The medicolegal challenges of spinal cord stimulation for the treatment of pain
Chronic pain affects millions of people worldwide and most adults will experience it at some point during their lifetime. It can have a significant cost, both in terms of healthcare expenditure and quality of life. However, current treatment options are limited in terms of their efficacy, particularly over the long term. This has led to an increase in research surrounding novel treatments. Of these, neuromodulation, including spinal cord stimulation, may offer genuine hope to patients with chronic neuropathic pain (1-3).
Neuromodulation involves electrical stimulation of the central or peripheral nervous system (1, 3). There are several techniques available; spinal cord stimulation (SCS) is the most commonly performed, but patients may also be offered peripheral nerve, dorsal root ganglion (DRG) or deep brain stimulation (1). SCS was first developed in the 1960s (1,2 4).. An SCS device consists of three components: a pulse generator which may be implanted into the patient’s body, one or two leads that deliver electrical impulses via electrodes that are placed in the epidural space and a hand-held remote controller used to switch the device on or off and to adjust the settings (3).
During traditional SCS, a current is applied to the spinal dorsal columns (1). It is known that these structures modulate pain signals to the brain via ascending pain pathways, although the precise mechanism remains unclear (1, 5). One suggestion is that a “gate” in the dorsal horn relays neuronal signals from sensory afferent fibres to the parts of the brain involved in pain perception (1, 6). Stimulation of faster Ab fibres leads to closure of these gates, which then blocks the transmission of pain signals by slower C fibres. This theory provides an explanation as to why non-nociceptive stimuli, such as massage, provide temporary relief from pain. In the spinal cord, the two types of fibres are segregated from the fibres controlling movement, making the dorsal column an ideal location for neuromodulation (1).
The original goal of SCS was to replace pain signals with a more tolerable tingling sensation. However, recently developed methods employing higher frequencies or bursts of stimuli have reduced or eradicated these sensations, while maintaining, or even improving, the pain relief associated with more traditional methods of SCS (1, 3, 7). In particular, patients report that activities such as sleeping and driving are less impacted by unpleasant tingling (3). Techniques that target the DRG or peripheral nervous system can reduce pain signals with greater anatomical specificity, as the treatment of a specific dermatome can provide focussed pain relief. The DRG has been a particular focus due to the discovery that rather than just a relay station, there is inherent modulation of pain signals at this site. Anecdotally, there have been technical issues with lead migration and fracture which is also seen in traditional epidural leads.. The lower level of cerebrospinal fluid at the DRG also reduces energy requirements, which improves battery life (1).
SCS is increasingly being recognised as a useful therapy in patients with chronic pain, particularly where pharmacological, physical and psychological treatments have failed to provide adequate relief from symptoms (1, 3, 7). It has been used successfully in the treatment of a number of conditions, which include complex regional pain syndrome, low back pain (especially where surgery has not provided relief) and post traumatic or surgical neuropathies(1-3). There is some evidence that the success of SCS is inversely proportional to the time between initial pain diagnosis and implantation of the SCS device. Studies have reported a success rate of 85% when SCS is used within 2 years of diagnosis, which declines to 9% if SCS is delayed by 15 or more years (7). Unfortunately, the reality is that access to timely SCS remains challenging.
SCS is generally considered to be asafe treatment, due in part to its minimal invasiveness and reversibility (1, 2, 7, 8). While serious complications following surgical implantation are rare (1, 8), minor complications, many of which are transient in nature, are reported by up to 40% of patients (4). They usually occur fairly soon after implantation [Han], and can be categorised as mechanical or medical (1, 4).
The commonest mechanical complication is lead migration, which occurs in around 14% of all patients (1, 3, 4, 8). As post-operative scarring keeps the implanted leads in the correct location, this complication usually arises within days of surgery, before the scar tissue has formed (1). It is associated more frequently with percutaneous electrodes than paddle-shaped ones (8). Revision surgery may be required if there is a loss of targeted stimulation (1, 8). The risk of lead migration can be reduced by sufficient anchoring of the leads during implantation, and recent improvements in the design of both leads and anchoring systems has resulted in a reduction in the incidence of lead migration (1). Other less common mechanical complications include hardware malfunction or lead breakage, both of which may require the removal of the SCS device (1, 4, 8).
