Keywords

1 Case Description

A 55-year-old male, with a past medical history significant for multilevel cervical and lumbar degenerative disease, presents to the pain clinic with chronic axial neck and radicular low back pain after cervical and lumbar spine surgery consistent with cervical and lumbar post-laminectomy syndrome. After successful trial, he had a spinal cord stimulation (SCS) system placed to treat his low back pain without complications. He reported alleviation of this pain using conventional paresthesia stimulation via two percutaneous eight contact leads positioned with the tips at the middle of the T6 vertebral body level. The patient had adequate analgesia for over a year and was able to return to employment, which he hadn’t been able to do for 6 months prior to the implant. One year later, he experienced a reduction in analgesia with inadequate coverage of his previously painful areas. In addition, he noticed increased paresthesias migrating to above the nipple line that were now bothersome. He also complained of insomnia refractory to conservative regimen. Reprogramming the stimulator did not resolve these issues. Imaging studies obtained at that time confirmed a cephalad and lateral migration of one of the stimulation leads. During the surgical revision to reposition the migrated lead, both leads were found to be frayed at the distal tips. The leads were replaced and repositioned to obtain adequate coverage of his painful areas. The patient reported adequate pain relief after the revision, without any adverse events.

2 Case Discussion

2.1 Overview of Spinal Cord Stimulation

SCS is a well-established neuromodulatory technique used to address refractory chronic pain. The implantable device is part of an overall pain treatment strategy and is implemented only after more conservative treatments have failed. It has been shown to decrease pain and improve functionality, enabling some patients to go back to work, and, ultimately, SCS implantation has proven to be cost-effective despite relatively high initial costs of implantation [1]. This invasive treatment typically involves two steps, first a trial of stimulation for one or more days (typically 4–7) to verify adequate pain relief and lack of adverse side effects, including unwanted paresthesias. During this step, one or more stimulation leads are placed percutaneously in the posterior epidural space and are driven by an external IPG. If successful (more than 50% reduction of the pain with the system on), the trial leads are removed and replaced by surgical implantation of a permanent, subcutaneous stimulation system. The system consists of stimulation leads, either paddle or percutaneous, that are placed into the posterior epidural space with or without laminectomy, respectively, and tunneled subcutaneously to an IPG secured in a subcutaneous pocket, typically on the posterior flank or buttock. SCS is FDA approved for the management of chronic, intractable pain of the trunk or limbs, including post-laminectomy syndrome, and the device has been shown to successfully treat various neuropathic pain syndromes [2, 3]. Although off-label, SCS has also been shown to successfully treat cancer-related pain in adults [4], ischemic pain from peripheral vascular diseases [5], chronic visceral abdominal pain [6], and even refractory angina, although recent randomized controlled studies have shown small improvement effects [7].

Despite clinically positive results, mechanisms supporting its efficacy have not been well elucidated. Parameters such as lead design, stimulation mode, and stimulation intensity can all effect outcomes with likely differing mechanisms, and the exact nerve fibers and neural pathways that are activated through the highly conductive cerebrospinal fluid remain unclear [8, 9]. SCS has been shown to inhibit both nociceptive and non-nociceptive myelinated sensory afferents at segmental spinal or supraspinal levels [10]. Several studies of SCS used resting-state functional MRI to investigate changes in cortical networks and cortical processing involved in stimulation-induced analgesia; they revealed that SCS can reduce the affective component of pain [11]. The sympatholytic effect of SCS is considered to be responsible for the effectiveness of SCS in peripheral ischemia [12] and complex regional pain syndrome [13]. This effect has also been considered part of the management of other chronic pain states such as failed back surgery syndrome, phantom pain, diabetic neuropathy, and post-herpetic neuralgia [13,14,15].

2.2 Factors Influencing Success of Spinal Cord Stimulator Implantation

2.2.1 Patient Selection

As with any pain treatment, proper patient selection includes a comprehensive history and physical examination. In addition, a detailed psychological assessment is necessary to identify psychosocial factors that could limit efficacy. Ultimately, the goal is for the patient to achieve therapeutic success while minimizing adverse events. Patients who meet the following criteria are most likely to benefit [16]:

  • Chronic, intractable pain for more than 6 months.

