other conditions also make the eye “pink.”  We diagnose this primarily by ruling out those other possibilities, which we’ll get to.  The basic features of Viral Conjunctivitis include:

  • Symptoms lasting less than 3 weeks
  • Both eyes eventually involved, but often begins on just one side
  • Discharge from the eye is clear.  There may be lots of yellow crusts in the morning, from normal bacteria that grew overnight, but once the patient washes their face, any discharge throughout the day is clear

**  Bacterial Conjunctivitis  —  This is quite uncommon, because the eye can protect itself from bacteria.  I only diagnose this if the patient describes constant, non-stop, yucky discharge.  I need to see it myself right then and there, in the office.  It can be green, yellow, or milky white — wipe it away, & more appears within minutes.  This is not simply yellow crusts on the lids when they wake up in the morning.

Bacterial conjunctivitis is usually one-sided, but may occasionally affect both eyes.

We give antibiotic eye drops for bacterial conjunctivitis.  Unfortunately, they tend to also be prescribed for the 99% of conjunctivitis cases that are really viral!  Antibiotics don’t touch a virus!  But patients (especially parents of young patients) want something.  In a busy practice, it takes much less time prescribing than explaining.  That’s not good for anyone — I’ve seen some reactions to antibiotic drops that were much worse than the original virus.

**  Herpes Conjunctivitis  —  This is the Herpes Type-1 that causes cold sores or fever blisters, not the STD.  It’s pretty rare.  The clue to diagnosis is one red eye with tiny blisters on the eyelid (any group of blisters anywhere should make us think Herpes in general).  A surer clue but not consistent is a history of having had the same thing before (with the tiny eyelid blisters).  If there’s real eye pain instead of just an icky feeling, we’d worry about Herpes on the cornea, which can cause blindness.

**  Gonorrheal Conjunctivitis  —  Very rare, but dramatic: an enormous amount of rapidly accumulating puss covers the eye.  Conjunctiva swell.  When the bacteria digs into the cornea, bad pain begins, until the eye is completely destroyed.  Try the link for a picture if you dare.

We’d suspect Gonorrhea if the patient also has discharge from their penis or vagina, since they may have spread it on up by touching themselves. It can also occur if a partner ejaculated in their face.  We refer to an ER, usually for hospitalization with IV antibiotics.

**  Chlamydia Conjunctivitis  —  This is like gonorrhea in that itโ€™s rare, and usually comes from the patient’s own genital disease, but can also occur from direct contact with semen.  Yet it’s very different, in that itโ€™s much more subtle and not destructive.  We suspect it by sexual history; we may also suspect it if we see conjunctival follicles inside the lower lid (see link for pictures, no trigger warning needed here).

Immigrants from very poor countries may have “Trachoma”, a different strain of chlamydia (not the STD).ย  We’d consider it if there’s chronic or frequently-recurring conjunctivitis that doesnโ€™t seem allergic.ย  An ophthalmologist can make a definitive diagnosis and treat before permanent scarring [and eventual blindness] happens.

**ย  Allergic Conjunctivitis (airborne) —ย  Caused by seasonal respiratory allergies, i.e. Hay Fever.ย  It’s very common.ย  Our main dilemma is to distinguish this from Viral Conjunctivitis.ย  We think Allergy if:

  • Runny nose, nasal congestion, and especially lots of sneezing
  • Symptoms going on over 3 weeks (viruses don’t last that long)
  • If thereโ€™d been several similar episodes before, and especially if a past episode lasted over 3 weeks, or was accompanied by lots of sneezing

The first time, at its onset, it’s impossible to distinguish allergy from virus.  But initial treatment is almost the same: a) Give it Time; b) Cool compresses; c) Maybe antihistamine eye drops; d) No antibiotics drops.

The main difference is that if we suspect allergic conjunctivitis, oral antihistamines are the drug-of-choice (see Allergy Medicines).  If the eyes feel real itchy with viral conjunctivitis, they may help as well.

