Muscle MEP Research Articles

The role of intraoperative extensor digitorum brevis muscle MEPs in spinal surgery.

Intraoperative muscle motor evoked potentials (m-MEPs) are widely used in spinal surgery with the aim of identifying a damage to spinal cord at a reversible stage. Generally, lower limb m-MEPs are recorded from abductor hallucis [AH] and the tibialis anterior [TA]. The purpose of this work is to study an unselected population by recording the m-MEPs from TA, AH and extensor digitorum brevis (EDB), with the aim of identifying the most adjustable and stable muscles responses intraoperatively. Transcranially electrically induced m-MEPs were intraoperative recorded in a total of 107 surgical procedures. m-MEPs were recorded by a needle electrode placed in the muscle from TA, AH and EDB muscles in the lower extremities. Overall monitorability (i.e., at least 1 Lower Limb m-MEP recordable) was 100/107 (93.5%). In the remaining 100 surgeries in 3 cases, the only muscle that could be recorded at baseline was one AH, and in other 2 the EDB. Persistence (i.e., the recordability of m-MEP from baseline to the end of surgery) was 88.7% for TA, 89.8% for AH and 93.8% for EDB. In our series, EDB m-MEPs have demonstrated a recordability superior to TA and a stability similar to AH. The explanations may be different and range from changes in the excitability of the cortical motor neuron to the different sensitivity to ischemia of the spinal motor neuron. EDB can be used alternatively or can be added to TA and AH as a target muscle of the lower limb in spinal surgery.

Surgery for intramedullary spinal cord ependymomas in the neuromonitoring era: results from a consecutive series of 100 patients.

The established treatment of intramedullary spinal cord ependymomas (ISCEs) is resection. Surgical series reporting treatment results often lack homogeneity, as these are collected over long time spans and their analysis is plagued by surgical learning curves and inconsistent use of intraoperative neurophysiological monitoring (IONM). The authors report the oncological and functional long-term outcomes in a modern series of 100 consecutive ISCEs that were resected between 2000 and 2015 by a surgically experienced team that consistently utilized IONM. In this retrospective study, the authors tailored surgical strategy and multimodal IONM, including somatosensory evoked potentials, muscle motor evoked potentials (mMEPs), and D-waves, with the aim of gross-total resection (GTR). Preservation of the D-wave was the primary objective, and preservation of mMEPs was the second functional objective. Functional status was evaluated using the modified McCormick Scale (MMS) preoperatively, postoperatively, and at follow-up. Preoperatively, 89 patients were functionally independent (MMS grade I or II). A GTR was achieved in 89 patients, 10 patients had a stable residual, and 1 patient underwent reoperation for tumor progression. At a mean follow-up of 65.4 months, 82 patients were functionally independent, and 11 lost their functional independence after surgery (MMS grades III-V). Muscle MEP loss predicted short-term postoperative worsening (p < 0.0001) only, while the strongest predictors of a good functional long-term outcome were lower preoperative MMS grades (p < 0.0001) and D-wave preservation. D-wave monitorability was 67%; it was higher with lower preoperative MMS grades and predicted a better recovery (p = 0.01). In this large series of ISCEs, a high rate of GTR and long-term favorable functional outcome were achieved. Short- and long-term functional outcomes were best reflected by mMEPs and D-wave monitoring, respectively.

Comparison of Muscle MEPs From Transcranial Magnetic and Electrical Stimulation and Appearance of Reflexes in Horses.

IntroductionTranscranial electrical (TES) and magnetic stimulation (TMS) are both used for assessment of the motor function of the spinal cord in horses. Muscular motor evoked potentials (mMEP) were compared intra-individually for both techniques in five healthy horses. mMEPs were measured twice at increasing stimulation intensity steps over the extensor carpi radialis (ECR), tibialis cranialis (TC), and caninus muscles. Significance was set at p < 0.05. To support the hypothesis that both techniques induce extracranially elicited mMEPs, literature was also reviewed.ResultsBoth techniques show the presence of late mMEPs below the transcranial threshold appearing as extracranially elicited startle responses. The occurrence of these late mMEPs is especially important for interpretation of TMS tracings when coil misalignment can have an additional influence. Mean transcranial motor latency times (MLT; synaptic delays included) and conduction velocities (CV) of the ECR and TC were significantly different between both techniques: respectively, 4.2 and 5.5 ms (MLTTMS–-MLTTES), and −7.7 and −9.9 m/s (CVTMS-CVTES). TMS and TES show intensity-dependent latency decreases of, respectively, −2.6 (ECR) and −2.7 ms (TC)/30% magnetic intensity and −2.6 (ECR) and −3.2 (TC) ms/30V. When compared to TMS, TES shows the lowest coefficients of variation and highest reproducibility and accuracy for MLTs. This is ascribed to the fact that TES activates a lower number of cascaded interneurons, allows for multipulse stimulation, has an absence of coil repositioning errors, and has less sensitivity for varying degrees of background muscle tonus. Real axonal conduction times and conduction velocities are most closely approximated by TES.ConclusionBoth intracranial and extracranial mMEPs inevitably carry characteristics of brainstem reflexes. To avoid false interpretations, transcranial mMEPs can be identified by a stepwise latency shortening of 15–20 ms when exceeding the transcranial motor threshold at increasing stimulation intensities. A ring block around the vertex is advised to reduce interference by extracranial mMEPs. mMEPs reflect the functional integrity of the route along the brainstem nuclei, extrapyramidal motor tracts, propriospinal neurons, and motoneurons. The corticospinal tract appears subordinate in horses. TMS and TES are interchangeable for assessing the functional integrity of motor functions of the spinal cord. However, TES reveals significantly shorter MLTs, higher conduction velocities, and better reproducibility.

