Pathophysiology of Post-Herpetic Neuralgia (PHN)1–4
Post-herpetic neuralgia (PHN) can occur as a consequence of infection with the varicella zoster virus (VZV), which is associated with childhood chickenpox.1
Following infection, the virus can remain dormant in neurons of the dorsal root ganglion for many years. The latency period depends on host cell-mediated immunity.1
The virus can be reactivated under certain conditions, for example, in response to trauma or following a decline in VZV-specific cell-mediated immunity during ageing, or as a result of immunosuppression.1,2
In a number of patients, reactivation leads to acute herpes zoster (or shingles),3 which is characterized by skin rashes and pain.
Painful sensations occurring as a result of acute herpes zoster continue to persist in some patients, leading to PHN.4
Why is PHN often misdiagnosed?
Many patients live with neuropathic pain conditions such as PHN for a long time before they are correctly diagnosed and receive appropriate treatment.5, 6-10
The symptoms of neuropathic pain may vary, and due to its complex pathophysiology, PHN is often difficult to treat.5,11,12 Therefore, without a clear diagnosis, patients may undergo 'trial and error' treatments or endure a step-wise treatment approach over months or even years.13,14 The pain associated with PHN may be intermittent and the patient may not be aware that pain can occur after the shingles rash has healed. An accurate evaluation of pain therefore has a critical role to play in the identification and treatment of PHN.
How can PHN be diagnosed?
PHN presents differently from patient to patient and there are a number of elements to an accurate diagnosis.
Firstly, the analysis of patient history has an important role to play. In particular, establishing if there has been an episode of shingles recently, and if so, where was this located?
In addition, gaining an accurate description from the patient of the nature of the pain can also prove useful. The pain associated with PHN is often described as 'shooting', 'stabbing' or 'burning'.5,11,12
A physical examination and tests for positive or negative sensory signs, allodynia, and hyperalgesia, can be carried out. These can include Q-tip or pinprick tests, hot and cold stimulus, vibration or pressure.
As PHN is a type of Localized Neuropathic Pain (LNP), the LNP Screening Tool, which was developed by international experts, can help to support doctors and patients in getting an early diagnosis leading to an adequate treatment.15 The screening tool is presented as a DIN A6 pocket card with four decision making questions about patient`s history, pain distribution, testing of symptoms, and the location and size of the painful area on the patient`s body. A short pictorial guidance shows how to conduct a neurological sensory examination.
References1 Gershon AA et al. Advances in the understanding of the pathogenesis and epidemiology of herpes zoster. J Clin Virol. 2010; 48:2-7.
2 Opstelten W et al. The impact of varicella zoster virus: chronic pain. J Clin Virol. 2010; 48:8-13.
3 Levin MJ, et al. J Clin Virol. 2010;48:S14-9.
4 Whitley RJ et al. Management of herpes zoster and postherpetic neuralgia now and in the future. J Clin Virol. 2010;48:20-28.
5 Dworkin RH et al. Pharmacologic management of neuropathic pain: evidence-based recommendations. Pain 2007;132:237-51.
6 Vadalouca A et al. Therapeutic management of chronic neuropathic pain: an examination of pharmacologic treatment. Ann N Y AcadSci 2006;1088:164-86.
7 Backonja MM. Defining neuropathicpain. AnesthAnalg 2003;97:785-90.
8 Hanna M et al. Prolonged-release oxycodone enhances the effects of existing gabapentin therapy in painful diabetic neuropathy patients. Eur J Pain 2008;12:804-13.
9 Oster G et al. Pain, medication use, and health-related quality of life in older persons with postherpetic neuralgia: results from a population-based survey. J Pain 2005;6:356-63.
10 van Seventer R et al. A cross-sectional survey of health state impairment and treatment patterns in patients with postherpetic neuralgia. Age Ageing 2006;35:132.
11 Rehm et al. Post-Herpetic Neuralgia: 5% lidocaine medicated plaster, pregabalin, or a combination of both? A randomized, open, clinical effectiveness study.Curr Med Res Opin 2010;26(7):1607-19.
12 Argoff CE. Conclusions: chronic pain studies of lidocaine patch 5% using the Neuropathic Pain Scale. Curr Med Res Opin. 2004;20(2):S29-S31.
13 Gore M et al. Selecting an appropriate medication for treating neuropathic pain in patients with diabetes: a study using the U.K. and Germany Mediplus databases. Pain Pract 2008;8:253-62.
14 Attal N et al. EFNS guidelines on pharmacological treatment of neuropathic pain.Eur J Neurol 2006;13:1153-69.
15 Mick G, et al. Is an easy and reliable diagnosis of localized neuropathic pain (LNP) possible in general practice? Development of a screening tool based on IASP criteria. Curr Med Res Opin 2014;30(7):1357-66.