Neurotherapeutics: The Journal of the American Society for Experimental NeuroTherapeutics

Spinal Cord Stimulation: An Update

Steven Falowski, Amanda Celii, and Ashwini Sharan

Department of Neurosurgery, Jefferson Medical College, Philadelphia, Pennsylvania 19107

Summary: Spinal cord stimulation has been used in the treat- the power source has changed from a radio frequency– driven
ment of many chronic pain disorders since 1967. In this update, system to a rechargeable impulse generator. These topics are
the indications for spinal cord stimulation are reviewed with covered, along with a short discussion of implant technique.
attention to recent publications. A focused review of the liter- Finally, we include a review of complications of such therapy.
ature on abdominal and visceral pain syndromes is also pro- SCS as a technology and therapy continues to evolve. Key
vided. Furthermore, the technology has evolved from the use of Words: Spinal cord stimulation, dorsal column stimulation,
monopolar electrodes to complex electrode arrays. Similarly, chronic pain, neuropathic pain, pacemaker.

INTRODUCTION the subarachnoid space6 or between the two layers of the

dura or in the epidural space either dorsal or ventral to
Spinal cord stimulation (SCS) is an adjustable, non-
the spinal cord.7–9 Subsequently, less invasive percuta-
destructive, neuromodulatory procedure that delivers
neous techniques were introduced.10
therapeutic doses of electrical current to the spinal cord
Great advances in technology have also driven the
for the management of neuropathic pain. The most com-
popularity of the field. The electrodes were initially all
mon indications include postlaminectomy syndrome,
unipolar; the bipolar arrays developed subsequently. Fur-
complex regional pain syndrome, ischemic limb pain,
thermore, the contact combinations could only be hard-
and angina. Scattered reports have also appeared regard-
wired, and could not be reprogrammed after the pulse
ing the treatment of intractable pain due to other causes
generator was implanted. In the beginning, only radio
including visceral and abdominal pain, cervical neuritis
frequency– driven (RF-driven) passive receivers were
pain, spinal cord injury pain, postherpetic neuralgia, and
available. In the mid-1970s, the first implantable pulse
neurogenic thoracic outlet syndrome. The procedures are
generator powered by a lithium battery was introduced
most commonly performed by neurosurgeons or anesthe-
into the market. In 1980, the first percutaneous quadri-
siologists specializing in pain management,1–3 but other
polar electrode was produced, one that could be repro-
specialties, such as rehabilitation medicine and orthope-
grammed noninvasively through an external transmit-
dic surgery, have also demonstrated interest in the pro-
ter.11 Subsequently, implantable pulse generators (IPG)
cedure.
have been developed that can be both transcutaneously
Enthusiasm for SCS began with the introduction of the
charged and programmed. This most recent advance is
gate-control theory for pain control by Melzack and
now leading a renewed interest in the utility of using
Wall4 in 1965. They noted that stimulation of large my-
special electrode arrays in the delivery of electrical stim-
elinated fibers of peripheral nerves resulted in paresthe-
ulation to the spinal cord.
sias and blocked the activity in small nociceptive projec-
tions. In 1967, Shealy inserted the first dorsal column
stimulator in a human suffering from terminal metastatic MECHANISMS OF ACTION
cancer.5,6 Subsequently, electrodes have been implanted
utilizing a variety of techniques: via a laminectomy in Although a large body of work has been published, the
exact mechanisms of action of SCS remain unclear. The
computer modeling studies of Holsheimer, Coburn, and
Address correspondence and reprint requests to: Ashwini D. Sharan, Struijk12–17 have shed some light, at least theoretically,
M.D., Department of Neurosurgery, Jefferson Medical College, 909
Walnut Street, 2nd Floor, Philadelphia, PA 19107. E-mail: ashwini. on the distribution of the electrical fields within the spi-
sharan@jefferson.edu. nal structures. It is clear that stimulation on the dorsal

86 Vol. 5, 86 –99, January 2008 © The American Society for Experimental NeuroTherapeutics, Inc.
 SPINAL CORD STIMULATION 87