  • Objective evidence of pathology concordant with pain complaint.

  • Lack of adequate relief from more conventional treatments.

  • Initial or further surgical intervention not indicated.

  • No contraindications to therapy or surgery.

  • Patient can properly understand how to operate the system and is able to operate it.

  • Patient understands therapy risks.

  • Therapy and function goals have been established.

  • Satisfactory results from the screening test.

  • Patient is not pregnant.

  • No untreated drug abuse.

  • Clearance and completion of psychological evaluation.

  • 18 years of age or older.

2.2.2 Perioperative Preparation and Procedure

Anatomical, medical, and psychological considerations are taken into account prior to considering SCS trial in all patients. Spinal abnormalities such as spondylosis, scoliosis, prior surgery, and spinal stenosis can pose technical difficulties for successful placement of the stimulator leads within the epidural space. Medical assessment includes evaluation of functional and neurologic status, excluding sources of active infection, and identifying coagulopathy, impaired immune response, and other factors that would affect tissue healing, such as diabetes and tobacco abuse [17]. Psychological evaluation is essential, as negative psychosocial factors have been found to predict poor outcome with SCS [18]. Patients benefit from being educated about the procedural and postoperative expectations of SCS prior to the trial [17]. Presurgical psychological preparation, detailed informed consent, and post-procedural information should also be given. Intraoperative medical considerations include holding anticoagulant medications for the appropriate length of time [19] and the administration of antibiotics.

2.2.3 SCS Device Comparisons

Various vendors have different hardware components that may affect treatment outcomes. Things to consider include:

  • System type (e.g., voltage, power sources)

  • Coverage (e.g., number of contacts, percutaneous versus paddle lead, maximum pulse width, maximum voltage, maximum frequency)

  • Software (e.g., programming algorithm, upgradeable, positional shock)

  • Battery (e.g., cordless recharge, battery life)

  • Warranty and accessibility to SCS device representative

2.3 Overview of Complications

As with any sophisticated medical device, complications can and do occur with SCS (Table 38.1). In a 2014 review of complications, electrode (lead) migration was found to be the most frequent complication of SCS, with a greater incidence involving percutaneous compared to paddle leads [20]. Other complications include infection, electrode fracture, extension wire or implantable pulse generator failures, cerebrospinal fluid leakage, pain over the stimulator components, and spinal epidural hematoma [20]. A recent retrospective study of 345 SCS patients concluded that it is a safe, minimally invasive procedure with good long-term outcomes but with high rates of hardware malfunction particularly, comprising 74.1% of all complications and leading to surgical revision and explant rates of 23.9% each [21]. In a different 5-year retrospective review, review of radiologic imaging studies obtained in patients with SCS systems found that hardware complications comprised 50% of all complications while infectious complications comprised 29.1% (Table 38.1) [22]. In a systematic review that analyzed the effectiveness and complications of this device for failed back surgery syndrome and complex regional pain syndrome, 34% of the patients experienced some type of an adverse event, and pain relief decreased over time [23].

Table 38.1 Approximate rate of complications of SCS: compilation of published data from Zan et al. [22] and Bendersky et al. [20]
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2.4 Surgical Complications

Complications may be avoided or at least diminished by good surgical technique and strict sterile precautions, as well as optimizing patient selection before the implantation according to published recommendations [20].

2.4.1 Seroma and CSF Hygromas

Seromas and hematomas can occur in up to 9% of patients with SCS implants [20]. Seromas, a collection of serous fluid beneath the wound, are one of the most benign surgical complications. Early detection is key in order to prevent infection complicating a seroma. Seromas present early after surgery similarly to hematomas with acute, afebrile, swelling and pain at the surgical site. If the seroma is very tender or large, it can be aspirated under sterile conditions. Empiric antibiotics are not recommended for seromas to avoid complicating the diagnosis: seromas can be managed conservatively with abdominal binder and serial aspirations if necessary, whereas infected hardware warrants explantation [17].

