Anodal Stimulation Latency Response Differences Between Ipsilateral and Contralateral Motor Evoked Potentials in Hand Muscles.

PURPOSE: Purpose of this study was to analyze the latency and amplitude of transcranial motor evoked potentials responses from the contralateral and ipsilateral muscle groups to the same stimulus. If responses are because of activation of deeper structures, the latency of both the ipsilateral and the contralateral responses should have no difference. However, a difference in latency would suggest that activation might be occurring at different subcortical levels. METHODS: Data regarding demographics, medical history, and neurophysiological parameters were collected retrospectively on patients undergoing lumbar spine surgeries. Each side transcranial MEPs was considered as an independent data. Latency and amplitude of motor responses were recorded from the hand muscles of the ipsilateral and contralateral side from the same transcranial stimulus at preincision baseline. Statistical data analysis was performed using SAS 9.4. Paired t test was used to identify mean of differences in latency and amplitude between contralateral and ipsilateral intrinsic hand muscle. RESULTS: Data on 54 patients (104 MEPs) were obtained. Using paired t test, mean of differences in latency between ipsilateral (crossover) and contralateral (desired) intrinsic hand muscle was 0.8967 milliseconds ( P < 0.0001) while median was 0.71 milliseconds. Using paired t test, mean of differences in amplitude between ipsilateral and contralateral hand muscles was -1,071 µV ( P = <0.0001). CONCLUSIONS: Significant latency differences were seen between the contralateral and the ipsilateral hand MEP responses using the same transcranial stimulus, suggesting a different subcortical activation. Understanding of this difference might help better in the selection of baselines, and whether to favor responses obtained under the anode or under the cathode.

Photostability of 2,6-diaminopurine and its 2'-deoxyriboside investigated by femtosecond transient absorption spectroscopy.

Ultraviolet radiation (UVR) from the sun is essential for the prebiotic syntheses of nucleotides, but it can also induce photolesions such as the cyclobutane pyrimidine dimers (CPDs) to RNA or DNA oligonucleotide in prebiotic Earth. 2,6-Diaminopurine (26DAP) has been proposed to repair CPDs in high yield under prebiotic conditions and be a key component in enhancing the photostability of higher-order prebiotic DNA structures. However, its electronic relaxation pathways have not been studied, which is necessary to know whether 26DAP could have survived the intense UV fluxes of the prebiotic Earth. We investigate the electronic relaxation mechanism of both 26DAP and its 2'-deoxyribonucleoside (26DAP-d) in aqueous solution using steady-state and femtosecond transient absorption measurements that are complemented with electronic-structure calculations. The results demonstrate that both purine derivatives are significantly photostable to UVR. It is shown that upon excitation at 287 nm, the lowest energy 1ππ* state is initially populated. The population then branches following two relaxation coordinates in the 1ππ* potential energy surface, which are identified as the C2- and C6-relaxation coordinates. The population following the C6-coordinate internally converts to the ground state nonradiatively through a nearly barrierless conical intersection within 0.7 ps in 26DAP or within 1.1 ps in 26DAP-d. The population that follows the C2-relaxation coordinate decays back to the ground state by a combination of nonradiative internal conversion via a conical intersection and fluorescence emission from the 1ππ* minimum in 43 ps and 1.8 ns for the N9 and N7 tautomers of 26DAP, respectively, or in 70 ps for 26DAP-d. Fluorescence quantum yields of 0.037 and 0.008 are determined for 26DAP and 26DAP-d, respectively. Collectively, it is demonstrated that most of the excited state population in 26DAP and 26DAP-d decays back to the ground state via both nonradiative and radiative relaxation pathways. This result lends support to the idea that 26DAP could have accumulated in large enough quantities during the prebiotic era to participate in the formation of prebiotic RNA or DNA oligomers and act as a key component in the protection of the prebiotic genetic alphabet.

Success Rate of Obtaining Baseline Somatosensory and Motor Evoked Potentials in 695 Consecutive Cranial and Spine Surgeries.

