Herpes simplex keratitis

Overview

•Almost always unilateral; young/middle-aged adults

•Painful red eye - circumcorneal (limbal) injection

•Foreign body sensation - can mimic corneal abrasion or foreign body

•Photophobia - due to corneal nerve irritation

•Watery (clear) discharge - distinguishes from bacterial keratitis (purulent)

•Periocular vesicles or crusted lesions on eyelid margin/surrounding skin

•Palpable preauricular lymph node - classic sign of viral aetiology

🎯
The dendritic ulcer (branching/tree-like pattern on fluorescein staining) is pathognomonic of HSV epithelial keratitis - its pattern reflects viral spread along corneal epithelial nerve endings.

Investigations

🏆 Gold standard

•slit-lamp examination with fluorescein staining - reveals dendritic ulcer; also identifies stromal haze, corneal vascularisation, iritis

🥇 First-line

•visual acuity - essential baseline at every assessment

•Corneal swab/scraping (PCR or viral culture) - reserved for atypical or recalcitrant cases; PCR detects HSV DNA and can identify aciclovir-resistant strains

Differential diagnosis

Key differentials for dendritic/corneal ulcer
ConditionKey distinguishing features
Herpes simplex keratitisTrue branching dendritic ulcer on fluorescein; clear discharge; periocular vesicles; preauricular node
Herpes zoster ophthalmicusElderly; dermatomal rash; pseudo-dendritic (non-branching) ulcers; Hutchinson's sign
Corneal abrasionHistory of trauma; no dendritic pattern; resolves rapidly
Corneal foreign bodyVisible on examination; no dendritic ulcer
Bacterial keratitisPurulent discharge; contact lens risk factor; different ulcer morphology

Condition	Key distinguishing features
Herpes simplex keratitis	True branching dendritic ulcer on fluorescein; clear discharge; periocular vesicles; preauricular node
Herpes zoster ophthalmicus	Elderly; dermatomal rash; pseudo-dendritic (non-branching) ulcers; Hutchinson's sign
Corneal abrasion	History of trauma; no dendritic pattern; resolves rapidly
Corneal foreign body	Visible on examination; no dendritic ulcer
Bacterial keratitis	Purulent discharge; contact lens risk factor; different ulcer morphology

Management

•All suspected cases require same-day urgent ophthalmology referral - do not initiate treatment before specialist assessment

•First-line (epithelial keratitis): topical aciclovir eye ointment (3%) or ganciclovir eye gel - equally effective; inhibit HSV DNA polymerase. Oral aciclovir is an alternative/addition

•Second-line (stromal keratitis): topical corticosteroids combined with antivirals - reduces immune-mediated stromal damage; must NEVER be used as monotherapy

•Concurrent iritis: cyclopentolate (cycloplegic) - relieves ciliary spasm and prevents synechiae formation

•Prophylaxis: long-term oral aciclovir for recurrent epithelial or stromal keratitis

⚠️
Topical corticosteroids are contraindicated as monotherapy in HSV keratitis - used alone they suppress immunity without controlling viral replication, leading to uncontrolled viral spread and stromal destruction.