PB6. A new motor threshold criterion for intraoperative corticobulbar MEP can predict postoperative facial nerve function

Objective Intraoperatively, facial nerve (FN) function is assessed with corticobulbar Motor Evoked Potentials (FN-coMEP). MEP-amplitude decrement and increase in motor threshold (MT) serve as warning criteria. A novel threshold criterion for extremity muscle MEP compares final-to-baseline motor threshold levels (BFB-MT) between the operated versus non-operated side. We applied BFB-MT for FN-coMEP with regard to postoperative FN-function. Methods 79 patients (45f; 48 ± 16 yrs., 68% vestibular schwannoma) undergoing cerebellopontine angle tumor surgery were analysed. FN-function was assessed with the House-Brackmann score (HB) pre- and postoperatively at day 1 (d1), 7 (d7), 3 months (3 m) and grouped in mild (HB score increase ⩽ 1) or marked deterioration (HB score increase ⩾ 2). FN-coMEP were elicited with anodal transcranial electric stimulation at C4;C3 referenced to Cz, and recorded from oris and mentalis muscles. A BFB-MT difference ⩾ 20% (operated vs. non-operated side) was considered significant. MT-increase of ⩾ 20 mA and amplitude decrement ⩾ 50% on the operated side served as reference criteria. FN function was correlated to all 3 criteria. Results At d1, 22% of patients (17/79) showed mild HB deterioration, 27% (21/79) marked. At d7, 25% (20/79) showed mild and 16% (13/79) marked deterioration. At 3 m follow-up (available in 68 patients), 94% showed recovery (64/68). Only 4/68 patients showed residual marked FN function deterioration. FN-coMEP were obtained in 95% cases (75/79). Changes according to BFB-MT (35 ± 17 mA) occurred in 17 patients, 16 of these patients suffered a marked deterioration of FN function at d1. BFB-MT correlated significantly with FN function at d1 and d7. The correlation was stronger using the BFB-MT (Spearman correlation: d1: p = 0.726; d7: p = 0.559; 3 m: p = 0.265) compared to absolute threshold increase ⩾ 20 mA (Spearman correlation: d1: p = 0.675; d7: p = 0.528; 3 m: p = 0.275) and amplitude reduction ⩾ 50% (Spearman correlation: d1: p = 0.305; d7: p = 0.378; 3 m: p = 0.118). Conclusion An increase of BFB-MT ⩾ 20% between the operated vs. non-operated side serves as a better predicator for postoperative FN function than absolute mA increase and amplitude reduction. Further studies have to validate this criterion intraoperatively for its real time applicability.

S113. Intraoperative recording of somatosensory evoked potentials from both peripheral median nerve and proximal upper trunk to assess C5, C6 brachial plexus injury

The prognosis of reconstructive surgery for brachial plexus injuries is related to the localisation of injury, whether the damage exists primarily in the brachial plexus, the spinal nerves distal to the dorsal root ganglia, or at the point of connection with the spinal cord (Jones, 1980). The use of somatosensory evoked potentials (SSEP) to monitor upper extremity nerves during surgery is becoming more accepted as a valid and useful technique to minimize intraoperative nerve injuries (Son et al., 2017). When SEP recording is used in an operating theater, it is valuable for evaluating proximal root lesions (Sugioka, 1984). We present a case illustrating the benefit of utilizing both SSEP from peripheral median nerve (MN) and from brachial plexus lateral cord (LC) and upper trunk (UT) for conducting the surgery procedure. Neurosurgeons decided to do nerve transfer, distal spinal accessory nerve to suprascapular nerve and ulnar nerve branch to musculocutaneous (MC) nerve, as it was found that the lesion might correspond to C5, C6 roots avulsion instead of nerve graft for the initially suspected lateral cord injury. EMG test before surgery and intraoperative muscle MEP mapping technique and SSEP from MN, LC, anterior division of the upper trunk (ADUT) and UT in a 66-year-old patient with total left forearm flexion and shoulder abduction paralysis after motorbike accident occurred 4 months earlier. SSEP protocols (4.1 Hz stimulus rate and 300 sweeps) were already installed in the IONM NIM-eclipse Medtronic device. We used subdermal paired electrodes for MN stimulation and two hooked monopolar electrodes for proximal stimulation (LC, ADUT and UT). The peripheral nerve distal SSEP and proximal brachial plexus SSEP were recorded from spinal Cs2-Fpz, inion-Fpz and C4’-Fpz channels. For muscle MEP and raw EMG were used paired subdermal electrodes in deltoid, biceps, EDC, APB and ADM muscles. EMG: 3 weeks after the accident and 4 days before surgery showed acute denervation in myotomes corresponding to C5, C6 root levels. SNAP of ulnar, median and radial nerves were present. Intraoperatively, muscle MEP did not show responses from LC to biceps and from posterior cord to deltoid muscle. SSEP from MN showed delayed N13 at 23 ms and P(N)20 at 27 ms. SSEP from LC showed N13 at 15 ms and P20 at 20 ms. From ADUT and from the UT SSEP was obtained at cervical level but not from the cortex. At this moment we added another channel Fpz-inion. The UT SSEP was absent in inion-Fpz, C4‘-Fpz while peripheral MN SSEP showed responses at three C2 spinal, inion and cortex levels. We proved the sensory-motor dysfunction of the ADUT, UT and proximally to the site of connection with the spinal cord, both with muscle MEP mapping and SSEP absence (ADUT and UT) at cortical level and neurosurgeons decided to perform a nerve transfer instead of a nerve graft. This report demonstrates a case in which IONM was useful in identifying the lesion site during brachial plexus surgery.

