Postherpetic neuralgia (PHN) is neuropathic pain that occurs due to damage to a peripheral nerve caused by the reactivation of the varicella zoster virus (herpes zoster, also known as shingles). PHN is defined as pain in a dermatomal distribution that lasts for at least 90 days after an outbreak of herpes zoster. Several types of pain may occur with PHN including continuous burning pain, episodes of severe shooting or electric-like pain, and a heightened sensitivity to gentle touch which would not otherwise cause pain (mechanical allodynia) or to painful stimuli (hyperalgesia).
A key factor in the neural plasticity underlying neuropathic pain is altered gene expression in sensory dorsal root ganglia neurons. Injury to sensory nerves induces neurochemical, physiological, and anatomical modifications to afferent and central neurons, such as afferent terminal sprouting and inhibitory interneuron loss. The zoster vaccine Shingrix provides around 90% protection from postherpetic neuralgia, and has been used in many countries since 2017. The earlier vaccine Zostavax provides lesser protection against shingles, and PHN.
The varicella vaccine is approved for infants to prevent chickenpox, which also protects against PHN from a herpes zoster infection. Vaccination decreases the overall incidence of virus reactivation, and also decreases the severity of disease development and incidence of PHN if reactivation does occur.
Secondary prevention
A 2013 Cochrane meta-analysis of 6 randomized controlled trials (RCTs) investigating oral antiviral medications given within 72 hours after the onset of herpes zoster rash in immunocompetent people for preventing postherpetic neuralgia (PHN) found no significant difference between placebo and aciclovir.
Additionally, there was no significant difference in preventing the incidence of PHN found in the one RCT included in the meta-analysis that compared placebo to oral famciclovir treatment within 72 hours of HZ rash onset. Studies using valaciclovir treatment were not included in the meta-analysis.
PHN was defined as pain at the site of the dermatomic rash at 120 days after the onset of rash, and incidence was evaluated at 1, 4, and 6 months after rash onset. Patients who are prescribed oral antiviral agents after the onset of rash should be informed that their chances of developing PHN are no different than those not taking oral antiviral agents.
Treatment
The pain from postherpetic neuralgia can be very severe and requires immediate treatment. There is no treatment which modifies the course of the disease and management primarily aims to control symptoms.
Low-dose capsaicin may be useful for reducing PHN-associated pain but is limited by side effects (redness and a burning or stinging sensation with application) and the need to apply it four times daily. Due to the need for topical anesthesia before application of the high-dose capsaicin patch, referral to a pain specialist is generally recommended if this approach is being considered. Similarly, treatment with gabapentin also leads to a 50% reduction in pain intensity in one person out of every 7-8 people treated (NNT=7.5). Acetaminophen and nonsteroidal anti-inflammatory drugs are thought to be ineffective and have not undergone rigorous study for PHN.
New medications
Pharmacological treatment of PHN is unsatisfactory for many patients and there is a clinical need for new treatments. Between 2016 and 2023, 18 clinical trials have been carried out evaluating 15 molecules with pharmacological actions on nine different molecular targets: antagonism of the angiotensin type 2 receptor (AT2R) (olodanrigan), inhibition of the α2δ subunit of the voltage-gated calcium channel (VGCC)(crisugabalin, mirogabalin and pregabalin), activated sodium channel (VGSC) blockade (funapide and lidocaine), cyclooxygenase-1 (COX-1) inhibition (TRK-700), adapter-associated kinase 1 inhibition ( AAK1) (LX9211), lanthionine synthase C-like protein (LANCL) activation (LAT8881), N-methyl-D-aspartate (NMDA) receptor antagonism (esketamine), mu opioid receptor agonism (tramadol, oxycodone e hydromorphone) and nerve growth factor (NGF) inhibition (fulranumab). Of them, some of them report promising results while others did not work.
Prognosis
The natural history of postherpetic neuralgia involves slow resolution of the pain syndrome. A subgroup of affected individuals may develop severe, long-lasting pain that does not respond to medical therapy.
Epidemiology
In the United States each year approximately 1,000,000 individuals develop herpes zoster, with nearly 1 in 3 people in the United States developing it in their lifetime. Of those individuals, approximately 10–18% develop postherpetic neuralgia.
The incidence of herpes zoster, and also developing postherpetic neuralgia, both increase with age.