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New technology in spinal cord stimulation

Discussion

Around the world more than 20,000 patients undergo spinal cord stimulator implants each year. First used to treat pain in 1967, spinal cord stimulation (SCS) delivers electrical stimulation to nerves along the spinal cord, modifying nerve activity to minimise the sensation of pain reaching the brain. The mechanisms of pain relief include:

Suppression of the neuroimmune response: Markers of glial and immune cell activity, up-regulated in neuropathic pain states, are down-regulated in SCS patients;

Activation of supraspinal mechanisms and descending inhibitory pathways;

Activation of GABAergic inhibitory interneurons in the dorsal horn; 

Inhibition of action of wide dynamic range (WDR) neurones: WDR neurones are important gate-keepers in the dorsal horn during pain transmission. They may be switched off when stimulated by A-fibres in the DC;

Modulation of central neuropeptide biosynthesis.

Since the therapy first entered routine use in the 1980s, improvements in programme generator and microprocessor power, plus the ability to integrate position sensors, are contributing to SCS systems that have a longer lifespan, lower maintenance, and more precise control.

Case report

AB was a 44-year-old fit male cattle farmer who experienced a cervical fracture with spinal cord damage while playing vets rugby. He was admitted to hospital and stabilised. After a while the pain subsided a little, but he began to notice that he was losing power in both legs and his right hand was also getting weaker. AB was right hand dominant.  Radiological assessment did not support surgical intervention. This was a devastating situation for the previously healthy cattle farmer, who now found himself wheelchair-bound and not able to provide for his family anymore. 

Spinal rehabilitation provided a small benefit only. AB was now strapped into a wheelchair. AB did not want to give up despite progressive weakness in both legs and right arm with loss of locomotion and physical independence. 

With great courage AB decided to retrain and despite all the odds he opened a small accountancy company at his own home. He not only learned all the intricacies of the Irish tax system, but also learned how to interact with the rest of the world using the computer mouse in the non-dominant hand. Several years passed and AB saw more clients coming for tax advice. AB developed a successful business and was able to provide for his family. Unfortunately, as he was busy filling the tables on his computer he began to notice that his left hand, through which he was able to provide for the family, started to swell on occasion, giving him significant discomfort.

Over the next few years he attended multiple consultants in different specialities in the region seeking help. The left hand was now beginning to lose power and was swelling up intermittently. Loss of function in this limb would be the final straw.

AB was referred to Dr Connail McCrory’s Pain Clinic at St James’s Hospital, Dublin, where he had two nerve blocks and pulse radiofrequency treatment performed on the left cervical nerve dorsal root ganglia. After a short-lived benefit to both the pain and swelling, symptoms recurred. Complex regional pain syndrome was the provisional diagnosis. In this particular case the traditional SCS system would expose AB to increased risk of complications; AB had experienced enough problems.

In September 2016, however, a new system became available in Ireland. The Stimwave Freedom-8A Spinal Cord Stimulation (SCS) System is the world’s first wireless, fully-programmable SCS neuromodulation device available, which presents a technological breakthrough. 

In October 2016 the lead was successfully implanted for AB and after 24 hours he was discharged home. The system is starting to work. The new system will require significant input from the pain team for this is the first system successfully implanted in Ireland and one of the first in Europe. As always, patient selection is crucial and we could not have been referred a more committed and courageous patient than AB.

Each system, however, up to very recently consisted of three basic elements: Electrodes, battery (IPG – internal pulse generator), and leads connecting the two elements. 

The Stimwave Freedom-8A Spinal Cord Stimulation (SCS) System is the world’s first wireless, fully-programmable SCS neuromodulation device available, which presents a potentially life-changing technological breakthrough. The traditional implanted programme generator, which drives the stimulator, is no longer implanted but is instead externalised. This avoids tunnelling a connection between the stimulator lead and programme generator. It also avoids the creation of a subcutaneous pocket, usually in the buttock, to house the programme generator.

One of the disappointments of SCS has always been the successful use of stimulation to treat the initial complaint, only to find that the patient has a complication from implantation, such as bleeding or infection in the pocket, which may result in explantation of the system. Another problem is “pocket pain” whereby the patient may get a good response to the initial complaint but finds the presence of the programme generator painful. Numerous strategies have been developed to deal with this, including revision of the pocket, however, this often does not work well.

The new system is an innovation as for the first time there are no internal leads required, as the electrodes are charged with the externally worn power source about the size of a mobile phone. There is no need for a pocket to be created to house the programme generator. It is also unique because the device can deliver all the available stimulation regimens:

Conventional or tonic stimulation: Uses frequencies of 60-100Hz. This produces paraesthesia in the target area. This is the most common mode of stimulation, but in the last few years, two more modes have gained in popularity.

