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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Conjunctivitis is common and usually benign, but red-eye red flags must be excluded first
- Viral: watery discharge, gritty sensation, preauricular lymph node, URI exposure; highly contagious
- Bacterial: mucopurulent discharge with eyelids stuck shut; hyperacute purulent disease suggests gonorrhea
- Allergic: bilateral itching, tearing, chemosis, seasonal/atopic history
- Pain, photophobia, decreased vision, corneal opacity, contact lens use, or fixed pupil = not simple conjunctivitis
Overview
Conjunctivitis is inflammation of the bulbar and/or palpebral conjunctiva. It is commonly infectious (viral or bacterial), allergic, toxic, or irritative. The key clinical task is distinguishing uncomplicated conjunctivitis from sight-threatening red-eye diagnoses such as keratitis/corneal ulcer, uveitis, acute angle-closure glaucoma, scleritis, chemical injury, and orbital cellulitis. Most viral and allergic cases are supportive-care conditions; bacterial disease may be treated empirically when clinically likely.
Epidemiology
Conjunctivitis is one of the most common causes of red eye in primary care, urgent care, and emergency settings. Viral conjunctivitis is most commonly adenoviral and spreads easily through contact and fomites. Bacterial conjunctivitis is more common in children than adults. Allergic conjunctivitis is common in patients with atopy, allergic rhinitis, or seasonal allergen exposure. Contact lens wearers require special caution because microbial keratitis can mimic conjunctivitis but threatens vision.
Clinical Features
Symptoms
Viral: watery discharge, burning/gritty sensation, recent URI, exposure to infected contact
Bacterial: mucopurulent discharge, eyelids stuck together on waking
Allergic: prominent itching, tearing, bilateral symptoms, seasonal trigger
Severe pain, photophobia, or foreign body sensation preventing eye opening suggests corneal disease
Reduced visual acuity is not typical of simple conjunctivitis and requires urgent evaluation
Contact lens use with red eye and pain suggests microbial keratitis until proven otherwise
Signs
Diffuse conjunctival injection with normal pupil and usually preserved visual acuity
Preauricular lymphadenopathy supports viral conjunctivitis
Papillae and chemosis support allergic conjunctivitis
Purulent discharge supports bacterial conjunctivitis
Ciliary flush, corneal opacity, dendritic lesion, hypopyon, or irregular pupil suggests alternative diagnosis
Proptosis or painful extraocular movements suggests orbital cellulitis
Investigations
First-line
Visual acuityRequired in red-eye assessment. Decreased acuity is a red flag for keratitis, uveitis, glaucoma, or retinal/optic nerve disease
Penlight/slit-lamp examinationAssess pattern of injection, cornea, anterior chamber, pupil, discharge, and foreign body
Fluorescein stainingIndicated with pain, photophobia, foreign body sensation, trauma, contact lens use, or concern for epithelial defect/keratitis
Second-line
Culture / NAATNot routine. Obtain for neonatal conjunctivitis, hyperacute purulence, suspected gonococcal/chlamydial disease, immunocompromise, or treatment failure
Adenovirus testingMay be used in clinics but does not usually change supportive management
IOP measurementIf headache, halos, fixed pupil, or concern for angle closure
Specialist
Ophthalmology referralUrgent for pain, photophobia, decreased vision, corneal involvement, contact lens keratitis concern, herpes simplex/zoster eye disease, or severe hyperacute purulent conjunctivitis
1
Viral conjunctivitis
- Supportive care: artificial tears, cold compresses, hand hygiene, avoid sharing towels, avoid touching eyes
- Usually self-limited; adenovirus can remain contagious for days to weeks
- Avoid routine topical antibiotics — they do not shorten viral disease
- Avoid topical steroids unless directed by ophthalmology because they may worsen HSV keratitis and raise IOP
2
Bacterial conjunctivitis
- Mild uncomplicated cases may resolve spontaneously, but topical antibiotics can modestly shorten symptoms and reduce transmission
- Options: erythromycin ointment, polymyxin B/trimethoprim drops, or fluoroquinolone drops for contact lens wearers when keratitis is not suspected
- Contact lens wearers: stop lenses immediately, discard lens case, and evaluate for corneal ulcer if pain/photophobia/opacity
- Hyperacute gonococcal conjunctivitis: urgent ophthalmology + systemic ceftriaxone; evaluate/treat sexually transmitted infections
3
Allergic conjunctivitis
- Avoid allergen exposure; artificial tears and cold compresses
- Topical antihistamine/mast-cell stabilizer such as olopatadine, ketotifen, or azelastine
- Oral antihistamines may help systemic allergy symptoms but can worsen ocular dryness
- Short steroid course only under ophthalmology supervision for severe disease
4
Public health and follow-up
- Advise school/work exclusion only when required locally or if hygiene cannot be maintained; avoid swimming pools during active discharge
- Return urgently for pain, photophobia, decreased vision, worsening swelling, corneal opacity, or failure to improve
Complications
- Keratitis: Especially HSV, adenoviral epidemic keratoconjunctivitis, or contact lens-associated bacterial infection
- Corneal ulcer/perforation: Risk with contact lens-related Pseudomonas or gonococcal disease
- Transmission outbreaks: Adenovirus spreads easily in clinics, schools, and households
- Medication toxicity: Overuse of topical antibiotics or vasoconstrictors can worsen irritation
- Missed alternative diagnosis: Angle closure, uveitis, scleritis, or orbital cellulitis can be mislabeled as conjunctivitis
USMLE Step 2 CK Exam Tips
- 1Watery discharge + preauricular lymph node + URI = viral conjunctivitis; treat supportively
- 2Itching is the key word for allergic conjunctivitis
- 3Mucopurulent discharge and eyelids stuck shut = bacterial conjunctivitis
- 4Contact lens wearer with painful red eye = think Pseudomonas keratitis/corneal ulcer, not simple conjunctivitis
- 5Do not give topical steroids for undifferentiated red eye — can worsen HSV and raise IOP
- 6Severe pain, photophobia, decreased vision, corneal opacity, or fixed pupil are red flags against simple conjunctivitis
- 7Hyperacute purulent conjunctivitis = Neisseria gonorrhoeae; systemic ceftriaxone and urgent ophthalmology
- 8Neonatal conjunctivitis has a separate STI differential and should not be managed like routine adult conjunctivitis
practicetest your knowledge on conjunctivitisApply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — ophthalmology and beyond.
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