PURPOSE: Intraoperative neurophysiological monitoring has been well documented as an adjunctive technique that significantly decreases the risk of developing inadvertent sensory and motor deficits during cranial and spine surgeries. The ability to detect neurologic problems intraoperatively depends largely on accurately identifying changes that occur in somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs) during each procedure. Therefore, obtaining accurate and reproducible SSEP and MEP data during the initial setup is paramount for intraoperative monitoring. In 2007, Chen et al. found the overall success rate for establishing reliable MEP responses to be 94.8% in the upper extremities and 66.6% in the lower extremities. Since then, the success rate of obtaining baseline sensory and motor evoked potential responses has not been specifically reevaluated. The main goal of this study was to evaluate the current success rates of obtaining adequate SSEP and MEP baseline data in the current era, as well as take a closer look into some of the factors that can reduce the success rates. METHODS: Somatosensory evoked potential and MEP monitoring was attempted in a total of 695 consecutive brain and spine surgeries performed by neurosurgeons and orthopedic surgeons between January 2010 and July 2011. Somatosensory evoked potential and MEP baseline data were obtained after initiation of general anesthesia and before skin incision. The primary measure is the ability to obtain adequate SSEP and MEP baseline in each extremity. A secondary measure was to stratify the success rate based on preoperative diagnosis. RESULTS: Six hundred ninety-five consecutive cranial and spinal cases that required intraoperative monitoring were reviewed. Baseline upper extremity SSEPs were successfully obtained in 679 cases (98.1%), and baseline lower extremity SSEPs were successfully obtained in 626 cases (90.1%). However, if the preoperative diagnosis was in the category spine trauma or spine infection, the success rate of obtaining adequate baseline in the lower extremities dropped to around 60% for both SSEPs and MEPs. CONCLUSIONS: The success rates of obtaining adequate baseline SSEP and MEP data are overall higher than previously reported. Preoperative diagnosis like spinal infection or trauma may predict lower success rates for acquiring adequate baseline SSEPs and MEPs.

Crossover Phenomena in Motor Evoked Potentials During Intraoperative Neurophysiological Monitoring of Cranial Surgeries.

PURPOSE: Transcranial motor evoked potentials (TcMEPs) are used to assess the corticospinal tract during surgery. Transcranial motor evoked potentials are elicited by preferentially activating the anode over the target cortex. Crossover occurs when stimulation also induces activation of ipsilateral motor evoked responses. These responses are believed to be generated by activation of corticospinal tract on more caudal neural structures. The presence of cross activation poses a problem in craniotomy surgeries because activation of neural structures occurs distal to the area of interest leading to false negatives. Eliminating crossover may lead to activation of the motor pathway proximal to the surgical site, thus potentially reducing false-negative responses. There are no data on how often crossover signals occur or the conditions in which they take place. This study examines the frequency of crossover, the surgical procedures in which they occur, and their stimulation parameters. METHODS: We reviewed all the TcMEP data files for intracranial procedures performed in 2016 at Keck Hospital of USC. We recorded demographic information about the surgical side, lobe, diagnosis, age, and sex. Only baseline TcMEPs were analyzed. Crossover responses were deemed present if recorded amplitudes were greater than 25µv on the ipsilateral side. We evaluated the rate of crossover presence, the lowest voltage associated with crossover, the highest voltage without crossover, if crossover resolved, and the last muscles to remain present when crossover is eliminated. Transcranial motor evoked potentials were divided into four groups. Group A: crossover present and was not resolved, group B1: crossover present but resolved with desired signals, group B2: no crossover seen with desired signals in both limbs, and group C: crossover resolved with loss of signals in either limb. The Difference between lowest amplitude with crossover and highest amplitude without crossover was obtained for each patient, and the mean of this difference was calculated using paired t-test. RESULTS: We analyzed 186 TcMEPs. Forty-four TcMEPs were in group A, 52 in B1, 68 in B2, and 22 TcMEPs were in group C. Of total crossovers (118), 63% resolved at baseline, whereas 37% did not resolve. The mean difference between minimum value with crossover and maximum value without crossover was 50 V (P < 0.0001). In five TcMEPs, this difference was 0 and the median was 250 V. There was no significant difference between surgical site, stimulation side, pathology, or sex between crossover (A) and noncrossover (B + C) groups. There was a significant association found between age group ≤50 years versus >50 years and being in crossover versus noncrossover groups (P = 0.01). For 95% of the cases in group C, the last muscles to stay were hand muscles. CONCLUSIONS: Transcranial motor evoked potential crossover may pose a problem during surgeries leading to false-negative results. Crossover is a frequent phenomenon that should not be overlooked. Stimulation intensity is the main factor contributing to the reduction of crossover. Crossover can be reduced in most TcMEPs performed (63%) leading to adequate monitoring in 76% of TcMEPs. Despite best efforts, there are still one quarter (24%) of TcMEPs where crossover cannot be eliminated. Newer strategies should be sought to reduce crossover. Teams should focus their efforts on reducing crossover of TcMEPs to make monitoring of intracranial surgeries more reliable.

