Chapter Overview
Chapter Synopsis: Spinal cord stimulation (SCS) is an adjustable, nondestructive, mode of neuromodulation that delivers therapeutic doses of electrical current to the spinal cord for the management of neuropathic pain. Stimulation can be provided by both implanted paddle electrodes and percutaneous electrodes. Although a large body of work has been published, the exact mechanisms of action of SCS remain unclear. Overall SCS has been shown to have an approximately 50% improvement in pain relief that persists over years of follow-up, which is superior to many other invasive modalities.
Important Points: It is important to remember that the goal of neurostimulation is to reduce pain rather than to completely eliminate it. It can be applied reliably to patients with postlaminectomy syndrome/failed back surgery syndrome (FBSS), complex regional pain syndrome (CRPS), chronic critical limb ischemia (CCLI), and pain.
Clinical Pearls: Growing experience with SCS and improvement of available hardware have not only improved the reliability of this modality but have also allowed for its use in other disease processes. Other applications of SCS include its use in patients with angina, abdominal/visceral pain syndromes, brachial plexitis/neurogenic thoracic outlet syndrome, phantom limb pain, intractable pain secondary to spinal cord injury, mediastinal pain, cervical neuritis, and postherpetic neuralgia.
Clinical Pitfalls: Although results are largely beneficial, appropriate patient selection is imperative; psychological screening is needed to detect mood disorders, which can alter pain reporting and perception by a patient. In addition, coverage of low back pain alone is often difficult and has a lower rate of success. Complications of SCS include electrode migration or breakage, other hardware failure, infection, epidural hematoma, cerebrospinal fluid leak, and rarely neurologic deficit.
Establishing Diagnosis
Spinal cord stimulation (SCS) is an adjustable, nondestructive, mode of neuromodulation that delivers therapeutic doses of electrical current to the spinal cord for the management of neuropathic pain. The most common indications include postlaminectomy syndrome, complex regional pain syndrome (CRPS), ischemic limb pain, and angina ( Table 5-1 ); but it has been applied to other causes of intractable pain.
| Indications | Probability of Success |
|---|---|
| Postlaminectomy syndrome/failed back surgery syndrome (FBSS) | 50%-60% with >50% pain relief |
| Complex regional pain syndrome (CRPS) | 67%-84% |
| Chronic critical limb ischemia (CCLI) and pain | 70%-80% with >75% pain relief |
| Angina | No data available |
| Abdominal/visceral pain syndromes | |
| Brachial plexitis/neurogenic thoracic outlet syndrome | |
| Phantom limb pain | |
| Intractable pain secondary to spinal cord injury | |
| Mediastinal pain | |
| Cervical neuritis | |
| Postherpetic neuralgia |
Anatomy
The spinal canal contains several neural and nonneural structures that can be stimulated electrically. The properties of the intraspinal contents are similar to an inhomogeneous conductor. Knowledge of the different type of responses and their correlation with the underlying anatomical substrate is extremely important in implementing strategies for SCS.
Stimulation of large myelinated afferent fibers may occur in four different areas: the dorsal root, the dorsal root entry zone (DREZ), the dorsal horn, or the dorsal columns ( Fig. 5-1 ). Electrical stimulation of these structures results in an ipsilateral tingling paresthesia, but changes to discomfort and pain with increasing stimulation voltage. Particularly, activation of the dorsal roots causes a radicular paresthesia at a particular dermatomal level, whereas stimulation of the dorsal columns causes paresthesia in areas of the body caudal to the level of the electrode. In the cervical and thoracic cord the dorsal columns are usually about 3 to 5 mm wide, largest at the level of the cervical enlargement, and smallest in the midthoracic spine.
The single most important factor in determining stimulation parameters is the width of the cerebrospinal fluid (CSF) space dorsal to the spinal cord ( Fig. 5-2 ). Increasing thickness of the dorsal CSF (dCSF) layer leads to early stimulation of the dorsal root fibers instead of the dorsal column fibers. This is largely because rootlet fibers, which are immersed in well-conducting CSF, have high conductivity at the DREZ. Therefore the stimulation threshold for the segmentary system ranges from 0.1 V to 0.5 V, whereas activation of the dorsal columns occurs at a threshold that is at least 0.5 V to 1 V higher.
