Andrea Szelényi

Intraoperative electrical stimulation in awake craniotomy: methodological aspects of current practice.

There is increasing evidence that the extent of tumor removal in low-grade glioma surgery is related to patient survival time. Thus, the goal of resecting the largest amount of tumor possible without leading to permanent neurological sequelae is a challenge for the neurosurgeon. Electrical stimulation of the brain to detect cortical and axonal areas involved in motor, language, and cognitive function and located within the tumor or along its boundaries has become an essential tool in combination with awake craniotomy. Based on a literature review, discussions within the European Low-Grade Glioma Group, and illustrative clinical experience, the authors of this paper provide an overview for neurosurgeons, neurophysiologists, linguists, and anesthesiologists as well as those new to the field about the stimulation techniques currently being used for mapping sensorimotor, language, and cognitive function in awake surgery for low-grade glioma. The paper is intended to help the understanding of these techniques and facilitate a comparison of results between users.

Functional magnetic resonance imaging-integrated neuronavigation: correlation between lesion-to-motor cortex distance and outcome.

OBJECTIVE:The integration of functional magnetic resonance imaging (fMRI) data into neuronavigation is a new concept for surgery adjacent to the motor cortex. However, the clinical value remains to be defined. In this study, we investigated the correlation between the lesion-to-fMRI activation distance and the occurrence of a new postoperative deficit. METHODS:fMRI-integrated “functional” neuronavigation was used for surgery around the motor strip in 54 patients. During standardized paradigms for hand, foot, and tongue movements, echo-planar imaging T2* blood oxygen level-dependent sequences were acquired and processed with BrainVoyager 2000 software (Brain Innovation, Maastricht, The Netherlands). Neuronavigation was performed with the VectorVision2 system (BrainLAB, Heimstetten, Germany). For outcome analysis, patient age, histological findings, size of lesion, distance to the fMRI areas, preoperative and postoperative Karnofsky index, postoperative motor deficit, and type of resection were analyzed. RESULTS:In 45 patients, a gross total resection (>95%) was performed, and for 9 lesions (low-grade glioma, 4; glioblastoma, 5), a subtotal resection (80–95%) was achieved. The neurological outcome improved in 16 patients (29.6%), was unchanged in 29 patients (53.7%), and deteriorated in 9 patients (16.7%). Significant predictors of a new neurological deficit were a lesion-to-activation distance of less than 5 mm (P < 0.01) and incomplete resection (P < 0.05). CONCLUSION:fMRI-integrated neuronavigation is a useful concept to assess the risk of a new motor deficit after surgery. Our data suggest that a lesion-to-activation distance of less than 5 mm is associated with a higher risk of neurological deterioration. Within a 10-mm range, cortical stimulation should be performed. For a lesion-to-activation distance of more than 10 mm, a complete resection can be achieved safely. The visualization of fiber tracks is desirable to complete the representation of the motor system.

Navigated transcranial magnetic stimulation and functional magnetic resonance imaging: advanced adjuncts in preoperative planning for central region tumors.

BACKGROUND:Tumor resection in the vicinity of the motor cortex poses a challenge to all neurosurgeons. For preoperative assessment of eloquent cortical areas, functional magnetic resonance imaging (fMRI) is used, whereas intraoperatively, direct cortical stimulation (DCS) is performed. Navigated transcranial magnetic stimulation (nTMS) is comparable to DCS in activating cortical pyramidal neurons. OBJECTIVE:To evaluate the reliability of nTMS compared with fMRI and DCS for preoperative resection planning of centrally located tumors. METHODS:In a prospective series, 11 patients (ages, 20-63 years; mean, 41.9 ± 14.9 years, 2 women) with tumors located in or adjacent to the motor cortex were evaluated for surgery. fMRI and nTMS were applied for preoperative assessment of the extent of tumor resection. A 3-dimensional anatomic data set with superimposed fMRI data was integrated in the eXimia Navigated Brain Stimulation station for ensuing motor cortex mapping by nTMS. Responses from nTMS were evaluated by electromyographic response. During surgery, the coordinates of each DCS site were unambiguously defined and integrated into neuronavigation. A post hoc comparison of the coordinates of nTMS, fMRI, and DCS was performed. RESULTS:Distances from nTMS to DCS (10.5 ± 5.67 mm) were significantly smaller than those from fMRI to DCS (15.0 ± 7.6 mm). CONCLUSION:nTMS anticipates information usually only enabled by DCS and therefore allows surgical planning in eloquent cortex surgery.