aspect of the epidural space creates complex electrical noted in the SCS group, compared with 0.2-cm increase
fields that affect a large number of structures. We do not in the physical therapy group; however, no functional
know whether activating afferents within the peripheral improvement was observed in either group. In 2006, in a
nerve, dorsal columns, or supralemniscal pathways share letter to the editor of the New England Journal of Med-
equivalent mechanisms of action. Furthermore, there icine, Kemler et al.25 recounted their 5-year follow-up on
may be antidromic action potentials passing caudally in the patients with SCS. Their major conclusion was that
the dorsal columns to activate spinal segmental mecha- the effects of SCS diminished over time for these pa-
nisms in the dorsal horns, as well as action potentials tients; they did not specify what impact reprogramming
ascending in the dorsal columns activating cells in the or modern devices might have on the long-term effects of
brainstem, which in turn might drive descending inhibi- SCS therapy.
tion. At the chemical level, animal studies suggest that Oakley and Weiner26 reported a prospective study of
the SCS triggers the release of serotonin, substance P, 19 patients with complex regional pain syndrome im-
and GABA within the dorsal horn.18 –20 planted with spinal cord stimulation systems. Of the 10
patients for whom detailed long–term efficacy data were
available, 3 reported full relief from their pain and 7
INDICATIONS reported partial relief.
SCS has been used for a variety of pain conditions and Including the Oakley and Weiner study, three prospec-
is particularly indicated for pain of neuropathic origin, tive studies without matched controls have been reported
including postlaminectomy syndrome, complex regional (total of 50 subjects).26 –28 Two of the studies reported
pain syndrome, phantom limb pain, spinal cord injury success rates, with an 84% overall success rate. The
pain, and interstitial cystitis. The indications have been study by Calvillo et al.27 reported a significant improve-
extended to include intractable pain due to abdominal or ment in pain scores (VAS) and a ⬎50% reduction in
visceral pain and neurogenic thoracic outlet syndrome. narcotic use by 44% of subjects. In eight retrospective
SCS has been used successfully to treat severe pain due studies, the overall success rate was 84% (192 pa-
to ischemic disease of the lower extremities and, more tients).29
recently, intractable angina pain. Experience suggests
that, in selected patients, SCS can produce at least 50% Postlaminectomy syndrome
pain relief in 50 – 60% of the implanted patients. Nota- Postlaminectomy syndrome (also called failed back
bly, with proper follow-up care, these results can be surgery syndrome) is vaguely defined. The term has in-
maintained over several years. cluded pain localized to the center of the lower lumbar
area, pain in the buttocks, persistent radicular pain, or
Complex regional pain syndrome diffuse lower extremity pain. Arachnoiditis, epidural fi-
The implementation of SCS in individuals with com- brosis, radiculitis, microinstability, recurrent disk hernia-
plex regional pain syndrome type I is more difficult than tions, and infections have been implicated in the etiology
with any other patient group. The possibility of aggra- of this syndrome. Most published series distinguish be-
vating the original pain or causing a new pain or allo- tween back and leg pain, but the details of the pain
dynia at the implanted hardware site is greater than with syndromes are seldom defined. SCS is accepted in the
any other diagnostic category mentioned. The pain may treatment of leg pain, but its widespread use for relief of
spread to other body parts, and it is challenging to be able pain in the lower lumbar area still remains to be defined.
to cover all the affected areas with stimulation. A great challenge in the treatment of postlaminectomy
In 1989, Barolat et al.21 reported reduction of pain in syndrome has been to obtain stimulation in the low back.
10 of 13 patients implanted. No patients in that series Even with direct stimulation to the low back, the pattern
were made pain-free but all 10 reported a definitive of paresthesia is often replaced in time by an unpleasant
difference when the stimulation was stopped. In 1997, segmental band of stimulation from the thoracic roots,
Kumar et al.22 presented a median follow-up of 41 which negates the benefits of the procedure. Previous
months on 12 patients with permanently implanted leads; pioneering work by Jay Law30,31 has shown that stimu-
8 patients reported near complete resolution of their lation in the low back can be obtained only if one uses
symptoms and the remaining 4 maintained good relief. multiple arrays of closely spaced bipoles at T9 –T10.
In another series, Kemler et al.23 reported 23 additional North et al.32 challenged the concept of the superiority of
cases, with 78% of the patients reporting improvement. centered dual electrodes by showing that one single
In 2000, Kemler et al.24 published work on a series of 54 quadripolar electrode in midline has the ability to stim-
patients who underwent randomization either to SCS ulate the axial low back. These were acute observations,
with physical therapy or to physical therapy alone. In the and no data exist as to the long-term behavior of single
SCS group, 67% patients experienced significant pain versus dual electrodes. The advent of tripole electrodes
relief, which persisted at 6 months. A 2.4-cm decrease and the ability to steer current has made it more plausible
and improvement in visual analog pain score (VAS) was to aim for low-back paresthesia. Further, flanking the
Neurotherapeutics, Vol. 5, No. 1, 2008
88 FALOWSKI ET AL.

cathode by lateral anodes also appears, in theory at least, parts of the myocardium has been demonstrated as a
to raise the discomfort threshold.33 long-term effect of spinal cord stimulation, both at rest
Marchand et al.34 conducted a prospective, random- and after pharmacologic stress induction.44
ized controlled study examining patients with at least one Nienke et al.45 conducted a prospective study on qual-
prior surgery for chronic back pain secondary to trauma. ity of life changes in patients with refractory angina
All the patients used a SCS and acted as their own pectoris implanted with SCS. They found that both the
control. Although a small trial (n ⫽ 8), pain scores were pain and the health aspects of quality of life improved
significantly reduced with SCS, compared with placebo significantly after 3 months of SCS. After 1 year of SCS,
stimulation. social, mental and physical aspects of quality of life also
Longitudinal studies by North et al.35 showed that, in were found to be improved.
patients with postsurgical lumbar arachnoid or epidural Hautvast et al.46 implanted SCS in patients with stable
fibrosis without surgically remediable lesions, SCS is angina pectoris and randomized them. One group’s stim-
superior to repeated surgical interventions on the lumbar ulator remained inactivated; the other group was in-
spine (for back and leg pain) and to dorsal ganglionec- structed to use the stimulator three times a day for 1 hour
tomy (for leg pain). That study comprised 50 patients and with any angina attack. At 6 weeks, compared with
with a postlaminectomy syndrome who averaged 3.1 controls, the treatment group had increased exercise du-
operations prior to SCS implantation. Successful out- ration and time to angina, and decreased anginal attacks
come (ⱖ50% pain relief and patient satisfaction with the and sublingual nitrate consumption. Also observed was a
result) was obtained in 53% of patients at 2.2 years. A decrease in ischemic episodes on EKG, as well as a
systematic review of the literature was conducted by decrease in observed ST segment depressions on exer-
Turner et al.,36 reviewing a total of 41 articles from 1966 cise EKG. There was an increase in perceived quality of
to 1994 that met their criteria. It was noted that for life and a decrease in pain. It was shown that a placebo
⬃50 – 60% of patients with postlaminectomy pain, effect from surgery in the treatment group was unlikely
⬎50% pain relief was attained from the use of SCS. In because all patients had implantation surgery at baseline.
1996, Burchiel et al.37 conducted a prospective multi- In the ESBY study, Mannheimer et al.47 randomized
center study with 1-year follow-up and also reported 104 patients accepted for coronary artery bypass graft
55% successful stimulation. Medication usage and work (CABG) to receive either CABG (n ⫽ 51) or SCS (n ⫽
status were not changed significantly. 53). This study demonstrated that patients randomized to
North et al.38 also conducted a prospective study ran- SCS showed a greater than 30% improvement in Not-
domizing patients with failed back surgery syndrome to tingham Health Profile NHP scores, compared with base-
either repeat their back surgery, or undergo SCS surgery. line, which was significant and comparable to the im-
Patients were allowed to cross over after 6 months. Ten provement shown by patients randomized to CABG.48
of 15 patients crossed over from back surgery to SCS, These results were consistent on follow-up after 4 years.
whereas only 2 of 12 patients crossed over from SCS to Notably, the 5-year mortality of 27.9% in the ESBY
back surgery. study was similar between those receiving SCS and those
Studies do not routinely differentiate between axial who received CABG, with no difference in the percent-
back and leg pain. What recent data there are on back age of cardiac deaths. The ESBY study showed that
pain remain inconclusive. In our experience, most im- cardiac events were similar across the groups, but that
planting physicians have found that SCS is far more there were significantly more cerebrovascular events ob-
effective for radicular pain than for axial low back pain. served in the CABG group. Both groups experienced a
significant reduction in both the number of angina at-
Angina tacks and the consumption of nitrates. There was no
The role of SCS in the management of refractory an- significant intergroup difference regarding these param-
gina pectoris seems to be a very promising. There are eters. In another prospective study of 104 patients who
well-documented reports in the literature of uniformly underwent SCS implantation for refractory angina pec-
good results in the relief of anginal pain.39 – 43 Further- toris, there was a significant decrease in angina episodes
more, the results have been maintained in long-term at rest, angina episodes with activity, and total angina
follow-up and have been substantiated also by a reduc- episodes.49
tion in the intake of nitrates. Notably, other findings have DeJongste et al.41 randomized 17 patients with angina
supported the evidence that SCS has effects that go be- to an active treatment group with SCS implantation and
yond pain relief. The observations that there is less ST a control group. The control group was followed for 2
segment depression and that the exercise capacity, the months and then received SCS implantation. Both groups
time-to-angina, and the recovery time all improve with were followed for a total of 12 months. This study also
stimulation suggest that there may be a reduction in revealed a significant reduction in the incidence of an-
ischemia. In a positron emission tomography study, a gina attacks and in the consumption of nitrates (p ⬍
redistribution of myocardial flow in favor of ischemic 0.05).
Neurotherapeutics, Vol. 5, No. 1, 2008
 SPINAL CORD STIMULATION 89

