In herpes zoster, which dermatome distribution is MOST commonly affected?

Question

Accepted Answer

B. Thoracic dermatomes (T1–L2). ## Dermatome Distribution in Herpes Zoster

**Key Point:** Thoracic dermatomes account for approximately 50–60% of all herpes zoster cases, making them the single most common site of involvement.

### Frequency of Zoster by Dermatome

| Dermatome Region | Frequency | Clinical Notes |
|---|---|---|
| **Thoracic (T1–L2)** | **50–60%** | Most common; typically unilateral, follows rib distribution |
| Cervical (C2–C4) | 10–20% | Second most common |
| Lumbar/Sacral (L1–S5) | 10–15% | Lower frequency |
| Ophthalmic (V1) | 10–15% | Risk of ocular complications; requires urgent ophthalmology referral |
| Other cranial nerves | 5–10% | Ramsay Hunt syndrome (CN VII), CN V2/V3 |

**High-Yield:** The thoracic distribution is so common that zoster should be suspected in any patient presenting with unilateral vesicular rash in a dermatomal pattern across the chest or trunk.

### Why Thoracic Dermatomes?

1. **Larger surface area:** Thoracic region covers a large body surface, increasing the probability of reactivation
2. **Dorsal root ganglia density:** The thoracic spinal ganglia have high numbers of latent VZV-infected neurons
3. **Mechanical factors:** Thoracic dermatomes are less mobile than extremities, potentially affecting viral reactivation kinetics

**Clinical Pearl:** Thoracic zoster typically presents as a unilateral vesicular rash following one or more ribs, often preceded by dermatomal pain or hyperesthesia. Post-herpetic neuralgia (PHN) is common in this distribution, especially in elderly patients.

**Mnemonic:** **THORAX** — **T**horacic zoster is most common, followed by **H**ead (V1/V2/V3), **O**ther cervical, **R**adicular pain precedes rash, **A**cute phase 7–10 days, **X**-ray (imaging) rarely needed for diagnosis.

[cite:Park 26e Ch 8]

![Herpes Simplex and Zoster — Skin diagram](https://mmcphlazjonnzmdysowq.supabase.co/storage/v1/object/public/blog-images/explanation/3541.webp)

Answer

A. Ophthalmic division of trigeminal nerve (V1)

Answer

C. Cervical dermatomes (C2–C4)

Answer

D. Sacral dermatomes (S1–S5)

In herpes zoster, which dermatome distribution is MOST commonly affected?

Explanation

Dermatome Distribution in Herpes Zoster

Frequency of Zoster by Dermatome

Why Thoracic Dermatomes?

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Dermatome Region	Frequency	Clinical Notes
Thoracic (T1–L2)	50–60%	Most common; typically unilateral, follows rib distribution
Cervical (C2–C4)	10–20%	Second most common
Lumbar/Sacral (L1–S5)	10–15%	Lower frequency
Ophthalmic (V1)	10–15%	Risk of ocular complications; requires urgent ophthalmology referral
Other cranial nerves	5–10%	Ramsay Hunt syndrome (CN VII), CN V2/V3