Varicella-zoster virus reactivation from dorsal root ganglia typically presents with dermatomal distribution. Which clinical feature best distinguishes herpes zoster from herpes simplex virus infection?

Explanation

## Distinguishing Herpes Zoster from Herpes Simplex **Key Point:** The most reliable clinical feature distinguishing herpes zoster (HZ) from herpes simplex virus (HSV) is the **unilateral dermatomal distribution with strict midline sparing**. ### Comparison Table | Feature | Herpes Zoster | Herpes Simplex | | --- | --- | --- | | **Distribution** | Unilateral, dermatomal, midline sparing | Often bilateral, non-dermatomal, crosses midline | | **Recurrence** | Single episode (usually), rare recurrence | Frequent recurrences at same site | | **Prodrome** | Pain/paresthesia 48–72 hrs before rash | Burning, tingling (variable) | | **Vesicle grouping** | Grouped on erythematous base | Grouped on erythematous base | | **Tzanck smear** | Multinucleated giant cells | Multinucleated giant cells | | **Viral source** | Reactivation from dorsal root ganglion | Reactivation from sensory nerve root | **High-Yield:** The **dermatomal distribution with midline sparing** is the single most important clinical discriminator. HSV can cross the midline and is not restricted to a single dermatome. **Clinical Pearl:** Herpes zoster involving the ophthalmic division (V1) with corneal involvement (zoster ophthalmicus) is a medical emergency requiring urgent ophthalmology referral and systemic antivirals. **Mnemonic:** **Z**oster = **Z**one (dermatomal, unilateral); **S**implex = **S**pread (can cross midline, bilateral possible). ### Why Other Features Are Non-Discriminatory - **Vesicular rash on erythematous base**: Both conditions present this way — not distinctive - **Tzanck smear**: Both show multinucleated giant cells; cytology cannot differentiate HSV from VZV - **Prodromal pain**: Both can present with prodromal symptoms, though zoster pain is typically more severe