T96. Neuromonitoring adolescent idiopathic scoliosis with pulse train technique

The placement of thoracic pedicle screw is technically difficult, and the medial mal position can result in a feared risk for the spinal cord. The effectiveness of detecting mal-positioning of pedicle screws varies widely in the literature, specially at thoracic levels, but in 2014, Calancie et al. described a technique with high accuracy in preventing that in adult patients. We prospectively studied the usefulness of the technique in adolescents with idiopathic scoliosis. We studied 51 adolescent patients (41 female), mean age of 14.8 y (11–18 y) diagnosed with idiopathic scoliosis, submitted to scoliosis correction with the free-hand technique for thoracic pedicle screw placement under total intravenous anesthesia (TIVA) and intraoperative neuromonitoring (IONM) with our standard protocol updated with the multi-train stimulation technique (MTST). MTST parameters consists of a repetitive train of electric pulses applied in the track and in the implanted pedicle screw. The warning criteria was a muscle MEP recorded in lower limb muscles with stimulus below 12 mA for the track and 25 mA for the screw. Intraoperative fluoroscopy was used to check screw position if the warning criteria was achieved and after the placement of all screws if there were no alarms. The final decision of the surgeon to a given alarm considered anatomical features, the threshold and the fluoroscopy images. We studied 210 tracks/screws from T2 to L1 in the first 13 cases and 390 from T2 to T10 in the remaining 38. Track/screw threshold was below the limit 49 times (8.2%), with higher incidence for T5 and T9 (10 times each), and for T7 and T8 (8 times each). Tracks/screws were redirected successfully in 33 cases and due to their persistent lower thresholds, withdrawn in 8, all of them from the apex of the scoliosis concavity. In the remaining 8 cases the surgeon decided to let the implant in the original position considering the imaged and the screw thresholds close to the limit. In 24 cases (47%) the warning criteria were not achieved neither for track or screw, and the images agreed with the IONM findings. In 3 cases the thresholds for the screws were below 4 mA and a complete loss of TcMEP in lower limb immediately followed the screw placement. In two cases the screws were removed immediately, and the patients have transitory motor deficit, but in one case the surgeon took a long time to remove the implants and the patient become paraplegic. MTST takes no more than 10 s for each screw. The technique showed a good relationship with medial mal-positioning of the track/screw. It was not time consuming and the surgeons felt confident with the warning criteria, reducing also the X-ray exposition. Very low thresholds can even suggest spinal cord lesion demanding the cross checking with TcMEP. The technique showed to be safe and trustful for younger patients (adolescents) with idiopathic scoliosis and should be considered a standard of care for thoracic pedicle screw placement.

P285 Postoperative clinical motor findings correlations with IONM MEP results during intramedullary spinal cord tumor removal. Presentation of 2 cases

Objectives Description of two cases of IONM in intramedullary spinal cord tumors muscle MEPs and D-wave recordings, and its immediate postoperative outcomes correlation. Methods Muscle MEPs, D-wave and SSEPs were recorded in both patients. D-wave amplitude and muscle MEPs presence-absence, were the critical parameters for MEP interpretation. Results Case 1. A 64-year-old male presented with a history of progressive tingling paresthesia with clumsiness to fine movements in both hands. A cervical MRI scan revealed a lesion from C3–C4 level, compatible with ependymoma. During surgery, loss of muscle MEPs in the left TA, significantly decreasing MEPs in right and left AH, right TA, left APB and EXT and slightly decreasing MEPs in right APB and EXT, occurred. D-wave amplitude showed no change. Postoperatively, the patient had a transient quadriplegia, as expected from MEP data. After a week, he was able to reach the vertical position with some help. Case 2. A 61-year-old male presented with a history of dysesthesias and tingling sensation of lower extremities, causing the patient to fall. Weakness for right hand movementsA cervical-dorsal MRI study revealed a lesion from C6-T1 level, compatible with an ependymoma. During surgery, right leg muscle MEPs completely disappeared, and right leg MEPs significantly decreased. Muscle MEPs in right EXT slightly decreased. A drop of D-wave amplitude of about 40% was noted. After surgery, the patient had a transient paraplegia. In the following days, there was a bilateral improvement of paraparesis. Discussion Intraoperative loss of muscle MEPs as long as D-wave remains above 50% of the baseline value, indicates a temporary loss of motor function in the corresponding limb. Further D-wave amplitude declining will translate in permanent plegia. Conclusions Correct prediction of the clinical status during surgery is possible with a high certainty. Muscle MEPs, combined with D-wave amplitude recordings, provides useful intraoperative information and correctly correlate with postoperative recovery. Using this information, surgical strategy can be adapted before irreversible neurological damage is caused. Significance Present and stable IONM recordings allow complete resection of intramedullary tumors with confidence about the integrity of the motor pathways.