An Alternative Transcranial Motor Evoked Potential Montage to Minimize Ipsilateral "Crossover" Motor Responses.

Transcranial electrical motor evoked potential (TcMEP) is a modality utilized in intraoperative neurophysiological monitoring to assess the integrity of the corticospinal tract. Traditionally, TcMEPs are obtained by anodal stimulation of the scalp over the motor cortex of the selected hemisphere and referenced to the contralateral hemisphere. Subsequent compound motor action potential responses (CMAPs) are recorded at various muscles. The muscle responses of interest are usually those recorded in the limb contralateral to the hemisphere of stimulation. However, TcMEPs may elicit simultaneous muscle responses in the limbs ipsilateral to the hemisphere of stimulation, otherwise defined as "crossover" responses. Crossover TcMEPs are thought to be generated when electrical stimulation reaches the corticospinal tract at intracranial structures as deep as the medullary pyramids. If electrical stimulation penetrates deeper than the at-risk structures, false negatives motor evoked potential monitoring may occur. Therefore, in surgeries where cerebral cortical structures may be at risk, TcMEPs may be elicited so that contralateral CMAP responses are present without crossover responses. We present three cases using an alternative TcMEP montage in which anodal stimulation of the target hemisphere is referenced to Fpz at midline. Compared with the C3-C4/C4-C3 montage, crossover responses are minimized with the modified montage. C3-Fpz/C4-Fpz TcMEP stimulation may be a potential tool to implement in certain intraoperative neuromonitoring cases.

Factors Associated With Inadequate Intraoperative Baseline Lower Extremity Somatosensory Evoked Potentials.

PURPOSE: Intraoperative neurophysiologic monitoring involves the use of various modalities, including somatosensory evoked potentials (SEP), to assess the integrity of the at-risk nervous system during surgeries. Reliable baseline tracings are important because they are data against which future tracings are compared to detect potential injury. In some cases, adequate baselines may be difficult to achieve. Therefore, we analyzed several patient-specific factors to determine which variables are associated with inadequate intraoperative SEP baseline signals. METHODS: This is a single-center, retrospective chart review of 631 consecutive patients who underwent spine or cranial surgeries between 2010 and 2011. Variables analyzed included age, glucose levels, diabetes mellitus type 2, hypertension, hyperlipidemia, height, weight, sex, smoking, preexisting neurologic conditions, surgical history, lower extremity edema, and neurologic examination findings. Association between these patient factors and baseline lower extremity SEP signals were analyzed. RESULTS: Height, weight, neurologic deficits, lower extremity edema, and history of neurologic disease are each associated with inadequate baseline lower extremity SEPs after controlling for confounding variables. Baseline signals were able to be acquired in 94.1% of patients. CONCLUSIONS: Adequate baselines are paramount for successful intraoperative neurophysiologic monitoring. However, certain patient-specific factors are associated with inadequate baseline SEP signals. Physical examination findings and a detailed chart review can be done to identify these factors and guide expectations during monitoring. Further research related to patient-specific factors amenable to modification can further improve our capacity to protect the nervous system during surgery.

Comparison of Transcranial Motor Evoked Potential Amplitude Responses Between Intramuscular and Subcutaneous Needles in Proximal Thigh Muscle.

PURPOSE: Successful intraoperative neurophysiological monitoring is predicated on the presence of adequate baseline-evoked potentials. We have observed that transcranial motor evoked potentials (TcMEPs) yield more robust responses in the distal muscles compared with proximal muscles. One possible explanation is the distance from the needle to the muscle generator. In this study, we investigate whether TcMEP amplitudes from the rectus femoris muscle are affected by changes in needle length. METHODS: We analyzed rectus femoris TcMEP responses in surgical patients undergoing lumbar spinal surgery. Needles of two different sizes were placed simultaneously. A shorter 13-mm subcutaneous needle was inserted into the rectus femoris muscle subcutaneous group in addition to a longer 25-mm intramuscular needle (intramuscular group). Each limb was used as an independent control. Transcranial motor evoked potential amplitude responses were obtained using both needles, and statistical analysis was calculated using the Wilcoxon signed-rank test for paired data. Secondary analysis was performed to correlate between TcMEP amplitude and skinfold thickness. RESULTS: Twenty-eight TcMEP responses from the rectus femoris (14 patients) were analyzed. We observed that TcMEP amplitude responses were higher in the intramuscular needle group compared with the subcutaneous group (N = 28, P < 0.0001). There was a mean difference of 604 µV between the intramuscular versus subcutaneous group (median 184 µV). There was also a significant correlation between TcMEP amplitude and skinfold thickness. CONCLUSIONS: Higher TcMEP amplitude responses are seen with longer needles compared with shorter needles placed in the same rectus femoris muscle. Transcranial motor evoked potential baselines may be optimized using longer needles. Skinfold thickness can be a good marker to determine appropriate needle size.