**  Contact Allergy  —  As opposed to typical allergic conjunctivitis caused by something carried in the air, a contact allergy is due to something that directly touches the eye.  Facial cosmetics are a possibility, or any eye drops the patient has used (or has been given).  Especially antibiotic drops, like sulfonamides, neomycin, etc.  History provides a clue.

**  Conjunctival Foreign Body  —  “Foreign body” means a “thing” that’s gotten in the eye & is stuck on the conjunctiva (if it’s stuck on the cornea, it causes real pain).  Patients with foreign bodies usually seek care immediately, because it’s very disturbing, although after 1-2 days it will likely be washed out by tears & thus cured.  But sometimes it gets stuck, especially under the upper eyelid, which we have to flip inside-out to find & remove the thing.

If someone happened to get a weird sense of โ€œsomething in my eyeโ€ while working in a metal shop, where they or others were cutting or hammering steel, itโ€™s possible that a tiny piece of metal entered the eyeball, which rapidly sealed itself.  See below under โ€œCorneal Abrasion.โ€

**  Episcleritis  —  As opposed to Scleritis, which is rare and painful, Episcleritis is uncommon & annoying.  It may be due to an overall inflammatory disease (like types of arthritis), but usually just occurs on its own (“idiopathic,” i.e. “unknown cause”), & gets better on its own.

Unless we think of it, the condition can easily imitate conjunctivitis.  Some key findings with Episcleritis:

  • The eyelid conjunctiva (linings of upper & lower lids) aren’t involved.  With conjunctivitis, theyโ€™re as red as the conjunctiva which covers the sclera
  • The redness is localized to one spot, not spread diffusely, and not affecting the inside of the eyelids
  • There’s no real watery discharge with morning crusting (maybe increased tears from rubbing)

When located between the iris & the nose, we need to distinguish it from a Pterygium [see below] by:

  • Episcleritis goes away on its own (a pterygium is permanent unless removed)
  • Tufts of blood vessels noted in episcleritis
  • Blood vessels in a pterygium run neatly longitudinally toward the cornea

See the link for pictures.

**  Inflamed Pterygium  —  Easily diagnosed by its location, extending from the nose running toward the cornea.  We distinguish it from episcleritis [see above] by:

  • Pterygia [plural] are much more common
  • Blood vessels run longitudinally down the long axis of a pterygium, whereas the vessels are more random and tuft-like with episcleritis

See the link for pictures, and more explanation of how we ultimately manage the harmless pterygium.

** Sub-Conjunctival Hemorrhage  —  Here the redness is socked-in, & not in scattered dilated blood vessels.  And thereโ€™s no other symptom like itchiness or “something-in-the-eye,” and certainly no pain.  There’s absolutely no tenderness on exam if we gently wiggle the eyeball through a closed eyelid. 

Subconjunctival Hemorrhages, literally “bleeding under the conjunctiva,” happen when tiny little veins rupture due to straining.  For example, they’re common when women push during labor.  Maybe we get a patient’s history of recent coughing spells, lifting a piano, etc. I reassure patients by explaining that a tiny vein burst, if it had happened elsewhere like on the arm there’d have been a tiny bruise nobody would ever worry about.

We just need to beware if the sub-conjunctival hemorrhage occurred right at the time of blunt trauma (like being punched in the eye), because then it could be a sign of major eye injury.  But in that case, we’d find something else wrong with the eye, like maybe a “pear-shaped” pupil (which would mean a ruptured eyeball; we’d call 911 to rush to surgery).  Even if our entire eye exam is normal, with a subconjunctival hemorrhage due to trauma, we’d gently wiggle the eyeball through a closed eyelid — if that’s painful, we’d refer right to an ophthalmologist.

See the link for pictures.

Letโ€™s go on to the much less common, but more serious, conditions.  Again, โ€œPainโ€ means true pain that might even need heavy narcotics for relief, not just an uncomfortable, scratchy, “something-in-my-eye” sensation.

