Can sustained nerve input sensitize pain circuits and keep pain going after a shingles rash heals?

Herpes zoster can injure sensory ganglia and peripheral nerves; in an 83-year-old, persistent pain after rash resolution is consistent with postherpetic neuralgia (PHN). The proposed pathway is biologically and clinically supported, with moderate confidence.

Mechanistic evidence

• Injured peripheral afferents can develop ectopic discharges and ongoing nociceptor activity, including sustained C-fiber input.
• Persistent afferent signaling can increase dorsal-horn transmission, drive NMDA-dependent synaptic plasticity, and reduce inhibitory control. These changes promote central sensitization in spinal pain circuits, with associated abnormalities in sensory and affective–cognitive brain networks.
• This does not mean ongoing peripheral input is necessary in every case once PHN is established; central and peripheral mechanisms can vary between individuals.

Clinical expression

• Sensitized processing can manifest as spontaneous pain, mechanical hyperalgesia, and tactile allodynia—pain from normally non-painful contact.
• In one PHN cohort, brush-evoked allodynia was present in 64% of assessed patients and was associated with greater pinprick pain and overall pain severity.
• Longitudinal sensory phenotyping found increased mechanical pain sensitivity among people with preserved or sensitized C-fiber function, consistent with a gain in pain responsiveness that can sustain symptoms after skin healing.

Interpretation

• “Lowered pain thresholds” is best understood as modality- and phenotype-specific. Some people have prominent allodynia or hyperalgesia, whereas others have sensory loss; one cohort found conventional thermal and mechanical thresholds largely unchanged despite frequent allodynia.

Bottom line

• Sustained signaling from zoster-injured nerves can sensitize spinal and brain pain systems, producing allodynia and hyperalgesia that contribute to persistent pain after the eruption resolves. This is a well-supported contributory mechanism, but it is not universal across all PHN presentations.