## Distinguishing HSV from Varicella-Zoster Virus (VZV) Infection ### Clinical Presentation Comparison | Feature | Herpes Simplex (HSV) | Herpes Zoster (VZV) | |---------|---------------------|---------------------| | **Distribution** | Non-dermatomal; clustered on lips, genitals, or random sites | **Strictly dermatomal** (unilateral, follows nerve distribution) | | **Vesicle appearance** | Grouped vesicles on erythematous base | Grouped vesicles on erythematous base | | **Recurrence pattern** | Frequent recurrences at same site | Rare recurrence (< 5% of cases) | | **Prodrome** | Tingling, burning (1–2 days) | Severe pain, burning (2–7 days) | | **Tzanck smear** | Positive (multinucleated giant cells) | Positive (multinucleated giant cells) | | **Immunology** | Primary infection or reactivation from latency in sensory ganglia | Reactivation of latent VZV from dorsal root ganglia | ### Key Point: **Dermatomal distribution is the single most reliable clinical discriminator between zoster and HSV.** Zoster ALWAYS respects dermatome boundaries; HSV does not. ### High-Yield: Remember: "**Z**oster follows a **Z**one (dermatomal)" — this mnemonic anchors the key distinction. ### Clinical Pearl: A patient with recurrent vesicular lesions on the lower lip at the same site over years = HSV. A patient with unilateral vesicles in a T4 distribution (thoracic) = zoster. The anatomical pattern is diagnostic. ### Why Tzanck Smear and Vesicles Are Not Discriminators Both HSV and VZV produce identical cytopathic changes (multinucleated giant cells) and identical clinical morphology (grouped vesicles). Tzanck smear cannot differentiate between them — it only confirms a herpesvirus. Viral culture, PCR, or serology is needed for definitive typing. [cite:Robbins 10e Ch 25]
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