6. Warning criteria in spine surgery: Source and evidence

Objective To discuss the importance, source and evidence that support warning criteria used in spine surgery. Methods Revision of the Literature. Co-Chairman of ISIN Rio 2015, meeting that joined for the first time in a common session with the authors of the most used warning criteria in Motor Evoked Potentials. Results You must tailor your warning criteria to different surgical procedures. I discuss the classical warning criteria for Somatosensory evoked potentials, explaining the value of amplitude decrease more than 50% and latency prolongation more than 10%. I discuss the four warning criteria for Motor Evoked Potentials, all or nothing, amplitude, threshold and morphology. I explain the strength of the all or nothing warning criteria. I stress the recommendation of starting all the restorative maneuvers in case of loss of the muscle MEPs and consider judiciously the drop of amplitude of 80% or increase of the threshold of motor evoked responses. Conclusions It is necessary to recognize the warning criteria and critically analyze them in order to have the opportunity to prevent or revert neurologic damage.

CLINICAL AND RADIOGRAPHIC EVALUATION OF ELBOWS FROM SPINAL CORD INJURIED PATIENTS.

Objectives: To evaluate clinically and radiologically the elbows of spinal cord injured patients and compare them to the control group. Methods: Twenty patients (10 paraplegics and 10 tetraplegics) were clinically evaluated through assessment of pain scale, measurement of active and passive range of motion, degree of muscle strength and MEPS score. They were also submitted to bilateral plain radiography of the elbows. Both groups were compared to the control group. Results: Four paraplegic and three tetraplegic patients referred mild to moderate, sporadic and motion related pain. The control group was asymptomatic. No statistic significant difference was found in passive range of motion among the three groups. The tetraplegic group showed a lower active range of motion as well as lower MEPS score as compared to the control group. Equal number of patients in the spinal cord injured patients had radiological abnormalities, but those were more severe in the tetraplegic group. Conclusion: Spinal cord injured patients presented clinical and radiological elbow abnormalities, which were more evident on tetraplegics. Level of Evidence III, Case Control.

S100B, intraoperative neuromonitoring findings and their relation to clinical outcome in surgically treated intradural spinal lesions

Neurophysiological monitoring (IOM) consisting of somatosensory (SEPs), muscle (MEPs) and spinal motor evoked (D-wave; spinal MEPs) potentials is used to indicate injury related to surgical treatment of intradural and intramedullary lesions. Combining spinal and muscle MEPs reliably predicts long-term motor deficit. If spinal MEPs recording is not possible, additional markers-e.g. S100B, a serum marker for glial injury-may be a helpful adjunct. Thus, serial serum S100B measurements were related to both the intraoperative IOM recordings and the long-term neurological outcome in patients surgically treated for cervical and thoracic intradural lesions. In 33 patients (9 men, 24 women, 54 ± 17 years) during intramedullary (8) or intradural (25) cervical or thoracic spinal surgeries significant intraoperative SEP-amplitude decrement >50 % or MEP loss and serial S100B serum concentration (perioperative days 0, 1-3, 5) were related to outcome (>1 year after discharge, grouped into improved and unchanged/altered neurological symptoms). Differences in S100B levels between patients with improved and unchanged/altered neurological outcome were significantly on postoperative days 2 (0.085 ± 0.08 μg/l vs 0.206 ± 0.07 μg/l, p = 0.005) and 3 (0.076 ± 0.03 μg/l vs 0.12 ± 0.05 μg/l, p = 0.007). All patients with permanent altered neurological symptoms developed S100B levels >0.08 μg/l (0.09-0.35 μg/l). Eighty-one percent of patients with improved neurological symptoms presented with S100B levels ≤0.08 μg/l (0.02-0.08 μg/l). Nine out of ten patients (90 %) without changes in EP and S100B had an improved long-term outcome, whereas 9/13 patients (69 %) with changes in EP and S100B had altered neurological symptoms in long-term outcome. Intraoperative stable EPs and S100B ≤0.08 μg/l may be used as a marker to predict long-term neurological improvement, whereas EP-changes and elevated S100B levels on the 3rd postoperative day may be useful as a marker to predict long-term neurological alteration. In summary, the combined use of S100B and EPs might be helpful in the prediction of the severity of adverse spinal cord affection following surgery and guidance of patients.

