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Brown: Atlas of Regional Anesthesia, 3rd ed., Copyright © 2006 Saunders, An Imprint of Elsevier
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PLACEMENT

Anatomy.

The epidural SCS lead is placed directly in the dorsal epidural space just to one side of the midline using a paramedian, interlaminar approach. Entry into the epidural space is performed several levels below the final intended level of lead placement. Typically, leads for stimulation of the low back and lower extremities are placed via the L1–2 interspace, and those for upper extremity stimulation are placed via the C7–T1 interspace. Investigators have mapped the patterns of electrical stimulation of the dorsal columns and the corresponding patterns of coverage reported by patients with leads in various locations. In general, the epidural lead must be positioned just 2 to 3 mm to the left or right of the midline on the same side as the painful region to be covered. For lower extremity stimulation, successful coverage is usually achieved by placing the lead between the T8 and T10 vertebral levels, whereas upper extremity stimulation usually requires lead placement between the occiput and C3 vertebral levels. If the lead ventures too far from the midline, uncomfortable stimulation of the exiting nerve roots results. If the lead is placed too low, overlying the conus medullaris (at or below L1–2), unpredictable patterns of stimulation may result. In the region of the conus, the fibers of the dorsal columns do not lie parallel to the midline; rather, they arc from the corresponding nerve root entering the spinal cord toward their eventual paramedian location several levels cephalad.

Position.

Placement of a percutaneous trial spinal cord stimulator lead can be carried out in any location that is suitable for epidural catheter placement. This may be done in the operating room but can easily and safely be carried out in any location that allows adequate sterile preparation of the skin and draping of the operative field and that has fluoroscopy available to guide anatomic placement. Using a strictly percutaneous trial, the trial lead is placed in the same fashion as that used for permanent lead placement, but the lead is secured to the skin without any incision for the trial period.

Before permanent spinal cord stimulator implantation, discuss with the patient the location of the pocket for the impulse generator. The most suitable regions are the lower quadrant of the abdomen and the lateral aspect of the buttock. Once the site is determined, mark the proposed skin incision with a permanent marker while the patient is in the sitting position. The position of the pocket is deceptively difficult to determine once patients are lying on their side. If the location is not marked, the pocket is often placed too far laterally in the abdominal wall. Placing the impulse generator in the buttock allows the entire procedure to be carried out with the patient in the prone position and simplifies the operation by obviating the need to turn from the prone to the lateral position halfway through implantation.

Implantation of a spinal cord stimulator lead and impulse generator is a minor surgical procedure that is carried out in the operating room using aseptic precautions, including skin preparation, sterile draping, and use of full surgical attire ( Fig. 46-1 A ). The procedure must be conducted using local anesthesia and light enough sedation that the patient can report feeling the electrical stimulation during lead placement.

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Figure 46-1a  Spinal cord stimulator trial and permanent implantation. A, View of a typical operating room arrangement during spinal cord stimulator implantation: The patient is placed in the prone position with the C-arm in place for an anteroposterior (AP) view of the thoracolumbar spine. B, Posteroanterior radiograph of a spinal cord stimulator lead in position for stimulation of the right lower extremity. The lead lies to the right of, with the tip over, the inferior endplate of the T9 vertebral body. During initial patient positioning, care must be taken to ensure that the image is in the AP plane without rotation by moving the image intensifier in the mediolateral direction until the spinous process is seen midway between pedicles. C, Initial epidural needle placement at the L1–2 interspace using a paramedian approach. The angle of entry into the epidural space must be less than 45 degrees relative to the plane of the epidural space to ensure that the lead can pass easily.


The patient is positioned on a radiolucent table in the prone position (see Fig. 46-1 A ). Initial lead placement can be carried out with the patient in a lateral decubitus position, but even small degrees of rotation along the spinal axis can make positioning the lead difficult. The arms are extended upward so they are in a position of comfort well away from the surgical field. The skin is prepared, and sterile drapes are applied. For stimulation in the low back and lower extremities, the radiographic C-arm is positioned directly over the thoracolumbar junction to provide an anteroposterior view of the spine. Care must be taken to ensure that the radiographic view is not rotated by observing that the spinous processes are in the midline, halfway between the vertebral pedicles ( Fig. 46-1 B ).

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Figure 46-1b  D, The electrode is advanced under continuous fluoroscopic guidance using a slight twisting motion to steer the catheter to the desired position just lateral to the midline on the side of the desired stimulation. E, For trial stimulation using a percutaneous lead, the lead is secured to the back using either sutures or sterile, adhesive strips; a sterile occlusive dressing is then placed. The intrathecal catheter is placed through the spinal needle under fluoroscopic guidance. F, A cephalad-caudad incision is made through the skin and subcutaneous tissues, with the incision extending above and below the needle entry point. Using blunt dissection, the skin and subcutaneous tissues are further divided until the lumbar paravertebral fascia is exposed.


