Description of the condition: Chronic pain is a common problem. When defined as pain of greater than three months duration, prevalence studies indicate that up to half the adult population suffer from chronic pain, and 10 to 20% experience clinically significant chronic pain (Smith 2008). In Europe 19% of adults experience chronic pain of moderate to severe intensity with serious negative implications for their social and working lives and many of these receive inadequate pain management (Breivik 2006). Chronic pain is a heterogenous phenomenon that results from a wide variety of pathologies including chronic tissue injury such as arthritis, peripheral nerve injury, central nervous system injury as well as a range of chronic pain syndromes such as fibromyalgia. It is likely that different mechanisms of pain production underpin these different causes of chronic pain (Ossipov 2006).

Description of the intervention: Brain stimulation techniques have been used to address a variety of pathological pain conditions including fibromyalgia, chronic post-stroke pain and complex regional pain syndrome (Cruccu 2007; Fregni 2007; Gilula 2007) and clinical studies of both invasive and non-invasive techniques have produced preliminary data showing reductions in pain (Cruccu 2007; Fregni 2007; Lefaucheur 2008b). Various types of brain stimulation, both invasive and non-invasive are currently in clinical use for the treatment of chronic pain (Cruccu 2007). Non invasive stimulation techniques require no surgical procedure and are therefore easier and safer to apply than invasive procedures. They include repetitive transcranial magnetic stimulation (rTMS), transcranial direct current stimulation (tDCS) and cranial electrotherapy stimulation (CES). rTMS involves stimulation of the cortex by a stimulating coil applied to the scalp. Electric currents are induced in the brain directly using rapidly changing magnetic fields (Fregni 2007). Trains of these stimuli are applied to the target region of the cortex to induce alterations in cortical excitability both locally and in remote cortical and subcortical brain regions (Leo 2007). A recent meta-analysis suggested that rTMS may be more effective in the treatment of neuropathic pain conditions with a central compared to a peripheral nervous system origin (Leung 2009). tDCS and CES involve the safe and painless application of low intensity (commonly ≤ 2 mA) electrical current to the cerebral cortex of the brain (Fregni 2007; Gilula 2007). tDCS has been developed as a clinical tool for the modulation of brain activity in recent years and uses relatively large electrodes that are applied to the scalp over the targeted brain area to deliver a weak constant current (Lefaucheur 2008a). Recent clinical studies have concluded that tDCS was more effective than sham stimulation at reducing pain in both fibromyalgia and spinal cord injury related pain (Fregni 2006a; Fregni 2006b). CES was initially developed in the USSR as a treatment for anxiety and depression in the 1950s and its use later spread to Europe and the USA where it began to be considered and used as a treatment for pain (Kirsch 2000). The electrical current in CES is commonly pulsed and is applied via clip electrodes that are attached to the patients earlobes. A Cochrane review of non-invasive treatments for headaches (Bronfort 2004) identified limited evidence that CES is superior to placebo in reducing pain intensity after 6 to 10 weeks of treatment.



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