Spinal Cord Stimulation

 

Background

The knowledge that electricity could be used to treat pain dates as far back as observations by Scribonius that the pain of gout could be relieved by contact with torpedo fish. [1Furthermore, multiple examples and proposed mechanisms exist that demonstrate the perception of pain is not strictly proportional to the intensity of the noxious neural stimulus. [2One postulate that pioneered our view of the spinal cord’s role in the bias of nociception is attributed to the gate theory as hypothesized by Melzack and Wall in 1965. [3]

The image below depicts cross-section anatomy of spinal cord.

Cross-section anatomy of spinal cord. Cross-section anatomy of spinal cord.

See Pain Management: Concepts, Evaluation, and Therapeutic Options, a Critical Images slideshow, to help assess pain and establish efficacious treatment plans.

Although the action of spinal cord stimulation (SCS) is ascribed to the direct inhibition of pain transmission in the dorsal horn, these theories do not fully explain the mechanisms by which SCS reduces pain. Before the complexity of the gate theory was realized, Dr. Norman Shealy, a Harvard-trained neurosurgeon at Case Western Reserve University, sought to show clinical support for this function by implanting the first unipolar SCS in 1967. [4]

Recent research has provided some insight into how such neuromodulation affects pain. The mechanisms of action may differ depending on the type of pain targeted for treatment. For example, its effect on neuropathic pain may be secondary to stimulation-induced suppression of central excitability, whereas the beneficial effect of SCS on ischemic pain may be related to stimulation-induced inhibition of sympathetic nervous system influences and antidromic vasodilation, which increases blood flow and reduces oxygen demand. [5]

The neurophysiologic mechanisms of SCS are not completely understood; however, some research suggests that its effects occur at local and supraspinal levels and also through dorsal horn interneuron and neurochemical mechanisms. [678Experimental evidence supports a beneficial SCS effect at the dorsal horn level, whereby the hyperexcitability of wide-dynamic-range neurons is suppressed. Evidence exists for increased levels of gamma-amino butyric acid (GABA) release and serotonin, and perhaps, for reduced levels of some excitatory amino acids, such as glutamate and aspartame. [678]

Despite our limited knowledge of the precise biological mechanisms responsible for the benefit of SCS, the estimated number of stimulators implanted each year has surpassed 20,000, and the annual revenue is in the excess of a half-billion dollars. [910]

However, after analysis of the medical literature, Boswell and colleagues concluded that evidence for the efficacy of SCS in the treatment of failed back surgery syndrome (FBSS) and complex regional pain syndrome (CRPS) was strong for short-term pain relief and moderate for long-term relief. [11Also, a 20-year literature review found evidence that revealed long-term safety and efficacy of SCS in FBSS, CRPS, peripheral neuropathy, and severe ischemic limb pain secondary to peripheral vascular disease. [12The primary purposes of SCS are to improve quality-of-life (QOL) and physical function by reducing the severity of pain and its associated characteristics [131415]

Indications

Spinal cord stimulation can be used to treat a variety of diseases that result in chronic pain. The most commonly treated diagnoses include failed back surgery syndrome (FBSS; 33%), complex regional pain syndrome type I (45%) and type II (4%), neuropathy (10%), visceral pain (5%), and peripheral vascular disease (3%). [16]

FDA-approved indications include the following:

Failed back surgery syndrome (FBSS)

The application of SCS for FBSS is indicated by first excluding the presence of a causative lesion that can be treated surgically or by other nonoperative therapies. Treatment of neuropathic pain symptoms due to FBSS is more likely to respond favorably. [131718In general, these diagnostic entities include radiculopathy or polyradiculopathies due to epidural fibrosis, arachnoiditis, and intrinsic nerve root damage, or radiculitis. [13The differential diagnosis must exclude transient metabolic processes, such as diabetes, infectious etiologies, entrapment neuropathies, and potential referred or radiating pain often associated with arthropathy (zygapophysial and sacroiliac joints), visceral pathology, or myofascial disorders.

Chronic painful peripheral neuropathy or plexopathy

Treatable causes of neuropathy must be excluded through appropriate diagnostic evaluation, including blood work, electrodiagnostic studies, and nerve biopsy, if necessary.

Multiple sclerosis (MS)

A 2006 study demonstrated good relief in 15 of 19 patients with lower extremity pain due to MS. [18]

Complex regional pain syndromes (CRPS) I and II

Of course, the goals in treating CRPS are reducing pain, improving function and blocking trophic changes that are associated with autonomic dysfunction. Patients should be diagnosed correctly, respond to sympathetic blockade (at least for diagnostic purposes), and demonstrate significant pain intensity (>5/10 on a visual analogue scale), along with physical findings that are diagnostic of this disorder. [131718]

Other approved disorders

The following disorders have approved indications, but reduced probability of a beneficial response: [1317181920]

  • Axial pain due to FBSS

  • Postherpetic neuralgia

  • Post-thoracotomy pain

  • Phantom limb pain

  • Intercostal neuralgia

  • Spinal cord injuries with most varied motor and sensory deficit

Off-label applications undergoing investigation

Although peripheral vascular disease, end stage (PVD), and refractory angina have shown strong literature support for SCS treatment efficacy, these indications are not yet FDA-approved. SCS has a profound effect on sympathetic vascular tone and promotes local blood flow and ischemic ulcer healing in patients with PVD. Due to the nature of pain and disability associated with these disorders, many US insurers now cover SCS treatment for this indication. [212223]

The treatment of axial and other musculoskeletal pain syndromes show some support in the literature when epidural placed SCS is coupled with subcutaneous peripheral nerve field stimulation. [24]

Nerve root stimulation has been identified as potentially useful in gastrointestinal motility disorders, as well as genitourinary and sexual dysfunction. [25262728Also, isolated subcutaneous peripheral nerve stimulation of is under investigation for treatment of focal neuralgias, [29especially occipital nerve stimulation for chronic migraine/headache. [30]