Painful Red Eyes

We also ask, โ€œWhen?โ€  Corneal abrasions heal completely in 24 hours (rarely by 48).  If the pain has lasted over a day without easing at all, itโ€™s either a deeper injury, or different condition.  The only other possibility, especially if we see vertical streaky abrasions, is that something is stuck under the upper lid, scratching the cornea with every blink.

**  Corneal Abrasion  —  This is a scratch on the very sensitive cornea, due to trauma, which is the key.  But Iโ€™ve had several patients say, โ€œSomething scratched my eye,โ€ simply because they guessed that something must have.  One had a rapidly-blinding corneal ulcer!  So I always ask, โ€œWhat were you doing when it happened?โ€  If theyโ€™d been hiking and a branch snapped in their face, fine.  But if no obvious injury occurred, I’d be very suspicious that there’s something else going on.

Oftentimes we can see corneal abrasions with a simple penlight.  But usually we have to stain the cornea with fluorescein and use an ultraviolet light in order to see the scratches.

Even if weโ€™re sure the patient has a corneal abrasion from same-day trauma, we want to be sure that the injury hasnโ€™t penetrated the cornea into the Anterior Chamber, causing a traumatic Iritis (see Diagram โ€“ Eye Anatomy).  One way we can tell is by placing a drop of topical anesthetic in the eye, which immediately relieves corneal pain, but not deeper injury.  Also, we test for cross-photophobia: shining a light in the healthy eye causes pain in the injured one!  Thatโ€™s because both irises constrict their pupils when light enters either eye (unfortunately, most clinicians arenโ€™t aware of these maneuvers).  See under Corneal Abrasion for more explanation.

Addendum:  Maybe the โ€œgot something in my eyeโ€ happened in a metal workshop, or any place people were cutting or hammering metal.  This raises the possibility that a tiny piece penetrated way into the eye, & got sealed over.  The fact that symptoms began in such a setting requires a referral to an Ophthalmologist, even if no treatment winds up necessary.

People can live & see just fine with tiny pieces of metal deep in their eyes.  But if they ever get an MRI for any reason, the magnet will whip that tiny unnoticed piece of metal back & forth [yuck!].  MRI techs always ask about the possibility first; Iโ€™ve always wondered if that got thought out ahead of time, or if it took a couple of bad outcomes to raise the query.

**  Corneal Infection / Ulcer  —   The cornea has lots of nerve endings, so it hurts like crazy with the least little irritation.  As such, corneal ulcers & infections (“Keratitis“) are painful!  We can often see them with a simple penlight if we know what we’re looking for, but sometimes may need to stain the eye as noted above.  On the one hand, these infections can progress fast and destroy the cornea.  Also, any scar they happen to leave will permanently interfere with vision.

Identifying the germ may be tricky, patients often need hospitalization to save vision.  That’s why we don’t mess with true eye pain — refer right to an ER, and telephone ahead to be sure they have an ophthalmologist on-call.

**  Scleritis  —  Rare, but very painful, in contrast to any mild discomfort of “episcleritis” mentioned earlier.  Pain may be felt in the face or as a behind-the-eye headache, exacerbated by moving the eye.  Underneath the redness, the sclera (the white of the eye) turns bluish.  Gentle wiggling the eyeball through the closed eyelid is very tender.  Scleritis requires aggressive treatment to prevent blindness, and also lots of tests to determine the cause (like different types of inflammatory arthritis).

**  Iritis / Uveitis  —  Iritis (serious) and Conjunctivitis (not serious) both cause red eyes.  The main way we distinguish them is pain.  Iritis hurts, no question about it, while conjunctivitis just creates that annoying, gritty or itchy sensation.  “Iritis” and “Uveitis” are essentially the same thing; the uveal tract (a.k.a. โ€œchoroidโ€) covers the eyeball, lying between between the sclera and the retina; the Iris is merely that part of it in the very front, which we can see (see Diagram โ€“ Anatomy of the Eye).

