Utility of motor evoked potentials to diagnose and reduce lower extremity motor nerve root injuries during 4,386 extradural posterior lumbosacral spine procedures.

BACKGROUND CONTEXT: Motor evoked potentials (MEPs) have excellent sensitivity for monitoring the functional integrity of the lateral corticospinal tract of the spinal cord. The sensitivity for nerve root function, however, is not as well established; consequently, MEPs are often not utilized for posterior extradural spine procedures distal to the conus. Spontaneous electromyography (sEMG) and somatosensory evoked potentials (SSEPs) are often included for these procedures, but their limited sensitivity has been well documented. Given the risk of motor nerve root injuries during spine procedures, and specifically increased vulnerability of the L4 and L5 nerves, the sensitivity of MEPs was evaluated for diagnostic accuracy and therapeutic impact. PURPOSE: To determine the diagnostic sensitivity of MEPs during lumbosacral spine procedures and the potential therapeutic impact of the resolution of MEP alerts. STUDY DESIGN: A total of 4,386 posterior extradural lumbosacral spine procedures utilizing multimodality intraoperative neuromonitoring (IONM) with sEMG, SSEPs, and MEPs were abstracted from a multi-institutional database. All cases took place between October 2015 and October 2017. No external funding was provided. OUTCOME MEASURES: Sensitivity and specificity, as well as positive and negative likelihood ratios for new postoperative neurologic deficits were calculated for each modality individually as well as when combined (multimodality). PATIENT SAMPLE: Age 18 and older METHODS: Data entered in the electronic medical record were analyzed. Alerts to sEMG activity, decreases in SSEP amplitude, or decreases in MEP amplitude were documented as well as the status of the alerts at closure: resolved or unresolved. The presence of an sEMG alert or an unresolved MEP or SSEP alert at closure was considered a positive diagnostic result, and these results were assessed relative to presence of new immediate onset neurologic deficits as documented in the electronic record. RESULTS: The sensitivity and specificity of multimodality IONM for new immediate-onset lower extremity motor deficits were 100.0% (95% confidence interval: [64.6, 100.0]) and 92.2% (91.1, 93.1), respectively. Looking at the modalities in isolation, the sensitivity of MEPs was considerably better than either lower extremity sEMG or posterior tibial nerve SSEPs: 100.0% (78.5, 100.0) versus just 14.3% (4.0, 39.9) and 28.6% (8.2, 64.1), respectively. Surprisingly, the specificity of lower extremity MEPs was better than sEMG, 97.9% (97.5, 98.3) versus 95.4% (94.7, 96.0) (χ2=43.0, p<.001). The specificity of lower extremity SSEPs was 99.0% (98.5, 99.3). Only 4.4% of all procedures had a lower extremity MEP alert. There were 14 significant new nerve root injuries and all 14 had unresolved MEPs at closure. Total 85.7% of those nerve root injuries were dorsiflexion foot drop injuries and all had unresolved tibialis anterior MEP alerts. Although the overall rate of nerve root injuries was 0.32% (14/4,386), the rate for procedures with unresolved isolated tibialis anterior MEP alerts was 44.4% (12/27). The therapeutic impact is evident in the 2.0% of cases (87/4,386) with lower extremity MEP alerts that were able to be fully resolved by closure and for which the rate of injury was zero. CONCLUSIONS: The diagnostic accuracy of MEPs for anterior tibialis-related nerve root dysfunction supports the inclusion of this modality during routine posterior extradural lumbosacral procedures, especially when the L4 or L5 nerve roots are at risk. Moreover, therapeutic interventions that lead to the resolution of MEP alerts avert postoperative neurologic injuries.

MIS Expandable Interbody Spacers: A Literature Review and Biomechanical Comparison of an Expandable MIS TLIF With Conventional TLIF and ALIF.