Differentiating between stimulation of the dorsal root, DREZ, or dorsal horn can be exceedingly difficult. One can expect dorsal root stimulation with laterally placed electrodes; stimulation of the DREZ or dorsal horn is more likely if the electrode is placed near midline. In the latter group stimulation leads to segmentary paresthesias that are rapidly followed by activation of the dorsal columns with a small voltage increment. Interestingly, minute changes in mediolateral position of the electrode have been shown to lead to movement of the paresthesias in a W two-dimensional pattern as shown in Fig. 5-3 .
If electrical stimulation reaches the ventral cord, motor structures are affected, and consequently muscle contractions are seen. Much like dorsal cord stimulation, activation of the ventral root or motor neurons causes muscular contractions in a myotomal distribution, whereas activation of the descending corticospinal pathways results in diffuse muscle contractions caudal to the level of the electrode. In the vast majority of patients stimulation over the dorsal columns is not complicated by simultaneous activation of the motor system unless a significantly higher voltage is implemented. Nevertheless, there have been cases in which activation of the motor structures has been seen to occur at the same threshold as the sensory system; therefore selective stimulation of the dorsal column could not be successfully maintained.
Understanding the somatotopy of the spinal cord is paramount to knowing the technical aspects of implantation. A basic tenet of SCS is to create an area of paresthesia that overlaps the patient’s region of pain. Therefore correlation of the somatotopy and the level of the spinal cord is necessary. Barolat and colleagues have published extensively on the mapping of the spinal structures, which led to a database correlating areas of sensory response to levels dorsal SCS.
High cervical regions such as C2 can cover regions of the posterior occiput and occasionally the lower jaw. C2-C4 stimulation provides coverage of the neck, shoulder, and upper extremities, as well as the face as a result of involvement of the descending nucleus of the trigeminal nerve. As one moves to the lower cervical region such as C5-C6, stimulation affects the entire hand. To cover the anterior chest wall or the axilla, an electrode placed toward C7-T1 is necessary.
More commonly an implanter seeks coverage of the lower extremities. Lateral placement at T11-T12 covers the anterior thigh, whereas placement at T11-L1 can cover the posterior thigh. Although coverage of the foot as a whole can be achieved by stimulating these same areas, it is difficult to cover the sole of the foot, which may require insertion on the lumbar L5 or S1 nerve roots. Low back pain is very difficult to cover because midthoracic stimulation can also affect the chest and abdominal wall. In general, midline placement of the electrode at T8-T10 seems to provide the best outcomes.
Recently this paradigm has been challenged by case series demonstrating successful four-limb paresthesia in patients receiving only cervical stimulation. In general, this effect was achieved with stimulation with longer pulse widths and lower frequencies. Although the mechanism is not yet well understood, some have hypothesized that thinner dorsal CSF thickness in the cervical spine may allow for the stimulation of deeper medial and midline fibers that correspond to the lower extremities.
Basic Science
The enthusiasm with SCS began with the introduction of the gate control theory for pain control by Melzack and Wall in 1965. They noted that stimulation of large myelinated fibers of peripheral nerves resulted in paresthesia and blocked the activity in small nociceptive projections. In other words, appropriate stimulation of a competing afferent signal can effectively block an existing pain signal.
Although a large body of work has been published, the exact mechanisms of action of SCS remain unclear. The computer modeling work of Coburn, Coburn and Sin, Holsheimer and associates, Holsheimer and Strujik, and Holsheimer and Wesselink have shed some light, at least theoretically, on the distribution of the electrical fields within the spinal structures. It is clear that stimulation on the dorsal aspect of the epidural space creates complex electrical fields that affect a large number of structures, including afferents within the peripheral nerve, dorsal columns, or supralemniscal pathways. Mechanisms at distinct levels of the central nervous system apart from the spinal cord are believed to contribute to the effects of SCS. Such mechanisms include antidromic action potentials that pass caudally in the dorsal columns to activate spinal segmental mechanisms in the dorsal horns and action potentials ascending in the dorsal columns activating cells in the brainstem, which in turn might drive descending inhibition. Recent work by Schlaier and associates demonstrated changes in cortical excitability with SCS, which is believed to be N- methyl- d -aspartate (NMDA)-related neuroplasticity at the supraspinal level. At the chemical level, animal studies suggest that SCS triggers the release of serotonin, substance P, and γ-aminobutyric acid (GABA) within the dorsal horn.
