A 34-year-old woman presents with a 3-day history of painful vesicular eruption in a dermatomal distribution on her left chest wall. She reports severe burning pain that preceded the rash by 2 days. On examination, grouped vesicles on an erythematous base are seen in the T4–T6 dermatome. She is afebrile and systemically well. She has no prior history of chickenpox but received varicella vaccine 10 years ago. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Herpes Zoster ### Key Distinguishing Features **Key Point:** Herpes zoster (shingles) is characterized by a unilateral, dermatomal distribution of painful vesicles, typically preceded by 2–3 days of prodromal pain (neuropathic burning or itch). ### Pathophysiology Herpes zoster results from reactivation of latent varicella-zoster virus (VZV) in dorsal root ganglia. The virus travels along a single sensory nerve, producing a dermatomal rash. Vaccination does not prevent reactivation but may reduce severity and post-herpetic neuralgia risk. ### Clinical Features Supporting This Case | Feature | Finding in This Case | Significance | |---------|----------------------|---------------| | **Distribution** | Unilateral, T4–T6 dermatome | Pathognomonic for zoster | | **Prodrome** | 2 days of burning pain before rash | Classic neuropathic pain precedes lesions | | **Lesion morphology** | Grouped vesicles on erythematous base | Identical to primary VZV (varicella) | | **Systemic signs** | Afebrile, systemically well | Typical for immunocompetent host | | **Age** | 34 years | Can occur at any age; risk increases >50 years | ### Diagnostic Approach **High-Yield:** Clinical diagnosis is usually sufficient. If confirmation needed: - **Tzanck smear:** Multinucleated giant cells (not specific for VZV vs. HSV) - **PCR or viral culture:** Gold standard; differentiates VZV from HSV - **Direct fluorescent antibody (DFA):** Rapid, specific for VZV ### Management Principles 1. **Antivirals** (within 72 hours of rash onset for maximal benefit): - Acyclovir 800 mg 5× daily × 7–10 days, OR - Valacyclovir 1000 mg 3× daily × 7 days (preferred; better bioavailability), OR - Famciclovir 500 mg 3× daily × 7 days 2. **Analgesia:** NSAIDs, pregabalin, or gabapentin for neuropathic pain 3. **Post-herpetic neuralgia (PHN) prevention:** Antivirals reduce PHN risk by ~50% if started early **Clinical Pearl:** Dermatomal distribution is the single most reliable clinical clue distinguishing zoster from HSV-1 (which is typically non-dermatomal, recurrent at same site, and preceded by prodrome at that site). ### Mnemonic: ZOSTER Features - **Z**oster = reactivation (vs. primary varicella) - **O**ne dermatome (unilateral) - **S**evere pain (neuropathic, often precedes rash) - **T**reatment window: <72 hours for antivirals - **E**ruption: grouped vesicles on erythema - **R**isk: age >50, immunosuppression, malignancy