A 32-year-old woman presents with unilateral eye pain, photophobia, and blurred vision for 3 days following a vesicular rash on the forehead. Slit-lamp examination reveals a dendritic ulcer with terminal bulbs and central stromal infiltration. Which single feature best distinguishes this presentation from Varicella Zoster Virus (VZV) keratitis?

Explanation

## Distinguishing HSV from VZV Keratitis ### Clinical Context Both Herpes Simplex Virus (HSV) and Varicella Zoster Virus (VZV) can cause viral keratitis with overlapping presentations. The stem describes a patient with a dendritic ulcer **with terminal bulbs** — this morphological detail is the single best discriminator between HSV and VZV keratitis at the slit-lamp. ### Comparison Table | Feature | HSV Keratitis | VZV Keratitis | | --- | --- | --- | | **Dendritic ulcer morphology** | True dendrites with **terminal bulbs** | Pseudodendrites — tapered ends, **no terminal bulbs** | | **Stromal involvement** | Common; recurrent immune-mediated stromal keratitis | Less common; usually epithelial | | **Recurrence rate** | Very high (50% within 5 years) | Low; typically follows single zoster episode | | **Recurrence triggers** | Stress, UV exposure, immunosuppression, fever | Rare; only with dermatomal zoster reactivation | | **Vesicular prodrome** | Rare; usually no skin lesions | Characteristic dermatomal distribution (forehead rash) | | **Ophthalmic nerve involvement** | Can occur | Classic in Herpes Zoster Ophthalmicus (HZO) | ### Key Point: **Terminal bulbs on the dendritic ulcer margin are pathognomonic for HSV keratitis.** VZV keratitis produces *pseudodendrites* — elevated, mucus-laden plaques with tapered or stellate ends but **without** the characteristic terminal bulbs seen in HSV. This morphological distinction is the single most reliable slit-lamp feature to differentiate the two entities at the time of presentation. ### High-Yield: - **HSV dendrite:** true epithelial ulcer, stains with fluorescein at the base, **terminal bulbs present** → this is the feature described in the stem - **VZV pseudodendrite:** elevated mucous plaque, stains with rose bengal at the edges, **no terminal bulbs** - Option A (stromal keratitis with AC reaction): occurs in both HSV and VZV — not a distinguishing feature - Option B (ophthalmic division involvement): classic for VZV/HZO but also occurs in HSV — not uniquely distinguishing - Option C (recurrence triggered by stress/sunlight): characteristic of HSV but is a historical/temporal feature, not a slit-lamp finding present at the time of examination; moreover, VZV can also rarely recur ### Clinical Pearl: The question stem explicitly states "dendritic ulcer **with terminal bulbs**" — this is the classic HSV morphology. VZV keratitis produces pseudodendrites without terminal bulbs. Per **Kanski's Clinical Ophthalmology (9th ed., Ch. 4)** and **Yanoff & Duker's Ophthalmology (5th ed.)**, the presence of terminal bulbs is the definitive slit-lamp feature distinguishing HSV from VZV keratitis. The question asks for the single feature that "best distinguishes" the presentation — the terminal bulbs described in the stem are that feature, making Option D the correct answer. [cite:Kanski's Clinical Ophthalmology 9e Ch 4; Yanoff & Duker Ophthalmology 5e]