The Changing Face of Neuromodulation

Neuromodulation is a relatively new discipline of medicine that encompasses several specialities including neurosurgery, anaesthetics, orthopaedics, neurology, neurology, neurophysiology, cardiology, urogynaecology and urology. Neuromodulation therapies are implantable therapies for the treatment of chronic pain and other disorders of the central nervous system. These therapies include spinal cord stimulation (SCS) systems and drug administration systems.

Neurostimulation is the application of low intensity electrical impulses to trigger selective nerve fibres to diminish or block the intensity of the pain message being transmitted to the brain, replacing areas of intense pain with a more pleasant sensation called parethesia.

Over the past two decades this reversible method of pain control has steadily increased in popularity with pain management specialists including neurosurgeons, anaesthetists, orthopaedic surgeons and rehabilitation physicians and is now commonly used in the management of neuropathic pain. Newer applications for neuromodulation techniques include the treatment of epilepsy, chronic angina, Parkinson’s Disease and other movement disorders and functional disorders.¹

The broad term ‘neuromodulation’ encompasses a number of techniques, including spinal cord stimulation (SCS), peripheral nerve stimulation (PNS), sacral root stimulation, motor cortex stimulation (MCS), deep brain stimulation (DBS).⁸ and intrathecal drug delivery.

The theoretical basis for these techniques was the gate-control theory of Melzack and Wall, who published their landmark study in 1965, explaining how stimulation was able to override the transmission of pain signals to the brain.⁴ This theory postulates that the stimulation of large-diameter, myelinated cutaneous afferent fibres is capable of inhibiting the transmission of nociceptive (pain) signals to the brain.^{1, 4} However, this theory has been found to be inadequate to explain the clinical effects of spinal cord stimulation.⁶ In particular, spinal cord stimulation is relatively ineffective in the treatment of nociceptive pain, but is effective for neuropathic pain.⁶

While the precise mode of action of peripheral nerve and spinal cord stimulation pain relief may have been poorly understood, the techniques were found to be effective. In particular, they offered a non-destructive and reversible approach to the management of severe chronic pain.⁴ This conferred a significant advantage over other, more traditional non-reversible neurosurgical techniques, including neurectomy, rhizotomy and sympathectomy.

Spinal cord stimulation and peripheral nerve stimulation were the first neurostimulation techniques to be developed. They were first employed for the treatment of intractable neuropathic pain in 1967.

Over time, computer modelling and an improved understanding of neurophysiology have allowed refinement of traditional neuromodulation techniques, as well as development of new applications for the technology.

Current Research and New Applications

Spinal Cord Stimulation

Spinal cord stimulation devices have been in use for 40 years. However, only recently has their true potential begun to be understood.²

Traditionally, spinal cord stimulation was used in the management of chronic, non-malignant neuropathic pain.⁴ However, SCS is also being used in the management of the following conditions:

• Peripheral nerve injuries: peripheral neuropathies or phantom limb pain
• Failed back surgery syndrome (FBSS)
• Complex regional pain syndromes (CRPS)
• Ischaemic pain due to peripheral vascular disease or cardiovascular disease, e.g. (angina pectoris)^{1, 7}

In recent years, the potential role of spinal cord stimulation devices in the management of conditions such as urinary incontinence (urge incontinence), and interstitial cystitis has also been studied.¹

Overall, approximately 15 000 patients worldwide are now treated with spinal cord stimulation each year.⁴ This is probably only a small fraction of the total number of patients who could benefit from these techniques.⁴

Peripheral Nerve Stimulation

Implanted peripheral nerve stimulation devices may be used in the management of peripheral neuropathic pain in the territory of a single sensory or mixed nerve and occipital neuralgia.⁴

Vagal nerve stimulation is a newer development in the field of peripheral nerve stimulation. It has been used in the treatment of epilepsy and depression.¹  

Deep Brain Stimulation

While originally developed for the treatment of epilepsy, deep brain stimulation is now used mainly for management of Parkinson’s-related movement disorders.¹ One major advantage of this technique over more traditional neurosurgical interventions (eg. thalamotomy) is its reversibility.

Excitingly, research continues into the possibility that early intervention with deep brain stimulation may even slow the progression of Parkinson’s disease.⁸

Motor cortex stimulation

Stimulation of the motor cortex for pain relief is practiced only in specialised neurosurgery centres,⁴ however, it may have a role in the management of special pain syndromes such as postherpetic neuralgia, spinal cord injury, limb stump pain, and trigeminal neuralgia.³

Despite this wide variety of clinical applications, the vast majority (85%) of neurostimulation electrodes are currently used in traditional spinal cord stimulation or extremity peripheral nerve stimulation sites.¹ A wider community awareness of neuromodulation techniques, as well as further research into new applications, will allow more patients to benefit from this technology.

Sources

1. Alo K.M. & Holsheimer J. 2002 ‘New Trends in Neuromodulation for the Management of Neuropathic Pain’, Neurosurgery, vol. 50, no. 4, pp 690 – 704
2. Barolat G. & Sharan A. D. 2000 ‘Future Trends in Spinal Cord Stimulation’, Neurological research, vol. 22, no. 3, pp. 279 – 285
3. Benabid A. L. et al 2000 ‘Future prospects of brain stimulation’, Neurological Research, vol. 22, no. 3, pp. 237
4. Gybels J, Erdine S, Maeyaert J, Meyerson B, et al. 1998 ‘Neuromodulation of pain. A consensus statement prepared in Brussels 16-18 January 1998, by the task force of the European Federation of IASP Chapters (EFIC)’ 1998 Eur J Pain vol. 2, pp. 203-9
5. Mekhail N. A. et.al. 2004, ‘Cost Benefit Analysis of Neurostimulation for Chronic Pain’, Clinical Journal of Pain, vol. 20, no. 6, pp. 462 – 468
6. Meyerson BA, Linderoth B. 2006 ‘Mode of Action of Spinal Cord Stimulation in Neuropathic Pain’, Journal of Pain and Symptom Management, vol 31, no. 4, pp S6-12
7. Murray S. et.al. 2000 ‘Neurostimulation treatment for angina pectoris’, Heart, vol. 83, pp.217-220
8. Simpson BA. 2006 ‘The role of neurostimulation: the neurosurgical perspective’, Journal of Pain and Symptom Management, vol. 31, no. 4. pp. S3-5