Procedures.

The L1–2 interspace is identified using fluoroscopy. The epidural needle supplied by the device manufacturer must be used to ensure that the lead can advance through the needle without damage. The needle is advanced using a paramedian approach starting 1 to 1.5 cm lateral to the spinous processes and somewhat caudad to the interspace to be entered. The needle is directed to enter the spinal space in the midline with an angle of entry no greater than 45 degrees from the plane of the epidural space ( Fig. 46-1 C ). If the angle of attack of the needle during the initial entry into the epiduralspace is too great, the epidural lead is difficult to thread as it negotiates the steep angle between the needle and the plane of the epidural space. The epidural space is identified using a loss-of-resistance technique. The electrode is then advanced through the needle and is directed to remain just to one side of midline as it is threaded cephalad under fluoroscopic guidance. The electrode contains a wire stylet with a slight angulation at the tip; gentle rotation of the electrode as it is advanced allows the operator to direct the electrode’s path in the epidural space ( Fig. 46-1 D ). For stimulation in the low back and lower extremities, the electrode is initially positioned 2 to 3 mm from the midline on the same side as the patient’s pain between the T8 and T10 vertebral levels (see Fig. 46-1 B ). Final electrode position is attained by connecting the electrode to an external impulse generator and asking the patient where the stimulation is felt. In general, cephalad advancement results in stimulation higher in the extremity, and caudad movement leads to stimulation lower in the extremity. However, if the lead is angled even slightly from medial to lateral, the pattern of stimulation may change less predictably with movement of the electrode; for example, cephalad advancement can lead to stimulation lower in the extremity under these circumstances. The final electrode position should be recorded using radiography so a permanent lead can be placed in the same position (see Fig. 46-1 B ). For trial stimulation, the needle is removed, the electrode secured to the back, and a sterile occlusive dressing applied ( Fig. 46-1 E ). The patient is instructed in the use of the external pulse generator and scheduled to return in 5 to 7 days to assess his or her response and to remove the trial lead.

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Figure 46-1c  G, The needle and stylet are removed together while holding the lead firmly in position. H, The lead is secured to the paravertebral fascia using an anchoring device provided by the manufacturer. I, A transverse incision is created in the abdominal wall midway between the umbilicus and the anterior axillary line or over the superolateral aspect of the buttocks. A pocket of sufficient size to accommodate the impulse generator is then created using blunt dissection, which can be accomplished using the fingertips or surgical scissors and a repeated spreading (rather than cutting) motion. J, A tunneling device provided by the manufacturer is used to position the catheter in the subcutaneous tissue between the paravertebral incision and the pocket.


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Figure 46-1d  K, After ensuring good hemostasis, the impulse is placed in the pocket, with any excess lead coiled loosely and inserted within the pocket, behind the impulse generator. L, The pocket and paravertebral incisions are then closed in two layers: a layer of interrupted, absorbable sutures in the subcutaneous tissue overlying the impulse generator and lead and a separate layer in the skin.


The procedure for initial lead placement for permanent implantation is identical to that for the trial stimulation. Once the final lead is in position and the optimal pattern of stimulation is confirmed, the lead is secured, a pocket for the impulse generator is created, and the lead is tunneled underneath the skin for connection to the impulse generator. Following initial lead placement, the epidural needle is withdrawn slightly (about 1–2 cm) but left in place around the lead in the subcutaneous tissues to protect the lead during the subsequent incision and dissection ( Fig. 46-1 F ). A 5- to 8-cm incision parallel to the axis of the spine is extended from cephalad to caudad to the needle, extending directly through the needle’s skin entry point (see Fig. 46-1 F ). The subcutaneous tissues are divided using blunt dissection until the lumbar paraspinous fascia is visible surrounding the needle shaft. The stylet is then removed from the lead, and the needle is withdrawn, using care not to dislodge the electrode ( Fig. 46-1 G ). The lead is then secured to the paraspinous fascia using a specific anchoring device supplied by the manufacturer ( Fig. 46-1 H ).

If lead placement has been carried out in the prone position and the impulse generator is to be placed in the abdominal wall, the lead must be coiled underneath the skin, the paraspinous incision temporarily closed using staples, and a sterile occlusive dressing applied. The sterile drapes are then removed, and the patient is repositioned in the lateral decubitus position with the side where the abdominal pocket is located facing upward. After repeat preparation of the skin and application of sterile drapes, attention is turned to creating the pocket in the patient’s abdominal wall or overlying the buttock (when the impulse generator is placed over the buttock, this site is included in the initial skin preparation and draping).