Burst stimulation: Involves five pulses in a burst (500Hz pulse frequency in each burst) with a burst frequency of 40Hz. Burst stimulation produces little or no paraesthesia.

High-frequency stimulation: Uses frequencies of 5-10kHz. This pattern does not generate paraesthesia, but may produce effective pain relief.

The major indications for SCS are:

Failed back surgery syndrome (FBSS)

This is the most common indication and also the most widely studied. Two randomised controlled trials (RCTs) compared SCS with either conventional medical management (CMM), CMM (PROCESS) or reoperation (North et al). On both occasions SCS was superior and also cost-effective. The above trials focused on patients with predominantly radicular lower limb pain. Patients with axial back pain are a more difficult group of patients to treat with conventional SCS. Recent data suggests that high-frequency SCS or burst stimulation may increase the efficacy of neurostimulation in this group.

Chronic regional pain syndrome (CRPS)

One RCT investigated the use of SCS in combination with physical therapy (SCS+PT) vs physical therapy only (PT) for the management of CRPS.

After six months, there was a significant decrease in pain in the SCS+PT group. The pain relief was sustained at two years. SCS is also a cost-effective option in CRPS.

Ischemic pain from peripheral vascular disease (such as chronic critical limb ischemia) – as well as from the chest pain of angina pectoris that has not responded to other treatment.

Pain associated with chronic leg ischaemia (CLI) is difficult to manage reliably with oral analgesics. Surgical restoration of adequate blood flow to the leg is desirable, but it is often impossible to achieve. Lower limb amputation is sometimes indicated.

In 2013, Ubbink and Vermeulen reviewed six controlled trials comparing the use of SCS+CMM with CMM. The primary outcome in most of the studies was limb salvage. The review demonstrated a significantly higher limb salvage rate in the SCS group. Also, patients managed with SCS had a significant reduction in pain score, analgesic use, and Fontaine stage (III to II). The cost associated with SCS at two years in one study was higher than the conservative management group. The mechanism of action of SCS in CLI may be modulation of nitric oxide or prostaglandin production, or modulation of the sympathetic nervous system.

Multiple case reports and small case series exist describing the use of SCS in other neuropathic conditions including diabetic neuropathy, post-herpetic neuralgia, and post-amputation pain syndromes. However, these indications currently lack evidence and implantation should only be considered on a case-by-case basis after a successful trial period.

Implantation

Implanting the stimulator is a two-stage process. The trial phase is performed under local anaesthesia to ensure adequate coverage of the affected area. The implantation phase is performed under general anaesthesia.

Under the fluoroscopy guidance the epidural needle is used to enter the space and through this passageway one or more thin wires are threaded, each carrying a number of small electrical contacts along the end. The leads are attached to a power supply that delivers a mild current. With feedback from the patient, the physician adjusts the position of the electrodes until the area of pain feels covered by a tingling sensation called paraesthesia. Once the position is chosen, the lead is secured in place.

The patient spends a trial period of variable duration (one to seven days) with a pulse generator that is carried on a belt or in a pocket. An extension wire conducts impulses to the temporary, implanted lead. Pain relief builds slowly and can last several hours past the period of active stimulation. The external control unit allows the patient to switch between programmes in order to obtain desired coverage in different postures, such as sitting or lying down.

Permanent implantation is the second phase. After trialling, a patient who has experienced a reduction in pain by at least half may choose to continue treatment with a permanent system. A new permanent lead is implanted through a small incision and sutured in place. Sometimes, permanent leads will have been implanted prior to the trial period. If the patient decides to continue, those original leads will remain in place. The surgical incision heals in approximately seven to 10 days.

For patients choosing a permanent SCS option, an implantable power generator or programme generator, slightly smaller than a computer mouse, is implanted in a surgically created pocket, most commonly in the buttock or abdomen.

Conclusion

The success of SCS is heavily dependent on appropriate patient selection. To date, RCTs in SCS involved patients with persistent pain (neuropathic or ischaemic) resistant to CMM for more than six months. They should have a definite diagnosis and an identifiable pain generator with a positive trial of stimulation. Patients with psychiatric disorders or unreasonable expectations are not suitable. For these reasons, a pre-operative psychological assessment is advised. The patient must have the cognitive capacity to give informed consent, demonstrate an ability to understand and use the device properly, and commit to weaning off inappropriate and/or ineffective medication.

The Stimwave system has given the case report patient, AB, new hope. The Stimwave family of wireless implantable neurostimulation devices represents a significant technological improvement. No tunnelling or surgical pocket requirement reduces the complication of this technique while its portability, minor surgical trauma, no requirement for staging, and flexibility in delivering various mode of stimulation, opens the application of the therapy to a much wider cohort of patients than ever before. 

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