P 185. Functional imaging of the M1 representation of the tongue, the hand and the foot in patients with eloquently localized intracerebral tumors. Discripancies and validity of navigated TMS and functional MRI results when compared to direct cortical stimulation

Introduction Functional magnetic resonance imaging (fMRI) and neuronavigated transcranial magnetic stimulation (nTMS) may be the most commonly used techniques for non-invasive mapping of the primary motor cortex (M1). The results of both techniques differ to a certain extent. However, which mapping technique displays a more realistic map of the M1 representation still remains unclear, especially when dealing with the tongue/face representation and under pathological conditions such us presence of a brain tumor adjacent to the M1 representation. We, therefore, evaluated both methods in patients presenting with intracerebral tumors adjacent to the cortical M1 representation or associated pyramidal fiber tracks, comparing the presurgical imaging results to the gold standard of monpolar direct cortical stimulation (DCS). Material and methods To date, 20 patients with primary brain tumors and cerebral metastases adjacent to the M1 representation and associated fiber tracks were prospectively investigated by anatomical MRI including DTI, fMRI and nTMS. NTMS and fMRI measurements were performed recording the following muscle MEPs and the corresponding movement paradigms: Abductor pollicis brevis (APB)/thumb abduction, plantaris medialis (PM)/toe flexion, anterior lateral tongue (LT)/tongue movements side-to-side. Morever, monopolar direct cortical stimulation (mDCS) was performed intraoperatively, applying a train of five on the hotspots of respective M1 representation, according to the fMRI and nTMS results (N = 8 patients with reliable results, according to DCS quality and registration mismatch). Euclidean distances (ED) and standard deviations (SD) between DCS and fMRI/nTMS coordinates were calculated. For analyses, iplanNet (Brainlab), MRICron, FSL, Matlab, Excel and SPSS (PASW 18) were used. For comparisons between means, Student’s T-test was computed. Results EDs between fMRI and DCS hotspots were within the same range (APB: 14.2 ± 6.4 mm SD; PM: 11.6 ± 9.2 mm SD; LT: 10.4 ± 4.4 mm SD) as the EDs between nTMS and DCS hotspots (13.7 ± 6.0 mm SD; PM: 12.6 ± 2.9 mm SD; LT: 10.2 ± 6.8 mm SD). However, matching between nTMS/fMRI and DCS results differed remarkably in some patients. Thus, combining both presurgical mapping techniques revealed lower EDs (APB: 10.9 ± 4.9 mm SD; PM: 8.1 ± 4.3 mm SD; LT: 9.2 ± 5.4 mm SD). Even when examining patients with symptomatic epilepsy, no side effects of nTMS were observed. Conclusion Overall, our preliminary results reveal no significant difference between both methods. Further analysis and recruiting a higher number of suitable patients may show which presurgical functional mapping technique amounts to the highest validity-compared to DCS-in specific subsets of patients. Preliminary results let suggest that clinical factors such as anticonvulsant drugs, motor deficits and alertness may considerably affect the results of both mapping techniques. Which presurgical functional mapping technique-fMRI or nTMS-may be eligible for certain patients, however, still remains to be further investigated.

Optimum interpulse interval for transcranial electrical train stimulation to elicit motor evoked potentials of maximal amplitude in both upper and lower extremity target muscles

ObjectiveThe aim of this study was to determine the optimum interpulse interval (OIPI) for transcranial electrical train stimulation to elicit muscle motor evoked potentials (TES-MEP) with maximal amplitude in upper and lower extremities during intra-operative spinal cord monitoring. MethodsIntraoperative spinal cord monitoring with TES-MEP was performed in 26 patients who had (corrective) spine surgery. Optimum interpulse interval (OIPI) were determined for the abductor pollicis brevis muscle (APB) representing the upper extremity and the anterior tibialis muscle (TA) representing the lower extremity. The IPI was varied between 0.5 and 4.0ms, where the OIPI was defined as the IPI with the highest muscle MEP amplitude for each muscle group. Differences between upper and lower extremity OIPIs were analyzed. Furthermore, the MEP amplitudes difference between the upper and lower extremity OIPIs and between the OIPI and IPI 2ms was determined. ResultsThe mean OIPIAPB representing the upper extremity was 1.78±1.09ms on the left side and 1.82±0.93ms on the right side. The lower extremity showed a mean OIPITA of 2.26±1.16ms on the left and 2.73±0.88ms on the right side. The mean differences between the OIPIAPB and OIPITA were significant for p=0.019 (Student’s T-test). No within patient differences in OIPIs between the left and the right side were found.The mean MEP amplitude reduction, the APB amplitude at OIPITA compared to the APB at OIPIAPB, was 32.5±27.9%. For the TA a mean amplitude reduction of 33.4±27.4% was found. The mean amplitude reduction for the OIPI amplitudes compared to the amplitudes at IPI 2ms was 53.6±25.5% for the APB and 45.8±28.3% for the TA. ConclusionLarge intra- and interindividual differences were found between the mean OIPIs of the TA and APB muscles (range 1.78–2.73ms) representing the upper and lower extremity. SignificanceBased on the results of this study, it is advisable to perform a set-up procedure for each individual patient undergoing TES-MEP to determine the optimal parameter settings when using supramaximal intensity of TES.

