Patient Selection

The most prudent physician will use good clinical judgment and critical analysis to anticipate and minimize perioperative and postoperative complications. Patient selection is of utmost importance, and multiple factors must be taken into account. Indications for the procedure should correlate with those diagnoses known to respond to SCS ( ).

Candidates for SCS therapy should be rigorously screened. Demographics such as age, sex, and number of previous operations were not shown to anticipate complications ( ), although one retrospective review reported that youth, male gender, high pain intensity, and associated musculoskeletal complaints had a higher revision rate when compared to other demographics ( ). Patients who are not candidates for SCS therapy are those with known uncorrectable coagulopathy or those unable to stop anticoagulant or antiplatelet therapy. Similarly, patients with active infections (including urinary and dental infections) or sepsis should not be considered for SCS trial or implantation. Other known risk factors such as uncontrolled diabetes, tobacco abuse, immune deficiency or ongoing immunosuppressive therapy, and poorly compensated cognitive impairment must be considered ( ).

The use of SCS to treat angina and heart failure in both clinical practice and research settings has changed the thought processes regarding the concomitant use of SCS with implanted cardiac devices such as pacemakers and defibrillators. In recent years there have been reports suggesting problematic interference interactions between cardiac devices and SCS devices ( ). More data is needed to quantify the risks of SCS and automatic implantable cardioverter-defibrillator (AICD) interactions. The use of SCS in patients with an implanted AICD device should be based on a careful discussion of risks, postoperative monitoring, care team assessment, and the relative contraindications ( ).

Clearance should be obtained from the cardiac care team when contemplating SCS placement in a patient with an implanted cardiac device. During the trial and in the postimplant period the device should be interrogated and the patient monitored.

Although the use of SCS in pregnancy is not sanctioned by device companies and is listed as a contraindication in the “Directions for Use” manuals, available data regarding its use in pregnancy is mostly limited to case reports that demonstrate no adverse effects on pregnancy outcomes ( ).

Preoperative radiological assessment should be used to improve safety of SCS and reduce risks to the patient. Patients who present with potential structural anomalies, such as spinal stenosis, should have preoperative radiographs and magnetic resonance imaging (MRI) or computed tomography (CT) examination of the cervical, thoracic, and/or lumbar spine, based on the planned implant site, in anticipation of difficulties in lead placement ( ). In patients with anomalies that narrow the spinal canal, such as thoracic or cervical spinal stenosis, lead placements can result in neurologic sequelae such as hemiparesis, hemiplegia, quadriparesis, and quadriplegia.

Psychiatric comorbidities may negatively affect outcomes of SCS therapy. While psychological screening by a licensed practitioner can identify psychiatric comorbidities that may be a contraindication for SCS, it is also useful for setting realistic expectations of the therapy between the patient and the physician. Although untreated or poorly treated psychiatric disease may pose a barrier to successful SCS therapy in some cases, there are recent studies suggesting that this patient profile may benefit from novel waveform technologies ( ). Please see Chapter 2 by Doleys in this book.

Postoperative Complications

Postoperative complications of SCS are divided into those that are hardware related and those that are biologically related ( ). While not considered a “true” complication, tolerance to stimulation is well known to lead to negative outcomes over time ( ). Hardware-related complications include hardware malfunction (lead migration or lead fracture ( )), lead disconnection, IPG displacement or flipping, IPG site pain, and battery-related charging problems. Biologic complications include infection, abscess, hematoma, seroma, spinal cord or nerve-root injury, dural puncture with associated headache, metal allergy and rejection, painful stimulation, and paralysis ( ).

