Introduction

Chronic pain poses a serious burden in American healthcare that ranks among cancer, heart disease, and diabetes. An estimated 100 million adults suffer from persistent pain at any point in time, leading to striking estimates ranging from $100 billion to $300 billion in annual economic losses. As opioids forfeit their place in the evidence-based treatment algorithms for neuropathic pain, pharmacotherapies available to chronic pain providers and patients become increasingly limited, and interventional approaches are more important than ever. Neuromodulation represents an essential, evolving approach to treating the chronic pain patient.

Interestingly, the concept of alleviating pain with electrical stimulation is not new and dates back to 2500 bc , when the ancient Egyptians attempted to treat pain by applying electrogenic fish to painful sites. However, the first written medical reports of this practice came from the ancient Romans, who credited the black torpedo fish with relieving pains through electrical discharges.

In 1965 Melzack and Wall proposed the gate control theory of pain, which links the central and peripheral nervous systems in the complex perception and processing of pain. It hypothesizes that Aδ fibers, C fibers, and Aβ fibers all synapse at the dorsal horn, where they must wrestle to pass through a single-lane physiologic “gate.”

Aδ fibers and C fibers transmit pain signals from mechanical, thermal, or chemical nociceptive inputs. Aδ fibers are myelinated and medium diameter, whereas C fibers are unmyelinated and small diameter. Thus Aδ fibers transmit pain more quickly than C fibers (40 mph vs. 3 mph), creating two stages in the pain perception separated by a brief interval. The immediate, sharp pain felt by an individual after touching a hot stove is the result of Aδ-fiber firing, whereas the deeper throbbing pain that persists after the hand is withdrawn is attributed to C-fiber conductance. Meanwhile, Aβ fibers are involved in transmission of touch, pressure, and proprioception and do not conduct pain.

Gate theory explains why rubbing over the site of a soft tissue injury or an insect bite may alleviate pain. If the “gate” is overwhelmed with nonpainful input from the Aβ fibers, it should fail to transmit input from the nociceptive Aδ and C fibers. Although the aforementioned description may be oversimplified, Melzack and Wall’s theory laid the foundation for spinal cord stimulation (SCS).

In 1967, 2 years after their theory was published, Norman Shealy successfully implanted a monopolar SCS lead in the intrathecal space near the dorsal column. The patient’s impressive neuropathic pain improvement brought neuromodulation into the limelight, and the field has evolved significantly ever since.

Mechanism of Action

The mechanism of SCS is only partially understood at this time, and more research is needed. SCS is used for a variety of indications, ranging from neuropathic pain to angina, and it has the ability to impact multiple organ systems by inducing changes in either the local autonomic nervous system or viscerosomatic reflexes. It is likely that the mechanism of pain relief varies according to the pathology being treated. For instance, SCS may work differently to relieve neuropathic pain than it does for ischemic pain. In neuropathic pain the suppression of central excitability is likely the key. However, for ischemic pain, vasodilation and inhibition of sympathetic outflow are more likely factors.

Animal studies of SCS have allowed for some hypotheses on pathophysiology. Because neuropathic pain is the most common current indication for SCS, animal model findings regarding neuropathic pain and SCS are detailed next.

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  Hyperexcitability of dorsal horn wide dynamic range (WDR) neurons after nerve injury has been found to be due to dysfunction of the local spinal gamma-aminobutyric acid (GABA) system and increased release of excitatory neurotransmitters in the dorsal horn (i.e., glutamate).

  
  
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  SCS inhibits dorsal horn WDR neuron excitability, increases release of GABA, and decreases release of glutamate.

  
  
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  SCS increases concentrations of serotonin, norepinephrine, and adenosine in the dorsal horn.

  
  
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  Segmental hypothesis: SCS requires the participation of only a few spinal segments, with minimal additive inhibitory signals from the brain.

  
  
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  Supraspinal hypothesis: A supraspinal pathway is activated. Dorsal horn neurons are inhibited by descending antinociceptive pathways.