CSF leakage can occur in 0.3–7% of patients [20]. If the fluid collection is due to CSF accumulation (hygroma), the initial care is to observe and treat similarly as a seroma, but if the wound is tense and painful, it should be aspirated under sterile conditions. If laboratory analysis of the aspirate is consistent with CSF and the hygroma does not respond to conservative management, an epidural blood patch can be performed near the site of catheter entry into the intrathecal space to theoretically seal the leak. Rarely, referral for surgical exploration is necessary for large or persistent hygromas. If the patient has systemic symptoms and signs of infection and hygroma is present, urgent evaluation and treatment for meningitis should be initiated [17].

2.4.2 Hematoma

A hematoma is a blood collection in the subcutaneous tissues. They are associated with an increased risk of infection compared to seromas [17], which can best be mitigated by careful surgical technique and meticulous attention to hemostasis, as well as appropriate perioperative management of coagulation issues; any anticoagulant should be held an appropriate amount of time prior to surgery to aid in their prevention. There is a greater risk in exploring a small, stable hematoma compared to watchful waiting, as usually a hematoma will usually resolve on its own. Larger volume or expanding hematomas should be evacuated under sterile conditions to prevent wound dehiscence [17]. Basic laboratory studies of aspirated fluid and ultrasound imaging can be helpful to differentiate between seroma, hematoma, and infection.

2.4.3 Wound Dehiscence

Wound dehiscence (Fig. 38.1) occurs when one or more layers of the surgical wound separate. This most often occurs between 5 and 8 days after surgery [17]. It is more common in patients prone to poor wound healing, such as patients with diabetes, immunosuppression, and cancer. Wound closure with excessive tension on the wound itself can lead to ischemia and subsequent separation of tissue layers due to necrosis. Failure to sufficiently close tissue layers will also lead to dehiscence. In the absence of infection, the patient with a partially dehisced wound can be managed conservatively with regular decontamination of the wound and dressing changes to allow the wound to heal by secondary intention.

Fig. 38.1
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Postoperative wound dehiscence. Photo courtesy of the University of California San Diego Anesthesia Department

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2.4.4 Infection or Epidural Abscess

Infection or epidural abscess after an SCS trial or implantation calls for explantation of the device as well as decompression and drainage of the abscess. Infection rates after SCS implantation range from 2.5 to 14% [21]. The symptoms of an epidural abscess can be similar to an epidural hematoma, including fever, new neurologic deficits, leukocytosis, and severe pain and are all strong indications for emergent surgical decompression. Suspicion of an epidural abscess warrants an emergent CT scan and an infectious laboratory workup, including ESR, CRP, CBC, and blood cultures.

A recent international survey examined current reported infection control practices for SCS trials and implants and compared them to evidence-based recommendations obtained from standard surgical guidelines and recommendations of the Centers of Disease Control and Prevention (CDC), the National Institute for Health and Care Excellence (NICE), and the Surgical Care Improvement Project (SCIP) [24]. The authors identified multiple areas with high levels of noncompliance, including weight-based antibiotic dosing, hair removal strategies, double gloving, surgical dressing, skin antiseptic agent selection, and postoperative continuation of antibiotics [24]. Improved compliance with established infection control practices can significantly reduce reported rates of infection after SCS procedures.

2.4.5 Dural Puncture and Spinal Cord Injury

Although dural puncture is a very rare complication of SCS, it can occur, as noted in a case study of electrode placement into the spinal cord itself causing tetraparesis in a patient [25]. One of the most common causes of a “wet tap” is an unrecognized blood clot obstructing the lumen of the Tuohy needle during lead placement. With a clot in the needle, it is difficult to appreciate the loss of resistance when entering the epidural space. If blood drips back out of the hub, the physician should stop, flush the needle with saline, and withdraw the needle to make sure it is patent prior to proceeding [17]. Judicious use of multiple fluoroscopic angles (AP, lateral, and oblique), cautious needle advancement, and entering the epidural space below the termination of the spinal cord can help to prevent inadvertent dural puncture and potential spinal cord injury.

2.5 Hardware Complications

Although the available SCS systems are very reliable and most hardware malfunctions can be readily corrected, device malfunctions and complications should not be trivialized, as surgery is still required to repair them. Complications also interfere with the patient’s pain therapy, are costly, and are associated with all the attendant risks of surgery and anesthesia.

