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The Use of Somatosensory Evoked Potentials to Determine the Relationship Between Patient Positioning and Impending Upper Extremity Nerve Injury During Spine Surgery: A Retrospective Analysis

Section Editor(s): Gelb, Adrian W.Kamel, Ihab R. MD*; Drum, Elizabeth T. MD, FAAP*; Koch, Stephen A. BS; Whitten, Joseph A. BA, MBS*; Gaughan, John P. PhD; Barnette, Rodger E. MD, FCCM*; Wendling, Woodrow W. MD, PhD*

doi: 10.1213/01.ane.0000198666.11523.d6
Neurosurgical Anesthesia: Research Report

Somatosensory evoked potential (SSEP) monitoring is used to prevent nerve damage in spine surgery and to detect changes in upper extremity nerve function. Upper extremity SSEP conduction changes may indicate impending nerve injury. We investigated the effect of operative positioning on upper extremity nerve function retrospectively in 1000 consecutive spine surgeries that used SSEP monitoring. The vast majority (92%) of upper extremity SSEP changes were reversed by modifying the arm position and were therefore classified as position-related. The incidence of position-related upper extremity SSEP changes was calculated and compared for five different surgical positions: supine arms out, supine arms tucked, lateral decubitus position, prone arms tucked, and the prone “superman” position. The overall incidence of position-related upper extremity SSEP changes was 6.1%. The lateral decubitus position (7.5%) and prone superman position (7.0%) had a significantly more frequent incidence of position-related upper extremity SSEP changes (P < 0.0001, Z-test for Poisson counts) compared with other positions (1.8%–3.2%). No patient with a reversible SSEP change developed a new postoperative deficit in the affected extremity. SSEP monitoring is of value in identifying and reversing impending upper extremity peripheral nerve injury.

IMPLICATIONS: The lateral decubitus and prone “superman” positions were identified as high-risk positions for upper extremity somatosensory evoked potential changes in spine surgery. Somatosensory evoked potential changes were reversible by upper extremity position modification and were not associated with postoperative neurological deficits.

Departments of *Anesthesiology, †Neuromonitoring, and ‡Biostatistics, Temple University Hospital and School of Medicine, Philadelphia, Pennsylvania

Accepted for publication November 15, 2005.

Address correspondence and reprint requests to Ihab R. Kamel, MD, Department of Anesthesiology (3rd Floor Outpatient Building), Temple University Hospital, 3401 N. Broad Street, Philadelphia, PA 19140. Address e-mail to ihab.kamel@temple.edu.

Peripheral nerve injury is a significant perioperative problem and is the second most common cause of professional liability and patient injury in the practice of anesthesiology (1). The etiology of perioperative nerve injury is not well understood (2). Somatosensory evoked potential (SSEP) monitoring has an established role in maintaining the integrity of the nervous system during spine surgery and in reducing the incidence of postoperative neurological deficits (3). Upper extremity SSEP conduction changes may indicate impending peripheral nerve injury (4–6).

Identifying upper extremity SSEP changes during surgery and modifying the arm position to reverse these SSEP changes may prevent peripheral nerve injury. Moreover, SSEP monitoring can be used to study the effect of intraoperative positioning on nerve function and may lead to a better understanding of perioperative nerve injury. We retrospectively investigated the effect of five different operative positions on upper extremity peripheral nerve function and the use of SSEPs to detect and prevent perioperative upper extremity peripheral nerve injury during spine surgery.

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Methods

After obtaining the approval of the IRB at Temple University, we retrospectively reviewed a computerized database of 1000 consecutive spinal surgeries from 1995 to 2001 during which SSEP monitoring was used. Patient demographics, indications for surgery, the areas of surgery (cervical, thoracic, and/or lumbar spine) and the types of surgery were noted.

After the induction of general anesthesia, patients were positioned in the operating room in one or more of five positions, supine arms tucked, supine arms out, lateral decubitus, prone “superman,” or prone arms tucked. A detailed description of each position is given in Table 1. The database and medical records were examined for three temporally related events: occurrence of a significant upper extremity SSEP change, position modification by the anesthesiology team, and the SSEP response to position modification. If the position modification led to resolution of the SSEP change, it was classified as a position-related SSEP change.

Table 1

Table 1

The changes in SSEPs in all surgical cases and all positions were reviewed and investigated. SSEP changes that returned to baseline after increasing mean arterial blood pressure, decreasing anesthetic depth, or interventions by the surgical team were considered to not be position-related and thus were excluded from consideration.

When a position-related SSEP change was identified, the modification in the position of the affected upper extremity was carefully noted. Position modification strategies used by the anesthesiology team included releasing shoulder traction on tucked arms, decreasing shoulder abduction, correcting extreme elbow flexion and extension, and returning of the arms and/or forearms to their original position if the position had already been modified. Re-evaluation of SSEPs was uniformly performed within 10 minutes of position modification.