Detection of positional brachial plexus injury by radial arterial line during spinal exposure before neuromonitoring confirmation: a retrospective case study

To demonstrate the potential usefulness of radial arterial line monitoring in detection of brachial plexus injury in spinal surgery. Multiple neuromonitoring modalities including SEPs, MEPs and EMG were performed for a posterior thoracicolumbar surgery. Radial arterial line (A-line) was placed on the right wrist for arterial blood pressure monitoring. Reliable ulnar nerve SEPs, hand muscle MEPs and arterial blood pressure readings were obtained after patient was placed in a prone position. A-line malfunction was noted about 15 min after incision. Loss of ulnar nerve SEPs and hand muscle MEPs with a cold hand on the right was noticed when neuromonitoring resumed after spine exposure. SEPs, MEPs, A-line readings and hand temperature returned after modification of the right arm position. Radial arterial line monitoring may help detect positional brachial plexus injury in spinal surgery when continuous neuromonitoring is interrupted during spine exposure in prone position.

Protection of the remaining spinal cord function with intraoperative neurophysiological monitoring during paraparetic scoliosis surgery: a case report

To demonstrate the usefulness of rectus femoris muscle MEPs monitoring in a paraparetic neuromuscular scoliosis case. Multiple monitoring modalities including SEPs, MEPs and EMG were performed for an anterior and posterior correction surgery for a neuromuscular scoliosis patient with no motor and sensory function below the knees. Bilateral tibial nerve SEPs were absent, and no MEPs were recordable from anterior tibialis and abductor hallucis muscles bilaterally at baseline. Robust MEPs were recorded on abductor pollicis brevis and rectus femoris muscles bilaterally. Spinal cord monitoring mainly relied on MEPs from bilateral rectus femoris muscles (RF-MEPs). Twice RF-MEPs were absent following deformity correction and returned after removal of both rods. The patient's remaining spinal cord function was preserved. Intraoperative neurophysiological monitoring should be used for neuromuscular scoliosis cases with paraparesis if proximal function, such as the rectus femoris muscle, exists.

Factors predicting the feasibility of monitoring lower-limb muscle motor evoked potentials in patients undergoing excision of spinal cord tumors

This prospective study on intraoperative muscle motor evoked potentials (MMEPs) from lower-limb muscles in patients undergoing surgery for spinal cord tumors was performed to: 1) determine preoperative clinical features that could predict successful recording of lower-limb MMEPs; 2) determine the muscle in the lower limb from which MMEPs could be most consistently obtained; 3) assess the need to monitor more than 1 muscle per limb; and 4) determine the effect of a successful baseline MMEP recording on early postoperative motor outcome. Of 115 consecutive patients undergoing surgery for spinal cord tumors, 110 were included in this study (44 intramedullary and 66 intradural extramedullary tumors). Muscle MEPs were generated using transcranial electrical stimulation under controlled anesthesia and were recorded from the tibialis anterior, quadriceps, soleus, and external anal sphincter muscles bilaterally. The effect of age (≤ 20 or > 20 years old), location of the tumor (intramedullary or extramedullary), segmental location of the tumor (cervical, thoracic, or lumbar), duration of symptoms (≤ 12 or > 12 months), preoperative functional grade (Nurick Grades 0-3 or 4-5), and muscle power (Medical Research Council Grades 0/5-3/5 or 4/5-5/5) on the success rate of obtaining MMEPs was studied using multiple regression analysis. The effect of the ability to monitor MMEPs on motor outcome at discharge from the hospital was also analyzed. The overall success rate for obtaining baseline lower-limb MMEPs was 68.2% (75 of 110 patients). Eighty-nine percent of patients with Nurick Grades 0-3 had successful MMEP recordings. Muscle MEPs could not be obtained in any patient in whom muscle power was 2/5 or less, but were obtained from 91.4% of patients with muscle power of 4/5 or more. Analysis showed that only preoperative Nurick grade (p ≤ 0.0001) and muscle power (p < 0.0001) were significant predictors of the likelihood of obtaining MMEPs. Responses were most consistently obtained from the tibialis anterior muscle (68%), but in the other 32% MMEPs could not be recorded from the tibialis anterior but could be recorded from another muscle. The ability to monitor MMEPs was associated with better motor outcome at discharge from the hospital (p = 0.052). The likelihood of obtaining lower-limb MMEPs is significantly greater in patients with better functional grades and higher motor power. Muscle MEPs are most consistently obtained from the tibialis anterior muscle but other muscles should also be monitored to optimize the chances of obtaining MMEP responses from the lower limbs.