Differences in the Transcranial Motor Evoked Potentials Between Proximal and Distal Lower Extremity Muscles.

PURPOSE: Transcranial motor evoked potentials (TcMEPs) are the preferred modality to monitor the integrity of motor pathways during surgery. Recently, it has also been used as a method to help with detection of nerve roots injuries. Adequate baseline muscle responses are vital to detect nerve injury. We have observed that TcMEP responses are not homogeneous across multiple myotomes, but this has not been studied systematically. Our objective is to determine whether there are any relative differences in amplitude or morphology of TcMEPs across various lower extremity muscles. METHODS: Clinical and neurophysiological monitoring data from patients who had lumbar spine surgery were obtained retrospectively. Transcranial motor evoked potential responses were evaluated for each limb in the quadriceps, tibialis anterior, and intrinsic foot muscles. We compared TcMEP responses between these muscle groups using paired t-test statistical analysis. Each limb was analyzed separately. Only limbs without deficit in the interested muscle groups were included for analysis. RESULTS: A total of 40 patients and 69 limbs were included for analysis. The mean TcMEP amplitude difference between the tibialis anterior and quadriceps muscles was 458 µV (P < 0.0001), and between intrinsic feet and quadriceps muscles was 541 µV (P < 0.0001). Proximal muscles also demonstrated a significantly smaller number of TcMEP phases than their distal counterparts. CONCLUSIONS: Transcranial motor evoked potential amplitudes are significantly smaller in proximal lower extremity muscles compared with distal lower extremity muscles. The observed difference might be due to cortical representation or higher subcutaneous tissue in thigh muscles.

Overview of Intraoperative Neurophysiological Monitoring During Spine Surgery.

Intraoperative neurophysiologic monitoring has had major advances in the past few decades. During spine surgery, the use of multimodality monitoring enables us to assess the integrity of the spinal cord, nerve roots, and peripheral nerves. The authors present a practical approach to the current modalities in use during spine surgery, including somatosensory evoked potentials, motor evoked potentials, spinal D-waves, and free-run and triggered electromyography. Understanding the complementary nature of these modalities will help tailor monitoring to a particular procedure to minimize postoperative neurologic deficit during spine surgery.

Posterior circulation cerebral hyperperfusion syndrome after high flow external carotid artery to middle cerebral artery bypass.

We present the first report, to our knowledge, in which revascularization of the middle cerebral artery (MCA) with a high flow extracranial-intracranial procedure resulted in symptomatic hyperemia of the posterior circulation. Cerebral hyperperfusion syndrome (CHS) is a poorly understood phenomenon that is classically seen in the distribution of a revascularized artery. A 37-year-old woman presented with a 3 month history of cognitive and speech difficulties, persistent headaches, weakness, numbness, and paresthesia which was worse in the right extremities and face. She was found to have bilateral watershed infarcts worse in the left cerebral hemisphere, severe bilateral stenosis of the supraclinoid internal carotid artery, and a small left superior hypophyseal aneurysm. The patient underwent left cerebral hemisphere revascularization with a high flow external carotid artery to MCA bypass with aneurysm trapping. During skin closure, significant changes were seen in her bilateral upper extremity motor-evoked potentials. The patient's postoperative exam was noted for an intermittent inability to follow commands, bilateral upper extremity weakness, vertical nystagmus, and alogia that all dramatically improved with strict blood pressure control. Postoperative perfusion imaging revealed posterior circulation hyperemia. This patient highlights the potential for hyperemic complications outside the revascularized arterial territory. Strict blood pressure control is recommended in order to prevent and manage hyperemia-associated symptoms. Improving our understanding of CHS may assist in identifying at risk patients and at risk arterial territories in order to optimize CHS prevention and management strategies.

Assessing fidelity of core components in a mindfulness and yoga intervention for urban youth: applying the CORE Process.

In the past years, the number of mindfulness-based intervention and prevention programs has increased steadily. In order to achieve the intended program outcomes, program implementers need to understand the essential and indispensable components that define a program's success. This chapter describes the complex process of identifying the core components of a mindfulness and yoga program for urban early adolescents through the systematic study of fidelity of implementation of the intervention. The authors illustrate the CORE Process [(C) Conceptualize Core Components; (O) Operationalize and measure; (R) Run analyses and Review implementation findings; and (E) Enhance and refine], based on data gained from a mindfulness and yoga intervention study conducted as a community-academic partnership in Baltimore city.