Spinal cord stimulation has also shown to improve locomotor behavior in Parkinson's disease. [31 Patients demonstrate improvements in pain control, gait, and posture; without inducing paresthesia. Additionally, SCS may have beneficial effects on mental status in patients with PD, with improvement in emtional symptoms. [32]

Contraindications

Relative contraindications [13include the following:

Anatomic conditions

See the list below:

  • Previous spinal surgery with epidural scarring

  • Severe spondylolisthesis with stenosis

  • Scoliosis that creates difficulty with lead steering

Medical comorbidities

See the list below:

  • Untreated infection

  • Implanted cardiac pacemaker were all or similar device

  • Coexisting additional major chronic pain condition

  • Anticoagulant or antiplatelet therapy

Psychosocial factors

See the list below:

  • Ongoing litigation

  • Operant factors (secondary gain)

  • Occupational discord or reduced functional capacity

  • Psychogenic factors that suggests a somatoform pain disorder

  • Significant psychological characteristics, including unstable axis I or II comorbidities

Absolute contraindications [133334include the following:

Anatomic conditions

See the list below:

  • Previous dorsal root entry zone surgery or disruption

  • Critical central canal stenosis

  • Serious neurological deficit with surgically correctable pathology

  • Anatomical spine instability or deformity at risk for progression

Medical comorbidities

See the list below:

  • Demand-type cardiac pacemakers

  • Need for future MRI studies or possible cardioverter defibrillators

  • Pregnant or pediatric patients

  • Coagulopathy, immunosuppression, or any medical condition that compromises surgical benefit over risk

  • Ongoing requirement for therapeutic diathermy

Occupational exposures

See the list below:

  • Theft detectors and metal detection devices

  • Operation of dangerous equipment or machinery

Psychosocial considerations

See the list below:

  • Severe cognitive impairment that interferes with evaluation or operation of the device

  • Unacceptable living situation or social environment

  • Active substance abuse

Numerous patient-characteristics must be scrutinized when evaluating candidates for SCS. Evidence of aberrant opioid-related drug use that suggests abuse or diversion should be considered when patients demonstrate behaviors such as unapproved dose escalation, lost prescriptions, frequent requests for early refills, and obtaining medications from multiple physicians.

Some Axis II diagnoses, perhaps, most obviously borderline or sociopathic personality disorders, and Axis I disorders, such as the presence of psychoses, often lead to treatment failure. Untreated mood disorders, anxiety/panic disorders, post-traumatic stress disorders, and somatization disorders may lead to errant surgical decisions and unsatisfactory outcomes. Medical conditions that have been reported to carry a higher risk for secondary morbidity or poor outcome include the presence of coagulopathy, implantable site or systemic infection, morbid obesity, diabetes, progressive arthropathy, and deteriorating neurological status. Certain patient characteristics that are generally regarded as predictors of favorable SCS outcomes include the cognitive abilities to understand the procedure, risks, and expectations of SCS treatment. [35]

Disease-specific entities that have demonstrated a high probability of successful pain reduction are most often seen with SCS treatment of chronic neuropathic pain, complex regional pain syndromes, refractory angina pectoris, or painful ischemic disorders. Disease states with a low probability of successful pain reduction that should be avoided include neuropathic pain due to spinal cord injury, central nervous system pain, or nerve root avulsion. Other disorders, such as postherpetic neuralgia, axial low back pain, and phantom limb pain present an unknown probability of pain reduction.

Furthermore, countless quality of life issues partner with chronic neuropathic pain. Reduced pain using SCS can prompt pain treatment physicians to eliminate polypharmacy or taper specific medications that cause cognitive dysfunction or blunt mental alertness and to avoid present or future adverse medication-related side effects. SCS may facilitate return to work or other life functions. These are additional influences that are considered when deciding upon whether a trial of SCS is indicated.

Technical Considerations

Lead placement

Cervical lead placement is effective in providing stimulation to the upper extremities. Entry point is typically in the upper thoracic spine, with advancement of the lead to the C2–3 level. [36Relief of lower extremity pain is achieved with lead placement at the thoracic T7–12 levels. Advancement of leads to the T6–T9 levels can be used to target low back pain. Furthermore, lead placement in the mid- to upper thoracic spine, T4–5, is effective for targeting visceral pain and postherpetic neuralgia. [36]

Percutaneous leads vs paddle electrodes

Spinal cord stimulation systems are available as percutaneous leads or paddle electrodes. Both are commonly used, each with their own advantages and limitations. Both percutaneous leads and paddle electrodes are effective in controlling chronic pain. [37Percutaneous leads are minimally invasive and can be placed by interventionists in an outpatient setting. However, percutaneous leads have a higher rate of migration and possible need for lead revision. Paddle electrodes are more invasive, requiring a laminectomy for placement. However, paddle electrodes can provide better coverage compared to percutaneous leads. Babu et al. compared outcomes of percutaneous leads to paddle electrodes for approximately 13,000 patients. Placement of paddle electrodes was associated with a slightly higher complication rate in the immediate postoperative period but had a significantly lower long-term re-operation rate. Long-term healthcare costs were similar between the two groups. [38]

Complication prevention

The most common causes of complications in SCS are lead migration, pain over the implant site, and infection. [39Over the past 10 years, better techniques in lead anchoring have decreased the incidence of lead migration. [36To prevent wound infections, patients are typically given perioperative IV antibiotics and can be discharged home on an oral antibiotic, such as Keflex, for one-week duration. The rate of SCS-related infections is approximately 4.5%. [16Patients with diabetes are at higher risk of SCS-related infection (9%). Infection typically requires hardware explant, followed by IV antibiotics for deep infections or oral antibiotics for superficial infections. Other complications include lead fracture, premature battery failure, and CSF leak from dural puncture. Serious adverse events, such as neurological injury are rare.

