Herpes Simplex Virus (HSV) Keratitis
A branching dendritic pattern staining of the cornea under cobalt blue light indicates herpes simplex virus (HSV) keratitis.
Diagnostic Reasoning
The distinctive clinical finding you describe—a branching dendritic pattern visible with fluorescein staining under cobalt blue light—is pathognomonic for HSV epithelial keratitis. The 2024 Conjunctivitis Preferred Practice Pattern specifically identifies "pleomorphic or excavated dendritic epithelial keratitis of cornea" as a distinctive sign of HSV infection 1.
Key Distinguishing Features
When examining fluorescein-stained corneas under cobalt blue light, HSV dendrites have specific characteristics that differentiate them from other conditions:
- Excavated dendrites: HSV produces true dendritic ulcers with terminal bulbs that are excavated (meaning they penetrate through the epithelium) 1
- Pleomorphic appearance: The branching pattern may vary in configuration 1
- Usually unilateral presentation: Though bilateral cases can occur in atopic, pediatric, or immunocompromised patients 1
Critical Differential: VZV vs HSV
The most important distinction to make is between HSV and varicella zoster virus (VZV), as both can present with dendritic-appearing lesions:
- HSV dendrites are excavated (true epithelial defects that stain brightly with fluorescein)
- VZV produces pseudodendritic keratitis that is non-excavated (raised epithelial lesions without true ulceration) 1
- VZV typically presents with a vesicular dermatomal rash and severe pain, which helps clinically differentiate it 1
Associated Clinical Context
Look for these accompanying features to support the HSV diagnosis 1:
- Bulbar conjunctival injection with watery discharge
- Mild follicular conjunctival reaction
- Possible palpable preauricular lymphadenopathy
- Vesicular rash or ulceration of the eyelids (when present)
- History of triggers: stress, febrile illness, UV exposure, recent surgery or trauma
Clinical Pitfalls to Avoid
Do not confuse this with Thygeson's superficial punctate keratitis (TSPK), which can rarely present with dendritic-appearing lesions 2. However, TSPK typically shows multiple fine epithelial lesions rather than true branching dendrites, and the clinical course differs significantly.
Avoid misdiagnosing diffuse punctate staining patterns seen in viral keratoconjunctivitis (adenovirus), dry eye, or medicamentosa as HSV—these show diffuse or exposure-zone patterns, not branching dendrites 3, 1.
Prognostic Implications
Without treatment, HSV keratitis typically subsides in 4-7 days, but complications are common and include 1:
- Progressive epithelial keratitis
- Stromal keratitis with neovascularization
- Corneal scarring, thinning, or perforation
- Endotheliitis and corneal edema
- Uveitis and elevated intraocular pressure
- Permanent vision loss
The presence of dendritic keratitis requires prompt antiviral therapy to prevent these vision-threatening sequelae 4, 5.