Shingles commonly affects one unilateral sensory dermatome, often thoracic, and persistent pain in the same distribution supports post-herpetic neuralgia.

Herpes zoster is particularly relevant in an 83-year-old because persistent neuropathic pain after an episode is clinically consequential, and the described distribution is a well-established diagnostic pattern.

Clinical pattern and distribution

• Shingles typically causes a painful vesicular eruption confined to a unilateral sensory dermatome, usually with a clear midline boundary; one or two adjacent dermatomes may be involved.
• Thoracic dermatomes are the most common location, accounting for approximately 50–55% of cases in guideline and clinical-review summaries. Trigeminal, lumbar, and cervical distributions occur less often.
• Bilateral, disseminated, recurrent, substantially midline-crossing, or otherwise atypical eruptions are not the usual pattern and should prompt consideration of atypical zoster or alternative diagnoses; VZV PCR can help when lesions are diagnostically uncertain.

Persistent pain and post-herpetic neuralgia

• Pain that remains confined to the dermatome previously affected by shingles strongly supports post-herpetic neuralgia (PHN), especially when it has neuropathic qualities: burning, stabbing, electric-shock sensations, itching, tingling, allodynia, or hyperalgesia.
• The most commonly used PHN definition is pain persisting for ≥90 days after rash onset; some definitions use three months after lesion healing or 120 days. Earlier persistent pain may be termed subacute herpetic neuralgia.
• Thoracic involvement is also among clinical factors associated with greater PHN risk.

Mechanistic basis

• Varicella-zoster virus reactivates in a sensory ganglion and travels along sensory axons to the skin, explaining the unilateral dermatomal eruption.
• Ganglionic inflammation and zoster-associated injury to sensory neurons and peripheral nerves can leave persistent neuropathic pain in that same territory.

Bottom line

• The claim is well supported: shingles usually follows one unilateral sensory dermatome, most often thoracic, and persistent neuropathic pain in the prior rash distribution—particularly beyond three months—is strongly consistent with PHN.