Outcomes

Failed back surgery syndrome (FBSS) or postlaminectomy syndrome

Persistent radicular pain after lumbosacral surgery can be managed either by reoperation or spinal cord stimulation. North et al. conducted a prospective, randomized clinical study that found SCS more effective in achieving pain control than reoperation as a treatment for persistent radicular pain after lumbosacral spine surgery. [40The PROCESS study, a multi-center randomized clinical study, compared SCS in combination with conventional medical management in patients with FBSS and predominantly lower extremity radicular symptoms. [41At 6-month follow-up, 48% of SCS patients achieved the primary outcome of >50% leg pain relief compared to only 9% of the medical management group. At 2-year follow-up, 42 out of 52 patients with SCS had less leg pain and improved functional capacity.

Complex regional pain syndrome (CRPS)

Patients with CRPS treated with SCS have demonstrated reduction in pain and allodynia with improvements in quality of life and limb function. [36Kemler et al. found patients with CRPS who underwent SCS demonstrated a 2.4 fold reduction in pain on the visual analog scale compared to a 0.2 fold increase in pain for patients who received physical therapy alone at 6-month follow-up. [42Long-term prospective data for spinal cord stimulation in CRPS indicates 63% of patients achieve long-term pain control. A 50% reduction in pain at 1 week post-implantation is associated with long-term success. [43The best outcomes are demonstrated in CRPS type 1 in patients under 40 years of age and those who receive SCS within one year of diagnosis. [44]

Painful diabetic peripheral neuropathy (PDPN)

SCS can be effective in patients with peripheral neuropathy secondary to diabetes. At 1-year follow-up, 63% of patients treated with SCS demonstrated a >50% reduction in lower extremity pain. Additionally, 60% of patients were weaned off analgesic medications. [45]

Postherpetic neuralgia (PHN)

Postherpetic neuralgia is a chronic neuropathic pain syndrome that can occur following a herpes zoster eruption. [36Spinal cord stimulation has been shown to be effective in the treatment of PHN, with 82% of patients experiencing a significant decrease in pain and improvement in quality of life. [46]

Visceral pain

Spinal cord stimulation has shown benefit in chronic visceral pain resulting from mesenteric ischemic pain, esophageal dysmotility, gastroparesis, inflammatory bowel syndrome (IBS), chronic pancreatitis, familial Mediterranean fever, post-traumatic splenectomy, generalized chronic abdominal pain, postgastric bypass chronic epigastric pain, and chronic visceral pain resulting from post-surgical intra-abdominal adhesions. [36Kapural et al. reported a study of SCS for chronic visceral abdominal pain. SCS resulted in 86% of patients achieving a >50% reduction in pain, along with a significant decrease in opioid requirements for pain control. [47SCS has also shown benefit in chronic pancreatitis, with 80% of patients experiencing >50% reduction in pain at 1-year follow-up. [48]

Peripheral vascular disease

Allodynia and painful cyanotic discoloration may be associated with the distal extremities Reynaud’s disease or ischemic peripheral vascular disease. SCS may result in decreased pain associated with ischemia. [36]

Dorsal Root Ganglion (DRG) Stimulation

Direct stimulation of the dorsal root ganglion is an emerging treatment in the field of spinal cord stimulation. A DRG stimulator consists of electrical leads, which are threaded through the epidural space into the intervertebral foramen and directly overlie the dorsal root ganglion. Leads are then connected to an implanted battery through extension wiring. The main difference between DRG-SCS and traditional SCS lies in the type of fiber activation associated with the two types of stimulation. SCS involves stimulation of the dorsal columns resulting in broad electrical stimulation of multiple dermatomes. DRG stimulation is more precise, directly activating the cell bodies of the very neurons that innervate the painful regions. [49DRG-SCS is as effective as traditional SCS in the treatment of pain due to failed back surgery syndrome, complex regional pain syndromes, and chronic postsurgical pain. [50DRG can be particularly effective in treating pain in areas difficult to treat with traditional SCS, including focal distal distributions such as groin and foot. Additionally, DRG stimulation is associated with a lower rate of lead migration and positional side effects associated with traditional SCS. Liem et el. treated 51 patients with DRG-SCS. In their cohort, patients reported a 56% reduction in pain 12 months post-implantation, with 60% of patients reporting greater than 50% improvement in their pain. Pain localized to the back, legs, and feet was reduced by 42%, 62%, and 80%, respectively. Patients also demonstrated a significant improvement in quality of life.


Spinal Cord Stimulation Periprocedural Care


Patient Preparation

Following completion of a SCS percutaneous trial, the leads are pulled, and the skin entry sites are cleaned with chlorhexidine and alcohol. A topical antibacterial agent can be used to cover the puncture sites, then a Tegaderm dressing is applied as an additional barrier.

The permanent trial is scheduled for no sooner than 2-3 weeks, after which any inflammatory or infectious residua should be resolved. This also allows time for recognition of all potential trial complications, so that they be treated and not increase the risk of complication of permanent SCS placement. The required lead length to the IPG pocket site is measured, any imaging studies that would expose an anatomical or technical barrier are requested, and pertinent preoperative blood work should be considered to preclude infectious or bleeding complications.

Limited research using computer modeling analysis suggested that the effect of spine flexion on the distance between an IPG site implanted in the buttock and the midline anchor requires an increased lead length of 9 cm. Therefore, strain relief loops 2.5-3 cm in diameter are placed at appropriate sites to compensate for spinal movement (adjacent the midline anchor and a second loop adjacent the IPG pocket). Each loop requires an additional 9-10 cm of lead length. These strain relief loops reduce kinking, damage, and breakage of the leads.

Conversely, modified direct tunneling techniques that travel diagonally, directly to the IPG pocket, require comparatively reduced lead length. Experts and the literature are unsettled as to a specific "best" approach (eg, gluteal, abdominal, or midline placement in lumbar cases or various axillary sites in cervical placements). Some implanters prefer to tunnel cervical placements to the gluteal, abdominal, or lateral hip region. A common lead length when tunneling directly to the IPG pocket while also allowing for adequate strain relief is 70-cm. For longer permanent leads, most manufacturers provide supplemental extensors.

