Medical Considerations in Spinal Cord Stimulation

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Chapter 3 Medical Considerations in Spinal Cord Stimulation

Chapter Overview

Chapter Synopsis: Chapter 3 deals with some of the perioperative considerations for surgery for electrical spinal cord stimulation (SCS). The invasive implantation procedure carries inherent risks, but these can be minimized with considerations specific to the patient. We can learn from the technically similar (and far more common) surgeries for implanted cardiac devices (ICDs), including pacemakers and defibrillators.

Surgical site infection (SSI) is perhaps the most common perioperative risk associated with SCS at around 3% to 8%; thus intravenous antibiotics should be used routinely. Implantation at the site of a previous incision increases the risk of infection; therefore previous surgery sites should not be used. Smokers carry a risk of wound infection as high as eight times that of nonsmokers. Smoking cessation within a few weeks before surgery can dramatically reduce the risks. Human immunodeficiency virus (HIV)–positive patients do not intrinsically face higher perioperative risk, but certain members of the population could. Similarly, obesity per se does not increase perioperative morbidity, but it can increase the likelihood of wound infection. Patients with rheumatoid arthritis face specialized risks that should be considered in coordination with their rheumatologist. Patients with diabetes make up a significant component of the implantation population. Surgery can induce a postoperative hyperglycemia that increases SSI risk; therefore postoperative glucose should be maintained below 200 mg/dL. Patients receiving anticoagulation therapy or with ICDs also warrant special consideration. As the population receiving SCS implantation grows, it is important to consider the specific conditions associated with each patient to minimize his or her perioperative risks.

Important Points:

Clinical Pearls:

Clinical Pitfalls:

Background

Electrical stimulation for the treatment of pain has been used for over 4500 years.1 In 1967 neurosurgeon Dr. C. Norman Shealy and colleagues from Case Western Reserve University were the first to implement spinal cord stimulation (SCS) in the treatment of chronic pain at University Hospitals of Cleveland.2 Shealy proved the clinical feasibility of SCS, and subsequently there has been tremendous growth in its application. Currently SCS is approved by the Food and Drug Administration (FDA) for chronic pain of the trunk and limbs, pain from failed back surgery syndrome (FBSS), and intractable low back pain. “Off label,” SCS has been used for neuropathic painful conditions and vascular and visceral pain, with diverse applications ranging from vulvodynia to cervicalgia. The full range of considerations for SCS is beyond the scope of this chapter.

As the role of SCS has expanded in the treatment of chronic pain conditions, the eligible patient population has grown as well. Patients who previously would not have been candidates are now able to benefit from neurostimulation. It is the responsibility of the implanting physician to consider and maximize the perioperative status of the patient to optimize outcome and minimize risks and complications.

Specific Considerations

Surgical Site Infection

Surgical site infection (SSI), in general, has an overall prevalence of 2% to 7%3; and, consistent with this, a rate of 3% to 8% has been found with SCS implantation.46 In expert panel recommendations, Kumar and colleagues state that the “use of antibiotics is recommended by the panel and others and should be started intravenously, 1.5 hours prior to surgery.”4

By comparison, infection rates for implanted cardiac devices (ICDs—pacemakers and defibrillators) were reported as 0.5% to 6% in early studies7,8 but have more recently been found to be as low as 1%.9 Although there have been no studies in SCS comparing infection rates in those with and without preoperative antibiotics, a prospective, randomized, double-blind, placebo-controlled trial evaluated infection risk for ICDs in those receiving either prophylactic cefazolin or a placebo.10 This trial was interrupted early by the safety committee because of the dramatically higher rate of infection in those who did not receive antibiotics vs. those who did (3.28% vs. 0.63%). The authors also found that the presence of postoperative hematoma and procedure duration were positively correlated with infection risk. A recent American Heart Association (AHA) scientific statement also identified ICD infection risk factors to include diabetes mellitus (DM), congestive heart failure (CHF), renal dysfunction, oral anticoagulation, revision surgery, hematoma formation, corticosteroid use, and surgeon inexperience.11 This statement also notes that “there is currently no scientific basis for the use of prophylactic antibiotics before routine invasive dental, gastrointestinal, or genitourinary procedures.” Although these findings are in the setting of ICDs, the similarity between minimally invasive surgeries such as these and SCS may provide guidance. There are no similar studies in the SCS population and until such a time this literature may be used as a prudent reference.

