Summary of Key Points
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Preoperative evaluation of the patient scheduled for spine surgery most commonly focuses on functional capacity, neurologic assessment, and upper airway examination.
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In patients undergoing cervical spine surgery, awake fiberoptic intubation may be the safest technique for upper airway management, especially in patients with significant spinal cord compression or an unstable cervical spine.
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Increased intraocular pressure during spine surgery may compromise the ocular perfusion pressure. Maintaining proper mean arterial blood pressure is important to avoid postoperative vision loss.
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For anesthetic management during intraoperative electrophysiologic monitoring, it is important to avoid muscle relaxants and high concentrations of inhalation anesthetics.
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Avoiding hypervolemia is prerequisite to avoid endothelial glycocalyx damage.
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Goal-directed fluid therapy is the ideal technique for fluid management during spine surgery.
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Albumin is the preferred colloid for spine surgery.
Spine surgery has evolved since the 1990s into a multidisciplinary specialty. This chapter presents the most recent evidence-based advances in the perioperative anesthetic management for spine surgery.
Preoperative Assessment
As in other surgeries, preoperative assessment should routinely include a history and physical examination as well as optimize the patient for surgery. Preoperative examination of the nontrauma patient’s ability to flex and extend the neck without symptoms while awake is a crucial step in assessing upper airway management during surgery. The anesthesiologist should examine the patient for the presence or absence of Lhermitte sign, sometimes called the “barber chair” phenomenon. Lhermitte sign is an electrical sensation that runs down the back and into the limbs from involvement of the posterior columns and is produced by flexing or extending the neck. This sign suggests compression of the spinal cord in the neck from any cause such as cervical spondylosis, disc herniation, tumor, or Chiari malformation. The presence of Lhermitte sign should alert the anesthesiologist to use extra caution in maintaining a neutral head position during intubation and throughout the procedure. Awake fiberoptic intubation is often used in patients with significant spinal cord compression or cervical spinal instability. Cervical spondylosis, the commonest indication for cervical spine surgery, can result in anterior cervical osteophyte formation, which may make direct laryngoscopy difficult. Of note, extension at the craniocervical junction is necessary for full mouth opening. Therefore, a small mouth opening should alert the anesthesiologist to the possibility of difficult intubation. Finally, radiologic diagnosis of atlantoaxial subluxation, often present in rheumatoid arthritis patients or Down syndrome patients, should be considered in the preoperative assessment, as direct laryngoscopy in patients with undiagnosed atlantoaxial subluxation has been reported to cause quadriplegia.
The risk for a perioperative cardiovascular event should be properly assessed in the preoperative evaluation. Identification of increased risk provides patients with information that may help them better understand the benefit-to-risk ratio of surgery, especially complex spine surgery; it may lead to interventions that decrease risk. The two most common risk models used in the preoperative assessment are revised cardiac risk index (RCRI) and the American College of Surgeons’ National Surgical Quality Improvement Program (NSQIP) risk model ( Table 196-1 ). The reported rate of cardiac death or nonfatal myocardial infarction is more than 5% in high-risk procedures, between 1% and 5% in intermediate–risk procedures, and less than 1% in low-risk procedures.
| History of ischemic heart | RCRI |
| History of heart failure | RCRI |
| History of cerebrovascular disease | RCRI |
| Insulin dependent diabetes mellitus | RCRI |
| Preoperative serum creatinine | ≥ 2 mg/dl (RCRI) or > 1.5 mg/dl (NSQIP) |
| Increasing age | NSQIP |
| American Society of Anesthesiologists class | NSQIP |
| Preoperative functional status | NSQIP |
National Surgical Quality Improvement Program Database Risk Model
The NSQIP database is used to determine risk factors associated with intraoperative/postoperative myocardial infarction or cardiac arrest (MICA). The NSQIP is composed of risk factors such as type of surgery, dependent functional status, abnormal creatinine, American Society of Anesthesiologists’ class, and increased age as predictors of MICA. The NSQIP risk model has higher predictive ability than the RCRI (0.884 versus 0.747).
