| 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 |
Dermatomal distribution is the single most reliable clinical discriminator between zoster and HSV. Zoster ALWAYS respects dermatome boundaries; HSV does not.
Remember: "Zoster follows a Zone (dermatomal)" — this mnemonic anchors the key distinction.
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.
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.
Robbins 10e Ch 25
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