Five studies are reported to be prospective but without low-intensity stimulation for several hours per day for
matched controls.45,50 –53 Each of these revealed signifi- prophylactic purposes.65 Recently, a randomized control
cant benefit from spinal cord stimulation. The benefit study demonstrated improvement in functional status and
indices ranged from reduction in angina attacks and de- symptoms in treatment arms with conventional or sub-
creased nitrate consumption to decreases in NYHA grade threshold stimulation, compared with a low-output pla-
and improvement in NHP grade. cebo treatment arm.66 This is the first blinded study in
The concern whether stimulation can conceal an acute which stimulation below the sensory threshold for par-
myocardial infarction was addressed by Andersen et al.54 esthesia demonstrated therapeutic efficacy, thus eliminat-
They reported on 3 out of 45 patients treated with SCS ing the possibility of a placebo effect.
for anginal pain who survived myocardial infarction. All The success of the procedure ultimately will be deter-
three patients noticed the pain to be different and unre- mined by cardiologists. The question arises of the actual
lieved with SCS and all patients correctly guessed that number of patients who continue, despite all treatment
the pain was due to myocardial infarction. The authors modalities (including coronary bypass), to have symp-
concluded that SCS for treatment of anginal pain does toms of such magnitude as to require a spinal cord stim-
not seem to conceal acute myocardial infarction. Ander- ulator. As with the indication of SCS for peripheral vas-
son followed this up in 1994, further concluding that cular disease, European physicians have demonstrated a
neurostimulation does not conceal the pain of an acute substantially greater interest in the modality than U.S.
myocardial infarction.55 In particular, SCS reduced the physicians. Similar to other applications, a substantial
severity of anginal attack but did not suppress conduc- amount of data from well-controlled clinical studies will
tion and perception of the cardiac pain signals that act as be necessary before the SCS procedure for angina pec-
alarm signals of cardiac distress.56 Murray et al.57 have toris will be fully endorsed by the medical community in
shown that SCS for refractory angina is effective in the United States.
preventing hospital admissions without masking isch-
emic symptoms or leading to silent infarction. Chronic critical limb ischemia and pain
The mechanisms of action of SCS are unclear. There Cook and Weinstein67 were the first to suggest, in
may be homogenization of myocardial blood perfusion 1973, that the indications for SCS might extend beyond
with SCS, and that this reduces myocardial ischemia.44,58 intractable pain control. They observed a group of pa-
Another study has demonstrated that SCS improved tients with multiple sclerosis who underwent SCS to treat
heart muscle lactate metabolism and oxygen demand and their chronic pain. Unexpectedly, the patients experi-
blood flow in the coronary sinus.59 Other studies show enced not only pain relief but also an improvement in
that SCS does not affect variability in heart rate or car- mobility and in sensory and bladder function. Cook et
diac arrhythmias.40,60,61 Hautvast et al.62 found no sig- al.68 noted apparent improvement in lower limb blood
nificant changes in heart rate variability after 6 weeks flow, and subsequently used SCS in patients whose pri-
and concluded that heart rate variability via autonomic mary problem was peripheral vascular disease (PVD).
modulation may not be the explanatory mechanism of He demonstrated relief of rest pain, increased skin tem-
action. perature, improved plethysmographic blood flow, and
Because the relation between pain and myocardial healing of small cutaneous ulcers. Subsequently, Meglio
ischemia has not been fully clarified, we do not know et al.69 in 1981 reported pain relief and ulcer healing in
whether the pain relief is due to direct depression of a patient with advanced peripheral arterial insufficiency.
nociceptive signals in the spinal cord or whether there is In 1988, Jacobs et al.70 published clinical evidence that
secondary gain from a reduction in the ischemia.63,64 A SCS improved the microcirculation as measured by cap-
significant amount of work by Foreman18 has shown that illary microscopy.
dorsal column stimulation inhibits the activity of spino- Klomp et al.71 randomized 120 patients with critical
thalamic tracts cells evoked by activation of the cardiac painful limb ischemia to receive either best medical ther-
sympathetic afferents or by intracardiac bradykinin. On apy alone or SCS in conjunction with best medical ther-
the other hand, the effects of stimulation might be equiv- apy. At a mean follow-up of 19 months, there was no
alent to those of a sympathectomy and may act by pro- significant difference in pain score improvement be-
ducing a prolonged inhibition of the hyperactive sympa- tween the two groups. Conversely, from a similar study
thetic system. Such mechanism has been shown in which 51 patients were randomized to receive either
experimentally in the rat by Linderoth et al.20 oral medication alone or SCS with oral medication,
The most appropriate electrode location for the treat- Jivegård et al.72 reported a significant improvement in
ment of angina pectoris is most likely the lower cervical pain scores of the SCS-treated group, compared with the
and upper thoracic region, although some have reported non-SCS group (p ⬍ 0.01).
successful higher cervical placements.47 Another consid- Four reported studies without matched controls reveal
eration is continuous versus cyclical use of SCS. In prac- an overall success rate of 78% (n ⫽ 271).73–78 Analysis
tice, patients using SCS for angina pectoris often use of seven retrospective studies found an overall success
Neurotherapeutics, Vol. 5, No. 1, 2008
90 FALOWSKI ET AL.