Operative strategies for minimizing hearing loss and other major complications associated with microvascular decompression for trigeminal neuralgia.

OBJECTIVE: To retrospectively assess the surgical outcomes and complication rates following microvascular decompression (MVD) for trigeminal neuralgia, using a targeted, restricted retrosigmoid approach. METHODS: During the period 1994-2009, a total of 119 patients underwent MVD for trigeminal neuralgia. A retrospective review was conducted in order to assess pain outcomes following surgery and at most recent follow-up. The intraoperative findings, Barrow Neurologic Institute (BNI) pain scores, medication usage, brainstem auditory evoked potential records, and complication rates (including postoperative hearing status) were reviewed and subsequently analyzed. RESULTS: Of the 119 patients who underwent MVD, 61 (51%) were male and 58 (49%) were female. The mean age was 60 years (range 22-86 years). Operative findings included 94 patients (79%) with arterial compression, 16 patients (13%) with isolated venous compression, 1 patient (1%) with a small arteriovenous malformation, and 8 patients (7%) with no obvious source of compression. No perioperative deaths or major complications, including hearing loss, occurred in any patients. Minor complications occurred in 9 patients (8%), including a transient trochlear nerve palsy in 1 patient, transient nystagmus in 1 patient, cerebrospinal fluid leak requiring revision in 1 patient, wound infections requiring revision in 3 patients, and wound infections requiring antibiotics alone in 3 patients. Follow-up data were available for 109 patients, of whom 88 (81%) had excellent outcomes (BNI Score I-II). Ninety-eight patients (90%) had good outcomes (BNI scores I-IIIb), 7 patients (6%) had persistent pain that was not controlled with medications (BNI Score IV), and 4 patients (4%) experienced no relief following surgery (BNI Score V). CONCLUSION: The use of a small craniectomy (<20 mm) in conjunction with a restricted retrosigmoid approach, inferolateral cerebellar retraction, and maintenance of the vestibular nerve arachnoid may minimize complications and optimize surgical outcomes associated with microvascular decompression for trigeminal neuralgia.

Intraoperative neurophysiological monitoring during spine surgery: a review.

Spinal surgery involves a wide spectrum of procedures during which the spinal cord, nerve roots, and key blood vessels are frequently placed at risk for injury. Neuromonitoring provides an opportunity to assess the functional integrity of susceptible neural elements during surgery. The methodology of obtaining and interpreting data from various neuromonitoring modalities-such as somatosensory evoked potentials, motor evoked potentials, spontaneous electromyography, and triggered electromyography-is reviewed in this report. Also discussed are the major benefits and limitations of each modality, as well as the strength of each alone and in combination with other modalities, with regard to its sensitivity, specificity, and overall value as a diagnostic tool. Finally, key clinical recommendations for the interpretation and step-wise decision-making process for intervention are discussed. Multimodality neuromonitoring relies on the strengths of different types of neurophysiological modalities to maximize the diagnostic efficacy in regard to sensitivity and specificity in the detection of impending neural injury. Thorough knowledge of the benefits and limitations of each modality helps in optimizing the diagnostic value of intraoperative monitoring during spinal procedures. As many spinal surgeries continue to evolve along a pathway of minimal invasiveness, it is quite likely that the value of neuromonitoring will only continue to become more prominent.

The use of transcranial magnetic stimulation for monitoring descending spinal cord motor function.

This report describes our initial clinical experience using transcranial magnetic stimulation for monitoring spinal cord motor function during surgical procedures. Motor evoked potentials were elicited using a cap shaped coil placed on the scalp of 27 patients while recording peripheral motor responses (compound muscle action potentials--CMAPs) from the upper (N = 1) or lower limbs (N = 26). Wherever possible, cortical somatosensory responses (SEPs) were also monitored by electrically stimulating the left and right posterior tibial nerve (N = 25) or the median nerve (N = 1). The judicious choice of anesthetic regimens resulted in successfully obtaining motor evoked responses (MEPs) in 21 of 27 patients and SEPs in 26 of 27 patients. Single pulse TMS resulted in peripheral muscle responses having large variability, whereas, the variability of SEPs was much less. Criteria based on response variability for assessing clinically significant changes in both MEPs and SEPs resulted in two false negative predictions for SEPs and none for MEPs when evaluating postoperative motor function. We recommend monitoring both sensory and motor pathways during procedures where placing the spinal cord at risk of damage.