Revised Cardiac Risk Index
The RCRI performs well in distinguishing patients at low compared to high risk for all types of noncardiac surgery, but it is less accurate in patients undergoing only vascular noncardiac surgery. Moreover, it does not capture risk factors for noncardiac causes of perioperative mortality. Of note, only one third of perioperative deaths are due to cardiac causes ( Table 196-2 ).
| No risk factors | 0.4% |
| One risk factor | 1.0% |
| Two risk factors | 2.4% |
| Three or more risk factors | 5.4% |
Patients whose estimated risk is less than 1% are labeled as being low risk and require no additional cardiovascular testing. However, patients whose risk is 1% or higher may require additional cardiovascular evaluation. Often, these are patients with known or suspected coronary artery or valvular heart disease. Further evaluation, in addition to consultation with a cardiologist, may include stress testing, echocardiography, or 24-hour ambulatory monitoring. In addition, no further tests usually are required in patients who can perform ≥ 4 metabolic equivalents (METs) of activity. However, for those whose functional capacity is lower or unknown, additional testing is indicated if it will influence perioperative care.
Biomarkers
Biomarkers brain natriuretic peptide (BNP) and N-terminal pro-BNP (NT-proBNP) have been used for perioperative evaluation. Those biomarkers are produced by myocytes in response to stress and important prognostic indicators of heart failure. Cuthbertson and associates demonstrated that a BNP > 40 pg.mL −1 has a sensitivity of 75% and a specific of 70% for predicting perioperative death or myocardial injury; it also performed better than the RCRI. The combination of C-reactive protein (CRP) and NT-proBNP had better predictive ability for perioperative major cardiovascular events in noncardiac surgery than other risk models.
Ankle Brachial Index
The Ankle Brachial Index (ABI) is valuable for cardiovascular risk qualification. ABI is easily done by measuring the systolic blood pressure (SBP) with a portable Doppler ultrasound machine on each arm and on dorsalis pedis and posterior tibial arteries of each ankle. The highest of the two arm pressures is selected, as is the highest of the two pressures of each ankle. The ABI is obtained by dividing the highest ankle SBP in each leg by the highest arm pressure. The normal ABI would be > 0.9 and ≤ 1.3. Values between 0.41 and 0.9 are associated with peripheral arterial disease (PAD) and severe PAD when ≤ 0.4. Therefore, the addition of ABI to traditional cardiovascular risk factors will improve the sensitivity, specificity, and predictive values for a future cardiovascular event.
Movement of the Cervical Spine with Intubation
Placement of the endotracheal tube (ET) requires a complex series of movements. The primary force applied by the laryngoscope is upward lift, which results in extension of the atlanto-occipital interspace. The lift also results in flexion at lower vertebrae. There is evidence that laryngoscope results in maximal extension at occiput and atlas, with flexion below C2. External stability methods may reduce movement during direct laryngoscopy (DL), but they will also make glottis visualization more difficult.
Atlantoaxial instability, observed in patients with rheumatoid arthritis (RA) and Down syndrome, holds important clinical significance for the anesthesiologists. In atlantoaxial instability, the odontoid process is no longer firmly held against the back of the anterior arch of C1, due either to disruption of the transverse ligament or to damage of the odontoid process itself.
Roughly 30% of patients with severe RA may exhibit some instability at C1-2, although few patients require surgical correction. Therefore, it is advisable for all patients with severe RA to have periodic flexion and extension radiographs, certainly prior to undergoing any surgical procedure. Similarly, roughly 15% of patients with Down syndrome patients may exhibit laxity in the transverse ligament. It may be advisable that Down syndrome patients have cervical dynamic radiographs before any surgical procedure that requires DL or extensive neck manipulation.