The Iris controls the size of the pupil; when it’s inflamed, it goes into spasm.  So maybe that pupil will be stuck, either very small or large.  But if the pupils are equal size, we have a wonderful test — shine a light in the non-affected eye, & see if it makes the affected eye hurt (“cross-photophobia”).  That’s because shining a light in one eye makes both pupils constrict.  If light in the left eye makes the right eye hurt, the only thing done to the right eye was to make the Iris move.  That’s Iritis.

Most Iritis occurs for unknown reasons, but a fair bit is due to other diseases such as syphilis, tuberculosis, types of inflammatory arthritis, etc.  So anyone with Iritis needs a work-up.

**  Optic Neuritis  —  Half the time this condition is “idiopathic,” a fancy term for “unknown cause,” but 50% of the time it’s due to Multiple Sclerosis.  It may be the very first manifestation of M.S., which can’t be diagnosed until 2 different nerves are affected.  So more tests & close follow-up are crucial. Ophthalmologists diagnose Optic Neuritis, but it’s the Neurologist (specialist in diseases of the nervous system) who deals with Multiple Sclerosis.

Inflammation of the optic nerve occurs deep in the retina.  Pain is the main symptom, especially with eye movement; there may not even be any redness.  Since the retina is what allows us to see, there is usually some vision loss, but it may be subtle.  When we look inside the eye, the optic nerve may appear swollen, or maybe very pale (“optic atrophy”).  But maybe not.

Key Test:  the “Swinging Flashlight“.  Normally, when we shine a light in an eye, both pupils constrict.  So in a completely dark room, we shine a light in the uninvolved eye — Pupil Constricts.

  • Shine it quickly over to the involved side, Pupil Dilates!
  • Swing back to the uninvolved, Pupil Constricts.
  • Back to the involved eye, Pupil Dilates!
  • etc.

If you want to understand why this happens, click for an Explanation of the Afferent Pupillary Defect.  This simple exam should be performed every time we examine pupils, & unfortunately most clinicians don’t remember or never learned it.

**  Closed-Angle Glaucoma  —  This is the rare type of glaucoma, which most people wouldn’t know they had until the sudden onset of severe pain in or behind the eye, often with vomiting (nothing subtle).  The eye is red, the cornea looks cloudy.  Immediate surgery can prevent blindness.  The main reason this gets missed is that the clinician thinks โ€œmigraineโ€ (one-sided headache with vomiting) & forgets to examine the eye.

Most glaucoma is “wide-angle,” which is a chronic painless condition, without any symptoms, until it gradually causes blindness after many years.  It can be detected by any kind of eye specialist who measures the eye pressure (intraocular pressure) & looks inside the eye.

**  Recurrent Corneal Abrasion  —  Uncommonly, some corneal abrasions never heal completely, and can recur spontaneously, even during sleep.  The onset is abrupt, they feel just like the original one did, but now thereโ€™s no trauma.  If we stain the cornea, it looks just like a fearsome corneal ulcer, so we send them same-day to an ophthalmologist, via an ER if necessary (telephoning ahead to be sure the ER has such a specialist on-call).

**  Contact Lens Wearer  —  The rate of potentially-blinding corneal infections with extended-wear lenses may be as high as one-in-a-thousand (0.1%) per year [which is 5% with 50 years of use!].  So it may eventually happen to 1 in 20 people.  Assuming the patient removed the lens, and pain has persisted, we presume there’s a Corneal Ulcer until proved otherwise, and refer for a same-day exam by an eye specialist.


Most red eyes are due to benign conditions which are not serious.  The majority get better on their own, some can benefit from specific treatments.  But we always have to be sure not to miss potentially blinding diseases.  These always include either true frank eye pain, and/or measurable decreased vision (which does not improve with pinholing).

See also Red Eye for the clinicianโ€™s condensed thought-process when face-to-face with a patient.

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