Indications/Contraindications
As mentioned previously, SCS has been used for a variety of pain conditions and is particularly indicated for pain of neuropathic origin rather than for nociceptive pain. It is important to realize that neurostimulation is a treatment option along the continuum of pain control.
Another important consideration is the need for careful psychological screening in the selection of candidates for SCS procedure. This becomes particularly important because mood disorders can alter pain reporting and perception by a patient. Patients who have frank psychiatric disorders, excessive depression, anger, or unrealistic expectations may be inappropriate for SCS; however, some of these patients may become favorable candidates for SCS after appropriate psychiatric therapy. The most common indications for SCS follow.
Postlaminectomy Syndrome/Failed Back Surgery Syndrome
Postlaminectomy syndrome is vaguely defined. The term has included pain localized to the center of the lower lumbar area, pain in the buttocks, persistent radicular pain, or diffuse lower extremity(s) pain. Arachnoiditis, epidural fibrosis, radiculitis, microinstability, recurrent disc herniations, and infections have been perpetrated in the etiology of this syndrome. Although SCS is accepted in the treatment of leg pain, its widespread use for relief of pain in the lower lumbar area still remains to be defined.
Complex Regional Pain Syndrome
CRPS is characterized by gradually worsening severe pain, swelling, and skin changes of the hands, feet, elbows, or knees. It may be secondary to injury (CRPS type II) or may not have a clear etiology (CRPS type I). Implementation of SCS in these patients may be more difficult than with any other patient group, partly because of the ill-defined nature of the pain. In addition, there is an increased possibility of aggravating the original pain or causing new pain or allodynia at the site of implanted hardware. Furthermore, the pain may also spread to other body parts, thus making lasting coverage extremely challenging.
Chronic Critical Limb Ischemia and Pain
In 1973 Cook and Weinstein were the first to suggest that the indications for SCS might extend beyond intractable pain control. They observed a group of patients with multiple sclerosis who underwent SCS to treat their chronic pain. Unexpectedly the patients experienced not only pain relief but also an improvement in mobility and sensory and bladder function. They incidentally found an apparent improvement in lower limb blood flow and subsequently used SCS in patients whose primary problem was peripheral vascular disease (PVD). These patients were found to have relief of rest pain, increased skin temperature, improved plethysmographic blood flow, and healing of small cutaneous ulcers.
Similarly, it is believed that SCS benefits patients with pain secondary to Raynaud phenomenon and diabetic neuropathy as well.
Angina
The role of SCS in the management of refractory angina pectoris seems to be very promising. There are well-documented reports in the literature revealing uniformly good results in the relief of angina pain. Further, the results have been maintained in long-term follow-up and have been substantiated by a reduction in the intake of nitrates as well. Interestingly, other findings have supported the evidence that SCS has effects that go beyond pain relief. The observations that there is less ST segment depression and that the exercise capacity, the time to angina, and the recovery time all improve with stimulation may suggest that there is a reduction in ischemia. In a positron emission tomography study, a redistribution of myocardial flow in favor of ischemic parts of the myocardium has been demonstrated as a long-term effect of SCS, both at rest and after pharmacological stress induction.
Moreover, since the relation between pain and myocardial ischemia has not been fully clarified, we do not know whether the pain relief is caused by direct depression of nociceptive signals in the spinal cord or whether there is secondary gain from a reduction in the ischemia. On the one hand, a significant amount of work by Foreman has shown that dorsal column stimulation inhibits the activity of spinothalamic tract cells, which are typically activated by cardiac sympathetic afferents or intracardiac bradykinin. On the other hand, the effects of stimulation might be equivalent to those of a sympathectomy in that it inhibits a hyperactive sympathetic system. This latter mechanism has been shown experimentally in the rat by Linderoth and associates.