Intraoperative neurophysiologic monitoring for intramedullary spinal-cord tumor surgery

During resection of intramedullary spinal-cord tumors intraoperative neurophysiological monitoring has become a true surgical technology. Motor evoked potentials are the most important modality for this purpose. Its use requires neurophysiological expertise from the surgeon, and a monitoring team in place able to handle the necessary equipment. Motor potentials are evoked by transcranial electrical motor cortex stimulation. A "single stimulus technique" evokes D-waves recorded from the spinal cord. The "multipulse (or train) stimulation technique" evokes electromyographic responses in peripheral muscles. These are optimally recorded from the thenar, hypothenar, tibialis anterior, and flexor hallucis brevis muscles, which are known to have strong pyramidal innervation. D-wave monitoring looks primarily at the peak-to-peak amplitude. When monitoring muscle MEPs, the presence or absence of the response irrespective of stimulation intensity is the important parameter. Preparations for neurophysiological monitoring fit quite well into a neurosurgical operating room environment. Recording and interpretation of MEPs is fast and straightforward. Pre- and postoperative clinical motor findings correlate with intraoperative MEP results. Thus correct prediction of the clinical status at a given time during surgery is possible with a very high certainty. The sensitivity of muscle MEPs for postoperative motor deficits is nearly 100%, its specificity is about 90%. Thus MEP data indeed reflect the clinical "reality". Present and stable recordings document intact motor pathways and allow the surgeon to confidently proceed with a tumor resection. Loss of muscle MEPs and/or decrease of the D-wave amplitude constitutes a "window of warning". It reflects a pattern of MEP change indicating a reversible injury to the essential motor pathways. Using this information, the surgical strategy can be adapted before irreversible neurological damage is caused by the surgical manipulation. Such adaptation comprises simply waiting for the recordings to spontaneously improve again, irrigating with warm saline solution to wash out blocking potassium. Other measures include the elevation of mean arterial pressure to improve local perfusion. Even staged resection can be considered if intraoperative measures do not sufficiently improve the recordings.

Monitoring of motor pathways during brain stem surgery: What we have achieved and what we still miss?

Intraoperative neurophysiological monitoring (IOM) has established itself as one of the paths by which modern neurosurgery can improve surgical results while minimizing morbidity. IOM consists of both monitoring (continuous "on-line" assessment of the functional integrity of neural pathways) and mapping (functional identification and preservation of anatomically ambiguous nervous tissue) techniques. In posterior-fossa and brainstem surgery, mapping techniques can be used to identify - and therefore preserve - cranial nerves, their motor nuclei and corticospinal or corticobulbar pathways. Similarly, free-running electromyography (EMG) and muscle motor-evoked potential (mMEP) monitoring can continuously assess the functional integrity of these pathways during surgery. Mapping of the corticospinal tract, at the level of the cerebral peduncle as well as mapping of the VII, IX-X and XII cranial nerve motor nuclei on the floor of the fourth ventricle, is of great value to identify "safe entry-zones" into the brainstem. Mapping techniques allow recognizing anatomical landmarks such as the facial colliculus, the hypoglosseal and glossopharyngeal triangles on the floor of the fourth ventricle, even when normal anatomy is distorted by a tumor. On the basis of neurophysiological mapping, specific patterns of motor cranial nuclei displacement can be recognized. However, brainstem mapping cannot detect injury to the supranuclear tracts originating in the motor cortex and ending on the cranial nerve motor nuclei. Therefore, monitoring techniques should be used. Standard techniques for continuously assessing the functional integrity of motor cranial nerves traditionally rely on the evaluation of spontaneous free-running EMG in muscles innervated by motor cranial nerves. Although several criteria have been proposed to identify those EMG activity patterns that are suspicious for nerve injury, the terminology remains somewhat confusing and convincing data regarding a clinical correlation between EMG activity and clinical outcome are still lacking. Transcranial mMEPs are also currently used during posterior-fossa surgery and principles of MEP monitoring to assess the functional integrity of motor pathways are similar to those used in brain and spinal-cord surgery. Recently, current concepts in muscle MEP monitoring have been extended to the monitoring of motor cranial nerves. So-called "corticobulbar mMEPs" can be used to monitor the functional integrity of corticobulbar tracts from the cortex through the cranial motor nuclei and to the muscle innervated by cranial nerves. Methodology for this purpose has appeared in the literature only recently and mostly with regards to the VII cranial nerve monitoring. Nevertheless, this technique has not yet been standardized and some limitations still exist. In particular, with regards to the preservation of the swallowing and coughing reflexes, available intraoperative techniques are insufficient to provide reliable prognostic data since only the efferent arc of the reflex can be tested.

Monitoring of scoliosis surgery with epidurally recorded motor evoked potentials (D wave) revealed false results