Patient Selection and Preoperative Considerations

Appropriate patients for neurostimulation implant must meet the following criterion: (1) the patient has a diagnosis amenable to this therapy, (2) the patient has failed conservative therapy for at least 6 months, (3) all significant psychological issues have been ruled out, (4) a history of illicit drug abuse is not present, and (5) a trial has demonstrated pain relief. However, pure neuropathic pain syndromes are relatively less common than the mixed nociceptive/neuropathic disorders, including failed back surgery syndrome (FBSS) ( Fig. 73.1 ). In addition, many patients with chronic pain will have some depressive symptomatology, and psychological screening can be extremely helpful to avoid implanting SCS in patients with major psychological disorders. An interesting study by Olson and colleagues revealed a high correlation between many items on a complex psychological testing battery and favorable response to a trial of SCS. It is recommended that extensive risk/benefit discussion and support for patients are ensured prior to implementing an invasive procedure.

(A and B) Ideal candidates: failed back surgery syndrome/complex regional pain syndrome. Note the radicular versus axial pain pattern.

Courtesy Medtronic Inc.

To maximize the probability of improvements in functional outcomes as well as pain, part of the preoperative evaluation is establishment of functional goals. Doing so will reinforce to the patient that improvement in pain is not the primary endpoint, but rather it is the return of functional activity. Patients are often fearful that pain is indicative of damage, and this belief may limit their rehabilitation even after pain improves. Counseling on functional goals has the potential to question or even break a detrimental belief system.

Preoperatively, images should be reviewed carefully for optimal procedure planning. Consideration should be given to anatomic changes in the spine and variations. Epidural scarring, previous laminectomy, age-related changes, degenerated discs, and tight interlaminar spaces have the potential to vastly complicate the placement of percutaneous leads—and in some cases may even make it impossible. Severe spinal stenosis or the presence of surgical hardware may result in an obliterated epidural space, increasing the risk of complications from stimulator implant, including cord compression from an implanted lead.

Equipment

SCS is a technically challenging interventional/surgical pain management technique. It merits extensive training and understanding of neuroanatomy, surgical technique, and perioperative patient care. Collaboration between the pain physician and spine surgeon is advocated for optimal success with neurostimulation. SCS involves the placement of platinum alloy electrodes into the posterior epidural space to electrically stimulate the dorsal columns of the spinal cord. It is important for the interventional pain physician to understand the equipment options available.

The SCS electrodes are of two types: percutaneous leads versus paddle electrodes ( Fig. 73.2 ). Percutaneous leads are flexible, cylindrical polyurethane catheters characterized at the distal tip by platinum alloy electrode contacts of variable numbers, lengths, and spacing patterns (depending on model and make). These leads are placed into the epidural space via needle insertion under fluoroscopy and are popularly placed by interventional pain physicians because they are minimally invasive when compared with surgical paddle leads. Percutaneous leads generally range from 8 to 16 contacts. Due to their cylindrical design, they generate circumferential current flow. This may cause stimulation of dorsal structures, such as the ligamentum flavum, and thereby lead to uncomfortable sensations.

Neurostimulator leads: (left to right) percutaneous type to paddle type.

Courtesy SJM Inc.

The paddle leads are flat, wide, and rectangular at the distal end. They have insulation on one side and electrical pads on the other, with up to 32 electrode contacts. Current flow is unidirectional towards the cord. Therefore the electric field generated by paddle leads is more efficient than that of percutaneous leads, and paddle leads may avoid dorsal structure stimulation. However, these leads require a more invasive approach and are placed under direct visualization into the epidural space after a laminotomy or laminectomy by the spine surgeon. They have been linked in a large retrospective survey by Rosenow and colleagues to increased complications, fractures, and infections when compared with percutaneous leads. However, no adequate prospective studies comparing the percutaneous and paddle leads have been performed to date. Paddle leads may be chosen when anatomy, postsurgical changes, or epidural scarring make percutaneous lead placement difficult. In addition, paddle leads should be considered if lead revision is needed because of lead migration, if the patient experiences undesirable positional stimulation, or if more lead stability is required.

There are three types of power source options: primary cell implantable pulse generators (IPGs), rechargeable IPGs, and radiofrequency (RF) units ( Fig. 73.3 ). RF units, the first of implantable SCS devices, are not limited by battery life but require an external power source. The patient must tape an antenna to the skin on top of the device receiver. This is inconvenient and may result in skin irritation. Thus RF units are rarely used nowadays. Primary cell IPGs were introduced in 1980, based on pacemaker technology. Although they tend to be larger and have a shorter life span of 3–4 years, they are low maintenance because they do not require charging or interfere with daily activities. Rechargeable IPGs have been introduced and contain lithium ion cells with a life span of 9 years.

(A) Schematic view of an implanted pulse generator system.

