Neuropathic pain local anaesthetics

The amide local anaesthetic lidocaine may also be used as an antianhythmic for ventricular tachycardia and exra-systoles after injection into the blood circulation. Drugs with high lipid solubility such as bupivacaine cannot be used for these purposes because their prolonged binding to the channel may induce dysrhythmias or asystolic heart failure [3]. Systemically applied lidocaine has also been used successfully in some cases of neuropathic pain syndromes [4]. Here, electrical activity in the peripheral nervous system is reduced by used-dependent but incomplete sodium channel blockade. 

This peripheral activity may be a rational basis for the use of systemic local anaesthetics in neuropathic states since ectopic activity in damaged nerves has been shown to be highly sensitive to systemic sodium channel blockers. This too is probably part of the basis for the analgesic effects of established effective anti-convulsants that block sodium channels such as carbamazepine, although central actions are important and may even predominate. The precise actions of excitability blockers therefore remains hazy as does any clear basis for the effectiveness of antidepressants and other adrenergic agents in the treatment of neuropathic pain as both central and peripheral actions, including sympathetic effects are possible. 

Strichartz GR, Zhou Z, Sinnott C, Khodorova A 2002 Therapeutic concentrations of local anaesthetics unveil the potential role of Na channels in neuropathic pain. In Sodium channels and neuronal hyperexcitabUity. Wiley, Chichester (Novartis Fund Symp 241) p189-205... 

Examples of cutaneous patches are lidocaine containing patches that sometimes contain a second local anaesthetic. After applying the patch, lidocaine penetrates deep into the skin where it has a local anesthetising effect. Capsaicin containing patches are used in the treatment of peripheral neuropathic pain in non-diabetic adults. Following exposure to the patch, capsaicin penetrates the skin and interacts with the cutaneous transient receptor potential vanilloid 1 receptor (TRPVl) resulting in pain relief. 

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