Objective To elucidate the mechanism behind D wave amplitude changes after surgical correction of scoliosis. Methods We collected D wave and muscle MEP data from 93 patients (78 female, 15 male, age range 4–19 years, mean age 15.9 years), who underwent surgical correction of scoliosis. D waves were recorded via a catheter electrode inserted epidurally through the flavectomy. Muscle MEPs from lower limb muscles were also recorded. Muscle MEPs/D wave were elicited by short trains/single transcranial electrical stimuli. SEPs were elicited through bilateral percutaneous stimulation of the tibial nerves at the ankle and an averaged response from 100 to 200 single sweeps were recorded over the scalp at Cz’/Fz. In addition, we analyzed intraoperatively obtained X-ray images of the spine in 9 patients and preoperative spinal MRI in two of those nine. Results After surgical correction of scoliosis in 25 of 93 (27%) patients, the D wave amplitude changed by more than 20% of its baseline value. A decremental change occurred in 21 (84%) and an incremental change in 4 (16%) patients. D wave decrements of more than 50% were observed in 5 patients without significant SEP changes in any of these cases. In 9 patients, intraoperatively obtained X-rays of the spine (before and after correction of spine curvature) showed no catheter displacement. Muscle MEPs did not change and postoperative sensory-motor status was normal. In 2 patients, preoperative MRI revealed displacement of the spinal cord towards the concave side of the scoliotic curvature. Conclusions During scoliosis surgery, D wave amplitude changes should be interpreted cautiously until the definitive cause(s) of these changes are found. One possible mechanism to explain D wave changes during scoliosis correction could involve rotation of the spinal cord within the spinal canal, and the relative position of the epidural recording catheter (ERC). Rotation of the spinal cord after correction of scoliosis could introduce a new relationship between the ERC and the corticospinal tracts (CTs). Due to high incidence of false D wave amplitude changes we suggest that this methodology should not be used to assess the functional integrity of the CTs during scoliosis surgery. Significance This study provides new insight into the methodology of D wave monitoring as well as strong evidence of a high incidence of false positive results using D wave monitoring during surgical correction of scoliosis.

Intraoperative corticomuscular motor evoked potentials for evaluation of motor function: a comparison with corticospinal D and I waves

The goal of this study was to compare motor evoked potentials recorded from muscles (muscle MEPs or corticomuscular MEPs) with corticospinal MEPs recorded from the cervical epidural space (spinal MEPs or corticospinal MEPs) to assess their efficacy in the intraoperative monitoring of motor function. Muscle and spinal MEPs were simultaneously recorded during surgery in 80 patients harboring brain tumors. Each case was assigned to one of four groups according to final changes in the MEPs: (1) Group A, in which there was an increased amplitude in the muscle MEP with an increased 13 wave amplitude (12 cases); (2) Group B, in which there was no significant change in the MEP (43 cases); (3) Group C, in which there was a decreased muscle MEP amplitude (< 35% of the control) with a decreased I wave amplitude but an unchanged D wave (15 cases); or (4) Group D, in which there was an absent muscle MEP with a decreased D wave amplitude (10 cases). In patients in Group A, the increase in the amplitude of the muscle MEP (range of increase 128-280%, mean increase 188.75 +/- 48.79%) was well correlated with the increase in the 13 wave in corticospinal MEPs. Most of these patterns were observed in patients harboring meningiomas (10 [83.3%] of 12 cases). Patients in Group B displayed no changes in muscle and corticospinal MEPs and no signs of postoperative neurological deterioration. Patients in Group C showed a substantial decrease in the amplitude of the muscle MEP (range of decrease 5.3-34.8% based on the control waveform, mean change 21.81 +/- 10.93%) without deterioration in the corticospinal D wave, and exhibited severe immediate postoperative motor dysfunction. This indicates dysfunction of the cortical gray matter, including the motor cortices, which are supposed to generate I waves. Patients in Group D exhibited decreases in the corticospinal D wave (range of decrease 21.5-55%, mean decrease 39.75 +/- 11.45%) and an immediate cessation of the muscle MEP as well as severe permanent motor paresis. These results indicate that, during surgery, monitoring of corticomuscular MEPs (which are related to I waves) is a much more sensitive method for the detection of immediate motor cortical damage than monitoring of corticospinal MEPs (D wave).

Intraoperative neurophysiologic monitoring for intramedullary spinal cord tumor surgery

Intraoperative neurophysiological monitoring is an integral part of surgery for intramedullary spinal cord tumors. Motor evoked potentials are the key technology for this purpose. Sensory evoked potentials play a limited role. Motor potentials are evoked by transcranial electrical motor cortex stimulation. The single-stimulus technique elicits D-waves, which are recorded from the spinal cord. The multipulse technique elicits muscle MEPs, which are recorded from limb muscles. The amplitude of the D-waves and the presence or absence of muscle MEPs are the critical parameters for MEP interpretation. The practical procedures for evoked potential recording are well suited to a neurosurgical environment, and the intraoperative interpretation and application of neurophysiological information is fast and straightforward, particularly for MEPs. MEPs can almost always be recorded in a patient who is not already severely disabled before surgery. Pre- and postoperative clinical motor findings correlate with the intraoperative MEP findings. As a result a patient's clinical status can be predicted any time during the operation with considerable certainty (the sensitivity of muscle MEPs for post-operative motor deficits is almost 100%; its specificity is about 90%). Therefore, MEP data reflect the clinical “reality” in an individual patient. Loss of muscle MEPs, interpreted in combination with an incremental decrease in the amplitude of the D-wave, provides a “window of warning.” It reflects a pattern of MEP change consistent with a reversible injury to the essential motor pathways. The surgical strategy can be adapted before irreversible neurological damage occurs. Present and stable recordings allow complete resection of tumors with confidence about the integrity of the motor pathways.