rate of 76% (n ⫽ 308).29 A review of the European scribed a patient treated for irritable bowel syndrome
literature shows that 70 – 80% of patients achieved sig- who was developing escalating pain and diarrhea. Al-
nificant (⬎75%) pain relief, and many other large studies though the pain treatment eventually required intrathecal
have been reported on the long-term results of SCS on therapy, there was a significant reduction in the amount
pain and ulcer healing.73–78 of diarrhea. The patient underwent a 2-week trial and
In a prospective randomized trial by Guarnera et al.,79 subsequently had implantation at the T8 level. In the first
comparison of the effectiveness of SCS versus distal 6 months, there was a subjective decrease in pain from
arterial reconstruction demonstrated a more favorable 9/10 to 2/10, with only two diarrheal episodes and sig-
outcome with SCS (72%) than with distal arterial recon- nificant reduction in pain medications. There was some
struction (40%). In a Cochrane review80 looking at six return of pain after 10-month follow-up, requiring in-
studies of SCS versus conservative treatment, it was crease in pain medications, but the significant reduction
determined that limb salvage after 12 months was sig- in diarrhea remained.
nificantly higher in the SCS group, and that significant In the largest series to date, Khan et al.89 reported on
pain relief occurred in both treatment groups but was nine patients with refractory abdominal pain. Five of the
more prominent in the SCS group. nine patients had nonalcoholic pancreatitis, three had
The mechanisms of action are unclear. The most likely presumed abdominal wall neuromas from frequent ab-
mechanism responsible for increased blood flow in sub- dominal surgery, and the last had postsplenectomy pain
jects with peripheral vascular disease is inhibition of the after trauma. At 6- to 8-month follow-up, with placement
sympathetic system. This phenomenon occurs within the of the leads at the T5–7 level, all patients had a signifi-
spinal cord at the local level and is not related to anti- cant improvement in VAS scores, as well as decreased
dromic activation of afferent fibers. Suprasegmentary in- narcotic use.
fluence on medullary vegetative centers does not need to Tiede et al.90 described treatment of refractory abdom-
be invoked. The possible role of locally released vaso- inal pain in two patients. Both patients had a significant
active peptides awaits elucidation. Whether the effects history including multiple abdominal surgeries and failed
on pain and blood flow are due to the same mechanisms conservative measures. Each patient had an element of
is unknown, although some evidence suggests that pain postprandial abdominal pain with associated nausea and
relief is secondary to the microcirculatory changes. Mul- vomiting. In both patients, the leads were placed at the
tiple mechanisms may be operating simultaneously.81– 85 T2 level with significant improvement in pain, decreased
narcotic use, and increased functioning, such as return to
Abdominal and visceral pain syndromes work. Kapur et al.91 recently described relief of abdom-
Approximately 20% of the population in the United inal pain associated with colchicine intolerance or resis-
States have abdominal pain. There are many etiologies tance in patients with familial Mediterranean fever, with
for abdominal pain, including gastrointestinal, genitouri- placement of the electrodes at the lower thoracic levels.
nary, musculoskeletal, and nervous system. Treatment More recently, studies have looked at the treatment of
modalities have included cognitive– behavioral, physical, visceral pelvic pain with reference to the dorsal columns
and pharmacological therapies. Other more invasive and spinal cord stimulation. Kapural et al.92 reported on
therapies include celiac plexus blocks and celiac ganglia the value of neurostimulation for chronic visceral pelvic
destruction. Some studies have demonstrated some lo- pain in six women with the diagnosis of long-standing
calization in the spinal cord for visceral pain secondary pelvic pain. These patients had a history of endometrio-
to malignancy. Midline myelotomy through the dorsal sis, multiple surgical explorations, and dyspareunia. At
columns at the level of T10 has shown success in eight an average follow-up of 30 months, there was a signifi-
patients with refractory pelvic cancer pain. This was also cant decrease in the VAS score and an average of ⬎50%
demonstrated in animal studies, in which dorsal column pain relief, with a decrease in opiate use.
activity was observed in pelvic visceral nociception.86 Visceral innervation follows the embryologic origin
Initially, there was lack of evidence for the application and location of the viscera and is arranged in viscer-
of spinal cord stimulation for visceral and somatic pain, otomes, analogous to cutaneous dermatomes.93 The vis-
secondary to the belief that nociceptive pain could not be cera obtain their innervation via the sympathetic and
modulated via stimulation. Several initial studies have parasympathetic pathways. The parasympathetics carry
since demonstrated the benefit of SCS in abdominal vis- their afferents to anterior and posterior vagal trunks and
ceral disease. Ceballos et al.87 demonstrated reduction in are therefore not as amenable to spinal cord stimulation.
pain scores and decrease in narcotic use in a patient The sympathetics carry nociceptive information from the
treated for mesenteric ischemia. Trial stimulation was 13 viscera to spinal nerve roots, which makes them a more
days, with implantation followed for 12 months and the viable target. The sympathetic afferents in the lower six
electrode placed at T6. The patient had only two small thoracic and the upper three lumbar spinal segments have
pain recurrences in that period, one of which was when been shown to transmit painful impulses from the vis-
the stimulator was stopped. Krames and Mousad88 de- cera.94
Neurotherapeutics, Vol. 5, No. 1, 2008
 SPINAL CORD STIMULATION 91