Proper placement of the IPG site is performed preoperatively. The site should be in a comfortable area within the patient’s reach for recharging. Postural change should be considered so that comfort is achieved both sitting and standing. A model of the IPG is used during pocket formation, a tight fit that requires some soft tissue stretching is preferred to minimize any dead space around the battery. For example, preparing a male patient for a gluteal pocket is best performed with the patient wearing pants so that the surgeon or technologist can locate a site using a model IPG placed over the skin. Preferred placement is below the belt line, yet high enough so that the unit does not impede sitting or reclining.

Similarly, female patients should wear a bra so that the IPG model can be located under the bra line. The IPG model is outlined using an indelible marker for precise location during the procedure (see the image below). The goal is to create a pocket that matches the size of the IPG as closely as possible. The incision site will be placed at the superior edge of the battery pocket. Location should be discussed with and approved by the patient.

Placement of the IPG. Placement of the IPG.

Positioning

For this procedure, the patient lies prone on the fluoroscopic table, which allows free access to the C-arm throughout the entire thoracolumbar spine. The patient’s head should be turned to the side so that most of the neck and back are relaxed. The vertebral column is positioned to minimize cervical or lumbar lordosis as described above, by placing pillows or specialized surgical bolsters or frames under the abdomen or chest that allow adjustable flexion for comfort. Additional pillows and modifications should be used to allow the patient to be comfortable and warm before the patient’s back is scrubbed with antiseptic and draped. [13]

Specific landmarks are used to identify the needle entry point. The most desirable oblique angle for needle placement is 30-45°. After fluoroscopic alignment of the pedicles and vertebral end plates, previously described, anatomic references, including the intralaminar entry site should be horizontal, crisply outlined, and clearly identified. For most percutaneous trials of the lower back and legs, the preferred entry for distal lead placement is usually between the T12-L1 and L2-3 intralaminar spaces.

The medial aspect of the ipsilateral pedicle 1-2 levels below the intralaminar entry target at the 9 or 3 o’clock position are marked as the skin entry site or sites.

In an average-sized adult, skin entry between L2 and L3 is technically safest because the conus medullaris and spinal cord are cephalad to this level in most adults. When the insertion point is selected, the tip of a metal marker is placed over the point to provide a fluoroscopic landmark. Adjustment of the entry point may be necessary in patients with extremes in body habitus. Entry will be more cephalad in very thin patients and caudal in very obese patients. [13]

Monitoring & Follow-up

The SCS system is retested in the recovery room to assure that no lead migration or fracture may have occurred and that coverage remains adequate. If coverage is not acceptable, then the patient is taken back to the OR for revision. Over the next 24-hours neurological signs and symptoms are monitored. Any postoperative observations, including pain, weakness, or numbness, should be investigated. Paresthesia should be evaluated despite the device being turned off.

Postoperatively surgical wounds are dressed with nonocclusive bandages. Dressing changes are indicated when they become wet or bloody. Wet dressings should raise concern for fat necrosis in obese patients. Although the scientific literature is conflicting, conventional instruction has been to keep the wound dry for 10-14 days or until 1-2 days following suture/staple removal.

The first postoperative check is usually indicated at 3-4 days, whereby, the outer dressing is removed for wound inspection. Loose Steri-Strips are removed, but intact strips are left in place. Loose materials are irrigated with peroxide, and then the wound is gently patted dry with a 4x4 sterile cotton gauze pad. Next, a 4x4 sterile cotton gauze pad is loosely taped over the healing wound. At postoperative days 7-10 days staples/ sutures are removed, the final postoperative visit is usually scheduled at 3 weeks. If the wound shows no signs of infection, the patient may bathe.

Any signs or suspicion of infection should raise caution, and baseline blood studies including a complete blood count, erythrocyte sedimentation rate, and C-reactive protein should be obtained.

Patients are advised to avoid bending, squatting, or reaching above the shoulders for 6-8 weeks following surgery. Patients are asked to avoid twisting, limit lifting to less than 5-8 pounds, and to refrain from sleeping on their stomach. Motor vehicle accidents and trauma, like falls, can threaten lead integrity. Confronting a magnetic environment may induce current flow through coiled electrodes or lead extensions. Magnetic fields are commonly encountered through theft-deterrent devices, metal detectors, airport security procedures, and with medically indicated magnetic resonance imaging studies. Diathermy can cause tissue damage through energy that is transferred into the implanted SCS components resulting in severe injury or death.

During the healing period, the relationship between the implanted electrode surface and adjacent tissues evolves through healing, scarring and systemic/ metabolic factors. Therefore electrode contact and impedance changes can be expected. Manufacturer representatives are expected to stay in touch with postoperative patients to determine SCS efficacy when the system needs to be recalibrated or reprogrammed for improved pain coverage. Over the 6-8 weeks following SCS placement, physicians must manage medication-intake or tapering and patient expectations regarding pain relief.

Spinal Cord Stimulation Technique


Approach Considerations

Preparation for placement of a permanent spinal cord stimulator usually necessitates a trial procedure, which allows both the SCS team and the subject to determine whether or not a permanent implant would provide substantial or adequate pain relief and improvements in QOL.

The SCS team is composed of a trained interventional pain physician, his/her medical team, a fluoroscopy technologist, and a manufacturer’s representative, who is a trained SCS technologist. The trial helps determine the optimal number of leads and electrodes and optimal placement for pain coverage. Usually, trial leads are cylindered or wire-like and have 4-8 electrode sites separated by ≥4mm. These 1-column leads with eight electrode sites are more versatile for trial use, because they allow more precise signal positioning across the dorsal columns and can span up to 2 vertebral body segments. Often, column leads with 8-electrode sites can provide concordant pain coverage with a single ipsilateral lead placement, and they can be reprogrammed to recapture paresthesia for pain coverage if lead migration occurs during the trial.