Courtesy Medtronic Inc.) (B) Schematic view of an implanted radiofrequency spinal cord stimulation system. (Courtesy SJM Inc.) (C) Representative implanted pulse generator neurostimulation units with leads. (Courtesy SJM Inc.

Procedure

An SCS trial is conducted using fluoroscopy, under sterile conditions, and typically one or two leads are placed percutaneously. Prophylactic antibiotics are advocated for infection prevention, and the current recommendation is a cephalosporin, such as cefazolin. Clindamycin is recommended when the patient is allergic to β-lactams, and vancomycin is the agent of choice if the patient is methicillin-resistant Staphylococcus aureus (MRSA) positive. After a 14-gauge 9-inch Tuohy introducer needle, supplied with the kit, is percutaneously placed in the dorsal epidural space, using the standard loss of resistance technique, the SCS lead is introduced into the epidural space through the epidural needle ( Fig. 73.4 ). Once in the epidural space, the SCS lead is steered under fluoroscopic guidance into the posterior paramedian epidural space up to the desired anatomic location. Although two SCS leads, juxtaposed in the posterior epidural space, are most common and give superior stimulation options, single lead placement may suffice if adequate coverage of the painful area is achieved. For traditional SCS (excluding high-frequency SCS), short-acting sedatives are preferred, so that the patient can be readily awakened after the lead placement for evaluation of paresthesia coverage over the area of pain. After the successful epidural lead placement, the epidural needle is carefully withdrawn avoiding any lead migration. After the final lead position is confirmed by fluoroscopy, the lead is firmly anchored to the skin using nonabsorbable sutures and/or proprietary anchoring device and a sterile dressing is applied.

(A) Percutaneous lead placement: marking the interspinous level. (B) Percutaneous lead insertion. (C) Dual lead trial.

Courtesy Medtronic Inc.

When the patient returns after a trial of percutaneously placed leads, the dressing is removed, the sutures are clipped, and the leads are removed and discarded. If the trial is successful, new leads are placed and connected to an implanted IPG when the patient returns for the SCS implantation.

Alternatively, SCS trial leads can be placed through an open wound, over the dorsal spine, using the technique similar to percutaneous lead placement. However, while the original SCS leads are buried subcutaneously, only the tunneled extensions exit the skin. In this circumstance, if the trial is successful, during permanent implantation, only the extensions are discarded and the original trial leads are connected to the implanted generator. This method has the advantage of retaining the same lead position in a successful trial, but it adds an incision at the time of the initial trial, which increases postoperative pain confounding the trial interpretation. Furthermore, trial leads implanted through an open incision may have greater risk for infection than the straight percutaneous method.

A careful trial period of 5–7 days is advocated to decrease the risk of a failed implant. Patients are encouraged to pursue normal activities, with the exception of aggressive bending or twisting to prevent lead migration. Despite careful patient selection and introduction of improved and redundant multilead systems, which gives a host of varied stimulation options, clinical failures of SCS trials remain all too common. Prudent pain practitioners therefore must critically evaluate the trial outcomes and adhere to strict selection criteria. Even though most practitioners consider 50% or more of pain relief as indicative of a successful trial, other factors, such as functional improvement, increased activity level, and reduced analgesic intake should also be taken into account.

If the trial succeeds, the patient returns for permanent lead and generator placement. The technical challenges of permanent lead placement depend on (1) proper fixation and (2) lead redundancy. Consistent and reliable stimulation depends on applying an electric field over a small area of the spinal cord. SCS leads have a limited capacity for stretch, and certain body movements can stretch leads significantly and prompt lead migration. Although sclerotic changes in the epidural tissue surrounding the implanted leads stabilize them in the long run, during the acute phase, proper anchoring is a major factor in successful lead placement ( Fig. 73.5 ). In the event of minor lead migration, electrode redundancy is used to accommodate for minor shifts by using alternative electrodes to accommodate the desired electric field.

Anchoring the lead.

Courtesy Medtronic Inc.

The generator unit is generally implanted in the lower abdominal area or in the posterior superior gluteal area ( Fig. 73.6 ). For cervical or occipital leads, generators are often placed in between the scapula. In general, generators should be in a location the patient can access with his or her dominant hand for adjustment of settings or charging.

(A and B) Permanent implant: pulse generator internalization.

Courtesy Medtronic Inc.

