A 32-year-old woman presents with a 5-day history of unilateral eye pain, photophobia, and blurred vision. Slit-lamp examination reveals a dendritic ulcer with terminal bulbs on the cornea, and Giemsa staining shows multinucleated giant cells. What is the drug of choice for this condition?

Explanation

## Diagnosis: Herpes Simplex Virus (HSV) Keratitis **Key Point:** The dendritic ulcer with terminal bulbs and multinucleated giant cells on Giemsa staining are pathognomonic for HSV keratitis. ## Drug of Choice: Acyclovir ### Mechanism - Nucleoside analogue that inhibits viral DNA polymerase - Requires viral thymidine kinase for activation (high selectivity for infected cells) - Prevents viral DNA chain elongation ### Dosing & Route - **Topical:** Acyclovir 3% ointment, 5 times daily for 7–10 days - **Systemic:** 400–800 mg orally 5 times daily for 10–14 days (for severe or disseminated disease) - Topical therapy is usually sufficient for uncomplicated epithelial keratitis **High-Yield:** Acyclovir is the first-line agent for HSV keratitis because it is effective, well-tolerated, and has the longest safety record in ophthalmology. ## Comparison with Other Antivirals | Agent | Mechanism | Use in HSV Keratitis | Limitation | |-------|-----------|----------------------|------------| | Acyclovir | HSV DNA polymerase inhibitor | First-line, topical & systemic | Requires viral TK; resistance rare | | Ganciclovir | CMV DNA polymerase inhibitor | CMV retinitis (immunocompromised) | Not preferred for HSV; more toxic | | Cidofovir | Nucleotide analogue | CMV retinitis | Nephrotoxic; not for HSV | | Foscarnet | Pyrophosphate analogue | Acyclovir-resistant HSV (rare) | Nephrotoxic; second-line only | **Clinical Pearl:** Acyclovir-resistant HSV is extremely rare in immunocompetent patients but may occur in severely immunocompromised individuals (e.g., advanced HIV). In such cases, foscarnet or cidofovir may be considered. ## Adjunctive Management - Topical cycloplegic agents (cyclopentolate 1%) to relieve pain and prevent posterior synechiae - Avoid topical corticosteroids in epithelial keratitis (risk of geographic ulcer) - Systemic NSAIDs for pain control - Lubricating drops for comfort **Warning:** Do not use topical corticosteroids alone in HSV keratitis—they can worsen the infection and cause a "herpetic geographic ulcer." Steroids may be used ONLY with concurrent antiviral therapy and under close ophthalmologic supervision.