Because lead migration is commonly encountered, a second epidural lead, placed by an ipsilateral skin-entry point 1-segment above the first lead’s skin-entry or at a contralateral skin-entry site at the same segmental level, creates redundant coverage should such slippage occur. Furthermore, a second lead may provide improved or more comfortable pain coverage, increased ability to try various programs, and improved pain reduction during the trial.

The trial allows the SCS team to evaluate patient anatomy and potential anatomic barriers, eg, the capacity of the epidural space, CSF depth (greater distance allows more dispersion of signal), and patient-response to trial leads. The patient’s response to trial stimulation may be optimized by changing electrode geometry or tissue surface impedance; changing the frequency or intensity of the signal, a function of higher voltage or current; or changing the applied pulse width or duration.

The latter factors are important for determining the battery-type, such as a primary cell battery for lower requirements, or a rechargeable battery when higher needs are required. Rechargeable batteries require better patient compliance with supporting technologists, because recharging visits will need to occur on a more frequent and regular basis. In these cases, more rigorous compliance requirements may adversely affect SCS outcome due to ongoing or progressive physical, cognitive or financial limitations (eg, transportation due to no vehicle or gas money).

Other advantages of percutaneous trials are that they can be performed less expensively outside of an operating room in relatively aseptic conditions. [51Trials typically last between 3-10 days, long enough to eliminate any possible placebo effect and with instructions that help the patient focus on pain reports specifically related to the pretrial targeted neuropathic pain, not related to any soreness or pain that was caused by trial needle and lead placement.

During the SCS trial, epidural leads are attached to an external programmable pulse generator, which operates on alkaline batteries. Because the trial is performed in an outpatient setting, eg, the world of physical exertion, work, play, and social demands that are specific to this patient-participant’s lifestyle, a more realistic evaluation of the patient’s pain relief and QOL improvements can be achieved.

Most of the trial implants are placed through percutaneous tissues into the epidural space. The patient is usually premedicated with a sedative, often from the diazepam family (examples include alprazolam or lorazepam) and an adequate fast, short-acting pain medication, typically hydrocodone or oxycodone preparations. Most experts feel that these medications should be dosed for patient comfort but should do not obtund the patient’s capacity to report pain during needle placement that may indicate possible tissue injury, or to blunt the patient’s capacity to determine whether or not the trial successfully produces paresthesia coverage of the painful body regions.

The administration of oral or parenteral prophylactic antibiotics 30-120 minutes before the procedure is advocated by some. [13A consensus of studies and experts suggest cefazolin 1-2 g or cefuroxime 1.5 g IV 30 minutes prior to incision or surgical implant is commenced. If the patient is allergic to beta-lactams, substitute clindamycin 600 mg IV (30 minutes prior to incision). If the patient is colonized with MRSA then the use of vancomycin 1 g IV 30-120 minutes prior to incision is recommended. [52Antibiotic coverage beyond 24 hours after administration has not been shown to provide additional benefit. [53]

A percutaneous trial should be aseptic. Aseptic techniques, sometimes called "clean technique," refer to methods that prevent microbial contamination of the environment. For example, the purpose of scrubbing for surgery is to render your hands and forearms as aseptic as possible. Asepsis should protect both the patient and caregiver from infection. A sterile field describes a state that is technically free of all living microorganisms, including spores. Some items, such as surgical tools, can be sterilized and reused, but they must be stored in sterile conditions. A sterile operating field and sterile technique are used to keep the number of microorganisms to a minimum in an otherwise clean and aseptic environment, such as a fluoroscopic suite or operating room. [13]

Hand washing with soap and water removes dirt, organic substances, and loose hand flora, but it is ineffective for reducing antimicrobial activity. [54Alcohol, especially isopropanol, ethanol, and n -propanol alcohol are highly effective against both gram-positive bacteria and gram-negative bacteria, including multi-drug resistant pathogens and fungi. Antiseptic agents containing 60-90% of alcohol are most effective.

However, their effectiveness against spores, protozoan oocytes, and some viruses is poor. Although these solutions have a rapid onset antiseptic effect, they also evaporate rapidly, thereby, providing only short-term efficacy. However, the eventual regrowth of bacteria is slow. Alcohol produces its antimicrobial effect due to its ability to denature proteins. Iodine is an effective antiseptic that quickly kills a broad range of microorganisms. Unlike antibiotics, iodine is not associated with the development of resistant strains of microbes. Betadine, a solution of povidone-iodine is the iodophor most commonly used as an antiseptic and scrubbing agent. [55Iodophors are effective against gram-positive and gram-negative bacteria, as well as some spore-forming bacteria, as well as mycobacteria, viruses, and fungi. [54]

The patient is placed in a prone position with the arms above the fluoroscopic field and a pillow or a surgically designed flexion frame placed underneath the abdomen to reduce lumbar lordosis and even produce slight lumbar flexion for lumbar and thoracic needle entry and lead placement. Similarly, cervical positioning is best achieved with a prone neutral lower thoracic and lumbar spine with a bolster or pillow(s) placed underneath the chest to increase cervical flexion for cervical lead placement.

Prior to lead placement, the skin over the lumbar and lower thoracic paraspinal muscles within the needle entry region is prepared, wiped, and draped in an aseptic fashion. Complete sterile technique, including mask and gown are next implemented. Fluoroscopic assistance is used to visualize the pedicles of the vertebral bodies in the targeted field of entry. The optimal level for midline epidural entry is determined. This is typically in the dorsal midline at the level that the spinal cord becomes the conus medullaris, where lumbosacral nerve roots disperse laterally to form the cauda equina, ideally at the level above or below L1. Anterior-posterior fluoroscopic imaging of the working site is first optimized by aligning the image so that the spinous processes bisect the pedicles. Then the C-arm is adjusted in a cephalad or caudal direction to square off the vertebral end plates.

