A 28-year-old woman presents with a 5-day history of unilateral eye pain, photophobia, and blurred vision. She reports a vesicular rash on her forehead and eyelid 2 days prior to ocular symptoms. On slit-lamp examination, she has conjunctival injection, corneal epithelial defect with a dendritic ulcer, and anterior chamber reaction. What is the most likely causative organism?

Explanation

## Clinical Diagnosis: Herpes Zoster Keratitis (Varicella-Zoster Virus) ### Key Clinical Features **Key Point:** The pathognomonic finding is the **vesicular rash on the forehead and eyelid preceding ocular involvement** — this is Hutchinson's sign and is virtually pathognomonic for varicella-zoster virus (VZV) keratitis. ### Viral Keratitis Comparison | Feature | HSV-1 | VZV | Adenovirus | Enterovirus 70 | |---------|-------|-----|-----------|----------------| | **Preceding vesicular rash** | Rare, lip/face | **Yes, Hutchinson sign** | No | No | | **Dendritic ulcer** | Yes (classic) | Yes | No | No | | **Anterior chamber reaction** | Mild–moderate | Moderate–severe | Minimal | Minimal | | **Unilateral involvement** | Yes | **Yes (dermatomal)** | Bilateral common | Bilateral | | **Recurrence rate** | High (30–50%) | Lower (5–10%) | No | No | | **Preauricular lymphadenopathy** | Common | Less common | Common | Rare | ### Pathophysiology of VZV Keratitis 1. **Reactivation of latent VZV** in trigeminal ganglion (CN V1 distribution) 2. **Anterograde axonal transport** → vesicular eruption along V1 dermatomal distribution 3. **Direct viral invasion** of corneal epithelium → dendritic ulcer formation 4. **Immune-mediated stromal inflammation** → potential for stromal keratitis and scarring **High-Yield:** VZV keratitis is **more aggressive** than HSV-1 and carries higher risk of permanent corneal scarring and vision loss if untreated. ### Clinical Pearl **Hutchinson's sign** (vesicular rash involving the tip and side of the nose, indicating nasociliary nerve involvement) strongly predicts ocular involvement in herpes zoster and warrants immediate ophthalmologic evaluation. ### Management Approach ```mermaid flowchart TD A[VZV Keratitis suspected]:::outcome --> B{Dendritic ulcer present?}:::decision B -->|Yes| C[Acyclovir 400-800 mg PO 5x daily]:::action B -->|No| D[Stromal involvement?]:::decision D -->|Yes| E[Add topical corticosteroid + systemic acyclovir]:::action D -->|No| F[Topical antivirals + supportive care]:::action C --> G[Continue 7-10 days]:::action E --> H[Taper steroids after epithelial healing]:::action G --> I[Monitor for stromal keratitis]:::decision ``` **Key Point:** Systemic acyclovir is **superior to topical antivirals alone** for VZV keratitis due to higher corneal penetration and better stromal penetration. [cite:Kanski Clinical Ophthalmology 9e Ch 4]