Atlantoaxial instability arises from the fact that C1 is a rigid ring affixed firmly to the base of the skull. If the transverse ligament is damaged, lifting the skull and C1 will result in an increase in the anterior atlantodental interval and hence a decrease in the posterior atlantodental interval. In other words, C2 remains fixed while C1 slides anteriorly, with the cord potentially becoming compressed or trapped in the space behind the odontoid.
Dynamic factors in the cervical spinal column affect the degree of cord compression. Hyperextension narrows the spinal canal by shingling the laminae and buckling the ligamentum flavum. Translation or angulation between vertebral bodies in flexion or extension can narrow the space available for the cord. Patients who lack cord compression statically may compress the cord dynamically, leading to the development of myelopathic symptoms during intubation with DL. Translation of adjacent vertebral bodies in flexion or extension may compress the spinal cord ( Fig. 196-1 ). Therefore, maintaining the neutral neck position during intubation is of utmost importance to avoid cervical spinal cord injury, especially during cervical spine surgery.
The primary force applied during DL is upward lift with a little angular force. This force can be as high as 50 to 70 Newton (N) (45 N is sufficient to lift 4.5 Kg or 10 lbs). The more difficult the exposure, as may occur with cervical spondylosis, the greater the force that may need to be applied, leading to a greater likelihood of cervical spinal cord injury during intubation. DL with the Macintosh (MAC) 3 blade results in near maximum extension at the occiput and C1. The use of the new videoscope Airtraq resulted in a 23% reduction in cervical spine motion in a cadaveric study; it exerted only 20% of the force by DL.
Fiberoptic intubation is an ideal method for intubation, resulting in the smallest degree of upper cervical spine motion (ideal especially when cervical spine movement is not feasible). The use of the laryngeal mask airway (LMA) or intubating laryngeal mask airway (ILMA) is not recommended for upper airway management in the patient with an unstable cervical spine, as these exert high pressure against the upper cervical vertebrae during insertion, during inflation, and while in situ.
Manual In-line Stabilization and Cricoid Pressure
The goal in manual in-line stabilization (MLS) is to apply force to the head and neck equal in magnitude and opposite in direction to those generated by DL so as to limit movement that might result during airway management. However, MLS not only failed to reduce movement at the site of instability in cadaver models but also limited the anesthesiologists’ ability to visualize the vocal cords. Cricoid pressure, as long as it was not excessive, did not result in movement in a cadaver model of an injured cervical spine.
Central Cord Syndrome and Direct Laryngoscopy
Central cord syndrome (CCS) was first described by Schneider in 1954. Classic CCS presents as a spinal cord injury with weakness in the upper extremities greater than the lower extremities in patients with underlying cervical spondylosis. The pathologic mechanism often involves hyperextension of the cervical spine with compression of the cord, by osteophytes anteriorly, and by enfolded ligamentum flavum posteriorly, impinging on the central core of the spinal cord and leading to ischemia, edema, or hematomyelia. Hyperextension may seem mild, as with direct intubation, but in the setting of cervical spondylosis it can result in marked neurologic injury. Younger patients with congenital cervical stenosis also are at increased risk of sustaining CCS as a result of hyperextension injury as during DL.
Patients with chronic renal failure are prone to spinal degenerative disease. The term destructive spondyloarthropathy (DSA) is used to describe a process occurring in hemodialysis patients, which can affect the cervical spine. Therefore, the head of a patient with chronic renal failure should be kept in neutral position during endotracheal intubation and during surgery.
Chin lift, jaw thrust, and DL can cumulatively cause movement of cervical spine and dynamic hyperextension injury to the spinal cord, thereby inducing CCS. Therefore, the use of either asleep or awake fiberoptic intubation, while keeping the patient’s head in neutral position especially during cervical spine surgery, may be the best way to avoid cervical spinal cord injury.