The skin entry point is paramedian, usually 2 levels below the desired midline epidural entrance, adjacent the medial border of the ipsilateral pedicle. A shallow angle of entry will facilitate lead advancement. For dual lead placement, pedicles are marked for 2 consecutive levels or on the contralateral side. Following application of 1% lidocaine without epinephrine or preservative, skin and subcutaneous analgesia is achieved. Thereafter, a 14-16 gauge epidural access needle is used to identify the epidural space using a loss of resistance or hanging drop technique.

Functional Anatomy

The epidural space is the target for threading the percutaneous trial electrode. The outermost membrane of the spinal cord is the fibro-elastic dura mater that extends from the foramen magnum to the second sacral vertebrae. The epidural space encircles the dura mater. It is ventral to the ligamentum flavum with its adjacent periosteum and dorsal to the posterior longitudinal ligament. Its lateral borders for needle placement are the vertebral pedicles and intervertebral foramina (see the image below).

Cross-section anatomy of spinal cord. Cross-section anatomy of spinal cord.

The cervical epidural space extends from the dura of the foramen magnum to the inferior border of C7. The thoracic epidural space begins at C7 and extends to the upper margin of the L1 vertebrae. The lumbosacral epidural space extends from the upper margin of the L1 vertebra.

The size of epidural space, as measured by the distance between the ligamentum flavum and the dura, varies with location in the spinal column. The largest distance is at L2, where it can measure up to 5-6 mm. However, in the thoracic spine, the distance is reduced to approximately 3- 4 mm, and at C7 the distance is only 1.5-2 mm. In 40% of patients the anatomic (vertebral) and physiologic (spinal cord) midlines may differ by as much as 2 mm at all spinal cord levels. [56For these reasons, the first lead should always be tested for paresthesia location before the second lead is placed.

To cover the desired dermatomal area with paresthesia, it is necessary to place electrodes over the dorsal columns several segments cephalad to that level.

With lead placement slightly off the midline between C2 and C4, stimulation can be obtained to alleviate shoulder pain. When coverage that encompasses the entire upper extremity is desirable, this is usually accomplished when leads are placed just off the physiologic midline between C3 and C4 and then moving inferiorly, as necessary, for stimulation of the more medial forearm and hand. Placing 2 leads, each slightly off the physiologic midline to the right and left between C4 and C6, is successful when bilateral upper extremity stimulation is desirable.

Lateral fluoroscopy allows the operator to document lead placement in the dorsal epidural space. [57Stimulation of the back and both lower extremities is often intended following failed lumbar surgery for treatment of lower torso and extremity neuropathic pain. Most patients can achieve stimulation coverage when the leads are placed in the midline between T8 and T9. [58The current medical literature supports the placement of a single midline lead for coverage of low back pain; however, many experienced practitioners place an additional lead, so paresthesia coverage can be achieved with reprogramming in cases when lead migration occurs.

Placing two leads slightly off the physiologic midline to the right and left between T8 and T10 will often allow for both lower back and lower extremity stimulation. Coverage of midline low back pain with current steering SCS systems can be achieved with leads that are less than 4 mm apart so that current can be directed to the midline to obtain low back stimulation. [58]

Often unilateral lower extremity stimulation can be produced by lead placement slightly off the midline between T9 and T11. Distal lower extremity stimulation (eg, the foot) can be achieved with leads placed as low as T12 and L1, usually 1- 4 mm off the midline. Retrograde lead placement is sometimes necessary to achieve foot or pelvic stimulation. Furthermore, numerous more complex techniques that require retrograde electrode placement for lumbosacral nerve root stimulation are sometimes necessary when attempting pain coverage in some cases of rectal, perineal, or coccygeal pain.

For more information about the relevant anatomy, see Topographic and Functional Anatomy of the Spinal CordCervical Spine Anatomy, and Lumbar Spine Anatomy.

Device Summary

A trial of spinal cord stimulation consists of 3 parts: The lead that will be implanted in the epidural space, the external pulse generator, and extension wires that connect the lead to the pulse generator. [13Programming units are necessary so that stimulation variables can be adjusted. Implantation kits include leads, equipment, and other tools necessary to implant the SCS system and are available from the manufacturers.

Epidural leads have stimulating electrodes at the distal end and a connector at the proximal end. Percutaneous leads are cylindrical catheters (1-column leads) with sequentially spaced electrodes at the distal end of the catheter. The number of electrodes and spacing between electrodes vary depending on the model and manufacturer.

A percutaneous electrode design allows current to flow symmetrically, thereby, creating 360° stimulation. Cylindrical spacers separate each electrode on the catheter. Percutaneous leads are placed through a Tuohy needle with a large flat bevel that is suitable for percutaneous trials, tunneled trials, or permanent implantation.

Placing more than one lead in order to provide adequate stimulation and provide redundancy in the event of minor lead migration is common.

Paddle leads deliver focused energy ventrally and contain up to 3 columns. When using 3-column leads, the outer columns can provide anodal blocking, whereby, stimulation is confined to the midline. This is especially useful in cases of refractory midline back pain. Some suggest that paddle leads are more efficient because the electrical energy is focused in 1 direction, perhaps, prolonging battery life. [51Paddle leads are placed using a small laminotomy so that the lead can be secured to the supraspinous ligament at the level determined most effective following a percutaneous trial. A head-to-head trial revealed comparable clinical outcomes at 3 years following the procedure. [59]

Programmable pulse generators and neurostimulators provide the electrical power for stimulation. Extension wires connect the lead(s) to the pulse generators. One extension wire is necessary per implanted electrode lead. Manufacturer representatives are usually available to assist in selecting a combination of anodes and cathodes, amplitude, pulse width, and stimulation frequency that produce a comfortable and concordant paresthesia and provides the best coverage of the targeted pain area with the highest degree of pain relief. At the present time, most test screeners are integrated into a laptop computer or PDA device.