2.5.1 Lead Migration

After epidural placement, percutaneous leads are typically secured into the prevertebral fascia. Nonetheless, lead migration commonly occurs after implant, resulting not just in loss of analgesia but also potential for onset of new pain due to electrical stimulation of other structures, including the ligamentum flavum and/or the dorsal root entry zone (DREZ) [26]. Percutaneous spinal cord stimulation (SCS) electrodes are prone to migration even after scar tissue encapsulation [27] and are the most common mechanical reason for SCS failure [28]. In a retrospective review that examined records of SCS implantation between 2008 and 2011, 2.1% of the patients required a surgical revision due to clinically significant lead migration. Some investigators have estimated a 13–22% revision rate due to lead migration [29], with others reporting up to 30–40% [30]. Proper anchoring of the lead will lessen the chance of lead migration. Different manufacturers provide several different types of lead anchors; a simple “figure-of-eight” anchoring suture tie is also effective for securing stimulation leads [17]. A recent retrospective review of a novel fixation device demonstrated no lead migration at extended follow-up (10–68 weeks), suggesting that these types of devices may reduce the incidence of lead migration [30]. Another group showed the utility of bone cement to prevent lead migration with minimally invasive placement of spinal cord stimulator leads via laminectomy [31] (Fig. 38.2).

Fig. 38.2
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Migration of occipital nerve stimulator leads. Panel a: proper intraoperative positioning of leads. Panel b: postoperative lead migration. Photo courtesy of the University of California San Diego Anesthesia Department

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2.5.2 Lead Fraying, Pulse Generator Failure, and Losing Coverage

As illustrated in the case presentation, lead fraying (Fig. 38.3) is a known complication of SCS systems. A patient may experience increased pain, dysesthesias, and/or paresthesias. Commonly, the patient may report a prior traumatic event causing strain on the lead itself. Spontaneous lead breakage and insulation failures have also been reported [32] (Fig. 38.4). On rare occasions the IPG can also fail. All of these scenarios require surgical replacement and revision of the system.

Fig. 38.3
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Frayed spinal cord stimulator leads. Arrows point to frayed ends. Photo courtesy of the University of California San Diego Anesthesia Department

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Fig. 38.4
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Defected spinal cord stimulator lead. Contact number 4 with charring and different color; when on, patient felt electric shock in the mid-thoracic area. Photo courtesy of the University of Chicago Medicine, Department of Anesthesia and Critical Care

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If the patient reports loss of analgesia, reprogramming of the device is a viable first option. This can be done in the outpatient clinic setting. There are several published cases where a patient treated with SCS develops a new pain complaint, and device reprogramming provides an additional pain control of the new region. For example, a patient with complex regional pain syndrome type I with lower extremity radiculopathy reported 1 month of pain relief with the use of an SCS. However, she developed slipping rib syndrome after thoracotomy, and reprogramming her SCS covered her newly developed pain condition [33].

2.5.3 Hardware Complications due to Perioperative Procedures: MRI and Electrocautery

Perioperative hardware complications can also occur in patients with SCS systems. Prior to July 2013, SCS systems were not MRI compatible [34], requiring explantation prior to imaging. Now, based on the hardware specifications, certain SCS systems are compatible with MRI imaging of the brain and full body (conditional) MRI. Before performing an MRI, it is important to know which device a patient has implanted and if it is compatible with an MRI and to assess that the device is functioning properly and fully intact prior to the procedure, as these issues may result in heating of the device and potential injury to the patient. Electrocautery is another process that may damage SCS systems, and it is generally not recommended. If cautery is deemed necessary by the surgeon, bipolar electrocautery is recommended, and the electrocautery units should be used with caution to avoid damage to the system and thermal injury to the patient. Similar to MRI, electrocautery use in patients with impaired SCS systems, such as systems with suspected breaks or abnormal impedances, is unsafe and may cause injury [35].

Key Concepts

  • Spinal cord stimulators are cost-effective in the long term and aid people with chronic, intractable pain.

  • Surgical complications include seroma, hematoma, hygroma, wound dehiscence, infection, dural puncture, and cord injury.

  • Hardware complications include lead migration (most common), lead fraying, and IPG failure.

  • Prompt workup of any suspected complication is based on history, physical exam, appropriate laboratory studies, and imaging.