Elena Montes

Safe pedicle screw placement in thoracic scoliotic curves using t-EMG: stimulation threshold variability at concavity and convexity in apex segments.

Study Design. A cross-sectional study of nonconsecutive cases (level III evidence). Objective. In a series of young patients with thoracic scoliosis who were treated with pedicle screw constructs, data obtained from triggered electromyography (t-EMG) screw stimulation and postoperative computed tomographic scans were matched to find different threshold limits for the safe placement of pedicle screws at the concavity (CC) and convexity (CV) of the scoliotic curves. The influence of the distance from the medial pedicle cortex to the spinal cord on t-EMG threshold intensity was also investigated at the apex segment. Summary of Background Data. Whether the t-EMG stimulation threshold depends on pedicle bony integrity or on the distance to neural tissue remains elusive. Studying pedicle screws at the CC and CV at the apex segments of scoliotic curves is a good model to address this issue because the spinal cord is displaced to the CC in these patients. Methods. A total of 23 patients who underwent posterior fusions using 358 pedicle thoracic screws were reviewed. All patients presented main thoracic scoliosis, with a mean Cobb angle of 58.3 degrees (range, 46–87 degrees). Accuracy of the screw placement was tested at surgery by the t-EMG technique. During surgery, 8 screws placed at the CC showed t-EMG threshold values below 7 mA and were carefully removed. Another 25 screws disclosed stimulation thresholds within the range of 7 to 12 mA. After checking the screw positions by intraoperative fluoroscopy, 15 screws were removed because of clear signs of malpositioning. Every patient underwent a preoperative magnetic resonance imaging examination, in which the distances from the spinal cord to the pedicles of the concave and convex sides at 3 apex vertebrae were measured. Postoperative computed tomographic scans were used in all patients to detect screw malpositioning of the final 335 screws. Results. According to postoperative computed tomographic scans, 44 screws (13.1%) showed different malpositions: 40 screws (11.9%) perforated the medial pedicle wall, but only 11 screws (3.2%) were completely inside the spinal canal. If we considered the 23 screws removed during surgery, the true rate of misplaced screws increased to 18.7%. In those screws that preserved the pedicle cortex (well-positioned screws), EMG thresholds from the CC showed statistically significantly lower values than those registered at the CV of the deformity (21.1 ± 8.2 vs 23.9 ± 7.7 mA, P < 0.01). In the concave side, t-EMG threshold values under 8 mA should be unacceptable because they correspond to screw malpositioning. Threshold values above 14 mA indicate an accurate intrapedicular position with certainty. At the convex side, threshold values below 11 mA always indicate screw malpositioning, and values above 19 mA imply accurate screw placement. At the 3 apex vertebrae, the average pedicle–spinal cord distance was 2.2 ± 0.7 mm at the concave side and 9.8 ± 4.3 mm at the convex side (P < 0.001). In well-positioned screws, a correlation between pedicle–dural sac distance and t-EMG threshold values was found at the concave side only (Pearson r = 0.467, P < 0.05). None of the patients with misplaced screws showed postoperative neurological impairment. Conclusion. Independent of the screw position, average t-EMG thresholds were always higher at the CV in the apex and above the apex regions, presuming that the distance from the pedicle to the spinal cord plays an important role in electrical transmission. The t-EMG technique has low sensitivity to predict screw malpositioning and cannot discriminate between medial cortex breakages and complete invasion of the spinal canal.