The patients’ medical records were reviewed for the presence of postoperative neurological deficits in all patients who had SSEP changes. The incidence of upper extremity position-related SSEP change was calculated for the five different operative positions. The time spent in the operative position in which the upper extremity SSEP change occurred was reviewed. For patients in the lateral decubitus position, the incidence of position-related SSEP changes in the nondependent versus the dependent arms was calculated and compared.

Standard SSEP monitoring and practice protocols during the time period under review included the following:

Patients were monitored with two Explorer 8 Channel IOM systems, software version 5.71, Model 755 (Bio-logic Systems Corp., Mundelein, IL). Stainless-steel 12-mm subdermal needle electrodes (Medtronic Xomed, Jacksonville, FL) were used for stimulation. Subdermal needle electrodes (Rochester Electro-Medical, Inc., Tampa, FL) were used for recording.

Median nerve stimulation at the wrist was the predominant SSEP modality. When the ulnar nerve was used, it was stimulated at the wrist or at the ulnar notch. A pre-existing deficit in one distribution prompted monitoring of both the median and ulnar nerve SSEPs to maximize sensitivity. Stimuli were routinely 200-μs constant current square pulses with amplitudes ranging from 5 to 50 mA presented at a rate of 4.9/s (range, 1.9–5.1 stimuli/s). These settings were chosen to be out of phase with room electrical noise. Sweep duration was 50 ms (or 100 ms on rare occasions). Upper extremity testing was performed approximately every 30 minutes. Cervical surgeries necessitated shorter testing intervals. The montage combination of choice was Fpz-C3′, Fpz-C4′, C3′-C4′, and Fpz-Crv5. Linked Erb’s point recordings were occasionally used. During posterior cervical surgeries, the chin was used (Fpz-Crv5 replaced with Fpz-Chin). Impedances were checked at baseline and were typically well balanced (<5 kΩ each with < 2 kΩ imbalance).

Upper SSEPs were evaluated for changes by measuring the cortical and cervicomedullary peak latencies and amplitudes (N13/N14 and N20 to the next down-going peak) and comparing them with the baseline. Baselines for comparison were routinely acquired before the use of electrocautery, but after patient positioning. A change in SSEP signal was considered significant if the signal amplitude decreased by 50% or more and/or latency increased by 10% or more. If upper extremity edema was suspected, stimulus levels were brought up to exclude reduced stimulus efficacy (2–10 mA increase).

Demographic data (age and sex) and the duration of surgery are presented as mean and standard deviation. The Z-test for Poisson counts was used to compare the incidence of position-related upper extremity SSEP changes among the five operative positions and to compare the incidence of position-related SSEP changes between dependent and nondependent arms in the lateral decubitus position. A P value of <0.05 was considered significant.

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Results

One thousand consecutive spine surgeries were performed on 929 patients. The patients were positioned in 1109 operative positions. Patient demographics are presented in Table 2 and the types of surgeries performed are included in Table 3. Indications for surgery included scoliosis, degenerative diseases, neoplasms, fractures, and infections. Surgeries were performed on the cervical, thoracic, and/or lumbar spine. In the 1109 positions, 74 (6.6%) upper extremity SSEP changes occurred. Of the 74 changes, 68 (6.1%) were position-related and six (0.5%) SSEP signal changes were not. All changes identified (position-related and not position-related) were reversible. The mean duration spent in the position in which the upper extremity position-related SSEP changes occurred was 425 ± 139 minutes. Of the 68 patients who developed position-related SSEP changes and the six patients who developed non–position-related SSEP changes, none developed a subsequent new postoperative neurological deficit in the affected upper extremity.

Table 2

Table 2

Table 3

Table 3

Position-related upper extremity SSEP changes in each of the five positions are presented in Figure 1 and Table 4. For the position-related SSEP changes in the lateral decubitus position, the side of SSEP change (dependent versus nondependent) was identified in 26 patients of the 29; 16 changes occurred in the nondependent arm and 10 changes in the dependent arm. The incidence of position-related upper extremity SSEP changes in the lateral decubitus position was significantly more frequent compared with the supine arms tucked, supine arms out, and prone arms tucked positions (P < 0.0001) but not the prone superman position (P = 0.26). The incidence of position-related upper extremity SSEP changes in the prone superman position was significantly higher compared with the supine arms tucked, supine arms out and prone arms tucked positions (P < 0.0001) but not the lateral decubitus position. There was no significant difference in position-related upper extremity SSEP changes among the supine arms tucked, supine arms out and prone arms tucked positions. The incidence of position-related SSEP changes in the nondependent arm (16 changes) was not significantly more frequent compared with the dependent arm (10 changes) in the lateral decubitus position (P = 0.24).

Figure 1

Figure 1

Table 4

Table 4

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Discussion

In this study, the overall incidence of position-related upper extremity SSEP changes during spine surgery was 6.1%. The lateral decubitus and the prone superman positions were identified as having significantly more frequent incidences of position-related upper extremity SSEP changes compared with other operative positions during spine surgery. None of the position-related SSEP changes that were identified and reversed were associated with new postoperative neurological deficits.

