Post-Herpetic Neuralgia Does Not Occur Beyond the Original Dermatome
Post-herpetic neuralgia (PHN) is defined as pain confined to the dermatome(s) affected by the original herpes zoster infection and does not manifest in other locations. By definition, PHN represents chronic neuropathic pain that persists in the same dermatomal distribution where the acute shingles rash appeared 1, 2, 3, 4, 5.
Understanding the Dermatomal Distribution
The clinical presentation is straightforward:
- PHN pain remains unilateral and confined to a single dermatome (or occasionally adjacent dermatomes if the original infection involved multiple areas) 2, 3, 4
- The pain location directly corresponds to where the vesicular rash occurred during acute herpes zoster
- This dermatomal restriction reflects the underlying pathophysiology: VZV reactivates from a specific dorsal root ganglion or cranial nerve ganglion where it remained latent after primary varicella infection 6, 4
Important Caveat: Bilateral Sensory Changes
While PHN pain itself remains strictly unilateral and dermatomal, research has demonstrated bilateral sensory neuron damage in patients with PHN 7. A study found that patients with PHN had lost approximately half of their epidermal neurites in the contralateral (opposite side) location, despite having no pain or rash on that side 7. However, this bilateral neuronal damage does not translate into bilateral pain—the pain remains confined to the original shingles-affected dermatome.
Clinical Recognition
When evaluating suspected PHN:
- Look for pain lasting ≥90 days after acute herpes zoster in the exact same location as the original rash 2, 3
- The pain should be unilateral and follow a clear dermatomal pattern 2, 4
- If pain appears in a different location or becomes bilateral, consider alternative diagnoses
Treatment Implications
The dermatomal localization of PHN has direct treatment implications:
- Topical therapies work precisely because PHN is localized: high-concentration capsaicin (8% patch) or lidocaine patches can be applied directly to the affected dermatome 8, 1
- First-line systemic treatments include gabapentin or pregabalin 8, 1
- Tricyclic antidepressants (particularly nortriptyline) are highly effective with NNT of 2.64 1
If a patient reports pain outside the original herpes zoster dermatome, this is not PHN and requires evaluation for other causes of neuropathic pain.