Gaynes and colleagues12 have also found that American Society of Anesthesiologists’ (ASA) classification, the National Nosocomial Infection Surveillance (NNIS) wound classification, and prolonged operative time—defined as ≥75th percentile compared to average duration of the operation—are associated with SSI. In a retrospective review of >10,000 patients over 6 years, Haridas and Malangoni13 identified several other significant risk factors for SSI: hypoalbuminemia (≤3.4 mg/dL), anemia (Hgb ≤10 g/dL), excessive alcohol use (not defined), history of chronic obstructive pulmonary disease, history of CHF, infection at remote site, and current operation through a previous incision (Box 3-1). Most of these risk factors can be identified through an appropriate history and physical examination and preoperative laboratory work and can be addressed in conjunction with the patient’s primary care physician or appropriate specialist. However, operation through a previous incision site may be of greater concern. One of the most common indications for SCS use is FBSS. Many times old scars are used as an entry points for the new procedure, either because they provide adequate anatomic access or to prevent further cosmetic disfiguration. Haridas and Malangoni13 suggest that using a previous incision may predispose to SSI because of the decreased vascularity of scar tissue.

The pathogen most commonly involved in SSI is Staphylococcus aureus, which is responsible for more than 50% of infections,1416 with most cases occurring in patients who are themselves carriers of the organism. The carriage site is most often the anterior nares,17 and multiple studies have shown that nasal carriage is one of the most important risk factors for the development of surgical site infection.14,18,19 Given this, there is a new body of research specifically focused on identifying and treating nasal carriers of Staphylococcus aureus, with resultant dramatic decreases in SSI. Studies in cardiothoracic,14 orthopedic,2022 and dialysis23,24 populations have shown that treatment is feasible and cost-effective, decreases infection rates by 57% to 93%, and reduces morbidity and mortality. A recent, randomized, double-blind, placebo-controlled, multicenter trial showed that treatment with mupirocin nasal ointment and chlorhexidine soap reduced the infection rate to 3.4%, compared to 7.7% in the placebo control group.25

Although different treatment protocols have been used, there is accumulating evidence for a combination of intranasal mupirocin and chlorhexidine showers preoperatively, and vancomycin intra-operatively. When patients are seen in presurgical screening (or during a routine office visit for potential SCS patients), a polyester (Dacron) nasal swab of the nasal passage may be taken. Polymerase chain reaction (PCR)-based rapid testing is used to identify methicillin-resistant Staphylococcus aureus, and standard cultures are used to identify methicillin-sensitive Staphylococcus aureus. If patients test positive for either strain, they are treated with 2% intranasal mupirocin (Bactroban) twice daily for a five-day treatment course prior to implant date and continued for two days post-implant. Additionally, a shower wash of 2% chlorhexidine (Hibiclens) is taken the evening prior to surgery.20 A combination of vancomycin and cefazolin dosed for weight can be used intra-operatively, as β-lactam antibiotics may provide better coverage for methicillin-sensitive Staphylococcus aureus strains.26,27 SSI can be particularly devastating and difficult to treat in patients with implanted hardware, and, although the ideal regimen has yet to be determined, these developments allow another opportunity to minimize patient morbidity.

Finally a great deal of research has been done about the increased risk of SSI with smoking, as discussed in the following paragraphs.

Tobacco Use/Smoking Cessation

It is now clear that smoking is an important and significant factor in perioperative complications.2831 Although there are no SCS studies that have looked at the increased risk of SSI in smokers, there is an abundance of evidence in the general surgery literature from which to draw conclusions. Smokers have up to eight times the risk of wound infection (≈8% vs. 1%) after surgery.32 Although the exact etiologic mechanism is unclear, carbon monoxide and the hypoxemic state it creates are likely important factors. The role of nicotine itself is unclear. It is a known vasoconstrictor that impairs tissue revascularization.33 However, nicotine replacement therapy (NRT) does not increase infection rates in experimental or clinical studies and there is no evidence that it adversely affects wound healing.3436