SSEPs are frequently used for major spinal surgeries (75%) and are available at most institutions (94%) in the United States (7). A significant change in SSEP signal is indicated by a decrease in amplitude and/or an increase in latency. Changes in SSEP signal may be due to spine instrumentation, hypoperfusion, hypothermia, anesthetic agents and depth, operating room noise, and positioning of body and extremities (8–10). The overall incidence of SSEP changes during surgery ranges from 2.0% to 65% (10). The SSEP signal change is often attributed to surgical causes or anesthetic technique; however, the change in SSEP signal may be due to impending peripheral injury.

The use of SSEPs to monitor upper extremity conduction changes related to positioning has been the focus of multiple investigations. Reversing upper extremity changes by adjusting the arm position establishes a cause-effect relation between positioning and the SSEP change. As changes in nerve conduction are anticipated to occur before permanent nerve injury, position modification that reverses SSEP conduction changes should prevent perioperative peripheral nerve injury (4–6). SSEP monitoring has been reported to predict or prevent upper extremity perioperative nerve injury (4–6,10–17).

Schwartz et al. (5) reported the value of SSEPs in preventing position-related brachial plexopathy during surgical correction of scoliosis. The incidence of impending brachial plexopathy during spine surgery in the prone position was reported as 3.6%. The authors retrospectively investigated SSEP-guided upper extremity position modification in 500 cases in which 30% reduction in amplitude was used as the cut-off value to define a significant change. Changes in the SSEP signal returning to baseline after modifying the position were considered position-related and were not associated with postoperative brachial plexopathy.

Lorenzini and Poterack (18) examined SSEP monitoring in 14 awake volunteers in the prone position. Their protocol involved voluntary gradual abduction of the shoulders. Recordings for SSEP signals were made 10–15 minutes after positioning the arm. A 60% decrease in amplitude and 10% increase in latency were judged significant. Three of seven patients reported upper extremity symptoms without SSEP changes being noted; however, the method used did not completely parallel the intraoperative conditions, in that symptoms were used as a surrogate for nerve injury and anesthetized patients can be placed in positions that a conscious patient may not tolerate.

Peripheral nerve injury is usually due to ischemia of the intraneural capillaries resulting from overstretch or compression of the peripheral nerves. Prolonged intraoperative hypotension may also contribute to the injury. In the upper extremity, the ulnar nerve is more sensitive to ischemia compared with the radial and median nerves (12). Certain medical diseases such as diabetes mellitus, hypoglycemia, atherosclerosis, and uremia may render the peripheral nerve more vulnerable to injury (19). Most peripheral nerves are intolerant to stretch beyond 10% of their normal length (19). General anesthesia, muscle relaxation, and certain operative positions may increase the risk of overstretching and compression. The prone position was linked to claims for nerve injury (1). Very thin and obese patients are more prone to persistent postoperative ulnar neuropathy (20). Simultaneous application of multiple factors contributing to stretch or compression of the brachial plexus could explain the more frequent incidence of impending upper extremity nerve injury in the lateral decubitus and the prone superman positions.

The lateral decubitus position may apply vertical forces that compress the brachial plexus between the clavicle and the first rib (21). Trendelenburg position and the use of shoulder braces may augment compression forces (1,5). Pronation of the nondependent forearm in the lateral decubitus position increases direct pressure over the ulnar nerve at the elbow (22). Supination of the dependent forearm and full extension of the elbow overstretches the peripheral nerves (23). Overstretch of the nondependent arm as a result of full elbow extension and shoulder abduction more than 90° may also contribute to nerve injury (23). However, compression is the leading etiology of impending nerve injury in the lateral decubitus position (21).

In the prone “superman” position, overstretch of the brachial plexus may occur throughout the entire length of the plexus and is the major etiology of nerve injury (24). Overstretch may occur with shoulder girdle depression, shoulder abduction more than 90°, lateral rotation of the arm, and full elbow extension (23). Direct pressure is significantly higher on the ulnar nerve at the elbow if both forearms are pronated (22). Vertical compressive forces may contribute to the nerve injury. The head of the humerus may compress the neurovascular bundle if the arm and axilla are not relaxed (24).

The limitations of this study include its retrospective nature, the lack of individual stimulation of multiple peripheral nerves, the lack of standardization of anesthetic and surgical technique, and a demographically diverse patient population. We cannot comment on peripheral nerves that were not stimulated and monitored, and the effect of operative positioning cannot be extrapolated to peripheral nerves that were not monitored.

Prevention of perioperative nerve injury is an important goal in the practice of anesthesiology. Understanding which positions place patients at risk for potential peripheral nerve injury is also important. This study identifies the lateral decubitus position and prone superman position as high-risk positions for impending upper extremity perioperative peripheral nerve injury compared with other standard operative positions. None of the position-related SSEP changes which were identified and reversed were associated with postoperative neurological deficits. SSEP monitoring and intraoperative position modification are of value in identifying and reversing impending upper extremity peripheral nerve injury during spine surgery.

The authors thank Nancy Kenepp, MD, Associate Professor of Anesthesiology, Temple University School of Medicine, Philadelphia, for valuable editorial comments and guidance.

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