A 28-year-old man with a history of chickenpox in childhood now presents with a painful vesicular rash in a dermatomal distribution on the left thoracic wall. What is the most common dermatome affected by herpes zoster in immunocompetent individuals?

Explanation

## Most Common Dermatome in Herpes Zoster ### Epidemiology of Zoster Distribution **Key Point:** Thoracic dermatomes account for approximately 50–60% of all herpes zoster cases, making them the most frequently affected region in immunocompetent individuals. ### Frequency of Zoster by Dermatome | Dermatome Region | Frequency | Clinical Features | |------------------|-----------|-------------------| | Thoracic (T5–T10) | 50–60% | Most common; unilateral band-like rash on trunk | | Ophthalmic (V1) | 10–15% | Second most common; risk of ocular complications | | Cervical (C3–C4) | 10–15% | Upper trunk and neck; Ramsay Hunt if facial nerve involved | | Lumbar (L4–L5) | 10–15% | Lower trunk and lower extremity | | Sacral (S1–S2) | 5–10% | Least common | **High-Yield:** Thoracic > Ophthalmic > Cervical = Lumbar > Sacral. This distribution reflects the relative size and surface area of dermatomes, with thoracic dermatomes being the largest. ### Pathophysiology Herpes zoster results from reactivation of latent varicella-zoster virus (VZV) in dorsal root ganglia. The virus replicates and travels along sensory nerve axons to the skin, producing a dermatomal distribution of vesicles. The thoracic region has the largest number of dorsal root ganglia and the greatest surface area, explaining its higher frequency of involvement. ### Clinical Features of Thoracic Zoster 1. **Prodrome:** Pain, burning, or paresthesia in the affected dermatome for 2–3 days before rash appears. 2. **Morphology:** Unilateral, vesicular rash in a band-like or girdle-like distribution. 3. **Duration:** Acute phase lasts 7–10 days; crusting by 2–3 weeks. 4. **Complications:** Post-herpetic neuralgia (PHN) is common, especially in patients >50 years; intercostal neuralgia may occur. **Clinical Pearl:** Thoracic zoster is rarely associated with serious ocular or neurological complications, unlike ophthalmic zoster (V1), which carries a 50% risk of ocular involvement (keratitis, uveitis, corneal scarring). ### Risk Factors for Zoster Reactivation - Age >50 years (incidence increases exponentially) - Immunosuppression (HIV, malignancy, immunosuppressive drugs) - Trauma or radiation to the affected dermatome - Psychological stress - Malignancy (especially lymphoproliferative disorders) **Mnemonic:** **TRIM** — Trauma, Reactivation (age-related), Immunosuppression, Malignancy — risk factors for zoster.