|a) Some types of pain do not respond to either opioid analgesics or NSAIDs. For example, neuropathic pain appears to be relatively insensitive to opioids. It can be significantly relieved with tricyclic antidepressants (e.g. amitryptiline) or anticonvulsant agents (e.g. carbamazepine). Carbamazepine can also be used to treat the paroxysmal pain experienced by patients who suffer from trigeminal neuralgia. Corticosteroids (e.g. dexamethasone) may produce substantial improvement in some cases in neuropathic pain associated with cancer.
b) Local anaesthetics (e.g. lidocaine, amethocaine, bupivacaine, prilocaine) are agents which are used to block the initiation and propagation of nerve action potentials, by blocking Na+ channels. Their mode of administration varies: surface anaesthesia, infiltration, spinal or epidural anaesthesia. They are used for pain associated with localised surgery, childbirth or in dentistry. However, newer drugs, such as tocainide and mexiletine, may be used in future as oral analgesics for neuropathic pain. The main problem associated with local anaesthetics is the risk of systemic toxicity (e.g. hypotension, bradycardia and respiratory depression).
c) The management of pain associated with migraine consists of the management of acute attacks, and prophylaxis. Acute attacks may respond to NSAIDs such as aspirin and paracetamol, or to agonists at 5-HT1D receptors, such as sumatriptan. Prophylaxis may be achieved by use of 5-HT2 receptor antagonists (methysergide, cyproheptadine), calcium channel blockers (e.g. verapamil), or tricylic antidepressants (e.g. amitriptyline).
Several new approaches in the management of pain are still at an experimental stage, such as use of antagonists of substance P receptors (i.e. NK1 receptors), inhibitors of the enzymatic degradation of enkephalins, analogues of adenosine or agonists at nicotinic receptors, agonists or antagonists at excitatory amino acid receptors. If proved active in the clinic, these new drugs may diversify the management of pain in the future.
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