Brachial Plexitis/Neurogenic Thoracic Outlet Syndrome
SCS has also been described for pain secondary to either brachial plexitis or neurogenic thoracic outlet syndrome. Most of these patients complain of pain that affects the upper extremity, the shoulder, the trapezius, the axilla, and/or the anterior upper chest wall. Unfortunately literature is relatively lacking for this indication.
Other Indications
Early studies in the 1980s demonstrated benefits of SCS for urinary incontinence, with consequent increases in continence and substantial reduction in residual urine volumes. These effects were found with stimulation of ventral sacral roots, a method that has also been shown to improve bowel function in patients with spinal cord injury. Trials in the 1970s reported initial benefits of this modality for phantom limb pain, but few of these patients experienced lasting results with long-term follow-up. Scattered reports also exist demonstrating the benefit of SCS in the treatment of intractable pain secondary to spinal cord injury pain, mediastinal pain, cervical neuritis, and postherpetic neuralgia.
Equipment
Electrical stimulation consists of rectangular pulses delivered to the epidural space through an implanted electrode via a power source. Electrodes were initially all unipolar in nature, but subsequently bipolar and tripolar arrays were developed. The initial radiofrequency (RF)-driven passive receivers have given way to implantable pulse generators (IPGs), which were first introduced in the mid-1970s. In 1980 the first percutaneous quadripolar electrode was produced; it could be reprogrammed noninvasively through an external transmitter. Subsequently IPGs that can be both transcutaneously charged and programmed have been developed. This most recent advance is now leading a renewed interest in the possibility of using special electrode arrays in the delivery of electrical stimulation to the spinal cord.
Percutaneous Electrodes
Percutaneous electrodes, otherwise known as wire electrodes, are particularly appealing because they can be inserted without much dissection and are more appropriate for performing a trial to assess candidacy for a permanent implant. During implantation these electrodes can be advanced over several segments in the epidural space, allowing testing of several spinal cord levels to assess for optimal electrode position. After the trial period these electrodes can easily be removed in the physician’s office. Contemporary percutaneous electrodes are slim (i.e., only a few millimeters in diameter) and contain four (quadripolar) or eight (octipolar) contacts with various spacings. These percutaneous electrodes also come in varying lengths, which differ by manufacturer. Last, extension cables are occasionally necessary to bridge the distance from the spinal entry point to the pocket in which the battery will reside.
Electrode choice generally depends on how many segments of the spinal cord are to be covered. Although electrodes with larger spacing allow for broader coverage, closer spacing permits better steering and electric field shaping. The general trend is to use one or two quadripolar electrodes for limb pain and one or two octipolar electrodes for axial pain. Even insertion of three electrodes is being explored for better steering of current.
A major disadvantage of percutaneous electrodes is their tendency to migrate—secondary to their inherent flexibility, which is necessary for their insertion through a Tuohy needle and their cylindrical shape. These factors can lead to migration even months after implantation. As a result, patients with percutaneous leads have reported a greater positional variance in their paresthesia. To address this issue, some percutaneous electrodes have been introduced that require a stiffening stylet for introduction. Another disadvantage is that percutaneous electrodes are less energy efficient than plate electrodes. The electrical current is distributed circumferentially around the electrode and consequently results in greater shunting of current.
Recently ANS (A St. Jude company, Plano, Tex) has introduced a slim-line plate type electrode that can be inserted percutaneously. The broader electrode base provides a surface in which fibrosis should lessen the risk of caudal electrode migration. The slimmer profile of the electrode might also have advantages in the cervical spine where spinal cord compression might be an issue. Finally, the design emulates that of a miniplate lead since the contacts are on one side with the other side being insulated, leading to a more energy efficient system ( Fig. 5-4 ).
Paddle Electrodes
Paddle or laminotomy electrodes are offered in many sizes, shapes, lengths, spacing, and configurations of electrodes. They are primarily offered as single- and dual-column configurations; with three (tripole)- and five-column models being introduced more recently ( Figs. 5-5 and 5-6 ). With regard to their shape, these electrodes may come with curved or hinged leads to help facilitate insertion.
The main advantage of these electrodes resides in their more inherent stability in the dorsal epidural space and lesser propensity to migrate. Some preliminary data by North and associates also suggest a broader stimulation pattern and lower stimulation requirements. As alluded to previously, paddle electrodes are also energy efficient in delivering electrical stimulation.
