What is Postherpetic Neuralgia (PHN)?

What is Postherpetic Neuralgia (PHN)?

Postherpetic neuralgia (PHN) is a chronic neuropathic pain condition characterized by severe pain that persists for 3 months or more after the herpes zoster (shingles) rash has healed. 1, 2

Pathophysiology and Mechanism

• PHN results from reactivation of the varicella zoster virus (VZV), which remains dormant in the dorsal root ganglia or sensory ganglia of cranial nerves after primary varicella (chickenpox) infection 3
• When the virus reactivates, it travels along nerve pathways causing inflammation and damage to nerves, resulting in the characteristic painful rash with dermatomal distribution 3
• The pathological process involves inflammatory necrosis of dorsal root ganglia, occasionally associated with neuritis, leptomeningitis, and segmental unilateral degeneration of related motor and sensory roots 4
• This nerve damage leads to persistent neuropathic pain that can continue long after the visible rash resolves 3

Clinical Presentation

Patients with PHN experience multiple distinct types of pain:

• Constant pain: A steady, deep, aching, or burning sensation that persists continuously 2, 5
• Paroxysmal pain: Intermittent, lancinating (sharp, shooting) pain episodes 2, 5
• Allodynia: Pain triggered by normally non-painful stimuli, such as light touch or clothing contact 1, 2
• Hyperalgesia: Exaggerated pain response to painful stimuli 1, 2
• Patients may describe the pain as a "cold burning pain" typically following a dermatomal distribution 1

Epidemiology and Risk Factors

• PHN affects approximately 50% of herpes zoster patients over 60 years of age and 15% of all HZ patients 4
• The condition occurs mainly in HZ patients 60 years and older, particularly those with more severe acute pain and rash 6
• Immunocompromised individuals, including those with HIV, have increased risk and more severe disease with higher rates of PHN 1, 3
• The incidence is likely increasing due to greater longevity and increasing numbers of patients receiving immunosuppressive treatments 6

Duration and Natural History

• By definition, PHN is pain persisting 3 months or more after healing of the herpes zoster rash 7, 8, 2
• The condition may persist for months to years, and some patients require long-term treatment with no specified maximum duration 9
• The pain can be incapacitating, particularly in patients with advanced disease or immunocompromised states 1

Impact on Quality of Life

• PHN can have a severe impact on patients' quality of life and functional ability 10
• The condition may induce psychosocial dysfunction and negatively impact overall well-being 5
• Herpes zoster causes considerable morbidity in adults aged >50 years, with PHN being the most painful complication 1

Prevention Strategies

The most effective strategy to prevent PHN is vaccination against herpes zoster:

• Two types of herpes zoster vaccines are available: zoster vaccine live and recombinant zoster vaccine 3
• Vaccination is particularly important for older adults and those with compromised immune systems 3
• A zoster vaccine licensed in 2006 reduces the incidence of both HZ and PHN, as well as the burden of illness associated with these conditions 1, 6

Early intervention during acute herpes zoster may reduce PHN risk:

• Antiviral therapy (acyclovir and related agents) during acute HZ reduces the duration of pain and may prevent PHN in up to 60% of patients 4
• Robust management of pain during the early stage of acute herpes zoster is important for prevention 5
• Antiviral treatment should be started promptly if varicella zoster infection occurs, particularly in immunocompromised patients 3

Common Pitfalls

• Do not confuse acute herpes zoster pain with PHN: The diagnosis of PHN requires pain persistence for at least 3 months after rash healing 7, 8, 2
• Recognize that PHN remains highly resistant to treatment: Even with optimal therapy, complete pain resolution may not occur, and treatment is often only partially effective 10, 5
• Be aware that immunomodulator therapy should not be commenced during active varicella zoster infection: In severe cases, immunomodulator therapy should be discontinued if possible 3