RELEVANT ANATOMY Contemporary percutaneous electrodes are slim elec-

trodes, only a few millimeters in diameter, that contain
Understanding the somatotopy of the spinal cord is
four or eight contacts with various spacings (referred to
paramount in understanding the technical aspects of im-
as either quadripolar or octapolar electrodes). Choosing
plantation. A basic tenet of SCS is to create an overlap-
the particular electrode entails deciding how many seg-
ping of paresthesia and pain region. To this end, corre-
lation of the somatotopy and the level of the spinal cord ments of the spinal cord are to be covered, with larger
is necessary. Barolat et al.95 have published extensively spacing allowing broader coverage. Alternatively, closer
on the mapping of the human spinal structures. A data- spacing allows better steering and electric field shaping.
base was created to suggest areas of sensory response to Additionally, multiple parallel electrodes and different
dorsal spinal cord stimulation. configuration matrices can be constructed, which can
High cervical regions such as C2 can cover the pos- create extremely focused electrical fields. The general
terior occipital region, and occasionally the lower jaw. trend is to use one or two quadripolar electrodes for limb
C2– 4 stimulation will provide coverage of the shoulder, pain and one or two octapolar electrodes for axial pain.
and stimulation in the lower cervical region (e.g., C5– 6) Even insertion of three electrodes is being explored for
will provide coverage for the entire hand. To cover the better steering of current.33 These percutaneous elec-
anterior chest wall or the axilla, an electrode toward C7 trodes also come in varying lengths, and these differ by
is necessary. manufacturer. At times, extensions cables are necessary
More commonly, an implanter will seek to cover the to bridge the distance from the spinal entry point to the
lower extremities. Lateral placement at T11–12 will pocket in which the battery will reside.
cover the anterior thigh, and placement at T11–L1 can
The technique of percutaneous implantation
cover the posterior thigh. Coverage of the foot as a whole
The patient is positioned in a comfortable prone posi-
can be achieved along these same areas, but it becomes
tion on a fluoroscopy table. A pillow underneath the
more difficult to cover the sole of the foot. Alternatively,
abdomen may create some kyphosis, which can facilitate
for coverage of the sole of the foot, a patient may require
electrode insertion.
insertion on the lumbar L5 or S1 nerve roots. Low back
The level of electrode insertion is guided by several
pain is very difficult to cover, because midthoracic stim-
factors. A fundamental consideration is that several cen-
ulation can affect the chest and abdominal wall. In the
timeters of the lead have to lie in the epidural space to
authors’ experience, the best localization has been with
assure maximal stability of the electrode and minimize
midline placement at T8 –9.
unwanted migration. To assure this, insertion must take
Finally, most patients prefer stimulation of the dorsal
place at least two spine segments below the desired
column from electrodes closer to the midline. Laterally,
target. For cervical placement, one must be aware of the
placed thoracic electrodes are more likely to stimulate
cervical cord enlargement and, when possible, electrode
the thoracic nerve roots and result in painful stimulation.
insertion should be performed below the T1–2 level.
Some surgeons advocate upper thoracic placement of
EQUIPMENT: ELECTRODES AND PULSE percutaneous electrodes through a limited spinous pro-
GENERATORS cess removal, to minimize the potential risk of spinal
The implanting physician should become familiar with cord damage. For practical purposes, implantation for
the various implantable technologies available for SCS. low-back problems necessitates electrode insertion at
These include trial percutaneous electrodes, permanent T12–L1 or L1–2 and implantation for an upper extremity
percutaneous electrodes, permanent plate electrodes, to- target requires insertion at T2–3 or T3– 4.
tally implantable rechargeable and nonrechargeable The fluoroscopy equipment must be ready to function
pulse generators (IPG), and RF-driven pulse generators. in both the anterolateral and lateral planes at the time of
needle insertion. The Tuohy needle is inserted with as
Percutaneous electrodes shallow an angle as possible. In the thoracic area, this can
Percutaneous electrodes (or wire electrodes) are par- be accomplished with either a midline or paramedian
ticularly appealing. Percutaneous electrodes can be in- approach; in the upper lumbar area, a paramedian ap-
serted without much dissection and offer a substantial proach is required. A steep angle increases the risk of
advantage when one is performing a trial to assess can- electrode fracture.
didacy for a permanent implant. After the trial period, the Several methods are available to identify the epidural
temporary percutaneous electrode can easily be removed space. Tactile feedback is important, but cannot be solely
in the implanting physician’s office. During implanta- relied upon. The most approach is the loss of resistance
tion, these electrodes can be advanced over several seg- method using a low-friction glass syringe. We prefer to
ments in the epidural space, allowing testing of several inject a small amount of air. Fluid injected in the epidural
spinal cord levels to assess for optimal electrode posi- space may later be aspirated through the needle and give
tion. the false impression of being in the subarachnoid space.
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92 FALOWSKI ET AL.

