Introduction
Post Herpetic Neuralgia (PHN) is a form of nerve pain (neuropathy, neuritis) that persists after an acute case of shingles.
Shingles 
is an acute viral infection of the peripheral and central nervous system by the herpes zoster (HZ) virus, usually affecting a single spinal nerve root. After causing chicken pox in childhood, the HZ virus lies dormant in the Dorsal Root Ganglion cells of the spinal nerves, constantly being suppressed by the immune system.
During times of stress (infection, trauma) or immuno-suppression, the virus multiplies being no longer suppressed by the immune system. This leads to an acute attack of HZ, and is more likely to occur in the following groups:-
- Cancer patients
- Immuno-compromised (HIV) and immuno-suppressed (chemotherapy) patients
- Hodgkin's disease (lymphoma)
- Kidney transplantation
- History of injury or infection
The overall incidence of acute HZ is 1.3 - 4.8 cases per 1,000 person-years. Children account for 5-8% of cases, with adults aged 50-70 years accounting for 40%. The highest incidence is in the over 80's having an incidence of 10 cases per 1,000 person-years.
During an acute HZ attack, the virus spreads from the dorsal root ganglion travelling upwards to the Dorsal Horn of the spinal cord, and down the spinal nerve towards to the skin. When the virus reaches the skin it causes the typical blistering rash of shingles which follows a Dermatome (the area of skin supplied by a single spinal nerve).
The clinical features of acute HZ are:-
- pain and/or itching typically precedes the appearance of rash by several days.
- pain has little or no bearing on the severity of lesions (and visa versa)
- location of the rash
- thoracic dermatomes 50%
- trigeminal 3-20% (mainly the ophthalmic division). With advancing age, the incidence of trigeminal HZ increases, and the incidence of PHN may also be higher with trigeminal eruptions.
- lumbar/cervical dermatomes 12-20%
- bilateral eruptions < 1%
- recurrent HZ 1-8% patients, of these 50% in same location.
- rash characteristics - red areas of skin --> raised red areas (papules) --> vesicles (blisters) within 24 - 36 hours --> crust formation (this can continue for 1 month)
The viral outbreak can cause local nerve tissue swelling and reduced blood flow, and permanent nerve destruction affecting the dorsal horn sensory processing centres, the dorsal root ganglion, and also the peripheral sensory receptors in the area of skin affected.
Destruction of these different nerve areas leads to the syndrome Post Herpetic Neuralgia (PHN), Whether or not PHN develops after an acute attack of HZ depends on age:-
| Age (yrs) |
PHN risk after acute HZ |
| <20 |
4% |
| 20 - 60 |
15% |
| >60 |
50% |
PHN is simply defined as persisting pain in the area of the rash once the HZ lesions have healed. It presents as:-
- extreme sensitivity of the skin in the area affected by the HZ rash, especially to light touch and brushing by clothing. The sensitivity seems to be worst in the whiter scarred areas of skin which have lost their pigmentation.
- spontaneous pain in the area which may be aching or stabbing in nature.
- secondary musculo-skeletal problems (muscles, joints) caused by excessive guarding of the affected area. This pain can then contribute to the overall clinical picture adding movement induced pain.
Treatment
Acute HZ Infection (Shingles)
The two most important reasons for treating acute HZ early are :-
- To prevent virus multiplication thereby reducing damage to the dorsal horn, dorsal root ganglion, and peripheral sensory receptors in the skin. The early use of anti-viral drugs like acyclovir and famcyclovir help to reduce the severity of acute HZ and the incidence and severity of PHN by limiting viral induced nerve damage.
- To prevent excessive degrees of dorsal horn sensitisation. Acute HZ pain has both somatic and nerve components. Early pain management with combinations of paracetamol, NSAIDs, opioids, and amitriptyline help to reduce severe pain and therefore reduce sensitisation of the dorsal horn. There is strong evidence that early use of amitriptyline reduces the incidence and severity of PHN.
Other treatments which have been recommended (but not proven) in acute HZ are:-
- Early sympathetic nerve blocks (most pain relief clinics have long waiting lists in the UK, and therefore this is unlikely to occur).
- Oral steroids (prednisolone 60-80 mg per day for 10 days then taper off) may help earlier resolution of HZ pain, but may not affect the development of PHN.
- TENS
Post Herpetic Neuralgia (PHN)
The following treatments have been used in the pain clinic to treat PHN. Those marked with ** are the most likely to be helpful :-
- Amitriptyline ** 10 - 50 mg per day
- Anti-convulsants - gabapentin or pregabalin** are the most popular of these. The use of all the others has been described.
- Topical Capsaicin ** (0.025% or 0.075%) can be useful in some patients.
- Herpetic scar desensitisation ** - some patients find that injecting the depigmented areas of skin with LA/steroids on several occasions radically reduces hypersensitivity.
- Lignocaine 5% patch ** - Versatis has just released for sale in the UK in January 2007
- TENS (not proven) - may aggravate the sensitivity rather than help it.
- Topical Aspirin (not proven)
- Topical NSAIDs (not proven)
- Combinations of analgesics including paracetamol, NSAIDs, weak opioids. These however are often found to be ineffective as the pain is mostly neuralgic in nature, and therefore fairly resistant to traditional analgesics.
- Peripheral Nerve Blocks
- Sympathetic Blockade (early)
- Dorsal Root Ganglion Block (early)
- Epidural steroid injections (early) at the correct spinal segment to reduce the degree of nerve damage.
- Physical therapy including spinal manipulation is important for those with secondary musculo-skeletal pain.
- Neuro-ablative procedures - these should be avoided, as few, if any trials have shown them to be helpful for long-term improvement. If considering, consider DREZ and deep brain stimulation (early studies show "some" promise).
