A 28-year-old man from Mumbai presents to the emergency department with a 5-day history of severe left-sided facial pain, followed by vesicular rash in the distribution of the trigeminal nerve (V1 and V2 branches). He has fever (39.2°C), photophobia, and mild conjunctival injection. On examination, vesicles are noted on the forehead, eyelid, and tip of the nose. He is otherwise healthy with normal immune function. Urgent ophthalmology consultation confirms no corneal involvement at present. What is the most appropriate immediate next step in management?

Explanation

## Clinical Presentation: Herpes Zoster Ophthalmicus (HZO) **Key Point:** Herpes zoster involving the ophthalmic division (V1) of the trigeminal nerve is a medical emergency because of the risk of serious ocular complications, including keratitis, uveitis, and vision loss. **High-Yield:** The presence of vesicles on the tip of the nose (Hutchinson's sign) is a strong predictor of corneal involvement and indicates high risk for ocular morbidity. ## Why IV Acyclovir at 10 mg/kg Is Correct **Clinical Pearl:** HZO requires IV acyclovir at **10 mg/kg every 8 hours** (not the 5 mg/kg used for HSV), because: 1. **Higher CNS penetration:** Zoster carries risk of postherpetic neuralgia (PHN), meningitis, and vasculitis; higher IV doses achieve better CSF levels 2. **Ocular involvement:** Severe keratitis and uveitis demand rapid, high-dose viral suppression 3. **Immunocompetent patients with V1 involvement:** Standard dosing is insufficient; escalation to 10 mg/kg 8-hourly is guideline-recommended **Mnemonic:** **HZO = HIGH DOSE** — **H**erpes **Z**oster **O**phthalmicus requires 10 mg/kg (not 5 mg/kg). ## Why Renal Function Must Be Checked First Acyclovir is nephrotoxic and renally cleared. Before IV infusion: - Measure serum creatinine and calculate CrCl - Ensure adequate hydration (IV fluids if needed) - If CrCl < 50 mL/min, dose adjustment is mandatory - Infuse over ≥1 hour to minimize crystalline nephropathy ## Management Algorithm for HZO ```mermaid flowchart TD A[Herpes Zoster Ophthalmicus diagnosed]:::outcome --> B{Renal function normal?}:::decision B -->|Yes| C[IV Acyclovir 10 mg/kg 8-hourly]:::action B -->|No| D[Dose adjust based on CrCl]:::action C --> E[Ensure adequate hydration]:::action D --> E E --> F[Urgent ophthalmology review within 24 hrs]:::action F --> G{Corneal involvement?}:::decision G -->|Yes| H[Topical antivirals + lubricants + cycloplegics]:::action G -->|No| I[Continue IV acyclovir, monitor daily]:::action H --> J[Assess for PHN risk]:::action I --> J J --> K[Consider gabapentin or pregabalin if PHN develops]:::action ``` ## Comparison: IV vs. Oral Acyclovir in HZO | Parameter | IV Acyclovir 10 mg/kg | Oral Acyclovir 800 mg 5× daily | |-----------|----------------------|--------------------------------| | Bioavailability | 100% | 15–20% | | CNS penetration | Excellent | Poor | | Corneal involvement prevention | Superior | Suboptimal | | PHN incidence | Lower | Higher | | Recommended in HZO? | **YES** (standard) | **NO** (inadequate) | **Warning:** Oral acyclovir monotherapy in HZO is associated with higher rates of ocular complications and PHN. IV therapy is non-negotiable in V1 involvement. ## Why MRI Brain Is Not Routine MRI brain is indicated only if there are signs of CNS involvement (meningitis, encephalitis, vasculitis). This patient has no meningeal signs or altered mental status; MRI is not part of the immediate management algorithm and would delay critical IV antiviral initiation.