Electromyographic thresholds after thoracic screw stimulation depend on the distance of the screw from the spinal cord and not on pedicle cortex integrity

BACKGROUND CONTEXT Present studies concerning the safety and reliability of neurophysiological monitoring during thoracic pedicle screw placement remain inconclusive, and therefore, universally validated threshold levels that confirm osseous breakage of the instrumented pedicles have not been properly established. PURPOSE The objective of this work was to analyze whether electromyographic (EMG) thresholds, after stimulation of the thoracic pedicle screw, depend on the distance between the neural structures and the screws. The modifier effect of different interposed tissues between a breached pedicle and neural structures was also investigated. STUDY DESIGN This experimental study uses a domestic pig model. METHODS Electromyographic thresholds were recorded after the stimulation of 18 thoracic pedicle screws that had been inserted into five experimental animals using varying distances between each screw and the spinal cord (8 and 2 mm). Electromyographic thresholds were also registered after the medial pedicle cortex was broken and after different biological tissues were interposed (blood, muscle, fat, and bone) between the screw and the spinal cord. RESULTS Mean EMG thresholds increased to 14.1±5.5 mA for screws with pedicle cortex integrity that were placed 8 mm away from the dural sac. After the medial pedicle cortex was broken and without varying the distance of the screw to the dural sac, the mean EMG thresholds were not appreciably changed (13.6±6.3 mA). After repositioning the screw at a distance of 2 mm from the spinal cord and after medial cortical breach of the pedicle, the mean threshold significantly slowed to 7.4±3.4 mA (p<.001). When the screw was placed in contact with the spinal dural sac, even lower EMG thresholds were registered (4.9±1.9, p<.001). Medial pedicle cortex rupture and the interposition of different biological tissues in experimental animals did not alter the stimulation thresholds of the thoracic pedicle screws. CONCLUSIONS In the experimental animals, the observed electrical impedance depended on the distance of screws from the neural structures and not on the integrity of the pedicle cortex. The screw-triggered EMG technique did not reliably discriminate the presence or absence of bone integrity after pedicle screw placement. The response intensity was not related to the type of interposed tissue.

Influence of hypotension and nerve root section on the ability to mobilize the spinal cord during spine surgery. An experimental study in a pig model

BACKGROUND CONTEXT The correction of severe spinal deformities by an isolated posterior approach often involves cord manipulation together with hypotensive anesthesia. To date, the efficiency of methods to increase the tolerance of the cord to displacement and the influence of hypotension on this tolerance is yet to be assessed. PURPOSE The objective of this study was to determine the limits of cord displacement before the disappearance of neurophysiologic signals. The influence of the type of force applied, the section of the roots, and the induced hypotension on the cords tolerance to displacement was also assessed. STUDY DESIGN Experimental study using a domestic pig model. OUTCOME MEASURES Successive records of cord-to-cord motor evoked potentials were obtained during displacement maneuvers. Displacing forces were released immediately after the absence of neurophysiologic signals. METHODS Surgical procedures were performed under conventional general anesthesia. The spinal cord and nerve roots from T6 to T10 levels were exposed by excision of the posterior elements, allowing for free cord movement. Three groups were established according to the method of spinal cord displacement: the separation (Group 1, n=5), the root stump pull (Group 2, n=5), and the torsion groups (Group 3, n=5). An electromechanical external device was used to apply the displacing forces. The three displacement tests were repeated after sectioning the adjacent nerve roots. The experiments were first carried out under normotension and afterward under induced hypotension. RESULTS In Group 1, evoked potential disappeared with a displacement of 10.1±1.6 mm with unharmed roots and 15.3±4.7 mm after the sectioning of four adjacent roots (p<.01). After induced hypotension, potentials were lost at 4.0±1.2 mm (p<.01). In Group 2, the absence of potentials occurred at 20.0±4.3 mm and increased to 23.5±2.1 mm (p<.05) after cutting the two contralateral roots. Under hypotensive conditions, the loss of neurophysiologic signals was detected at 5.3±1.2 mm (p<.01). In Group 3, the cord allowed torsion of 95.3±.2° that increased to 112.4±7.1° if the contralateral roots were cut. Under hypotension, the loss of potentials was found at 20±6.2° (p<.01). CONCLUSIONS In this experimental model, it was possible to displace the thoracic spinal cord by a distance superior to the spinal cord width without suffering neurophysiologic changes. The limits of cord displacement increase when the adjacent nerve roots are sacrificed. Induced hypotension had a dramatic effect on the tolerance of the spinal cord for displacement. This work has an important clinical significance because induced hypotension during specific spine surgery procedures requiring spinal cord manipulation in humans may increase the risk of neurologic spinal cord injury.

