A 28-year-old man with a history of recurrent HSV-1 keratitis presents with acute epithelial keratitis. On slit-lamp examination, the ulcer shows a classic branching dendritic pattern with terminal bulbs. Which single feature best distinguishes primary HSV epithelial keratitis from recurrent HSV stromal keratitis in this patient?

Explanation

## HSV Epithelial vs. Stromal Keratitis: Clinical Discrimination ### Overview Both epithelial and stromal HSV keratitis can occur in the same patient, but their clinical features, pathogenesis, and treatment differ fundamentally. The question asks which single feature **best distinguishes** HSV epithelial keratitis from HSV stromal keratitis. ### Comparison Table | Feature | HSV Epithelial Keratitis | HSV Stromal Keratitis | | --- | --- | --- | | **Ulcer morphology** | **Dendritic ulcer with terminal bulbs** (pathognomonic) | Stromal infiltration/edema; epithelium often intact | | **Anterior chamber reaction** | Minimal to mild | Marked; keratic precipitates common | | **Antiviral response** | Rapid (1–2 weeks with topical agents) | Slow; may worsen with antivirals alone | | **Pathogenesis** | Active viral replication in epithelium | Immune-mediated (viral antigen–antibody complex) | | **Treatment** | Topical antivirals (acyclovir, ganciclovir) | Topical steroids + systemic antivirals | | **Scarring** | Minimal if treated early | Significant; may cause visual impairment | ### Key Point: **The dendritic ulcer with terminal bulbs is the single pathognomonic feature of HSV epithelial keratitis.** This morphology — a branching, linear ulcer with characteristic terminal end-bulbs staining with fluorescein and rose bengal — is not seen in stromal keratitis, which instead shows stromal edema, infiltration, and disciform changes with an intact overlying epithelium. No other condition produces this exact pattern (pseudo-dendrites of Acanthamoeba or herpes zoster lack true terminal bulbs). ### Why Option B is Incorrect: Rapid response to topical antivirals within 1–2 weeks is a *consequence* of epithelial disease, not a distinguishing morphological feature. It cannot be used at the time of initial presentation to discriminate between the two forms — it is a retrospective observation. Moreover, stromal keratitis may also partially respond to antivirals when combined with steroids. ### Why Option C is Incorrect: Anterior chamber reaction and keratic precipitates are characteristic of **stromal** keratitis, not epithelial keratitis. They do not distinguish epithelial from stromal disease as a feature of epithelial keratitis. ### Why Option D is Incorrect: Unilateral presentation with photophobia can occur in both epithelial and stromal keratitis and is not discriminatory. ### High-Yield: The **dendritic ulcer with terminal bulbs** is the morphological hallmark that definitively identifies HSV epithelial keratitis at the slit lamp. Stromal keratitis lacks this epithelial ulceration pattern. ### Clinical Pearl: Rose bengal stains the devitalized epithelial cells at the edges of the dendrite, while fluorescein pools in the ulcer base — together confirming the diagnosis of HSV epithelial keratitis at the bedside. [cite: Kanski's Clinical Ophthalmology 9e Ch 4; Yanoff & Duker Ophthalmology 5e]