Placement of leads in the cervical spine should account for the fact that the spinal cord in the mid-to-lower cervical spine has high mobility, making it difficult to maintain constant paresthesias with tonic stimulation as the neck changes position. This increased mobility also increases lead migration risk in the cervical region. In general, cervical lead placement involves entry into the epidural space between T1 and T4, ¹¹ with percutaneous leads advanced to the appropriate level in the cervical spine.

In the thoracic region the spine is more fixed and immobile. This allows less vulnerability to lead migration. At the T5 level the CSF diameter is largest dorsally, and the spinal cord is smallest. Thus stimulation thresholds are higher and postural changes are problematic when leads are placed at this level.

Leads may be placed in the lumbar spine, and electrical currents directed to conus medullaris or cauda equina, when attempting to provide relief for pelvic, sacral, or foot pains. With spinal nerves free-floating at these vertebral levels, stimulation is vulnerable to patient movement and can be suboptimal.

Complications

Complications with SCS range from minor problems, such as lack of appropriate paresthesia coverage, to devastating complications, such as paralysis, nerve injury, and death. Overall complication rates from SCS range from 28% to 42%. In a systemic review by Cameron the most common complication was found to be lead migration or breakage, which occurred in 22% of implanted cases. Studies by Barolat and May reported lead revision rates due to lead migration of 4.5% and 13.6% and breakage of 0% and 13.6%, respectively. The generator can also be a source of revision if changes in body habitus affect the source position.

Studies have demonstrated superficial infectious rates ranging from 2.5% to 7.5%, but fortunately only rarely these progressed to more serious infections (<0.1%). To avoid infectious complications, proper sterility should be observed in the operating environment. In addition, the patient should be instructed on wound care and on recognition of signs and symptoms of infection. Many superficial infections can be treated with oral antibiotics, but more serious infection may require surgical exploration and removal of the device. Although less common, abscess of the epidural space can lead to paralysis and death if not identified quickly, so a high index of suspicion is warranted. Staphylococcus aureus is the predominant pathogen for percutaneous epidural catheter–related infections.

Programming and Technical Overview

A full understanding of SCS requires appreciation of Ohm’s law: I = V / R, current (I) equals voltage (V) divided by resistance (R). In SCS circuit, voltage is supplied by the IPG and resistance is the impedance between the lead electrodes (determined by the tissues surrounding the electrodes including the dorsal columns), with these two variables determining the flow of current between the SCS electrodes. Factors such as epidural fibrosis, a large epidural space or small spinal cord diameter at any particular level, and a thick CSF layer may increase resistance or impedance to current flow between the electrodes, requiring higher voltages by the IPG to maintain adequate stimulation. An understanding of the anatomy of the spinal cord and particularly of the size/shape of the epidural space at varying levels is therefore important. Studies confirm that a lead placed in the cervical spine, where the spinal cord is larger and the epidural space narrow, has lower impedance than a lead placed in the lower thoracic spine, where the epidural space is large and the spinal cord diameter is small. Increased resistance will require higher voltage to maintain optimal current flow. As power is directly proportional to voltage and current, appropriately manipulating these variables may allow for better power performance. To date, the following factors have not been correlated with changes in impedance: age, spine surgery, number of leads, or model. However, males were found to have 21% higher impedance than women in the thoracic spine, without clear physiologic explanation.

There are four basic parameters in traditional tonic neurostimulation, which may be adjusted to create stimulation paresthesia in the painful areas, thereby mitigating the patient’s pain ( Fig. 73.7 ). They are amplitude, pulse width, rate, and electrode selection.

Typical patterns of coverage using different anodal and cathode combinations.

Courtesy Medtronic Inc.

Amplitude is the intensity or strength of each individual pulse and can be controlled by voltage or current. There is no evidence of superiority for voltage or current control. However, current-control systems are theoretically more immune to changes in electrical resistance in the tissue due to sclerosis and patient positional changes. As such, mathematical modeling predicts more even paresthesia. Amplitudes are variably optimized for each individual patient, but typical initial settings are 60%–90% of motor threshold. Pulse width is the duration of a pulse measured in microseconds (μs). It is usually set between 100 and 400 msec. Similar to increasing the amplitude, a larger pulse width delivers more energy per pulse and typically provides broader coverage. Common initial settings are 0.2 msec. Rate is measured in hertz (Hz) or cycles per second, between 20 and 150 Hz. At lower rates, the patient may feel myoclonic vibrations, whereas at higher frequencies, the feeling is more of a buzzing sensation. Higher frequencies (>500 Hz) are suggested to increase blood flow and decrease vascular resistance.

