Postherpetic Neuralgia: What Causes It and How It Is Treated

What Is Postherpetic Neuralgia?

Postherpetic Neuralgia (PHN) is a chronic neuropathic pain that persists or arises in the area affected by herpes zoster (shingles) after resolution of the cutaneous lesions. It is defined as pain that persists more than 90 days after the onset of the herpes zoster rash.

Herpes zoster is caused by reactivation of the varicella-zoster virus (VZV), which remains latent in the sensory ganglia after varicella (chickenpox). Reactivation causes inflammation and neuronal damage in the ganglion and nerve, potentially resulting in persistent neuropathic pain even after healing of the skin lesions.

PHN is the most common complication of herpes zoster and represents one of the most studied models of neuropathic pain in medicine. Pain may be extremely debilitating, affecting sleep, mood, and functionality, especially in older adults.

Epidemiology

PHN incidence rises dramatically with age. While fewer than 5% of herpes zoster patients under 60 develop PHN, the rate rises to 20-30% in patients over 70. Other risk factors include acute-pain intensity, rash severity, immunosuppression, and a painful prodrome before lesions appear.

Pathophysiology

PHN results from inflammatory and neurodegenerative damage caused by the varicella-zoster virus to the sensory neurons of the dorsal root ganglion and to peripheral nerve fibers. Viral reactivation provokes intense ganglionitis, with neuronal necrosis, demyelination, and fibrosis of the affected nerve.

Three neuropathic mechanisms converge: peripheral sensitization (remaining nociceptors become hyperexcitable), central sensitization (dorsal horn neurons of the spinal cord become hyperactive owing to prolonged nociceptive input), and deafferentation (loss of sensory fibers leads to aberrant reorganization of spinal circuits).

Allodynia (pain provoked by normally innocuous stimuli, such as the touch of clothing) is explained by synaptic reorganization: when C fibers (pain) are destroyed, A-beta fibers (light touch) form synapses in dorsal horn laminae normally destined for pain, causing light touch to be perceived as pain.

Symptoms

PHN manifests as pain in the dermatomal distribution of the nerve affected by herpes zoster. Thoracic dermatomes are the most common, followed by the ophthalmic branch of the trigeminal nerve. Pain takes different patterns, which often coexist in the same patient.

Diagnosis

Diagnosis of PHN is clinical, based on the history of prior herpes zoster and persistent neuropathic pain in the corresponding dermatomal distribution. The presence of residual scars from herpes zoster in the painful area confirms the diagnosis.

In atypical cases or without a clear rash history, diagnosis can be more difficult. Rarely, herpes zoster causes pain without visible skin lesions (zoster sine herpete), requiring serologic or PCR confirmation. Additional tests help exclude differential diagnoses.

Differential Diagnosis

PHN must be distinguished from other causes of regional neuropathic pain, especially when the herpes zoster history is unclear or symptoms are atypical.

PHN vs. Active Herpes Zoster: The Temporal Transition

The distinction between active herpes zoster pain and PHN is temporal: active herpes zoster lasts days to weeks (acute phase), while PHN is defined as pain persisting more than 90 days after rash onset. However, clinical practice recognizes an intermediate zone called "subacute pain" (30 to 90 days). Early antiviral treatment (within the first 72 hours) is crucial for the acute phase and may modestly reduce PHN risk. Aggressive acute-phase pain control also matters — evidence shows that severe uncontrolled acute pain promotes central sensitization and raises PHN risk.

Zoster sine herpete is a rare presentation in which the virus causes dermatomal neuropathic pain without visible skin lesions. Diagnosis is difficult and requires confirmation by CSF PCR or specific serology. Consider it in older or immunosuppressed patients with subacute-onset dermatomal neuropathic pain.

Visceral Pain and Peripheral Neuropathy: Avoiding Diagnostic Confusion

Thoracic PHN can be mistaken for cardiac, pleural, or gastric pain. With no active rash or visible scars but band-like chest pain, ruling out visceral causes is mandatory — especially unstable angina when the distribution is in the left T1-T4 dermatome. ECG and targeted clinical evaluation are indispensable. Allodynia (pain to light touch of clothing), concomitant hypoesthesia, and residual scars in the painful area strongly favor PHN.

Diabetic or alcoholic peripheral neuropathy has a symmetric "glove-and-stocking" distribution, without PHN's unilateral dermatomal pattern and without a herpes zoster history. Fibromyalgia causes diffuse, generalized pain, without a specific dermatome and without the characteristic viral history. In PHN patients with concomitant generalized hypersensitivity, consider central sensitization and secondary fibromyalgia.

Acupuncture in the Context of PHN

Medical acupuncture is especially relevant in PHN because it can modulate multiple neuropathic mechanisms simultaneously: peripheral sensitization, central sensitization, and dysfunction of descending inhibitory pathways. Perilesional needling (around the affected area, avoiding very sensitive scars) combined with distal points on the corresponding meridians is a common approach. Low-frequency electroacupuncture (2 Hz) promotes beta-endorphin release, particularly useful for constant burning pain.

Acupuncture may be especially valuable for older PHN patients, in whom conventional medications (tricyclics, gabapentinoids) often cause problematic adverse effects such as sedation, dizziness, falls, and urinary retention. As complementary therapy integrated into the multimodal plan, it may contribute to better symptom control — any medication dose adjustment must be discussed and conducted by the attending physician (neurologist/pain physician), never by the patient alone nor by the acupuncturist. Acupuncture does not replace gabapentin, pregabalin, tricyclics, or topical lidocaine in PHN treatment.

Treatment

Treatment of PHN frequently requires combination of multiple therapies, since no single agent provides complete relief in most patients. The goal is to reduce pain to tolerable levels and improve functionality and sleep.

Acupuncture as Treatment

Acupuncture is a complementary option for PHN, with mechanisms that include activation of descending inhibitory pain pathways, release of endorphins and enkephalins, modulation of central sensitization, and reduction of local neuroinflammation.

The approach combines perilesional needling (around the affected area, not directly over sensitive scars), distal points on the corresponding meridians, and electroacupuncture at specific frequencies. Low-frequency electrical stimulation (2 Hz) promotes beta-endorphin release, while high frequency (100 Hz) stimulates dynorphin release.

Acupuncture may be particularly useful for PHN patients who do not tolerate or respond well to conventional medications — in this scenario, it acts as an adjuvant within the multimodal plan led by the attending physician. Any dose reductions or medication switches must be made by the neurologist/pain physician after clinical evaluation; acupuncture does not replace gabapentinoids, tricyclics, topical lidocaine, or topical capsaicin prescribed for PHN.

Prognosis and Prevention

PHN is self-limited in most cases, but duration varies. About 50% of patients resolve within 1 year, but 10-25% may have persistent pain for more than 5 years, especially older adults with more severe initial presentations.

Prevention is the most effective strategy. The recombinant herpes zoster vaccine (Shingrix) has demonstrated efficacy near 90% in preventing herpes zoster in pivotal studies (ZOE-50/ZOE-70, follow-up of 2-4 years), with similar magnitude of efficacy in preventing PHN. Extended follow-up data show gradual reduction of protection over the years. Early antiviral treatment of herpes zoster (valacyclovir, famciclovir, within the first 72 hours of the rash) reduces the duration and severity of acute pain and may modestly reduce the risk of PHN.

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