Pathophysiologic Changes with Prone Position
Cardiovascular Changes
Using a noninvasive cardiac output monitor, both cardiac index (CI) and venous return decreased in unanesthetized, healthy volunteers in the prone position. CI decreased compared with the supine position as follows: knee-chest position (20%), on pelvic props from a modified Relton-Hall frame under the anterior superior iliac spines and padded support under the chest (17%), on an evacuatable mattress (11%), and on pillows (3%; one pillow under the thorax and one under the abdomen, leaving the abdomen free to move). Toyota and Amaki studied transesophageal echocardiograms in 15 healthy patients undergoing prone-position lumbar laminectomy. The prone position caused left ventricular volume and compliance to decrease. These changes were attributed to a decrease in the venous return due to inferior vena caval compression, and decreased left ventricular compliance due to increased intrathoracic pressure in the prone position. These results had been confirmed by other studies using thermodilution pulmonary artery catheters to measure the cardiac index when transferring from the supine to the prone position. Cardiac output decreased in these studies by 17% to 24%. The reduction in cardiac output in the prone position also leads to a decrease in the metabolism of propofol. A reduction in propofol metabolism while in the prone position could also explain the results of Sudheer and colleagues, who showed a significant reduction in cardiac output in the prone position during maintenance of anesthesia using propofol compared with isoflurane. Pearce observed vena caval pressures to be 0 to 40 mm H 2 O in patients in the prone position with the abdomen hanging free. In contrast, patients with abdominal compression had vena caval pressures greater than 300 mm H 2 O. Increased venous pressure not only increases bleeding during spine surgery owing to congestion of vertebral veins but also can impair spinal cord perfusion.
The use of the prone position with abdominal compression was identified as a plausible cause of spinal cord ischemia leading to neurologic deficits after cervical laminectomy. The authors of this case series recommended the avoidance of abdominal compression and hypotension, especially in myelopathic patients for whom maintenance of spinal cord perfusion pressure is of paramount importance.
Hemodynamic and Fluid Management during Spine Surgery in the Prone Position
The aim of fluid management is to maintain normovolemia and adequate tissue perfusion while avoiding tissue edema. The discovery of the endothelial glycocalyx (EG) has changed our understanding of tissue perfusion. The EG consists of membrane-bound proteoglycans and glycoproteins building up a network in which plasma proteins are retained. The main constituents of the glycocalyx are syndecan, heparan sulfate, and hyaluronan. EG plus bound fluids and plasma proteins form the endothelial surface layer (ESL) with a thickness of about 1 µm. The noncirculating part of the plasma fixed within the ESL is approximately 700 to 1000 mL in humans.
With the high extravascular colloid osmotic pressure (COP), the intact EG is the key factor in maintaining an intact vascular barrier, proper filtration rate, and avoidance of tissue edema despite high COP in the interstitial tissues. EG retains plasma and generates the endothelial surface layer with its own high COP. In a small gap below the EG, the concentration of proteins is lower than in the interstitial space, allowing small net fluid filtration into the interstitial tissue. The EG structure makes the arteriolar and capillary domains relatively impermeable. However, venules represent a suitable site for fluid filtration through their gaps and pores. Because colloids are able to escape through venular pores, there are low osmotic pressure differences in addition to low hydrostatic differences. The result is a slow net filtration through venular pores. The latter property is in accordance with the newly appreciated fact that there is no net reabsorption of fluid in the venular segments of the microcirculation.
In summary, a small fluid and protein shift out of the blood vessels occurs at all times, but it is disposed of in a timely manner from the interstitial space via the lymphatic system under normal physiologic conditions.