Complications Related to Hardware

While the overall incidence of complications is rather high (32%–38%), the most common complications are hardware related and associated with lead migration ( ). Leads over the dorsal column may displace from their initial position in a cephalad, caudal, or lateral direction with a displacement incidence of 13.2%–27.0% ( Fig. 51.1A–C ) ( ). The most common reason for SCS reoperation is lead revision or replacement, with an incidence of 23.1% ( ). Cervical lead displacement occurs more frequently than the corresponding thoracolumbar lead displacement ( ). Technology advances have reduced the need for SCS reoperation due to lead displacement. The introduction and use of multipolar multichannel devices ( ) and improvements in surgical technique and anchoring devices have contributed to a reduction in lead displacement and/or the need for reoperation ( ). Studies by Liem et al. showed a much lower migration rate with dorsal root ganglion stimulation than with conventional SCS. This is most likely due to the introduction of a strain relief loop within the epidural space during the procedure ( ).

(A) Inferior migration of the lead. Note the lack of strain relief loop in the lead remaining in the epidural space. (B) Inferior migration of the lead. Lateral view. (C) Lateral lead migration. This may present as painful rib stimulation or unilateral loss of coverage.

Lead Migration and Fracture

Lead migration usually occurs early postoperatively. Migration may present as loss of paresthesia coverage and/or aberrant stimulation patterns. Lead migration occurs when the longitudinal stresses on the lead overcome the stress limits of the lead anchor and/or the anchor is not well secured to the surrounding tissue ( ). Newer anchoring systems have been developed to strengthen the hold on the lead to the surrounding tissue ( Fig. 51.2 ). Diagnosis of lead migration is made clinically and confirmed by radiological studies. Management of lead migration begins in the preoperative setting. The initial placement of the lead should take an individual patient’s body habitus and spinal structure into account. Successful placement of the lead should minimize longitudinal tension along the lead. For thoracolumbar lead placement, the patient should be placed in such a position as to minimize lumbar lordosis. For cervical placement, cervical flexion, but not full flexion, is recommended. Placing at least two anchor sutures is recommended should one fail ( ), but three should be adequate for any anchor type.

Novel anchoring systems. (A) Swiftlock (Abbott/St. Jude Medical), (B) ClikX (Boston Scientific), (C) Injex with bumpy anchor (Medtronic).

Lead migration has also been correlated with the site of IPG implantation ( ). In a review of complications, studied bench data and determined that a 9 cm shift occurs between the buttocks and thoracic spine during flexion and extension. This extension/flexion shift is significantly reduced when the IPG is implanted in the abdominal wall. When a surgical strain relief loop is placed, tension is further reduced during changes in body position ( ). Some implanters promote anchor sutures directly on to the strain relief loop itself ( ), but this could result in damage to the internal lead fibers or insulation of the lead. One option for reducing migration is to place the IPG in the same anatomical plane as the anchor and entry point, ensuring that both points remain in the same plane no matter what the body position is. However, this technique requires care to ensure that the IPG position is below the 12th rib, above the posterior iliac crest, and not along the belt line. The technique is facilitated by the introduction of rechargeable batteries that allow for significantly smaller IPGs.

Lead fracture is also a well-documented ( ) complication that is mostly reported in cylindrical-type leads/electrodes. The most common site of fracture occurs just cephalad to the anchor as the lead enters the fascia ( ). When the lead is allowed to buckle, fatigue occurs with repetitive flexion and extension, leading to fracture and loss of conductivity. In a consensus statement, provided recommendations that minimize the risk of lead fracture. These authors recommend that no more than a 30° angle of entry to the skin should be used to access the epidural space prior to placing the lead. This can be achieved by entering the skin in a paramedian fashion, tight to midline ( ), and 1.5 levels below the intended epidural access point. This approach must be modified in patients with significant kyphosis or obesity. A shallow angle of entry improves the bend radius within the epidural space, reduces dural displacement, minimizes dysesthesia, and improves lead steering ( ). It is also prudent to use a needle entry point at a level significantly below the intended targeted vertebral level, maximizing the amount of lead within the epidural space and allowing for a shallow angle of entry. The medial-to-lateral trajectory should be approximated between the lateral pedicle shadow and the corresponding spinous process. A biologically inert (silastic) anchor is preferred, and the cephalad tip of the anchor should be advanced into the fascia to maximize the bend radius ( ). Newer anchors have been designed to support the bend radius as the cephalad portion of the anchor tracks into the underlying fascia ( Fig. 51.2 ). A figure-of-eight suture has been recommended to secure the anchor to the dorsal lumbar fascia.