Percutaneous Trial of SCS Placement

SCS trials are typically performed through percutaneous lead placement. Trial success is measured by >50% pain relief over 1 week of stimulation. Patients should keep a log of improved functional capacity or activities of daily living during the trial period. [36]

As noted previously, percutaneous trial electrodes are placed through a 14-gauge needle. When oral analgesic and antianxiety agents are adequate, local anesthesia should be administered with preservative-free lidocaine with or without epinephrine. Lidocaine is preferred over bupivacaine because of reduced cardiotoxicity should an inadvertent intravascular injection occur. Minimal sedation is used during the procedure since the patient needs to be alert and communicative during the intraoperative stimulation procedure to ensure proper positioning of the leads. Airway equipment and resuscitation drugs, oxygen, and resuscitation equipment must be maintained and readily available. [133551]

Following intradermal and subcutaneous injection of local aesthetic, some use a small skin incision with a #11 blade; however, with adequate anesthesia and multiple needle penetrations, using up to an 18-gauge needle, the area can be adequately anesthetized for the necessary 14-guage needle puncture radius that is required. The skin is entered at the medial aspect of the pedicle with the Tuohy needle (see image below) usually angled 30-45° toward the midline for optimal entry into the epidural space at the interlaminar edge 1 or 2 levels of cephalad.

Tuohy needle and stylet.Tuohy needle and stylet.

The 14-guage needle is then passed with the bevel facing up at a 30-45° oblique angle to reach the depth of the midline laminar target.

After contact between with the interlaminar bone, the needle stylet can be removed and a loss of resistance (LOR) syringe attached. The Tuohy can next be angled more steeply, walking down the bone to enter the epidural space (see the image below). Once LOR is achieved, the syringe is gently removed, and a trial electrode lead is slowly advanced through the needle into the epidural space. AP and lateral fluoroscopic guidance is used to verify correct course of the lead cephalad in the dorsal epidural midline. With dual leads, the second lead is usually placed opposite to the side of the same spinous process or on the same side at a higher or lower level.

Spine and epidural space. Courtesy of Wikimedia CoSpine and epidural space. Courtesy of Wikimedia Commons.

Although significant variability exists from patient-to-patient with regard to ideal lead positioning, most commonly, patients with lower back and leg back find adequate coverage with a lead placed midline at T8. Placing 2 leads, each slightly off the physiologic midline to the right and left, between T8 and T10 will allow for both back and lower extremity stimulation. Leads should be no more than 2-4 mm off the midline. During the procedure, and at the end, AP and lateral fluoroscopic records should be obtained to document the level and correct placement of the leads within the epidural space.

When electrodes are satisfactorily placed and the manufacturer’s representative has calibrated the SCS to achieve the most comfortable and concordant paresthesia with the best possible coverage of the targeted pain area, with the highest degree of pain relief at an acceptable voltage, then the lead stylets are carefully removed and a fluoroscopic record of final electrode placement is obtained. The Tuohy needle is then carefully removed and the operative site is cleaned with chlorhexidine and alcohol.

Next, the leads must be anchored externally. Lead migration is a common cause for an inadequate or unsuccessful trial. Most manufacturers provide cylindrical sleeves that are designed to improve skin fixation and to reduce lead migration. Recent anchor designs are under analysis to determine whether newer designs can reduce lead migration and breakage. The lead is passed through the anchor sleeve, sometimes with silicone adhesive to assure a firm bond within the anchor.

Before the Tuohy needle is removed, a surgeon’s knot can be used to suture the lead to the skin and to assure fixation before passing the lead through the anchor (see the image below).

Throughout this process of securing the lead externally, fluoroscopic imaging is used to confirm that lead manipulation does not result in any positional change of the electrode tip. Next, the anchor can be attached to the skin using a figure-of-8 stitch with 2-0 non-absorbable sutures. The leads and anchors can be further secured by carefully applying Steri-Strips to the lead or to its anchor. Some use Steri-Strips alone to fixate the leads.

A tension loop is usually placed, and then a sterile occlusive dressing is applied. Each lead is attached to an extension wire, which connects the trial lead(s) to the pulse generators. One extension wire per implanted electrode lead is necessary. The leads are attached to a laptop or PDA, which can be programmed to adjust the aforementioned variables.

After the procedure, the manufacturer’s representative performs final programming, and the patient is instructed to minimize general activities on the day of the procedure. Thereafter, the patient is asked to minimize activities that might encourage lead migration, such as bending, squatting, or stooping. Some physicians insist that patients wear a cervical collar or lumbar brace to prohibit precarious trunk movements. As mentioned, the trial length is determined by the physician’s experience and practice, usually, with durations that vary between 3-8 days. During the trial, the responsible company’s representative/ technologist should maintain 24 hours/ day availability for continued optimal programming and for any emergency physician actions that might be necessary, including lead removal. Patients are usually given written instructions that include precautions such as getting the SCS entry site wet or symptoms that represent complications (eg, infections at the needleentrysite,chills, fever, headache, or increasing neck or back pain). [13]

Trial Assessment

The goal of the trial is to evaluate the level of pain relief and any perceived positive changes in quality of life that can be potentially recaptured by electrical spinal neuromodulation. Diaries often help patients chronicle their pain scores, using various validated pain scales; reduction of medication usage, especially fast-acting opioids used for break-through pain; and functional improvements, such as daily activities of living. The goal of the trial is to evaluate the level of pain relief and any perceived positive changes in QOL that can be potentially re-captured by electrical spinal neuromodulation.

Current criteria used to proceed with a permanent implantation of an SCS device demand at least 50% reduction in pain intensity and reported improvement in the patient’s QOL during the SCS trial. [3551606162In addition to global impressions regarding outcome, a battery of physical and psychological self-assessment tests or psychometrics may allow better or more objective parameters by which to assess the failure or success of a trial.

Before the trial electrodes are removed, the position of the lead tips are reassessed by fluoroscopy and a visual record is established for permanent placement if the patient feels the trial was successful and chooses to proceed.