After multiple passes at one spine level have been per-

formed, the loss of resistance method may lose its reli-
ability. Inserting a Seldinger wire through the needle can
provide invaluable information as to the degree of pen-
etration into the spinal canal. If the needle tip is in the
interspinous ligament and has not penetrated the liga-
mentum flavum, the wire cannot be advanced. The wire
can be advanced only if the needle tip is in the paraspinal
muscles or within the spinal canal. The pattern of ad-
vancement and the location of the wire under fluoro-
scopic imaging can further clarify its position.
Once the electrode is in the spinal canal, one has to be
certain that it is positioned in the epidural space and not
within the subarachnoid space. Even though it may seem
obvious and easily recognizable, this can become a very
difficult task and require multiple attempts at needle
placement. This is particularly true if the arachnoid has
been previously pierced and cerebrospinal fluid has es-
caped and pooled in the dorsal epidural space. In the
subarachnoid space, much less resistance is encountered
when moving the electrode, particularly for lateral move- FIG. 1. Titan anchor (Medtronic, Minneapolis, MN; used with
ments. The wire seems almost to be floating and under- permission).
goes large shifts of direction, whereas in the epidural
space movements are more discrete and obtained only around the dural sac. Absolute confirmation of the ven-
with specific manipulations. The same type of wire or tral location arises from the stimulation generating vio-
electrode movement can, however, be experienced epi- lent motor contractions or observation in the lateral plane
durally if the dural sac has significantly collapsed sec- that readily discloses the anterior position of the elec-
ondary to loss of cerebrospinal fluid. Electrical stimula- trode tip. If more than one electrode is inserted, it is wise
tion will clarify the position, because a subarachnoid to insert the other needles before inserting the electrodes.
placement can elicit motor or sensory responses at much Needle insertion might shear an already implanted elec-
lower thresholds than epidural placement. trode. Besides, it is often possible to insert two electrodes
When the epidural space is satisfactorily identified, the simultaneously and advance them synchronously in the
electrode is gently inserted under fluoroscopic guidance epidural space while maintaining their relative position
in the anteroposterior view. Removal of the electrode and spacing.
once it has been inserted through the tip of the needle has Once in place, the electrode must be secured to the
to be accomplished without stripping the insulation on interspinous ligament to minimize dislodgment. An X-
the lead. If the electrode does not slide with minimal ray image is obtained to document electrode level and
resistance, the needle and the electrode should be re- position. We have found that securing loops at the elec-
moved together. Every time the electrode is withdrawn trode insertion site serves to relieve the strain and re-
through the needle, it should be inspected for minute duces migration during bending. Frequently, anchors are
breaks in the insulation, which would necessitate its dis- used to secure the electrode and the implanter should
posal. Alternatively, a sleeve can be inserted over the remember that anchoring is a two-step process: first se-
guide wire in the epidural space. The guide wire is then curing the anchor to the electrode and then securing the
removed and the electrode is inserted through the sleeve. anchor to the fascia. At present, there is only one anchor
This reduces the risk of shearing the electrode during available that can be detected on radiographs: the TITAN
manipulation. The electrode is then steered in the epi- anchor (Medtronic, Minneapolis, MN) (FIG. 1).
dural space to the desired location. Should the targeted A major disadvantage that has been cited with percu-
location prove to be less than two spinal segments from taneous electrodes is their tendency to migrate. This is
the electrode insertion, the electrode should be with- related to their inherent flexibility, necessary for inser-
drawn and repositioned at a more caudal level. tion through a Tuohy needle, and to their cylindrical
Frequently, the electrode curves around the dural sac shape, which does not prevent migration even months
and ends in the ventral epidural space. In the anteropos- after implantation. Some percutaneous electrodes require
terior projection this might be indistinguishable from a a stiffening stylet for introduction. Also, percutaneous
proper midline dorsal location. A gentle lateral curve of electrodes are less energy efficient than plate electrodes.
the electrode shortly after its entry in the epidural space The electrical current is distributed circumferentially
should arouse suspicion that it is directing ventrally around the electrode and is expected to result in greater
Neurotherapeutics, Vol. 5, No. 1, 2008
 SPINAL CORD STIMULATION 93

FIG. 2. Slim-line electrode (ANS–Advanced Neuromodulation Systems, Plano, TX; used with permission).

shunting of current. Patients with percutaneous leads also trical stimulation. We advocate plate electrodes as the
describe a greater positional variance in their paresthesia. only option in the case of previous spine surgery at the
Recently, Advanced Neuromodulation Systems (ANS, implant levels.
Plano, TX) has introduced a slim-line plate-type elec- Plate electrodes come in many sizes, shapes, spacing,
trode that can be inserted percutaneously. The broader and configurations. There are single-column and dual-
electrode base provides a surface such that fibrosis column electrodes. As with percutaneous leads, there are
should lessen the risk of caudal electrode migration. The varying lengths and shapes (such as curved leads and
slimmer profile of the electrode might also have advan- hinged leads), all designed to help facilitate insertion and
tages in the cervical spine, where spinal cord compres- tailor the electrode selection to the patient.
sion could be an issue. Finally, the design emulates that There is some literature describing the advantages of
of a miniplate lead, in that the contacts are on one side plate leads. North et al.96 have compared plate and per-
with the other side being insulated. This will be more cutaneous electrodes. Laminectomy electrode placement,
energy efficient than a percutaneous lead, which allows although more invasive than percutaneous placement,
delivery of current circumferentially (FIG. 2). yielded significantly better clinical results in patients
Plate electrodes with failed back surgery syndrome at up to 3-year fol-
Plate-type electrodes (also known as ribbon electrodes, low-up. Clinical success was defined as ⱖ50% pain re-
paddle electrodes, or laminotomy electrodes) require a lief and patient satisfaction with treatment. Secondary
surgical procedure, laminotomy, and implantation under outcome measures were ability to perform various activities
direct vision.28 Implantation under direct vision may be of daily living, neurological function, and analgesic use.
safer in the upper thoracic and cervical areas, where there There is some theoretical evidence that shaping of the
is a risk of damaging the spinal cord with the large-bore electrical field is possible with even more complex elec-
Tuohy needle. Most implants can be done through a skin trode arrays. Holsheimer et al.97 concluded that the trans-
incision between 2.5 and 4 cm long, depending on the verse tripolar system enabled finer control of paresthesia.
size of the patient and spinal anatomy. The amount of Electrical field steering could change the paresthesia area
bony removal is usually minimal. completely. When the transverse tripolar configurations
Multiple arrays or different electrode configurations are used, the threshold for stimulation of dorsal roots is
can be also constructed with plate electrodes. The main higher, compared with the dorsal column threshold. This
advantage of plate electrodes lies in their more inherent results in a wider therapeutic range, wider paresthesia
stability in the dorsal epidural space and lesser propen- coverage, and a greater probability to fully cover the
sity to migrate. Some preliminary data by North et al.96 painful area with paresthesia.
also suggest a broader stimulation pattern and lower One must bear in mind that the increasing number of
stimulation requirements with plate electrodes. Plate contacts brings with it a significant increase in power
electrodes are more energy efficient in delivering elec- consumption. The complexity of programming rises in
Neurotherapeutics, Vol. 5, No. 1, 2008
94 FALOWSKI ET AL.