There is some theoretical evidence that shaping of the electrical field is possible with more complex electrode arrays. Current available tripole electrodes and their dimensions are shown in Table 5-2 . Holsheimer and colleagues concluded that the transverse tripolar system enabled finer control of paresthesia because of its ability to more accurately shape the stimulating electromagnetic field. Specifically the transverse tripolar configurations allow for stimulation of the dorsal columns at higher amplitudes before the dorsal roots begin experiencing clinically significant stimulation. These electrodes are composed of a central cathode flanked by lateral anodes, which modulate the field medially or laterally, depending on their programmed voltage. In other words, the current delivered to the spinal cord can be steered not by moving the electrode but instead by simply modifying the voltage ratio between the central cathode and two flanking anodes. The lateral anodes create a shielding effect such that the dorsal roots are protected from stimulation; deeper activation of the dorsal columns is achieved with higher voltages ( Fig. 5-7 ). This feature ultimately results in a wider therapeutic range, wider paresthesia coverage, and a greater probability to fully cover the painful area with paresthesia. Possible programming configurations of these tripole electrodes are shown in Fig. 5-8 .
Although increasing the number of contacts allows for finer control of the electromagnetic field, there is also a significant increase in power consumption and an even greater increase in the complexity of programming. The number of possible configurations is n 2 n , where n is the number of electrodes. Therefore with two contacts the total number of configurations possible is eight; with four contacts, sixty-four; and with sixteen contacts, it increases exponentially to reach a number in the millions.
Implantable Pulse Generators
The totally implantable pulse generator contains its own lithium battery. Activation and modification of stimulation parameters can occur through an external transcutaneous telemetry device, which the patient can carry. More extensive programming of the system can be achieved through a small portable unit that can be managed by the physician. These generators allow stimulation with fine resolution increments of 0.05 V and with varying rates and pulse widths.
Some systems also use multiple channels, which allow for treatment of multiple, distinct areas of the body using a single IPG. Once programmed, the patient can also select a particular channel at any particular time to best cover his or her symptoms.
Battery life varies with usage and with the used parameters such as voltage, rate, and pulse width. These factors, particularly voltage, are affected by patient factors such as epidural scarring and accommodation to stimulation. With typical use these batteries last between 2.5 to 4.5 years. Replacement of the battery requires a surgical procedure, which is usually performed on an outpatient basis.
IPG selection is based on many variables. From a practical standpoint, the first and foremost reason might be the size of the patient. Although larger batteries have a longer life, they obviously have more bulk and can often become the source of significant patient complaints. Although the IPG can be implanted into the buttock, abdomen, or subclavicular area, implantation in the buttock has multiple advantages. This location makes it easier to tunnel the electrode leads to the battery; with a patient placed prone, repositioning is not required to reach the buttock region.
Rechargeable Implantable Pulse Generators
Rechargeable systems have now become available. Medtronic’s device, known as the Restore Rechargeable Neurostimulation System , uses a battery with an estimated 9-year total life span and takes about 6 hours to fully recharge. Advanced Neuromodulation Systems’ Eon device has a battery life that is currently estimated to be 7 years. Boston Scientific’s Precision device has a battery life estimated to be 5 years. A detailed comparison of the features of these rechargeable batteries is shown in Table 5-3 .
In an analysis of cost between rechargeable and nonrechargeable systems, Hornberger and associates found that the former would require 2.6 to 4.2 fewer replacement procedures, resulting in a total lifetime savings ranging from $104,000 to $168,833. With regard to overall efficacy, a 12-month study looking specifically at the Restore rechargeable IPG demonstrated that the system was easy to use and that all patients were able to recharge it successfully. Moreover, both patient and physician satisfaction was high, with significant improvements in pain, quality of life, and functional status.
Technique
In 1967 Shealy and Cady and Shealy, Mortimer, and Reswick inserted the first dorsal column stimulator in a human suffering from terminal metastatic cancer. Subsequently electrodes have been implanted using a variety of techniques: via a laminectomy in the subarachnoid space ; between the two layers of the dura, or in the epidural space, either dorsal or ventral to the spinal cord. Subsequently less invasive percutaneous techniques were introduced. Current practice indicates the placement of electrodes in the dorsal epidural space and connecting them to an implantable pulse generator usually placed in the buttock. However, before a permanent system is implanted, patients typically undergo a trial period of 3 to 10 days, during which time stimulation is provided by percutaneous electrodes to determine their overall efficacy.