Permanent SCS Placement

Percutaneous circumferential electrode leads are placed with the same preparation, patient positioning, and technique described for the trial with some exceptions. Parenteral access is placed with a 22-guage heplock. IV prophylactic antibodies are administered at least 30 minutes prior to skin entry. An anesthesia care provider should be present to provide light sedation and cardiopulmonary monitoring during the procedure. Sedation can be lightened appropriately for intraoperative programming.

Following confirmation of lead placement and stimulation coverage, the lead stylets and Tuohy needles are removed. Next, skin along the superior portion of the pocket is infiltrated with local anesthetic, and a 4-cm scalpel incision forms the superior border of the IPG pocket. A subcutaneous pocket approximately 2 cm deep is formed using bipolar electrocautery and blunt dissection. Monopolar electrocautery should not be used and care should be taken not to damage SCS system components. Local anesthetic is infiltrated prior to additional incisions.

After infiltration of local anesthetic, the leads are secured by manufacturer-supplied anchors in the midline to the interspinous ligament (see the image below).

Midline pocket suture.Midline pocket suture.

Next, a strain relief loop is placed, and a diagonal subcutaneous tunnel is created to pass the leads to the planned IPG pocket-site. Local anesthetic should be placed along the tunnel route, and IV sedation should be increased. A preassembled, manufacturer-supplied tunneling tool consists of a sharply tipped malleable metal shaft that is enclosed in a plastic cannula sleeve.

The tunneling tool is passed subcutaneously between IPG pocket-site and midline lead-anchor site above superficial muscular tissues. The handle of the tunneling tool is removed from its shaft, leaving the plastic sleeve within the tunnel.

The ends of the leads are introduced into and passed through the plastic sleeve. The plastic sleeve is withdrawn from the subcutaneous tunnel, and the ends of both leads are adjacent the IPG site. They should be irrigated with sterile water rather than saline to prevent corrosion and then securely connected to the pulse generator.

Following successful SCS testing, the set screws are tightened to secure the lead fixation to the IPG. When the pocket is clear of "bleeders" and irrigated, the battery is placed with the noninsulated lettered side of the unit facing out toward the skin. Both incision sites are irrigated with Bacitracin, and the wound is closed in layers with a 3-0 absorbable multifilament suture. Subcutaneous suturing is accomplished with a 4-0 absorbable monofilament suture and cyanoacrylate skin adhesive, followed by application of Steri-Strips. Meticulous hemostasis and suturing techniques prevent dead-space that predispose to seroma or hematoma formation and secondary infection. All surgical wounds are secured by sterile dressings, and the patient is taken to the recovery room.

When the patient is awake and alert, the unit is again activated, tested and programmed. The patient is instructed regarding limiting activities and braced per physician preference similar to post-trial instructions.


Spinal Cord Stimulation Medication


Medication Summary

The goals of pharmacotherapy are to reduce morbidity and prevent complications.

Benzodiazepines

Class Summary

These agents depress all levels of the central nervous system (eg, the limbic and reticular formations), possibly by increasing the activity of gamma-aminobutyric acid (GABA). Benzodiazepines also increase the frequency of chlorine channel opening in response to GABA binding. GABA receptors are chlorine channels that mediate postsynaptic inhibition, resulting in postsynaptic neuron hyperpolarization. The final result is a sedative-hypnotic and anxiolytic effect.

Benzodiazepines have been used in children for various indications, including the reduction of anticipatory or acute situational anxiety. Note the importance of using caution, and use these drugs only in conjunction with psychotherapy aimed at reducing the length of time on benzodiazepines.

Alprazolam (Xanax, Niravam)

Alprazolam binds receptors at several sites within the central nervous system (CNS), including the limbic system and reticular formation. Effects may be mediated through the gamma-aminobutyric acid (GABA) receptor system. It has a short half-life (< 12 h).

Lorazepam (Ativan)

Sedative hypnotic with short onset of effects and relatively long half-life. By increasing the action of GABA, a major inhibitory neurotransmitter in the brain, may depress all levels of CNS, including limbic and reticular formation. Excellent choice when patient must be sedated for longer than 24 h.

Antibiotics, Other

Class Summary

Empiric antimicrobial therapy should cover all likely pathogens in the context of this clinical setting. The administration of oral or parenteral prophylactic antibiotics 30-120 minutes before the procedure has been advocated.

Cefuroxime (Ceftin, Zinacef)

Cefuroxime is a second-generation cephalosporin that maintains the gram-positive activity of first-generation cephalosporins; it adds activity against Proteus mirabilis, Haemophilus influenzae, E coli, Klebsiella pneumoniae, and Moraxella catarrhalis.

Cefazolin

First-generation semisynthetic cephalosporin that by binding to 1 or more penicillin-binding proteins arrests bacterial cell wall synthesis and inhibits bacterial replication. Poor capacity to cross blood-brain barrier. Primarily active against skin flora, including S aureus. Regimens for IV and IM dosing are similar. Primarily active against skin flora, including S aureus. Typically used alone for skin and skin-structure coverage.

Clindamycin (Cleocin)

Clindamycin is a lincosamide is effective against aerobic and anaerobic streptococci (except enterococci). Clindamycin inhibits bacterial growth, possibly by blocking dissociation of peptidyl transfer ribonucleic acid (tRNA) from ribosomes, causing RNA-dependent protein synthesis to arrest.

Vancomycin

Vancomycin acts by inhibiting proper cell wall synthesis in gram-positive bacteria. It is indicated for the treatment of serious infections caused by beta-lactam–resistant organisms and in patients who have serious allergies to beta-lactam antimicrobials.

Local Anesthetics, Amides

Lidocaine (Xylocaine)

Lidocaine 1-2% is used. Lidocaine is an amide local anesthetic used in 1% concentration. The 1% preparation contains 10 mg of lidocaine for each 1 mL of solution. Lidocaine inhibits depolarization of type C sensory neurons by blocking sodium channels. Following application of 1% lidocaine without epinephrine or preservative, skin and subcutaneous analgesia is achieved.

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