Important Functions of Glycocalyx
Endothelial glycocalyx plays a very important role in maintaining the proper functions of immune and coagulation systems. Normally, the small endothelial adhesion molecules are within the EG. Degradation of EG as by cytokines, especially by tumor necrosis factor (TNF-α) and ischemic reperfusion injury, exposes adhesion molecules for immunocompetent cells, which enhances leukocyte and platelet adhesion. After shedding of the EG, circulating glycocalyx components like heparan sulfates have a direct chemotactic effect on leukocytes and increase their presence at the site of inflammation. Consequently, the destruction of EG can trigger the inflammatory cascade. Therefore, maintaining the integrity of EG might represent a promising therapy for inflammation and ischemic/reperfusion injury. EG has an important mechanosensory role by translating intravascular sheer stress into biochemical activation of endothelial cells to release nitric oxide (NO). EG is a crucial component for binding and regulating enzymes involved in the coagulation cascade. Furthermore, the most important inhibitor of thrombin and factor Xa the antithrombin III is firmly attached to the EG. It is therefore not surprising that hyperglycemia-induced loss of EG is accompanied by activation of coagulation and vascular dysfunction in diabetic patients ( Fig. 196-2 ).
Perioperative Fluid Management and Glycocalyx
Perioperative fluid management is one of the key factors in maintaining the integrity of EG. It is well known that iatrogenic acute hypervolemia can lead to the release of atrial natriuretic peptide (ANP). ANP induces shedding of EG components, mainly syndecan-1, thereby increasing shifts of fluid and macromolecules into the interstitial space. Thus, the ability of ANP to increase capillary permeability to water, solutes, and macromolecules might be at least partially explained by its capacity to disturb the EG structure. The average insensible fluid loss is only about 0.5 mL/kg/h via skin and airways in the awake adult. During abdominal surgery, insensible fluid loss increased to only 1 mL/kg/h. Avoiding hypovolemia and hypervolemia, which includes a careful indication for perioperative fluid management, is an important element to maintain a healthy EG and thereby to limit perioperative fluid and protein shifts into the interstitial space. An intact ESL is essential to avoid excessive tissue edema. Therefore, a goal=directed fluid approach is essential to maintain normovolemia and ESL integrity, reducing postoperative complications like anastomotic leaks, nausea and vomiting, infections, and pulmonary complications.
Many of these complications may result from excessive use of crystalloids. Of note, 80% of the infused crystalloids are distributed into interstitial tissues under all conditions. Therefore, the use of crystalloid at the rate of 1 to 2 mL/kg/h for maintenance and the use of iso-oncotic colloid like albumin for the replacement of blood loss may be the ideal for fluid management during spine surgery.
Suitable and Practical Techniques for Goal-Directed Fluid Therapy and Perfusion Pressure during Spine Surgery
Fluid management during spine surgery in the prone position represents a challenge due to decreased right ventricle preload induced by intrathoracic pressure, and consequently decreased stroke volume. The use of goal-directed fluid therapy (GDFT) is considered the preferred method to maintain proper intravascular filling without fluid overload. Fluid overload is a major problem resulting in increased facial edema, delayed postoperative extubation, and even in increased length of hospital stay.
During spine surgery, GDFT can be easily conducted using either stroke volume variation (SVV) or pulse pressure variation (PPV) and can be measured by pulse contour analysis using the Flo Trac/Vigileo system. The other technique for measuring SVV is measuring dynamic changes in descending aortic blood flow and stroke volume (SV) by an esophageal Doppler probe. The Doppler probe is used to guide the fluid boluses to maintain FTc > 0.35 s or to keep giving fluid boluses as long as SV continues to increase by more than 10%. However, in the PPV method for GDFT the fluid boluses are usually given when PPV is > 15% (sensitivity = 100%, specificity = 80%).
In our practice, we prefer to use an esophageal Doppler probe or PPV to guide fluid management during spine surgery, especially in the prone position. It should be remembered that tidal volume should be 8 to 10 mL/kg during the measurement period. If the blood pressure remains lower than the required target even after fluid supplementation, we administer vasopressors or inotropes to reach the target blood pressure. Of note, the use of vasopressors constricts the precapillary sphincter and decreases the perfusion pressure in the capillaries, thereby reducing the filtration pressure and the tissue edema ( Table 196-3 ).
Commonly Used Colloids during Spine Surgeries
Albumin
Albumin, a natural plasma protein with a molecular weight of 69000 KDa, accounts for the greatest proportion of plasma colloid osmotic pressure. For preparations commonly used in clinical practice, albumin is isolated from pooled human plasma and has been considered to be the gold standard solution for fluid resuscitation in the critically ill population.