Placement of Percutaneous Electrodes
To place percutaneous electrodes, the patient is positioned in a comfortable prone position on a fluoroscopy table with a pillow underneath the abdomen. This position creates some kyphosis, which may facilitate electrode insertion.
A fundamental consideration in choosing the level of entry is that several centimeters of the lead have to lie in the epidural space to ensure maximal stability of the electrode and minimize unwanted migration. To ensure this, insertion must take place at least two spine segments below the desired target. For cervical placement one must bear in mind the cervical cord enlargement and insert the electrode below the T1-T2 level when possible. Insertion is performed under fluoroscopy; this equipment must be ready to function in both the anteroposterior and lateral planes at the time of needle insertion. A Tuohy needle is inserted with as shallow an angle as possible to minimize the risk of electrode breakage and a paramedian approach to avoid friction with the spinous process.
Although tactile feedback is important in locating the epidural space, it should not be the sole source of information. The most common method is feeling a loss of resistance while injecting fluid with a low-friction glass syringe; however, injecting a small amount of air may be superior because fluid injected in the epidural space may later be aspirated through the needle and give the false impression of being in the subarachnoid space. After multiple passes at one spine level have been performed, the loss-of-resistance method may lose its reliability. Therefore inserting a Seldinger wire through the needle can provide invaluable information about the degree of penetration into the spinal canal. If the needle tip is in the interspinous ligament and has not penetrated the ligamentum 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 advancement and the location of the wire under fluoroscopic imaging can further clarify its position.
If more than one electrode is inserted, it is wise to insert the other needles before inserting the electrodes since subsequent needle insertion might shear an already implanted electrode. Moreover, it is often possible to insert two electrodes simultaneously and advance them synchronously in the epidural space while maintaining their relative position and spacing.
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. Although this may seem obvious, it may require multiple attempts at needle placement. This is particularly true if the dura has been previously pierced and CSF has escaped and pooled in the dorsal epidural space. One major difference is that in the subarachnoid space much less resistance is encountered when moving the electrode, particularly for lateral movements. The wire seems to be “floating” and undergoes large shifts of direction in the subarachnoid space; whereas in the epidural space movements are more discrete and obtained only with specific manipulations. Nevertheless, unfortunately the same type of wire/electrode movement can be experienced epidurally if the dural sac has collapsed significantly secondary to CSF loss. In such instances electrical stimulation clarifies the position; a subarachnoid placement elicits responses at much lower thresholds than an epidural placement.
When the epidural space is satisfactorily identified, the electrode is inserted gently under fluoroscopic guidance in the anteroposterior view. Once it has been inserted through the tip of the needle, the electrode must be removed without damage to the insulation surrounding the electrical contacts. If the electrode does not slide without minimal resistance, the needle and the electrode should be removed together. Every time the electrode is withdrawn through the needle, it should be inspected for minute breaks in the insulation, which would necessitate its disposal. Alternatively, a sleeve can be inserted over the guidewire in the epidural space. The guidewire is then removed, and the electrode is inserted through the sleeve. This obviates the risk of shearing the electrode during manipulation.
During placement the electrode may curve around the dural sac to the ventral epidural space. In the anteroposterior projection this might be indistinguishable from a proper midline dorsal location. A gentle lateral curve of the electrode shortly after its entry in the epidural space should raise the suspicion that it is actually lying ventrally. Absolute confirmation of the ventral location arises from stimulation generating violent motor contractions or observation in the lateral plane. An x-ray film should always be obtained to document electrode level and position. Care must be taken to identify either the bottom most visualized rib as the twelfth rib or to use the sacrum to count the number of lumbar vertebra to document the level of implant.