Targets for lead placement is a complex topic. Barolat and colleagues mapped data of sensory response and coverage patterns based on lead location in the epidural space in 106 patients. Most patients’ stimulators are programmed with electrode selection such that the patient obtains anatomic coverage, and then the pulse width and rate are adjusted for maximal comfort.

Placement of the electrodes for a few important and frequently used targets includes the following:

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  C2: lower half of face

  
  
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  C2-C4: neck, shoulder, hand

  
  
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  T5-T6: abdomen

  
  
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  T7-T9: back

  
  
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  T10-T10: leg

  
  
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  T12-L1: foot

  
  
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  L1: pelvis

The lowest acceptable settings on all parameters are generally used to conserve battery life. Other programming modes that save battery life include a cycling mode during which the stimulator cycles full on/off at patient-determined intervals (minutes, seconds, or hours). The patients’ programming may change over time and reprogramming needs are common.

Scs Types: Traditional, High-Frequency, and Burst Scs

Outcomes: Clinical Studies of Traditional Scs

Outcomes research for SCS is evolving. Greater research is needed to refine the technology, pursue increased efficacy, and limit complications. Much of the initial evidence is observational in nature or are comparative effectiveness studies. This is due to the invasive nature of the modality and due to the fact that blinded or placebo treatment, especially in the case of traditional tonic stimulation with the necessity for paresthesia, is difficult. Important studies on outcomes of traditional SCS are detailed later and in Table 73.1 .

Clinical Studies of Burst Scs

The literature for burst SCS is evolving because the burst mode of SCS allows for placebo-controlled, double-blinded trials more readily than traditional SCS due to the lack of paresthesia. Important studies are detailed next and in Table 73.2 .

In 2010 De Ridder and colleagues published prospective data on 12 patients with neuropathic pain, the majority having FBSS. The patients were trialed in random order with two, 1-hour sessions in tonic mode and an equal number/length of sessions in burst mode on a separate day. All patients preferred the burst mode. The patients reported significantly better suppression of axial and limb pain with burst mode than tonic mode based on VAS. Additionally, only 17% of the patients experienced paresthesia with burst mode of SCS compared to 91% of the patients with tonic SCS. At 1-year follow-up, the patients still experienced significant axial and limb pain reduction with the burst mode. While this study had a small sample size and was not a placebo controlled, double-blinded trial, it highlighted the potential for paresthesia-free stimulation with burst SCS as well as the potential for axial pain relief.

In 2013, De Ridder and colleagues performed a prospective, randomized, double-blinded, placebo controlled study in 15 patients with mixed limb and axial back pain. Burst SCS was studied against tonic SCS and placebo. Patients received each mode for 1 week, in a randomized sequence and VAS scores were measured. The authors showed that burst SCS was superior to placebo in decreasing VAS scores for axial, limb, and general pain. It was also superior to tonic SCS in suppressing axial and general pain, but there was no significant difference in limb pain. Given that each mode was evaluated for only 1 week, the conclusions are restricted to short-term evaluation. The study was also limited by the small sample size and a skewed gender demographic because the sample consisted of 4 men and 11 women.

In 2014 de Vos and colleagues prospectively studied 48 patients with neuropathic pain secondary to FBSS or painful diabetic neuropathy (PDN). All had used tonic SCS for at least 6 months before the study. Three groups were created: (1) FBSS with good response to tonic SCS, 24 patients, (2) FBSS with poor response to tonic SCS, 12 patients, and (3) PDN, 12 patients. All groups were switched to burst mode for 2 weeks. Overall, 60% of the patients experienced better pain relief with burst SCS as compared with tonic SCS. Subgroup analysis showed that burst SCS allowed 44% more pain relief in PDN patients and 28% better relief in FBSS. The study was not a randomized, placebo-controlled, double-blind study. In addition, it was limited by only 2 weeks of follow-up, failure of the cohort to isolate to one neuropathic pain state, and questionable washout interval between modes, potentially allowing for carry-over effects.