O39: Intraoperative neurophysiological monitoring in patients with spinal cord injuries. Experimental study

Question: What is the agreement in spindle scoring within, between and among experts? How does spindle scoring by humans compare to automated spindle scoring algorithms? Methods: We crowd-sourced the collection of spindle scorings from 24 experts in a large and varied dataset of EEG (C3-M2) from 110 middle-aged sleeping subjects. Epochs were scored by an average of 5.3 unique experts. Two experts scored parts of the dataset multiple times. We developed a simple method to build a large gold standard by establishing group consensus among expert scorers. We tested the performance of six previously published automated spindle detectors against the gold standard and refined methods of performance analysis for event detection. Results: We found an interrater agreement (F1-score) of 61±6% (Cohen’s Kappa (κ): 0.52±0.07) averaged over 24 expert pairs and an intrarater agreement of 72±7% (κ: 0.66±0.07) averaged over two experts. We tested the performance of individual experts to a gold standard compiled from all the expert scorers and found average agreement of 75±6% (κ: 0.68) over the 24 experts. We recompiled the gold standard and excluded the single expert whose performance was being assessed, and found an average agreement of 67±7% (κ: 0.59). Overall, we found the performance of human experts to be significantly better than the automated sleep spindle detectors we tested (maximum F1-score of detectors: 52%). Conclusions: Sleep spindle characteristics between subjects are very diverse which makes the scoring task difficult. The low interrater reliability suggests using more than one expert when scoring a dataset.

P31-18 Spinal cord lesions during scoliosis surgery: our experience and neurophysiologic identification of the lesion level

Objective: In animal studies, it is shown that proximal (target-reaching) and distal (grasping) movements of the upper extremity are differently affected by spinal pyramidotomy. We analyzed the target-reaching movement in patients with cervical myelopathy, and examined if it is useful to evaluate symptoms, and to predict recovery after the decompression surgery. Methods: Twenty-eight pre-operative patients and 15 age-matched controls participated in the experiments. Some patients returned to the experiments, up to 1 year after the surgery. They performed targetreaching movements, cued by sound. Three-dimensional positions of the index finger were sampled by an electromagnetic motion tracking system. By analyzing the movement, reaction time, movement time and accuracy of touch position (TP) were obtained, as well as time for online correction (CT) induced by sudden target jump. The parameters were compared with scores of conventional tests; the JOA score, 10-second grip and release test, the manual muscle testing, and motor evoked potential induced by transcranial magnetic stimulation of the motor cortex. Results: All patients mainly claimed clumsiness in the hand movement pre-operatively, and showed significantly deteriorated scores in conventional tests. They also showed poor online adjustments of the reaching movement. It was reflected in long CT and variable TP, though the other parameters were not affected significantly. However, CT was not correlated with any score from conventional tests. After the surgery, CT immediately returned to the normal level, while the JOA score, which mainly evaluate hand functions, improved gradually during months. When including post-operative data, CT and JOA score were correlated, and recovery of the latter could be predicted better if including CT immediately after the surgery. Conclusions: The target-reaching movement is useful to evaluate deficits that cannot be detected by conventional tests. Post-operative JOA score suggests that recovery in hand functions partly depends on mechanisms for the reaching movement.

P31-21 Recording triggered EMG thresholds from axillary chest wall electrodes. A new refined technique for accuracy of upper thoracic pedicle screw placement

MEPs remained above 50% of baseline in all of 23 patients and no patients had postoperative vagus nerve deficits. Conclusions: Reliable facial and vagus nerve MEPs could be recorded throughout the procedure during skull base procedures. CB MEP monitoring can circumvent difficulties of standard EMG monitoring techniques, provide ongoing evaluation of facial and vagus nerve function and predict outcome with sufficiently useful accuracy.