Pain From Implanted Hardware

Pain associated with the implanted SCS system can occur. Pain may come in the form of aberrant stimulation other than from lead migration, mechanical hardware-related pain, or thermal pain.

Aberrant or unwanted stimulation occurs when the patient reports pain or paresthesia occurring in an unintended intensity or distribution. This can occur early, but there have been reports of delayed onset ( ). Early forms of aberrant stimulation may occur due to localized tissue being stimulated by a misplaced lead, anatomical variations in the patient, epidural scarring ( ), or patient position. The potential for aberrant stimulation underscores the importance of SCS trials and intraoperative testing of traditional tonic SCS systems (tonic electrical stimulation or conventional stimulation is electrical stimulation produced by a charge-balanced wave form in a tonic or continuous manner at frequencies less that 1200 Hz).

Untoward positional stimulation is a known complication of tonic stimulation. Natural patient behaviors lead to gravity-related changes of their internal anatomy. As the anatomy migrates relative to a change in body position, the relative distance between the lead and the underlying targeted tissue (spinal dorsal horns in SCS) changes accordingly. The threshold of perception to the delivered paresthesia may decrease in the upright position or painfully intensify in the supine position. Some pulse generators have been developed with internal accelerometers that adapt to body position changes and deliver the appropriate amount of energy accordingly ( ).

Similarly, stimulation of the ligamentum flavum (LF), particularly with cylindrical electrodes that produce an electrical field in a circular direction, can evoke significant patient discomfort that may be intractable, regardless of body position. Early signs of LF stimulation may occur during the trial period and even months after implantation ( ). LF stimulation may be due to central sensitization, synaptic plasticity, or both ( ). Directional leads used to stimulate the thalamus in deep brain stimulation therapies may eventually be used to reduce this problem, but current cylindrical-lead technology does not allow for this. Paddle leads promote unidirectional delivery of energy away from the LF, thereby avoiding LF irritation, but implantation of paddle leads is often precluded by a successful cylindrical-lead trial.

High impedance values may also lead to abnormally high energy requirements ( ) and early battery failure. Painful paresthesia can be localized or segmental, even in the presence of normal or low impedance values. Strong consideration should be given to lead misplacement or migration when pain is associated with low energy requirements. Intrathecal lead placement may result in intense pain that is associated with both low impendence and low energy requirements, while subdural placement may be associated with low energy requirements and normal impedance values ( ). In both cases, removal of the lead is indicated.

Hardware-related pain might also occur at the anchor or the IPG pocket. In most cases anchor- and IPG-related pain is transient and may resolve over time as the IPG pocket becomes encapsulated and the anchor site scars. However, continued pain over the IPG has been reported as common ( ), and in some cases has been associated with thermal injury during the transcutaneous charging process ( ).

The IPG may also be subject to delayed complications that are related to patient body habitus or lifestyle. Preoperative planning for IPG placement should anticipate delayed IPG complications. The physician should be aware of weight distribution and skinfolds in the upright position, as these may not be apparent intraoperatively. Proper IPG depth is important, as late extrusion of the IPG through the skin may occur from chronic pressure ( ). Similarly, patient body habitus may predispose the IPG to movement, and rotation of the IPG within its pocket can move it to a painful perpendicular position within the pocket. When pain is associated with the implanted SCS system and cannot be mitigated otherwise, revision surgery is warranted ( ).

Anchor-related pain is often due to a patient being too thin, with minimal subcutaneous adipose tissue. This patient presentation may require anchor revision, with a change to an older anchor with no mechanical titanium components, or surgical revision to place the anchor under a tissue area that provides some reduction of pressure on the anchoring materials.