Also in 2014 Schu and colleagues performed a study that was similar in design to De Ridder’s 2013 study but looked at FBSS patients who already had received tonic SCS stimulation for 3 months. This was a prospective, randomized, double-blind, placebo-controlled study and tested burst SCS against tonic SCS and placebo. The patients received each mode for 1 week, in a randomized sequence. Numeric Rating Scale (NRS) pain intensity scores were measured, as were short-form McGill pain questionnaire (SF-MPQ) pain quality scores. Burst SCS significantly improved NRS and SF-MPQ scores compared with tonic SCS and placebo in FBSS patients who were already using tonic SCS. However, it should be noted that each mode was only evaluated for 1 week, possible carry-over effects existed from the baseline tonic stimulation mode, and the study was small in scale.

Finally, De Ridder and colleagues published a retrospective, multicenter study on 102 patients with FBSS or PDN who had all used tonic SCS for at least 6 months prior to the study. The patients were categorized into responders to tonic SCS group and nonresponders to tonic SCS group. Both groups were switched to burst mode SCS. Interestingly, 94.8% of responders to tonic SCS had improved pain suppression with burst stimulation. In addition, 62% of the nonresponders to tonic SCS were “rescued” with burst mode SCS. Thus it was concluded that (1) burst SCS may salvage nonresponders to tonic SCS and that (2) burst SCS may provide greater relief than tonic SCS in both tonic responders and nonresponders. This study was limited due to its retrospective nature, the lack of control group, and short-term 2-week follow-up.

Clinical Studies of High-Frequency Scs

Notable studies for high-frequency SCS are discussed next and summarized in Table 73.3 . Most recently, in 2016, Kapural and colleagues published a multicenter, prospective, randomized, controlled trial of 198 patients with back and leg pain who were naïve to SCS. The patients were randomly assigned to receive either tonic SCS or HF-10 in a 1:1 ratio, and success was defined as ≥50% axial pain reduction. At 2 years, the HF-10 SCS suppressed back and leg pain in more subjects than tonic SCS did. Limitations of the study included the lack of a placebo group and its nonblinded nature. This would have been difficult to accomplish with only one mode of SCS generating paresthesia.

TABLE 73.3

Clinical Outcome Studies on High-Frequency Stimulation

Study	Design Details	Findings	Analysis
Kapural and colleagues (2016)	• n = 198 • Cohort: back ± leg pain • n = 171 successful trial and implant • Naive to SCS • Multicenter, prospective, randomized, controlled trial • Follow-up: 24 months	• Responders—HF vs. tSCS: • leg pain: 76.5% vs. 49.3% • back pain: 72.9% vs. 49.3% • P < .001 • Degree of response—HF vs. tSCS: • back pain: 66.9% ± 31.8% vs. 41.1% ± 36.8% • leg pain: 65.1% ± 36% vs. 46% ± 40.4% • P < .001	• HF at 10 kHz better suppressed back and leg pain for more subjects than tonic SCS over long term. Limitations: • Not blinded • No placebo group
Tiede and colleagues (2013)	• n = 24 • Cohort: back pain > leg pain • Multicenter case series • Prospective, open-label • Trialed with tSCS (4 days) and then with HF (4 days)	• 88% preferred HF to tSCS • 77% reduction of VAS for back pain and overall pain from baseline with HF ( P < .001)	• HF at 10 kHz may be preferred by patients and effective for axial pain Limitations: • Not randomized or controlled • No placebo arm • Short duration • Small sample
Van Buyten and colleagues (2013)	• n = 83 • Cohort: low back pain (81% FBSS) • n = 72 for implant • Prospective, multicenter, open-label, observational • Follow-up: 6 months	• VAS ↓ from baseline at 6 months: • Back pain: 8.4–2.7 • Limb pain: 5.4–1.4 • P < .001 (both) • ODI ↓ by 17 points from baseline at 6 months ( P < .001) • At 6 months, 75% with >50% pain relief • 11/14 who failed tSCS were rescued with HF • Similar safety to conventional SCS	• HF at 10 kHz improved pain control for back/limb pain from baseline, and improved disability scored. May also salvage tSCS nonresponders. Limitations: • Not randomized or controlled • No placebo arm
Perruchoud and colleagues (2013)	• n = 40 • Cohort: stable relief with tSCS • Results for n = 33 • Randomized, controlled, double blind • HF at 5 kHz (instead of 10 kHz) vs. placebo • 2-week duration	• Mean benefit of HF at 5 kHz not statistically significant over placebo • Strong “period effect” ( P < .006)	Possible that HF at 10 kHz is efficacious, but HF at 5 kHz is not Limitations: • Therapy applied subthreshold kHz • Short duration • Does not address patients naïve to SCS

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