This intrinsic effect of albumin is most likely based on its electrostatic binding properties. The charges exposed by the molecules forming the EG are mainly negative, whereas an albumin molecule carries not only negative charges (carboxylate groups) but also positive charges (arginine, lysine) at the physiologic pH. Therefore, the presence of positive charges in albumin enable it to attach to the EG and provide intact ESL. It has been shown in an isolated perfused heart model that providing albumin to the endothelium, before and after ischemia, maintained vascular integrity during reperfusion and alleviated the development of tissue edema. In contrast to the other synthetic colloids molecules, which expose only negative charges on their surfaces, they are not be able to maintain the integrity of the ESL and vascular barrier like albumin. Thereby, using albumin instead of synthetic colloids better reduces interstitial tissue edema. The EG prefers albumin for evoking NO-mediated coronary dilatation. Therefore, albumin helps to reverse stagnation, thrombosis, and corpuscular adherence within cortical venules in the reperfusion phase after focal brain ischemia. One meta-analysis included 17 studies that randomized 1977 participants with sepsis; the use of albumin to resuscitate patients with sepsis was associated with lower mortality compared with other fluid resuscitation regimens.
Hydroxyethyl Starch
Hydroxyethyl starch (HES) is a hydrolyzed and hydroxyethylated derivative of the natural cornstarch amylopectin, dissolved in normal saline or other balanced solvents, and has been developed to serve as an alternative colloid to albumin. HES solutions are characterized by their molecular weight, degree of molar substitution (referring to the average number of hydroxyethyl residues per glucose subunit), and the C 2 /C 6 ratio (referring to the site of hydroxyethylation on the glucose constituent). The use of HES in critical care patients can be associated with severe complications. In a severe sepsis study, the 537 patients who were randomized to low-molecular-weight hydroxyethyl starch (HES 200/0.5) had higher rates of acute renal failure and renal replacement therapy (RRT) than with Ringer lactate. In the Scandinavian Starch for Severe Sepsis/Shock (6S) trial, the patients who were randomized to HES (130/0.4) had an increased risk of death and RRT as compared with those receiving Ringer acetate. In the Crystalloid versus Hydroxyethyl Starch (CHEST) trial, the patients who received HES (130/0.4) had a higher incidence of RRT than those patients were treated with normal saline (NS).
In a meta-analysis including 38 trials comparing HES to crystalloids (albumin or gelatin), the risk ratio (RR) for death among patients randomized to receive HES was 1.07. When seven retracted trials by Bolt were excluded from the analysis, the RR of death increased to 1.09 among 10,290 patients, renal failure increased to 1.27 among 8725 patients, and RRT increased to 1.32 among 9258 patients. The authors concluded that clinical use of HES for acute volume resuscitation is not warranted due to serious safety concerns. In another meta-analysis with 10 trials of 4624 patients, HES was associated with an increased incidence of acute kidney injury (AKI) and the need of RRT, red blood cell (RBC) transfusion, and 90-day mortality in patients with sepsis.
In a post hoc analysis of the Scandinavian Starch for Severe Sepsis/Shock (6S) trial database, treatment with HES increased the risk of bleeding, which was associated with an increased risk of death. In a systemic review of studies assessing 130/0.4 effects on hemostasis by thromboelastograph (TEG) or Sonoclot in comparison with crystalloid or albumin, HES 130/0.4 administration resulted in a weaker and smaller clot. Moreover, an in vitro study on human proximal tubular cells (PTCs) found that the HES molecule itself is cytotoxic to the PTCs but not the carrier, origin, or molecular size.
