A 58-year-old woman presents with a 3-day history of severe burning pain and tingling along the left T4 dermatome. On examination, she has grouped vesicles on an erythematous base confined to the left chest wall, sparing the midline. She denies fever or constitutional symptoms. She had chickenpox at age 6. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Herpes Zoster ### Key Clinical Features **Key Point:** Herpes zoster (shingles) is reactivation of latent varicella-zoster virus (VZV) in sensory nerve ganglia, typically occurring decades after primary varicella infection. ### Diagnostic Hallmarks | Feature | Herpes Zoster | Herpes Simplex | Disseminated Varicella | |---------|---------------|-----------------|------------------------| | **Distribution** | Dermatomal (unilateral, respects midline) | Non-dermatomal, often perioral/genital | Scattered, bilateral, multifocal | | **Prodrome** | Burning pain 2–3 days before rash | Mild or absent | Systemic symptoms (fever, malaise) | | **Vesicle grouping** | Grouped on erythematous base | Grouped, often recurrent site | Crops appearing over days | | **Age of onset** | Typically >50 years (reactivation) | Any age (primary or recurrent) | Primary infection, any age | | **Systemic symptoms** | Usually absent (unless complications) | Minimal | Prominent fever, lymphadenopathy | ### Why This Case Is Zoster 1. **Dermatomal distribution** — vesicles confined to T4 dermatome, respecting midline (pathognomonic for zoster) 2. **Prodromal pain** — severe burning dysesthesia preceding rash by 3 days 3. **Age and history** — 58-year-old with prior chickenpox (source of latent virus) 4. **Absence of systemic illness** — no fever or constitutional symptoms (typical for zoster in immunocompetent host) **High-Yield:** The **unilateral dermatomal distribution** is the single most discriminating feature that distinguishes zoster from HSV and other viral exanthems. ### Pathophysiology ```mermaid flowchart TD A[Primary varicella infection]:::outcome --> B[Virus enters sensory nerve endings] B --> C[Retrograde transport to dorsal root ganglion] C --> D[Latent infection established] D --> E{Immune decline<br/>or reactivation trigger}:::decision E -->|Reactivation| F[Virus replicates in ganglion] F --> G[Anterograde transport along nerve] G --> H[Dermatomal vesicular rash]:::outcome H --> I[Unilateral, respects midline]:::outcome ``` ### Clinical Pearl **Clinical Pearl:** Post-herpetic neuralgia (PHN) — persistent pain after rash resolution — is the most common complication in patients >50 years. Early antiviral therapy (within 72 hours) reduces PHN risk by ~50%. ### Management Implications - **Antiviral therapy:** Acyclovir 800 mg 5× daily × 7–10 days (or valacyclovir 1 g TID) — most effective if started within 72 hours of rash onset - **Analgesia:** NSAIDs, gabapentin, or pregabalin for neuropathic pain - **Vaccination:** Zoster vaccine (Shingrix, recombinant zoster vaccine) recommended for adults ≥50 years to reduce incidence and severity [cite:Robbins 10e Ch 8]