Infection is the most commonly encountered medical complication, affecting around 4% of patients (1, 3, 4, 8). It is usually superficial and easily treated with a short course of antibiotics. While there is no evidence to suggest that routine post-operative antibiotic use reduces the risk of infection (1), it is known that comorbidities such as diabetes, immunosuppression and tobacco smoking may impact wound healing and increase the likelihood of an infection developing (3, 7). Ideally, these must be well-controlled before surgery takes place (7). Pain at the site of the battery is common but usually tolerated well by patients and managed conservatively. Less frequent complications include haematoma, seroma, epidural fibrosis and late spinal compression due to fibrous encapsulation of the implanted leads (1, 3, 4, 8). Serious adverse events, such as dural puncture headache or neurological damage, are extremely rare (1, 4), with the latter occurring in only 0.25% of cases. The usual causes are the formation of epidural haematomas or damage to the spinal cord itself (1).
The incidence of both complications and treatment failures can be reduced by appropriate patient selection (1, 2). Potential candidates for SCS must undergo a thorough behavioural assessment to ascertain their psychological suitability before the procedure is contemplated. This is because a number of conditions, including untreated depression and major psychiatric illness, as well as unrealistic expectations, a lack of engagement, inadequate daily physical activity, and a lack of social support, are associated with lower rates of symptom improvement and higher disability scores (1-3, 7). Other factors that predict treatment failure include obesity, younger age and male gender (1).
Following this assessment, patients typically embark on a trial period, lasting around 7–14 days (3, 7). In this procedure, the electrical leads are inserted into the epidural space, but the pulse generator remains outside the body. The trial period allows the expected reduction in pain to be assessed, along with any other symptoms that may arise, without the risks of permanent implantation. In addition, multiple stimulation variables, including intensity, frequency, pulse width and pattern of stimulation, can be manipulated to obtain the optimum therapeutic effect (3). It therefore provides a means of identifying patients who are the most likely to benefit from permanent implantation of an SCS device (2, 3). It also allows the patient to experience the effects of full implantation. The trial is considered a success if the patient reports a reduction in pain level of at least 50%, although other markers, such as reduced reliance on pharmaceutical medication and improved functionality, are also indicators of a positive result (3, 7).
As well as the risks associated with surgical implantation, at the time of consent for the procedure, patients should be made aware of several other issues associated with SCS. While not a complication as such, the presence of an SCS device has traditionally precluded the patient from undergoing magnetic resonance imaging (MRI) at a later date (1, 2, 9), although recent technological developments may allow the use of MRI in specific circumstances (1, 9). Patients should also be informed of the need for battery replacement. Non-rechargeable batteries usually require replacement every 2–5 years, while rechargeable ones may last up to 10 years. Fewer replacement procedures means that the risk of associated complications is reduced, but some patients prefer to avoid the recharging process and opt for a non-rechargeable power source. Battery size has an effect on the recharge interval rather than the time-to-replacement (3).
SCS is not a cheap treatment, which has led to discussions about its cost-effectiveness. The high initial costs of SCS are offset by the decreased use of other therapies, both pharmaceutical and non-pharmaceutical. There is also a benefit in terms of increased functionality, which may include a return to work, and improved quality of life. Long-term studies have shown SCS to be cost-effective. A study in patients with unresolved low back pain following surgical treatment reported a decrease in associated annual healthcare costs of an average of 40% (1, 2). Recent developments, such as rechargeable battery systems, should improve this cost-effectiveness still further (1).
SCS offers a safe and minimally invasive additional treatment option for chronic neuropathic pain. Although it is not suitable for all patients, it is likely to be particularly beneficial for those who have experienced issues with side effects from conventional pharmacological interventions or for whom surgery has been unsuccessful. Recent developments have refined the technique, leading to improved outcomes and a reduction in risks, with a subsequent increase in patient satisfaction. However, in order to reduce the incidence of treatment failures, it is important that potential candidates are carefully screened and undergo a trial treatment period. This will ensure the best possible chance of successful treatment.
References
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