Once in place, the electrode must be secured to the interspinous ligament to minimize dislodgment. An important measure is to secure the wire as loops at the electrode insertion site to relieve the strain and reduce migration during bending. Applying suture directly to the leads has been shown to be safe since it does not cause substantial physical damage or any electrical impairment. Nevertheless, manufacturers still do not recommend tying directly to the lead and continue to recommend the use of anchors to secure leads. When anchors are used to secure the electrode, one must remember to secure the anchor to both the electrode and the fascia.
Placement of Paddle Electrodes
Paddle electrodes require a surgical procedure for implantation under direct vision. The patient may be placed in one of two basic positions: prone or semilateral. The prone position allows a more intuitive understanding of the spatial relations, which is more familiar to surgeons. At the same time, it can be difficult to obtain adequate sedation and maintain the airway in this position. In the semilateral position the surgeon has access to the spine and the flank, abdomen, or buttock for the implant of the pulse generator. The patient is asked to place himself or herself in the most comfortable position. If the pain is predominantly on one side, the patient is asked to lie on the less affected side. In this position airway management is safer than in the prone position, and the anesthesiologist is more comfortable in keeping the patient deeply sedated. For cervical placement the neck is slightly flexed but not excessively rotated laterally because extreme rotation substantially increases the difficulty of electrode placement. In general the variable degree of rotation with the patient in the semilateral position makes it more difficult to determine the location of the midline, which may be a significant problem in the cervical area.
A laminotomy is performed approximately one level below the intended level to allow the plate electrode to rest at the planned target for stimulation. Localization is performed with fluoroscopy or with a plain x-ray film, using metallic markers placed on the skin at the level of the planned incision. In a thin patient the incision is about 1 inch in length; and even in large individuals the incision seldom exceeds 2 inches because three to four levels can be accessed by extending the dissection and stretching the skin edges with a Gelpi retractor. Subperiosteal dissection is performed to expose the upper half of the inferior spinous process and the entire superior spinous process. Parts of the superior spinous process are then incrementally removed to expose the interposing ligamentum flavum. In the lower thoracic and upper lumbar area this usually requires the removal of the inferior third of the spinous process. However, in the midthoracic area the acute angle and significant overlapping of the spinous processes necessitates the removal of the whole spinous process. Following removal of the ligamentum flavum, the electrode is inserted into the dorsal epidural space; and its position is confirmed with fluoroscopy and by functional testing.
Such testing previously consisted of waking the patient and determining efficacy of stimulation from patient responses. This has gradually been replaced with intraoperative stimulation with electromyography (EMG) stimulation coupled with somatosensory evoked potential (SSEP) testing, which helps detect proper positioning of the electrode. EMG stimulation is performed with a signal consisting of a greater than 310 microseconds pulse width at a frequency of 5 Hz; the amplitude is gradually increased until EMG signal changes are detected. Alternatively, awake testing is commonly performed with 50-Hz stimulation. As one would expect, bilateral extremity stimulation suggests midline placement, and early root onset implies too lateral of a placement. In addition, stimulation of the dorsal columns results in a greater area of coverage with paresthesia—the percentage of body surface covered is inversely proportional to the distance between two electrodes. To achieve this, most of the current has to be directed to an area within 2 to 3 mm on each side of the physiological midline. It is important to note that the physiological midline may differ from its anatomic counterpart by up to 2 mm ; in such instances one should rely on the former since it has a higher clinical correlation. With this feedback, the implant should be placed midline in cases of axial symptomatology or with a bias to one side for patients with unilateral pain. By correlating paddle position to the prior position of trial electrodes and testing motor stimulation responses, one can effectively find the proper location for implantation without waking the patient.
As with percutaneous leads, it is important to secure the leads with a strain relief loop with a diameter no less than 2.5 cm to minimize lead migration with movement. This loop can be sutured directly to the paraspinal muscles before closing the fascial layer.
Implantable Pulse Generator Placement
The IPG should be placed in an area where the unit will have enough overlying soft tissue but also not be pressing against bone, usually the abdomen or buttock. To find the appropriate area of implantation in the latter, three boney prominences are used as landmarks. The posterior superior iliac crest marks the medial border; the greater trochanter of the femur and the apex of the iliac crest define the lateral border. The IPG is implanted approximately 2 to 3 cm deep in the lateral aspect of this triangle ( Fig. 5-9 ).