Therefore, the European Medicine Agency’s Pharmacovigilance Risk Assessment Committee recommended that all HES solutions must no longer be used in patients with sepsis, those with burn injuries, or critically ill patients. HES can used to treat hypovolemia caused by blood loss if the use of crystalloids is contraindicated. HES should not be used for more than 24 hours, and kidney functions should be monitored after HES administration. In addition, the U.S. Food and Drug Administration issued a boxed warning against the use of HES in critically ill patients and those at high risk of bleeding. Given all of the potential problems with using HES, it seems the use albumin is the preferred way to maintain intravascular volume during spine surgery, especially during extensive surgical procedures.
In Conclusion
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The goal of fluid management is to maintain an adequate mean arterial blood pressure (MAP) after adequate resuscitation of intravascular volume and stroke volume, allowing for a more restrictive use of fluids.
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About 80% of infused crystalloids are distributed into interstitial tissues under all conditions.
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Arteriolar pressors lower capillary pressure and reduce filtration, keeping the extracellular volume intravascular.
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Slower infusion rates reduce capillary pressure peaks, minimize hyperfiltration, and maximize intravascular volume.
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Albumin is the preferred colloid to use during spine surgery.
Changes in Respiratory Physiology
In an elegant study, Nyren and colleagues examined the regional distribution of pulmonary blood flow in 10 healthy volunteers. The subjects were studied in both prone and supine positions with and without lung distention caused by 10 cm H 2 O of continuous positive airway pressure. The results demonstrated that ventilation-perfusion matching during both normal breathing and positive pressure is more favorable in the prone than in the supine position. Because perfusion is more evenly distributed in the prone position, the recruitment of dorsal airways results in an increase in lung units and consequently increased functional residual capacity, with near-normal ventilation-perfusion matching and a reduction in shunt. By turning the patient prone and recruiting airways in the dorsal lung, prone positioning achieves similar beneficial effects as positive end-expiratory pressure ventilation but without the risks of barotrauma or interference with cardiac function. Of note, the prone position is sometimes used in patients with acute respiratory distress syndrome to improve oxygenation and decrease shunt. Similar findings were confirmed by Pelosi and coworkers during general anesthesia. Therefore, prone positioning during general anesthesia did not negatively affect respiratory mechanisms, and it improved lung volumes and oxygenation.
Poisson Effect and Prone Position
Breig and colleagues showed that the spinal cord stretches with flexion of the cervical spine and shortens and thickens with extension: the “Poisson effect.” Thickening of the cord in extension makes it more susceptible to pressure from the infolded ligamentum flavum or lamina. In flexion the stretched cord may be prone to higher intrinsic pressure if it is abutting against a disc or vertebral body anteriorly. Prone positioning is often associated with modest degrees of extension; therefore, the cord might sustain excessive pressure induced by soft encroachment on the spinal canal with extension and aggravated by the preexisting canal compromise. The clinical relevance of these findings is that a persistent malposition of an abnormal neck may result in cord ischemia and neurologic injury. Prone positioning is also associated with increases in vena caval pressures, which may further reduce cord blood flow, already compromised by cord compression, by increasing resistance in the venous outflow channels.
Intraoperative Neurophysiologic Monitoring
Intraoperative neurophysiologic monitoring (IONM) refers to the various neurophysiologic tests used to assess functional integrity of the central and peripheral nervous system during surgical procedures that place these structures at risk for iatrogenic injury. In particular, the reported incidence of paraplegia after thoracic and thoracoabdominal aortic surgery remains high, ranging from 2.4% to 40%. The incidence of neurologic complications after the correction of scoliosis with spinal instrumentation has been shown to range between 0.4% and 1.6%. The purpose of IONM is to provide feedback regarding changes in neural function before the development of irreversible neural injury, thereby permitting intervention to prevent or minimize postoperative neurologic deficits.
