Clinical Features

Spinal Cord Stimulation – Advances in Effectiveness and Tolerability

Written by Professor Connail McCrory and Dr Deborah Galvin, Trinity Translational Medicine Institute, Trinity College Dublin

Chronic neuropathic pain (CNP) is a specific form of chronic pain. It is caused by a dysfunction of the somatosensory nervous system. It affects approximately 8% of the population. In addition to trauma and stroke, a number of medical conditions contribute to activation of CNP including diabetes, multiple sclerosis, complications of common surgeries such as hernia repair, breast surgery, vasectomy and neurodegenerative disorders. It is associated with cancer chemotherapy also. This is a common and difficult problem to treat.

It has been recognized that spinal cord stimulation in appropriately selected patients is more effective than pharmacological therapy and avoided the daily use of medication with associated compliance issues and development of tolerance and side-effects. However, spinal cord stimulation was initially very expensive with a significant workup preplacement, a requirement of two visits to the operating theatre with associated inpatient stay and significant complication rates. In addition to this, the most expensive component of the spinal cord stimulator, the implantable pulse generator (IPG) needed to be replaced every 3 to 5 years depending upon usage and this required a further visit to the operating theatre.

IPG replacement was usually performed in the day-care unit. Therefore, spinal cord stimulation was seen as extremely expensive and access to it was limited primarily by funding concerns. A significant expenditure both financially and in terms of the pain service time was required to employ the therapy for one patient. Other concerns expressed by funders related to lack of knowledge regarding how this system actually worked.

However, clinical researchers in this field over the last 15 years in particular have made significant improvements in our knowledge regarding the initiating, maintenance and recovery factors involved in the development of chronic neuropathic pain and also developed a greater understanding of the functioning of spinal cord stimulation in vivo. This has been married with successful technological developments over a similar time period.

There are now many more options in terms of programming the IPG which means the system is much more versatile. Traditionally lowfrequency stimulation was used which was paraesthesia-based. This has significant limitations and effectiveness in addition to the fact that some patients found paraesthesia was uncomfortable. The development of higherfrequency stimulation and removal of paraesthesia-based stimulation has made this treatment more acceptable to the majority of patients. Furthermore, the development of burst stimulation, which more closely mimics physiological neural function is improving efficacy.

There is growing evidence that CNP is initiated, maintained and recovered by changes in the neural proteome and cellular immune function. There is a growing body of clinical work that demonstrates spinal cord stimulation in vivo alters the proteome within the cerebrospinal fluid and t-cell phenotype. There is therefore growing information regarding how spinal cord stimulation works and this tends to support higher frequencies of stimulation.

The mode of action of spinal cord stimulation has developed beyond the idea that the resultant electrical field caused the interneurons to modulate pain signalling in the dorsal columns – the gate theory of pain control. There is a growing body of clinical evidence which identifies that spinal cord stimulation can modulate peptides implicated in CNP pathophysiology in the cerebrospinal fluid including VEG-F, IL-10, in addition to the cellular constituents including CD4/ CD8 ratios and glial cell activation favouring constituent relationships that promote recovery from chronic neuropathic pain.

Other factors that promote spinal cord stimulation include the removal of the necessity of the “trial of stimulation,” in addition to the technical improvements. After a patient had been deemed suitable for spinal cord stimulation the traditional approach was that the patient underwent a trial whereby the electrode lead was inserted under sedation and connected to an externalized power source. The patient went home for a period of time which could range from 2 to 3 days to one month in some European countries. The rationale for trial was an attempt to prove the system would help the patient prior to implantation of the IPG which was the most expensive part of the system.

However, it also opened up the possibility of infection due to the externalized lead and did not mimic accurately the situation of having an implanted stimulator. Numerous restrictions were required including being unable to shower or drive as well as appropriate attention being paid to this bulky system which meant that it did not accurately reflect an implanted system. There was no comprehensive evidence supporting this and recently the International Neuromodulatory Society has very much challenged this mandated approach.

As always in medical practice, patient selection is absolutely crucial and this is more relevant than carrying out a trial. Today it is acceptable to proceed straight to implantation of the spinal-cord stimulator if the clinician deems this is the best treatment for a given patient. This change in practice has significant implications in terms of the cost of providing the service and potential complications by reducing the number of operating theatre visits from two to one.

The use of combination prophylactic antibiotic therapy at implantation has also reduced infection rates. Other technological improvements apart from the programming include superior anchors for stabilizing the leads, reducing the chances of lead migration, which was a significant problem in the past, and much smaller rechargeable IPG’s. The historical larger IPG’s could cause significant problems in terms of pouch pain whereby the patient found the IPG implantation site to be problematically painful.

The situation could arise whereby the chronic neuropathic pain complaint had improved however the patient was now experiencing a surgical issue which was pouch pain. This was very unsatisfactory for both the clinician and patient. The smaller modern IPG’s have significantly reduced this complication. Furthermore, the modern IPG’s can last from 8 to 10 years depending upon usage.

This has significantly prolonged the lifespan of the IPG. This again is a significant cost saving in terms of reducing the number of visits to theatre in the patient’s lifetime and the number of IPG’s required in a patient’s lifetime. Spinal cord stimulation has developed into a much more effective, affordable and user-friendly therapy. Traditionally funding bodies expressed serious concerns about the cost of this therapy for one patient who had a disease that was not seen as life-threatening.

The new developments in terms of removal of the mandate to trial, the smaller IPG with a much longer lifespan requiring fewer replacements and reduction in complication rates make spinal-cord stimulation more affordable. The advent of new programming paradigms has made the system more effective. Therefore, the future for spinal cord stimulation is bright.

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