Chronic pain can be defined as pain lasting longer than the expected duration of the disease. It represents a major health and socio-economic burden, with a UK prevalence rate of up to 20 per cent.
Neuropathic and nociceptive pain
In general, chronic pain may be neuropathic or nociceptive. Neuropathic pain results from damage to nervous tissue and nociceptive pain is due to noxious stimuli. Examples of neuropathic pain include: failed-back surgery syndrome, pain related to neuropathy, phantom-limb pain, post-herpetic neuralgia and complex regional pain syndrome, including causalgia and reflex sympathetic dystrophy. Nociceptive pain may arise from thermal, chemical and mechanical stimuli.
SCS for neuropathic pain
In October 2008, NICE issued a technology appraisal guidance recommending spinal cord stimulation (SCS) for the management of patients with medically refractory chronic neuropathic pain.1 This is defined as neuropathic pain with a severity of at least 50mm on a 0-100mm visual analogue scale which lasts at least six months despite conventional medical therapy.
NICE further emphasised the importance of a multidisciplinary team approach to the management of patients with chronic pain and their subsequent selection for SCS. Recognition and management of psychological dimensions of chronic pain syndrome remain crucial to the success of any pain-management programme, including SCS.
The technique of SCS involves implanting electrodes epidurally dorsal to the spinal cord, hence the term occasionally used 'dorsal column stimulation'. Depending on the distribution of the pain in the limbs and trunk, the electrode may be inserted in the thoraco-lumbar or cervical spine. There is now a wide variety of electrodes with different lengths or configurations to choose from. The electrodes may be inserted percutaneously or surgically and will need to be connected to an implantable pulse generator (IPG battery), similar to a cardiac pacemaker, to allow chronic stimulation.
The choice of technique very much depends on the performing clinician. By and large, a percutaneous technique has the advantage of avoiding the wound-related complications of the open surgical approach, whilst the latter is associated with lower electrode migration rates. The IPG may be inserted in a subcutaneous pocket in the chest, back or abdomen.
Newer, rechargeable IPGs have now reduced the need for frequent IPG changes, even in patients requiring high-energy stimulation. The NICE guidelines recommend trial stimulation, through externalised cables connected to an external stimulator, to ascertain efficacy before connection to a permanent IPG. Following implantation, the stimulating parameters such as the voltage or current, polarity, frequency and pulse width are set transcutaneously using an external controller. The aim of the stimulation setting exercise is to replace the pain in the affected area with a pleasant paraesthesia. It may take several attempts and varying length of time to achieve the eventual effective settings.
The advantages of SCS compared with the previous ablative surgical pain management techniques such as cordotomy, tractotomy and commissural myelotomy is that SCS is non-destructive, largely reversible and adjustable. Complications do, however, occur with SCS and include electrode migration, disconnection and hardware infection. Epidural haematoma or neurological deficits are rare.
Evidence for the clinical effectiveness of SCS continues to accumulate. The PROCESS trial, a study of 100 patients with neuropathic pain randomised to either SCS or conventional medical management, reported significantly improved quality of life in the SCS group.2
North et al randomised 45 patients with failed-back-surgery syndrome to undergo repeat spinal surgery or SCS and found the latter to be significantly more effective in treating the symptoms.3 Despite these trials and the NICE guidance, relatively few patients are referred for consideration for SCS and often after disease durations which are much longer than the recommended six months. This warrants further attention, given the potential impact of this treatment on patients' quality of life. The GP's role remains crucial, both in early recognition of chronic pain syndromes and timely referral to specialist centres capable of offering the full range of pain-management modalities.
- Mr Ashkan is a consultant neurosurgeon and senior lecturer at King's College Hospital, London
1, NICE. Spinal cord stimulation for chronic pain of neuropathic or ischaemic origin. TA159 London, NICE, 2008.
2, Manca A, Kumar K, Taylor R, et al. Quality of life, resource consumption and costs of spinal cord stimulation versus conventional medical management in neuropathic pain patients with failed back surgery syndrome (PROCESS trial). Eur J Pain. 2008; 12(8): 1047-58.
3, North R, Kidd D, Farrokhi F, et al. Spinal cord stimulation versus repeated lumbosacral spine surgery for chronic pain: a randomized, controlled trial. Neurosurgery 2005; 56(1): 98-106.