Mechanisms and Pathophysiology of Spinal Monitoring Changes
Neurologic complications may arise as the result of mechanical or vascular etiologies. Mechanical causes include direct injury (e.g., contusion), distortion of neural elements following the application of corrective forces to the spinal column, and injuries secondary to patient positioning. Spinal cord ischemia may result from the following: stretching of critical spinal cord vascular supply (e.g., following correction of spinal deformity or placement of an anterior graft or cage), prolonged hypotension, or ligation of anterior segmental arteries in the critical vascular zone of the spinal cord (T4-9). Evoked potential latency rarely changes in the absence of an amplitude loss except for latency shifts associated with an increased concentration of inhalational or intravenous agents, lowering of core body or limb temperature or perhaps hypercarbia.
Spinal cord contusion causes a transient spinal cord conduction block, resulting in marked amplitude suppression (50% to 75%) of somatosensory evoked potentials (SSEPs) or transcranial electrical motor evoked potentials (tceMEPs), which typically resolves within 15 to 20 minutes. Reversal of these changes may be aided by increasing mean arterial blood pressure to improve spinal cord perfusion and by temporarily stopping further surgical maneuvers. More serious concussive injury will obliterate both SSEPs and motor evoked potentials (MEPs) entirely.
Prolonged hypotension, whether local or systemic, can result in spinal cord vascular injury. TceMEPs are particularly sensitive to blood pressure changes and can be used effectively to titrate the degree of the hypotensive state the spinal cord will withstand. An area of hypovascularity exists between the proximal and middle third of the dorsal and ventral roots where the nerve is susceptible to both mechanical and ischemic insult. If microtrauma leads to mechanical or metabolic nerve root irritation, the nerve root will depolarize. An action potential results that can be identified both visually and acoustically using electromyographic (EMG) monitoring techniques.
Somatosensory Evoked Potentials
SSEPs are cortical or subcortical responses to repetitive electrical stimulation of a mixed peripheral nerve. SSEPs represent signal-averaged data in which multiple small amplitude responses are recorded over repeated trials via instrumentation that amplifies and filters the signal to provide a measurable response. Typical stimulation sites include the posterior tibial nerve (ankle), the peroneal nerve (fibular head), and the ulnar or median nerves (wrist). The ulnar nerve is the preferred stimulation site for upper extremity SSEPs because the lower spinal nerve entering between C7 and T1 permits assessment of the entire cervical neural axis. The general consensus is that the posterior column spinal pathways are the sites of primary mediation of SSEPs. After synapsing in the medullary nuclei, the neural signal crosses the brain stem and ascends in the medial lemniscal pathways.
It synapses once again in the thalamic nuclei and projects to the sensorimotor cortex. The cortical response for the lower extremity is called the P37, and the cortical response for the upper extremity is called N20. With posterior tibial nerve stimulation, cortical response of the P37 is at its maximum, typically at C7 or CP7 (ipsilateral to the side of stimulation). This ipsilateral orientation of the P37 response has been referred to as “paradoxical lateralization.” Data including signal amplitude (power) and latency (velocity) are recorded continuously during surgery and compared with baseline and recently acquired data. Of these two parameters, amplitude is more relevant. Sustaining a spinal cord injury without amplitude changes is unlikely. Changes in latency during surgery are common. Criteria for surgeon notification vary from center to center but generally include an intraoperative unilateral or bilateral amplitude loss of at least 50% to 60%. Factors other than neurologic injury that may compromise SSEP recordings include halogenated anesthetic agents, N 2 O, hypothermia, and hypotension. In addition, electrical interference from a variety of sources may contaminate the SSEP by affecting amplitude and can lead to interpretation ambiguity.
Limitations of Somatosensory Evoked Potentials
SSEPs monitor only the ascending pathways, which are located in the dorsal parts of the spinal cord and are supplied by the posterior spinal arteries. Therefore, false-negative results (postoperative paraplegia despite unchanged intraoperative SSEPs) have been reported. In contrast, the spinal motoneuronal system is located in the anterior horn of the gray matter and is supplied by the anterior spinal artery. Therefore, SSEPs cannot reflect motor function and motor tract blood supply. To assess the functional integrity of motor tracts during surgery, monitoring of MEPs is therefore required.
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