67. Post-herpetic neuralgia

Question 1

What do you understand by the term neuralgia?

Answer 1

This is simply a mononeuropathy of a named nerve

Question 2

Can you tell me something about the pathogenesis of post-herpetic neuralgia?

Answer 2

Following chicken-pox,

the varicella-zoster virus lies dormant in the dorsal
horn of the spinal cord.

Shingles develops after reactivation of the dormant
varicella zoster virus causing infection of a nerve.

The infection causes nerve fibre damage by inflammation and ischaemia
in both sensory and motor (usually subclinical) nerves.

These lesions are at the
dorsal root, dorsal root ganglion
dorsal horn.

Post-herpetic neuralgia is a persistent nerve pain after the rash of shingles has disappeared.

Question 3

What are the clinical features?

Answer 3

1. The initial herpes zoster is painful and has a variable time course.
2. There is no set definition of post-herpetic neuralgia,
   but some authors use the presence of
   pain persisting at 1 month as diagnostic.
3. The syndrome occurs predominantly in patients over the age of 50 and is
   normally isolated to a single dermatomal segment, frequently unilateral.
4. Thoracic dermatomes and the ophthalmic division of the trigeminal nerve are
   common sites.
5. The pain itself is severe with constant aching, burning or itching. There may
   be superimposed bouts of stabbing pain. Pigmentation and scarring may also occur.

Question 4

What are the treatment options available?

Answer 4

1. Simple analgesics
2. Tricyclic anti-depressants
3. Capsaicin
4. Anti-convulsants

Phenytoin and carbamazepine have both been used successfully.
More recently, gabapentin has been used with some success.

5 Opioids

6. TENS

Question 5

2. Tricyclic anti-depressants

Answer 5

These currently appear to be the most effective drugs used.

Amitriptyline is commonly used as first line and changed or added to if not wholly effective.

Tricyclics usually begin to ease the pain within a few days,
but they may take several weeks to gain maximum benefit.

They should ideally be continued for a month after the pain has gone.

The doses needed are lower than those for depression.

Question 6

Capsaicin cream

Answer 6

Acts by depleting the neurotransmitter
substance P in small afferent fibres.

The 0.075% cream is applied 3–4 times per day for 6 weeks.

Capsaicin cream should not be used on broken or inflamed skin.

Even on healthy skin, it may cause an intense burning feeling when it is applied.