The risks of smoking have been unequivocally shown, and evidence continues to accumulate that smoking cessation can drastically reduce perioperative morbidity. In one study preoperative smoking cessation before joint replacement surgery reduced wound infection rates from 27% in smokers to 0% in those who quit.37 At this time there is no consensus on the duration of smoking cessation for maximum benefit before surgery. Increased length of abstinence is certainly beneficial for a patient’s overall health, and the ideal situation would be for this to continue permanently after surgery. However, given the difficulty most patients experience with quitting smoking, the search continues for the shortest amount of time that will still yield clinical benefit operatively. Initial studies showed clear benefit from smoking cessation for 6 to 8 weeks before surgery, consistent with physiological improvements in pulmonary and cardiac function.32,38,39 Moller and associates37 found a 65% decrease in postoperative complications with 6 to 8 weeks of preoperative smoking cessation before orthopedic surgeries. Even 4 weeks of smoking cessation reduced wound infection rates to that of nonsmokers in those having skin biopsies.34 The 3-week mark may be the cutoff point to see benefit from smoking cessation. One study found that the complication rate for colorectal surgery was unchanged with smoking cessation ≤3 weeks,40 whereas two separate studies found a reduction in complications in head and neck and breast reduction surgery with cessation ≥3 weeks.41,42

With the clear and proven increased risks from continued smoking, discussing smoking cessation with patients considering SCS may be an important part of preoperative education and teaching. Perioperative intervention can directly and dramatically decrease complication rates and can lead to sustained smoking cessation for up to 1 year after surgery.43,44 Peters and colleagues45 gave important perspective to the need for smoking cessation: “the adverse effect of failing to quit smoking is similar to that of omitting antibiotic prophylaxis.” Unfortunately, despite this overwhelming increase in risks, many patients still continue to smoke.

Human Immunodeficiency Virus+

Advances in the treatment of human HIV/AIDS in the last 20 years have changed the disease course from a rapid and progressive affliction to a manageable chronic illness. With HIV/AIDS patients living longer and a general paradigm shift away from the focus on acute management, a greater percentage of HIV/AIDS patients are being seen for chronic pain states, whether specific to the condition or similar to those of the general population.

Currently there is the misperception that HIV-positive status alone increases the risk of postoperative complications. With the exception of certain transoral procedures,46,47 review of the literature does not support this belief.4851 The most important risk factor for postoperative complications in the HIV+ patient is the one routinely assessed in all patients: ASA classification.49 However, there are markers used to monitor disease status that are predictive of increased risk (Box 3-2). Increased morbidity and mortality rates are associated with CD4 count ≤200 cell/mm3 and viral load >10,000 copies/mL.50,5255 In addition, a postoperative CD4 percent of ≤18 ± 3 and a decrease in percent CD4 of ≥3 are associated with increased morbidity.54 All these values can easily be tested for, and any physician operating on an HIV+ patient should strongly consider ordering these laboratory values routinely. If there are abnormalities, both SCS trial and implant should be delayed, and the patient referred to an infectious disease specialist. To date there are no SCS studies that have specifically looked at the increased risk of infection in HIV+ patients.

Thrombocytopenia (platelets <50,000/µL) is a frequent finding in HIV+ patients, with prevalence rates from 9% to 37% in various study populations.56 Therefore thorough preoperative evaluation of platelet count and correction of a possible coagulation disorder is mandatory before proceeding with surgical intervention. Most implanters believe that an implant should be delayed until platelets are above 50,000 by either disease correction or platelet infusion.

Obesity

There is considerable stigma associated with obesity (body mass index [BMI] >30 kg/m2), and outcomes are impacted in many areas of medicine. Most physicians are aware of the deterioration of cardiac, pulmonary, and immunological function associated with obesity.5759 Obesity is also associated with decreased quality of life and life expectancy.60,61 The co-morbidities of obesity are well known, and the list of associated disease states continues to grow annually. Given this, there is the commonly held deduction that obesity is a significant risk factor for perioperative complications. Although there is an increased risk of wound infections,62,63 Dindo and colleagues64

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