even a greater magnitude. With 2 contacts, the total patient fully in the prone position under general anesthe-
number of configurations possible are 8, with 4 contacts, sia. Fluoroscopy should be used to identify the same
64; and with 8 and with 16 contacts, it increases expo- level where the active contacts were placed during the
nentially to reach a number in the millions. trial. A laminotomy is performed approximately one
Two basic positions can be used for the laminotomy level below this point, to allow the plate electrode to
and implantation: prone or semilateral. The prone posi- reach up to the intended level. After placing the elec-
tion allows a more intuitive understanding of the spatial trode, intraoperative stimulation with electromyographic
relations and is one that more surgeons are familiar with. correlation will be able to detect stimulation in the ex-
In this position, however, it can be difficult to obtain tremity and lateralization of the electrode. We stimulate
adequate sedation for the surgical exposure and also the electrode with 5-Hz stimulation at ⬎310 ␮s pulse
maintain the airway. In the semilateral position, the pa- width and ramp up the amplitude until electromyo-
tient lies comfortably in a park bench–type position, graphic signal changes are detected. Bilateral extremity
allowing access to both the spine and the flank, abdomen, stimulation suggests midline placement, and early root
or buttock for the implant of the pulse generator. The onset implies too lateral a placement. We have had in-
patient is asked to place him or herself in the most stances in which the physiological midline differed from
comfortable position. If the pain is predominantly on one the anatomic midline; in such cases, we are more apt to
side, the patient is asked to lie on the less affected side. rely on the intraoperative physiology. When treating ax-
In this position, airway management is safer than in the ial symptomatology, the lead is placed to straddle the
prone position, and the anesthesiologist is more comfort- midline. For patients with unilateral pain, the lead is
able in keeping the patient deeply sedated. Given the placed so that one array is on the side of the pain and the
variable degree of rotation of the body, however, it can other is on the midline.
be difficult for the surgeon to determine the location of
the midline. This might constitute a significant problem Rechargeable and nonrechargeable pulse generators
in the cervical area. and radio-frequency receivers
The planned level is localized either with fluoroscopy Electrical stimulation consists of rectangular pulses
or with a plain X-ray with metallic markers placed on the delivered to the epidural space through an implanted
skin at the level of the planned incision. In a thin indi- electrode via a power source. Two basic types of systems
vidual, the incision is ⬃1 inch in length (⬃2.5 cm); even are currently available: an Internal Pulse Generator (IPG;
in large individuals, the incision seldom needs to be more also called the battery) or a radio-frequency (RF) cou-
than 2 inches long. Different considerations apply if one pled pulse generator with an implantable receiver. The
is implanting through a level previously operated on. latter has largely fallen out of favor, because of the
For cervical placement, the patient is placed in the inconveniences of the external power source. The advent
semilateral position with the neck slightly flexed. Even of the totally implantable, rechargeable pulse generator
with a short skin incision, one can reach three to four has surmounted the power requirement issues, which
levels by extending the inside dissection and stretching were previously the real RF advantage (FIG. 3).
the skin edges with a Gelpi retractor. The neck should be The totally implantable pulse generator contains a lith-
flexed, but not excessively rotated laterally; even though ium battery. Activation and control occur through an
some neck rotation is inevitable, extreme rotation sub- external transcutaneous telemetry device. The IPG can
stantially increases the level of difficulty. be turned on and off through a small controller which the
Subperiosteal dissection is usually limited to the upper patient can carry. The controller also allows some control
half of the spinous process inferior to the addressed over the stimulation parameters. More extensive control
ligamentum flavum and to the whole spinous process of the unit can be achieved through a small portable unit
superior to it. Parts of the superior spinous process are that can be programmed by the physician. Life span of
incrementally removed until the ligamentum flavum is the battery varies with usage and with the parameters
exposed. In the lower thoracic or upper lumbar area, this used (i.e., voltage, rate, and pulse width). Most patients
usually results in removal of the inferior one third of the can expect the battery to last, under average usage, be-
spinous process. In the midthoracic area, due to the acute tween 2.5 to 4.5 years. Available lithium-powered pulse
angle and significant overlapping of the spinous pro- generators allow stimulation with fine resolution incre-
cesses, the whole spinous process must be removed. ments of 0.05 V and with varying rates and pulse widths.
After removal of the ligamentum flavum, the electrode or Replacement of the battery requires a surgical procedure,
electrodes are inserted in the dorsal epidural space; the one that is usually performed on an outpatient basis.
electrode position is then confirmed with fluoroscopy A particular IPG is selected based on many variables.
and test stimulation is performed with the patient awak- From a practical standpoint, the first and foremost reason
ened and able to report where he or she is feeling par- might be the size of the patient. Although larger batteries
esthesias. will have longer life, the site of insertion of the IPG
Alternatively, the electrode can be placed with the (either the buttock, abdomen, or the subclavicular re-
Neurotherapeutics, Vol. 5, No. 1, 2008
 SPINAL CORD STIMULATION 95