An 8- to 10-cm transverse incision is made along the previously marked line, and a subcutaneous pocket is created using blunt dissection ( Fig. 46-1 I ). The pocket should always be created caudad to the incision; if the pocket is placed cephalad to the incision, the weight of the impulse generator on the suture line is likely to cause wound dehiscence. In many patients, the blunt dissection can be accomplished using gentle but firm pressure with the fingers. It is simpler and less traumatic to use a small pair of surgical scissors to perform the blunt dissection using repeated opening (not closing or cutting) motions. After creating the pocket, the impulse generator is placed in the pocket to ensure that the pocket is large enough. The impulse generator should fit completely in the pocket without any part of the device extending into the incision. With the device in place, the wound margins must fall into close apposition. There should be no tension on the sutures during closure of the incision lest the wound dehisce.

After the pocket creation is completed, a tunneling device is extended in the subcutaneous tissues between the paraspinous incision and the pocket ( Fig. 46-1 J ). The electrode is then advanced through the tunnel. (Tunneling devices vary and are specific to each manufacturer.) The means by which the electrode is connected to the impulse generator also varies by manufacturer; some devices use a lead extension that connects the impulse generator and the lead, and others use a one-piece lead that is connected directly to the impulse generator. After tunneling, the lead and/or lead extension is connected with the impulse generator. Any excess lead is coiled and placed behind the impulse generator in the pocket ( Fig. 46-1 K ). This loop allows patient movement without placing tension on the distal electrode, causing it to be pulled from the epidural space. The skin incisions are then closed in two layers: a series of interrupted subcutaneous sutures to close securely the fascia overlying the impulse generator in the pocket and the electrode over the paraspinous fascia, followed by skin closure using sutures or staples ( Fig. 46-1 L ).

Potential Problems.

Bleeding and infection are risks inherent to all open surgical procedures. Bleeding in the impulse generator pocket can lead to a hematoma surrounding the pump that may require surgical drainage. Bleeding along the subcutaneous tunneling track often causes significant bruising in the region but rarely requires treatment. Similar to other neuraxial techniques, bleeding in the epidural space can lead to significant neural compression. Signs of infection in the impulse generator pocket typically occur within 10 to 14 days following implantation but may occur at any time. Some practitioners have reported successful treatment of superficial infections of the incision overlying the pocket with oral antibiotics aimed at the offending organism and close observation alone. However, infections in the pocket or along the lead’s subcutaneous course almost universally require removal of all implanted hardware and treatment with parenteral antibiotics to eradicate the infection. Lead and deep tissue infections can extend to involve the neuraxis and result in epidural abscess formation, meningitis, or both.

There is a significant risk of dural puncture during initial localization of the epidural space using the loss-of-resistance technique. The epidural needle used for electrode placement is a Tuohy needle that has been modified by extending the orifice to allow the electrode to pass easily. This long bevel often results in equivocal loss of resistance; it is not uncommon to have minimal resistance to injection along the entire course of needle placement. To minimize the risk of dural puncture, the needle tip can be advanced under fluoroscopic guidance and first seated on the margin of the vertebral lamina (taking care to place additional local anesthetic during advancement). In this way, the depth of the lamina is certain and the needle need be advanced only a small distance over the lamina, through the ligamentum flavum, and into the epidural space. Loss of resistance is used only during the final few millimeters of needle advancement over the lamina. If dural puncture does occur, there is no clear consensus on how to proceed. Some practitioners abandon the lead placement and allow 1 to 2 weeks before reattempting it; this approach allows the practitioner to watch and treat postdural puncture headache, which is nearly certain to occur. Other practitioners proceed with lead placement through a more cephalad interspace; if postdural puncture headache ensues and fails conservative treatment, an epidural blood patch is placed at the level of the dural puncture. Spinal cord and nerve root injury during initial lead placement have been reported. Placing the epidural needle and lead in the awake, lightly sedated patient able to report paresthesias should minimize the risk of direct neural injury.

The most frequent complication following spinal cord stimulator placement is lead migration. The first line of defense is to ensure that the lead is firmly secured to the paraspinous fascia. Suturing the lead to loose subcutaneous tissue or fat is not adequate. Postoperatively, the patient must be clearly instructed to avoid bending and twisting at the waist (lumbar leads) or bending and twisting the neck (cervical leads) for at least 4 weeks after lead placement. Placing a soft cervical collar on those who have had a cervical lead placed provides a ready reminder to avoid movement. Lead fracture may also occur, often months or years after placement. Avoiding midline placement or tunneling the lead across the midline reduces the incidence of fracture caused by compression of the lead on bone. Lead fracture is signaled by a sudden loss of stimulation and is diagnosed by checking lead impedance using the spinal cord stimulator programmer.

Wound dehiscence and impulse generator migration are infrequent problems. Ensuring that the size of the pocket is sufficient to prevent tension on the suture line at the time of wound closure is essential for minimizing the risk of dehiscence.

Subcutaneous collection of fluid surrounding the impulse generator (seroma formation) can be problematic and typically follows generator replacement. Percutaneous drainage of the sterile fluid collection is often successful in resolving the problem.

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