Firstly and importantly, paracetamol and NSAIDs are virtually useless in Post Herpetic Neuralgia (PHN).
The FIRST-LINE AGENTS for pain-relief in PHN are TCAs – tricyclic antidepressants.
The two most commonly used TCAs are AMITRIPTYLINE and NORTRIPTYLINE. Both are also useful in the pain of acute zoster.
Some common brand names of amitriptyline are ‘Tryptomer’, ‘Amitryn’, ‘Amitone’. Available in 10, 25, 50 and 75 mg strengths.
Some common brand names of nortriptyline are ‘Sensival’, ‘Primox’. It is available in only 25 mg strength.
Amitriptyline can be begun at a dose of 10 or 25 mg h.s. with escalations every 3 to 4 days till a maximum (in the elderly) of 75 mg per day.
It can be given as a single bedtime dose or in divided doses if daytime sedation is not a problem.The dose of nortriptyline follows the same pattern. It can be initiated as 12.5 mg hs and increased to 75 mg/d.
There is a lag of up to 3 weeks before TCAs begin to relieve pain.
As we know, PHN is commoner in the elderly. Only about 2% of herpes zoster patients under age 60 get PHN while nearly 20% of those above age 70 get PHN.
Side effects of TCAs can be bothersome in the elderly. These include constipation, sedation, confusion, blurred vision, dryness of mouth and urinary retention.
Nortriptyline has lesser anticholinergic side effects than amitriptyline and hence should be preferred in the elderly.
The duration of therapy of TCAs in PHN is as variable as the natural history of the neuralgia. The therapy can last from months to being lifelong.
The second line agents for PHN, if TCAs don’t work or are not tolerated, are anticonvulsants. The most commonly recommended are GABAPENTIN, PREGABALIN and DULOXETINE.
I personally prefer gabapentin because of easier dose titrability. One can start with a subtherapeutic dose of 100 mg hs and increase as tolerated. Gabapentin is available in 100 and 300 mg strengths and the dose in the elderly should not exceed 900 mg/d. Dose upto 300 mg may be given as a single nighttime dose. Higher doses should be split into bds or tds.
Do note that maximum doses of anticonvulsants for PHN recommended in literature are much higher e.g. 2400 mg/d for gabapentin.
The starting dose of pregabalin is 75 mg hs and the maximum dose in the elderly would be 300 mg/d. Duloxetine is available in 20 and 30 mg capsules. The starting dose is 20 mg hs and the max dose is 90 mg/d.
The principal ADRs of all the above anticonvulsants are dizziness, falls, somnolence, constipation, oedema and weight gain. In the elderly, the tolerance for these agents is poor, worse than for TCAs.
Another group of drugs used for PHN is opioid agonists. The two drugs available are TRAMADOL and TAPENTADOL. These drugs are considered third line agents for PHN because of potential for habituation. I personally prefer them over the more toxic anticonvulsants.
Tramadol is a norepinephrine and serotonin inhibitor. It also acts via a major metabolite that is an opioid agonist.
Dosage: 50 mg tds to start with. Maximum 400 mg/d.
Precautions: Avoid in those with seizure history.
Interactions: Increased seizure risk in those taking TCAs concurrently.
Adverse effects: Dizziness, nausea, constipation, somnolence, orthostatic hypotension.
Tapentadol (Vorth TP is one brand name) is also an opioid agonist. Doses of 50, 75 or 100 mg are given every 4 to 6 hours depending upon the intensity of pain and tolerance to the drug. It is said to cause less nausea than tramadol.
Topical pharmacotherapy options in post-herpetic neuralgia include CAPSAICIN and LIGNOCAINE. These are options in patients who do not tolerate the oral medicines, something that happens commonly
CAPSAICIN ointment (some brand names include Capsain P and Myolaxin) can give significant relief in neuralgia but has the paradoxical side effect of causing local burning. It has to be applied using a gloved hand so that the applying fingers are spared the burning. Capsaicin causes relief by depleting substance P and may take several days to do this.
Another topical therapy underused by us is the topical lignocaine patch 5% (available as LIDOPATCH by Cadilla, Rs.130/- per patch). Upto three patches may be applied simultaneously for twelve hours a day. It may take up to two weeks for significant effect. No significant side effects have been reported.
Please remember that topical treatments CANNOT be used in acute phase of zoster with active lesions.
This will be a good time to revise the concept of NNT, an important statistical tool in the age of evidence based medicine. NNT refers to ‘NUMBERS NEEDED TO TREAT’. The NNT is the number of patients who need to be treated to prevent one additional bad outcome.
In post herpetic neuralgia the NNT to achieve 50% reduction in pain for various drugs is as follows-
Lignocaine patch 2.0
Tricyclic antidepressant nortryptiline 2.6