FIG. 3. Rechargeable batteries.

gion) is often the source of significant patient complaint. patient to wear the external system in order to receive the
We prefer to implant the IPG in the buttock because of stimulation. RF-driven systems can deliver stimulation
ease in tunneling from the electrode insertion. Further- with rates up to 1400 Hz, and can be customized to
more, with a patient placed prone for electrode insertion, deliver high power levels.
there is no repositioning required to reach the buttock RF systems involve the inconvenience of having to
region. We identify three bony prominences (the poste- wear the antenna and the radio receiver. The problem
rior superior iliac crest, the greater trochanter of the might go beyond pure inconvenience for individuals who
femur, and the apex of the iliac crest) and implant the have handicapped motor function in the upper extremi-
IPG in the lateral aspect of this triangle (FIG. 4). ties and cannot properly go through all the steps required
RF-driven systems consist of a passive receiver, im- to make the external unit function properly. Other pa-
planted subcutaneously, and a transmitter that is worn tients, particularly those who have reflex sympathetic
externally. An antenna applied to the skin in correspon- dystrophy (RSD), may not tolerate an antenna taped to
dence of the receiver is connected to the transmitter, the skin. Obtaining adequate contact of the receiver with
which sends the stimulation signals transcutaneously. the skin may be difficult secondary to swelling at the site.
For the system to function, the transmitter has to contain The equipment cannot be worn while swimming or
charged alkaline batteries and the antenna must make showering, and severe perspiration, as with exercise and
adequate contact with the receiver. This requires the physical therapy, might make proper contact of the an-

FIG. 4. Battery placement. In our technique for the placement of the battery in the buttock, the bony prominences are marked: the
greater trochanter (lateral femur), the apex of the iliac crest, and the posterior superior iliac spine (PSIS). A triangle is created, and the
battery incision is made parallel to the top rung of the triangle.

Neurotherapeutics, Vol. 5, No. 1, 2008

96 FALOWSKI ET AL.

TABLE 1. Specifications for Three Rechargeable Batteries

Precision Plus Restore Advanced Eon

Volume, cm3 22 39 42
Dimensions, mm 55 ⫻ 45 ⫻ 11 65 ⫻ 49 ⫻ 15 59 ⫻ 58 ⫻ 16
Weight, g 36 72 75
Stimulation Output Multiple current sources Constant voltage Constant current
Amplitude, mA or V 0-20 mA 0-10.5 V 0-25.5 mA
Frequency, Hz 2-1200 2-130 2-1200
Pulse Width, ms 20-1000 60-450 50-500
Battery Capacity, mA h 200 300 325
Recharging Type Cordless Cordless Connect to Outlet
Wireless Communication ⱕ30 (ⱕ76) ⱕ4 (ⱕ10) not applicable
Distance, inches (cm)
Maximum Recharge 2 1 2.5
Depth, cm
Manufacturer Boston Scientific, Natick, MA Medtronic, Minneapolis, MN Advanced Neuromodulation
(http://www.bostonscientific.com) (http://www.medtronic.com) Systems, Plano, TX
(http://www.medical.com)

tenna problematic. Furthermore, the patient has to re- of the electrode, has also been reported in a number of
place batteries on a regular basis and make sure that cases.29
proper coupling exists between the antenna and the re-
ceiver at all times. What one loses in convenience, how- CONCLUSIONS
ever, is gained in power and flexibility. Currently, only
RF systems can provide a stimulation rate up to 1400 Hz. The treatment of chronic pain remains challenging.
This might be beneficial in some patients with neuro- Spinal cord stimulation has been performed for more
pathic chronic pain syndromes,98 as well as in patients than 30 years, and slow but steady progress with this
with extrapyramidal motor disorders. technology has been made. As the equipment and stim-
Rechargeable systems have now become available. A ulation parameters are improved, selection criteria have
Medtronic device, known as the Restore rechargeable been better defined and are slowly being expanded. More
neurostimulation system, uses a battery with an esti- important, experience in the technique and the equipment
mated 9-year total life span. It takes ⬃6 hours to fully has made SCS a much more reliable and safe modality.
recharge the batteries. The Advanced Neuromodulation As with all the modalities performed for chronic pain
Systems Eon device has a battery life that is currently management, its results are favorable. It is important to
estimated at 7 years. The Boston Scientific Precision remember that the goal of neurostimulation is to reduce
device has a battery life estimated at 5 year. A detailed pain, rather than to eliminate pain. SCS has been shown
comparison of the features of the rechargeable batteries to have a 50% improvement in pain relief. Very few
is given in Table 1. other invasive modalities can claim this success rate with
a few years of follow-up.
Careful follow-up of patients is necessary for success-
COMPLICATIONS ful long-term satisfaction. Equipment-related problems
With the proper expertise, permanent complications of can arise at any time after implantation, such as discom-
SCS are rare.70 The most serious complication, which is fort at the pulse generator or radio receiver site, electrode
shared with any type of spine surgery, is paralysis or breakage or migration, infection, and the like, and an
severe neurological deficit. This can occur during spinal open dialog with patients is vital for the continuing suc-
cord stimulation procedures, both with percutaneous and cessful implementation of the modality. Spinal cord
plate electrodes. Infection of the implanted hardware has stimulation has earned a firm and well-established role in
occurred at a 3–5% rate.99,100 Persistent pain at the im- contemporary chronic pain management.
plant site has been seen in ⬃5% of patients.99,100 Recal-
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