Topical Agents
Capsaicin (8-methyl-N-vanillyl-6-nonenamide) is an active component of chilli peppers. It binds to TRPV1 (transient receptor potential channel) receptors which is a heat-activated calcium channel. It causes depletion of substance P and decrease the densit
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Capsaicin
Capsaicin (8-methyl-N-vanillyl-6-nonenamide) is an active component of chilli peppers. It binds to TRPV1 (transient receptor potential channel) receptors which is a heat-activated calcium channel. It causes depletion of substance P and decrease the density of epidermal nerve fibres. The effect takes 3–4 weeks to manifest. It is available in 0.025 %, 0.075 % cream and 8 % patch. 0.075 % cream applied 3–4 times/day has shown efficacy in painful diabetic neuropathy and postherpetic neuralgia (NNT for 50 % pain relief is 6.7). It is associated with burning sensation which can be reduced by co-administration of glyceryl trinitrate or EMLA (eutectic mixture of local anaesthetics). Eight percent patch (Qutenza): cutaneous patch containing 179 mg of capsaicin/280 cm2 patch. It should be applied for 60 min and provides pain relief for up to 12 weeks. Adverse effects include redness, itching, blisters, papules and skin dryness.
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Lidocaine Patch
It works by decreasing the activity of sodium channels and pain transmission. It is available as 5 % transdermal patch. It has efficacy in postherpetic neuralgia. It is also of benefit in focal peripheral neuropathies, low back pain and myofascial pain. A maximum of three patches can be applied every 12 h. It may cause mild irritation. NNT is 4.4 for treating neuropathic pain. The patch in itself is protective against allodynia. EMLA (eutectic mixture of local anaesthetics – lidocaine and prilocaine) may provide some relief in postherpetic neuralgia. Long-term usage may cause methaemoglobinaemia.
R. Gupta, Pain Management, DOI 10.1007/978-3-642-55061-4_22, © Springer-Verlag Berlin Heidelberg 2014
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Topical Agents
Topical Nitrates
They cause the release of nitric oxide from the endothelium leading to the increase in 3′–5′ cyclic guanidine monophosphate (cGMP). This stimulates ATP-sensitive K channels causing peripheral antinociception. The efficacy is seen in supraspinatus tendonitis, musculoskeletal pain and osteoarthritis. The patch is applied for 12 h, and a drug-free period is given for 12 h. It can be used as a co-analgesic with capsaicin patch. The main side effect is headache which can be avoided by starting the patch on a small dosage.
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Topical Tricyclic Antidepressants
They act on sodium and adenosine receptors to produce analgesia. 5 % doxepin has found some efficacy in complex regional pain syndrome type 1. The cream is applied four times daily and pain relief is apparent 2–3 weeks. An oral rinse preparation is also available for mucosal pain in cancer patients.
Further Reading McCleane GJ. Topical Doxepin hydrochloride reduces neuropathic pain: a randomised double blind, placebo controlled study. Pain Clinic. 1999;12:47–50.
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