STUDY DESIGN: Biomechanical study and review of literature on expandable lumbar interbody fusion constructs. OBJECTIVE: To evaluate the biomechanical stability of expandable interbody devices. SUMMARY OF BACKGROUND DATA: Lumbar interbody implants placed from an anterior or lateral approach are desirable due to their large size, providing a stable fusion environment. Posterior implants are typically limited by their access corridor. Expandable footprint transforaminal lumbar interbody fusion (TLIF) interbodies may allow for a minimally invasive TLIF approach with the biomechanical benefits of an anterior lumbar interbody fusion (ALIF)-sized graft; however, this requires experimental investigation. METHODS: Six cadaveric L1-sacrum segments were tested intact with pure moments of  ± 7.5  N m in flexion-extension, lateral bending, and axial rotation. Specimens received at L4-5 either a medial-lateral expandable TLIF cage (MLX-TLIF) or a conventional polyether ether ketone (PEEK) banana-shaped TLIF cage (Conv-TLIF) first. Both were tested with unilateral and bilateral pedicle screw (PS) fixation. Testing was repeated with the alternate cage and fixation. Motion marker arrays were fixed to L4 and L5 to assess range of motion. Results were compared with published data for a PEEK ALIF cage with anterior plate and a PEEK ALIF cage with bilateral PS fixation, tested under the same conditions. RESULTS: The most rigid construct was ALIF with bilateral PS fixation in flexion-extension and axial rotation, whereas MLX with bilateral PS was most rigid in lateral bending. Conv-TLIF with unilateral PS was the least rigid construct. MLX-TLIF with unilateral PS provided similar range of motion to Conv-TLIF with bilateral PS in flexion-extension and lateral bending, and ALIF with anterior plate in lateral bending. CONCLUSION: The MLX-TLIF cage with unilateral PS fixation provided comparable stability to conventional TLIF with bilateral PS fixation and ALIF with anterior plate treatments. The large footprint of the expandable cage may reduce the TLIF supplemental fixation demands and facilitate minimally invasive single-position surgery. If needed, additional stability may be achieved by using bilateral PS. LEVEL OF EVIDENCE: N/A.

Functional magnetic resonance imaging and optical imaging for dominant-hemisphere perisylvian arteriovenous malformations.

OBJECTIVE: In this study, we developed an a priori system to stratify surgical intervention of perisylvian arteriovenous malformations (AVMs) in 20 patients. We stratified the patients into three categories based on preoperative functional magnetic resonance imaging (fMRI) language activation pattern and relative location of the AVM. METHODS: In Group I (minimal risk), the AVM was at least one gyrus removed from language activation, and patients subsequently underwent asleep resection. In Group II (high risk), the AVM and language activation were intimately associated. Because the risk of postoperative language deficit was high, these patients were then referred to radiosurgery. In Group III (indeterminate risk), the AVM and language were adjacent to each other. The risk of language deficit could not be predicted on the basis of the fMRI alone. These patients underwent awake craniotomy with electrocortical stimulation mapping and optical imaging of intrinsic signals for language mapping. RESULTS: All patients from Group I (minimal risk) underwent asleep resection without deficit. All Group II (high-risk) patients tolerated radiosurgery without complication. In Group III (indeterminate risk), three patients underwent successful resection, whereas two underwent aborted resection after intracranial mapping. CONCLUSION: We advocate the use of fMRI to assist in the preoperative determination of operability by asleep versus awake craniotomy versus radiosurgery referral. In addition, we advocate the use of all three functional mapping (fMRI, electrocortical stimulation mapping, and optical imaging of intrinsic signals) techniques to clarify the eloquence score of the Spetzler-Martin system before definitive treatment (anesthetized resection versus radiosurgery versus intraoperative resection versus intraoperative closure and radiosurgery referral).

Variability of intraoperative electrocortical stimulation mapping parameters across and within individuals.

OBJECT: Electrocortical stimulation mapping is regarded as the gold standard of intraoperative mapping for predicting functional outcomes. Nevertheless, methodologies across institutions are inconsistent. Although many vary and maximize stimulation currents at each cortical site, some use a single current level to map the entire exposed cortex. The former comes at the cost of possibly inducing additional afterdischarge activity. The authors retrospectively reviewed their eperience with intraoperative electrocortical stimulation mapping to characterize variability of both mapping and afterdischarge thresholds. METHODS: Seventeen patients satisfied the study inclusion criteria. Significant variability in mapping thresholds was identified within individuals and across the patient population. Moreover, a statistically significant difference in mapping thresholds was demonstrated between the frontal and parietal/temporal lobes (p = 0.007, one-way analysis of variance). The authors report a surprisingly high incidence of afterdischarge during mapping, wide variability in afterdischarge thresholds within individuals and across the study population, and mapping thresholds regularly exceeding afterdischarge thresholds in neighboring cortex. Differences in afterdischarge thresholds across lobes only approached significance (p = 0.086). CONCLUSIONS: To maximize identification of eloquent cortices in some clinical situations, it may be advantageous to maximize currents at each cortical site regardless of adjacent afterdischarge threshold rather than to map the entire exposed cortex at a single current level. Moreover, the current findings highlight the need for electrocorticography during electrocortical stimulation mapping, both to identify when afterdischarges occur and to verify stimulation by recording stimu lation artifacts. The advantages and limitations of maximizing currents at each cortical site as well as mapping at a single current level are discussed.

Functional assessment of Broca's area using near infrared spectroscopy in humans.

