1. Spinal Cord Stimulation Mechanisms and Indications
Douglas Dobecki, M.D.
San Diego Pain Institute

2. Gate Control Theory of SCS
Originally derived from gate control theory by Melzack and Wall
Peripheral stimulation of Aβ fibers leads to activation of inhibitory interneurons and subsequent inhibition of second order nociceptive neurons in the dorsal horn
Expanded to electrical stimulation of the dorsal column with production of paresthesia
Melzack et al, Science, 1965, Vol 150
Shealy, et al, Anesthesia and Analgesia, 1967, Vol 46

3. Neurophysiologic Mechanisms of SCS
SCS increases dorsal horn inhibitory neurotransmitter GABAB, while decreasing excitatory amino acids Glutamate and Aspartate
Activation of descending somatosensory control pathways through release of Serotonin, Norepinephrine, Adenosine
Suppression of sympathetic activation by modulation of α-adrenoreceptors and antidromic release of calcitonin gene-related peptide (CGRP) and Substance P
Modulation of WDR neurons
Meyerson et al, Journal of Pain and Symptom Management, April 2006, Vol 31, No 4S

4. Electrophysiologic Mechanisms of SCS
Current flows from Cathode (-) to Anode (+) resulting in neuronal depolarization at Cathode (-) and hyperpolarization at Anode (+)
Electrical parameters are adjusted during programming including electrode polarity, Frequency (Hz), Amplitude (V or ma), and Pulse Width (μs)
Potential segmental conductance blockade of spinothalamic tracts

5. SCS Mechanism Summary
Many theories on the mechanisms of action of spinal cord stimulation have been suggested, including gate inhibition, activation or release of purported neuromodulators, conductance blockade of spinothalamic tracts, blockade of sympathetic mechanisms, and activation of supraspinal mechanisms
The mechanism of spinal cord stimulation cannot completely explained by one model
It is likely that multiple mechanisms and neurophysiologic pathways operate sequentially or simultaneously

6. SCS Indications
Indicated for the management of chronic and intractable pain of the trunk or extremities
Patients have failed adequate trial of conservative and conventional therapies
Patients have passed psychological screening
Common conditions include
Failed Back Surgery Syndrome/Post-Laminectomy Syndrome
Complex Regional Pain Syndrome (CRPS)
Arachnoiditis
Chronic Radiculopathy
Epidural Scarring or Fibrosis
Chronic Neuropathy or Neuralgia
Post-Thoracotomy Pain

7. Failed Back Surgery Syndrome
Defined as persistent or recurrent complaints of low back and/or leg pain in patients who have undergone operative procedures intended to relieve those complaints
Can occur in up to 10-40% of spine surgeries
$20B annually in direct health care costs
5th most common reason for MD visit
Important to identify and treat other etiologies of FBSS that are not neuropathic and not candidates for SCS
Foraminal stenonsis, discogenic pain, recurrent disc herniation, pseudoarthrosis, facetogenic pain, sacroiliac syndrome
Common psychological disorders in FBSS
Depression, anxiety, personality disorders, and secondary gain issues
North et al, Neurosurgery, 2005, Vol 56, No 1
Follet et al, Neurosurgery Quarterly, March 1993, Vol 3, Issue 1

8. Failed Back Surgery Syndrome Etiology
Genesis is multifactorial
Improper patient selection
Inadequate operations
Operative complications
Epidural scarring, fibrosis, arachnoiditis,
Pseudoarthrosis
Hardware malposition or failure
Progression of degenerative processes
Onset of new pathology
Altered joint mobility
Spondylolisthesis
Adjacent segment disease
Follet et al, Neurosurgery Quarterly, March 1993, Vol 3, Issue 1
Park et al, Spine, September 2004, Vol 29, Issue 17

9. Clinical Effectiveness of SCS in FBSS
SCS was more successful than re-operation in giving selected FBSS patients at least 50% pain relief
Minimally invasive therapeutic trial is an important advantage versus reoperation
In most cases, SCS eliminated the need for further spine surgery in patients identified as reoperation candidates by standard criteria
Battery life is a major SCS cost driver. Lifetime savings of rechargeable SCS systems can exceed $300,000 for average patient.
24 month outcomes demonstrate significant improvements of SCS patients compared to conventional medical management (CMM) group. Significantly more SCS (47%) patients versus CMM (7%) achieved the primary outcome of greater than 50% pain relief (P=0.02).
North et al, Neurosurgery, January 2005, Vol 56, No 1
North et al, Neuromodulation, 2004, Vol 7
North et al, Neurosurgery, August 2007, Vol 61, Issue 2
Kumar et al, Neurosurgery, October 2008, Vol 63, No 4

10. Complex Regional Pain Syndrome
Describes a myriad of pain symptoms which bridge neuropathic and vasculopathic pain conditions
Symptoms are typically regional and out of proportion to the clinical course of the inciting event
CRPS Type 1- pain symptom development without evidence of nerve injury
CRPS Type 2- pain symptom development in setting of obvious nerve injury
Sympathetically Maintained Pain (SMP)
Sympathetically Independent Pain (SIP)
Bennett et al, Neuromodulation, July 1999, Vol 2, Issue 3

11. Complex Regional Pain Syndrome
Diagnosis based on history and physical exam
Report 1 symptom in 3 of 4 categories:
* Sensory- hyperesthesia and/or allodynia
* Vasomotor- temperature asymmetry and/or skin color changes
* Sudomotor /Edema- edema and/or sweating changes
* Motor/Trophic- decrease ROM and/or motor dysfunction and/or trophic changes
Must display at least 1 sign in 2 or more categories:
* Sensory- hyperesthesia (pin prick) and/or allodynia (mechanical,
Temperature) and/or deep somatic pressure/joint movement
* Vasomotor- temperature asymmetry (>1 C) and/or skin color changes
* Motor/Trophic - decrease ROM and/or motor dysfunction
(weakness, tremor, dystonia, and/or trophic changes)
Hayek et al, The Physician and Sports Medicine, May 2004, Vol 32, No. 5

12. Evidence Supporting SCS for CRPS
Reference
Type of Study
(Level of Evidence) N
Outcome
Kemler et al. 2000
RCT (level 2)
36 pts.
Improved pain and quality of life Successful in 56% of patients at 6 months
Kemler et al. 1999
Retrospective (level 3)
23 pts.
Successful in 57% of patients
Kemler et al. 2004
Successful in 63% of pts; 2 yr follow-up of Kemler et al. 2000
Bennett et al. 1999
101 pts.
70% pt satisfaction for 1 lead
91% pt satisfaction for 2 leads
Kumar et al. 1997
Nonrandomized (level 3)
12 pts.
Good relief in 4 pts.
Excellent relief in 8 pts.
*RCT = randomized controlled trial

13. Summary of Mechanisms and Indications
An understanding of the mechanisms of spinal cord stimulation continues to evolve
SCS has been proven to be sustainably effective in the treatment of chronic neuropathic pain in several modern randomized clinical trials versus conventional therapies or reoperation
Routine trial stimulation and an improved understanding of appropriate indications are associated with increased patient satisfaction, functional capacity and quality of life
Rational use of SCS will continue to expand and continuing technological advancements will continue shape treatment algorithms