We used near-infrared spectroscopy (NIRS) to compare functional hemoglobin concentration changes (delta[oxy-Hb] and delta[deoxy-Hb]) over human language and motor cortices. Eight subjects performed finger opposition, tongue movement, and covert visual object naming in an interleaved block paradigm design. NIRS revealed paradigm specific patterns of delta[oxy-Hb] and delta[deoxy-Hb] providing cortical localization of each function. During each task, significant response overlap was observed when comparing the [oxy-Hb] signals, whereas delta[deoxy-Hb] seemed more localized. Furthermore, by applying magnitude and time to significance measures to the delta[deoxy-Hb] response profile, Broca's area was easily distinguished from neighboring tongue (and hand) motor representation. Delta[oxy-Hb] did not provide this level of specificity. These findings suggest delta[deoxy-Hb] as the preferential NIRS parameter to map language cortices.

Multiwavelength optical intrinsic signal imaging of cortical spreading depression.

Cortical spreading depression (CSD) is an important disease model for migraine and cerebral ischemia. In this study, we exploit the high temporal and spatial resolution of optical imaging to characterize perfusion-dependent and -independent changes in response to CSD and to investigate the etiology of reflectance changes during CSD. In this experiment, we characterized the optical response to CSD at wavelengths that emphasize perfusion-related changes (610 and 550 nm), and we compared these results with 850 nm and blood volume data. Blood volume changes during CSD were recorded using an intravascular fluorescent dye, Texas Red dextran. We observed triphasic optical signals at 850 and 550 nm characterized by spreading waves of increased, decreased, then increased reflectance (Fig. 1) which expanded at a rate of approximately 3-5 mm/min. The signal at 610 nm had a similar initial phase, but the phase 2 response was slightly more complex, with a parenchymal decrease in reflectance but a vascular increase in reflectance. Reflectance values decreased in phase three. Blood volume signals were delayed relative to the optical intrinsic signals and corresponded temporally to phases 2 and 3. This is the first study to characterize optical imaging of intrinsic signal responses to CSD, in vivo, at multiple wavelengths. The data presented here suggest that changes in light scattering precede perfusion responses, the blood volume increase (phase 2) is accompanied by a reduction in deoxyhemoglobin, and the blood volume decrease (phase 3) is accompanied by an increase in deoxyhemoglobin. Previous studies have suggested the oligemia of spreading depression was a result of decreased metabolic demand. This study suggests that during the oligemic period there is a greater reduction in oxygen delivery than in demand.

Spatial/temporal correlation of BOLD and optical intrinsic signals in humans.

Comparing the BOLD signal with electrophysiological maps and other perfusion-dependent signals, such as the optical intrinsic signal (OIS), within subjects should provide insight into the etiology of the BOLD signal. Tongue activations were compared in five human subjects using BOLD fMRI, 610-nm OIS, and the electrocortical stimulation map (ESM). Robust fMRI activations centered on the lateral inferior aspect of the central sulcus and extended into pre- and post-central gyri, adjacent to ESM tongue loci. OIS and fMRI maps colocalized, although optical responses were spatially larger (P <.001 across multiple thresholds) and contained more gyral components. The timecourses of the fMRI and OIS signals were similar, appearing within 2.5 s and peaking 6-8 s after task onset. Although many processes contribute to increased 610-nm reflectance, optical spectroscopy and fluorescent dye imaging suggest that a significant part of this signal is due to a concomitant decrease in deoxyhemoglobin and increase in oxyhemoglobin concentrations. The spatial/temporal correlation of BOLD and the positive 610-nm response within subjects suggests that the two signals may share similar etiologies. The OIS/fMRI inconsistencies may be due to cell swelling and light-scattering contributions to OIS and fMRI sensitivity. This study also demonstrates that fMRI maps do not precisely colocalize with ESM, rather they emphasize changes in adjacent venous/sulcal structures.

Intraoperative optical intrinsic signal imaging: a clinical tool for functional brain mapping.

Optical imaging of intrinsic signals (OIS) is a well-established neuroimaging modality by which functional cortical activity is mapped by detecting activity-related changes in cortical light reflectance. Light reflectance changes are detected by a charged-coupled device camera that captures images of the exposed cortex both at rest and during activity. Although to date OIS has only been used for research purposes, intraoperative OIS (iOIS) holds promise as a clinical mapping tool. In general, iOIS demonstrates good spatial correlation with electrocortical stimulation mapping (ECSM) and other electrophysiological modalities. Additionally, iOIS offers high spatial resolution (in microns), does not make contact with the surface of the brain, and introduces no potentially harmful compounds. Moreover, mapping is relatively rapid. The authors review the potential contribution of iOIS to the intraoperative environment. Specifically, they review iOIS methodology, discuss signal origin, compare OIS with other functional mapping modalities, and explain its potential benefits and limitations. They propose that iOIS may, in the future, be used in conjunction with ECSM to improve the resolution and accuracy of intraoperative mapping, decrease total time of intraoperative mapping, and possibly improve neurological outcomes. Additional studies will be required to quantify the sensitivity and specificity of optical maps relative to ECSM before it can be implemented clinically.