Sentinel Headache and the Risk of Rebleeding After Aneurysmal Subarachnoid Hemorrhage

Background and Purpose— The clinical significance of sentinel headaches in patients with subarachnoid hemorrhage (SAH) is still unknown. We investigated whether patients with a sentinel headache (SH) have a higher rate of rebleeding after SAH. Methods— An SH was defined as a sudden, severe, unknown headache lasting >1 hour with or without accompanying symptoms, not leading to a diagnosis of SAH in the 4 weeks before the index SAH. Age, sex, smoking status, clinical grade, computed tomography (CT) findings, angiographic findings, placement of an external ventricular drain, and time to aneurysm obliteration were prospectively recorded. All rebleeding events were confirmed by CT. Outcome was assessed at 6 months according to the modified Rankin Scale. Results— Of 237 consecutive patients with SAH, 41 (17.3%) had an SH. Rebleeding occurred in 23 (9.7%) of all patients. Patients with an SH had a 10-fold increased odds of rebleeding compared with patients without SH. Aneurysm size and the total number of aneurysms were also significantly associated with rebleeding. There were no differences in age, sex, smoking, CT or angiographic findings, external ventricular drain placement, or time to aneurysm obliteration between groups. Patients with rebeeding had a significantly worse outcome. Logistic regression revealed the presence of an SH as an independent risk factor for rebleeding. Conclusions— In our study, patients with SAH who had an SH constituted a special group of patients with a 10-fold odds for early rebleeding. The presence of an SH may select candidates for ultraearly aneurysm obliteration or drug treatment.

Low field intraoperative MRI-guided surgery of gliomas: A single center experience

INTRODUCTION The aim of this article is to report on our experience in using a low field intraoperative MRI (iMRI) system in glioma surgery and to summarize the hitherto use and benefits of iMRI in glioma surgery. PATIENTS AND METHODS Between July 2004 and May 2009, a total of 103 patients harboring gliomas underwent tumor resection with the use of a mobile low field iMRI in our institution. Surgeries were performed as standard micro-neurosurgical procedures using regular instrumentarium. All patients underwent early postoperative high field MRI to determine the extent of resection. Adjuvant treatment was conducted according to histopathological grading and standard of care. RESULTS All tumors could be reliably visualized on intraoperative imaging. Intraoperative imaging revealed residual tumor tissue in 51 patients (49.5%), leading to further tumor resection in 31 patients (30.1%). Extended resection did not translate into a higher rate of neurological deficits. When analyzing survival of patients with glioblastoma, patients undergoing complete tumor resection did significantly better than patients with residual tumor (50% survival rate at 57.8 weeks vs. 33.8 weeks, log rank test p=0.003), while younger age did not influence survival (p=0.12). CONCLUSION Low field iMRI is a helpful tool in modern neurosurgery and facilitates brain tumor resection to a maximum safe extent. Its use translates into a better prognosis for these patients with devastating tumors. Future studies covering the use of iMRI will need to be conducted in a prospective, randomized fashion to prove the true benefit of iMRI in glioma surgery.

Intraoperative motor evoked potential alteration in intracranial tumor surgery and its relation to signal alteration in postoperative magnetic resonance imaging.

OBJECTIVETo determine the degree to which the pattern of intraoperative isolated, unilateral alteration of motor evoked potential (MEP) in intracranial surgery was related to motor outcome and location of new postoperative signal alterations on magnetic resonance imaging (MRI). METHODSIn 29 patients (age, 42.8 ± 18.2 years; 15 female patients; 25 supratentorial, 4 infratentorial procedures), intraoperative MEP alterations in isolation (without significant alteration in other evoked potential modalities) were classified as deterioration (> 50% amplitude decrease and/or motor threshold increase) or loss, respectively, or reversible and irreversible. Postoperative MRI was described for the location and type of new signal alteration. RESULTSNew motor deficit was present in all 5 patients with irreversible MEP loss, in 7 of 10 patients with irreversible MEP deterioration, in 1 of 6 patients with reversible MEP loss, and in 0 of 8 patients with reversible MEP deterioration. Irreversible compared with reversible MEP alteration was significantly more often correlated with postoperative motor deficit (P < .0001). In 20 patients, 22 new signal alterations affected 29 various locations (precentral gyrus, n = 5; corticospinal tract, n = 19). Irreversible MEP alteration was more often associated with postoperative new signal alteration in MRI compared with reversible MEP alteration (P = .02). MEP loss was significantly more often associated with subcortically located new signal alteration (P = .006). MEP deterioration was significantly more often followed by new signal alterations located in the precentral gyrus (P = .04). CONCLUSIONMEP loss bears a higher risk than MEP deterioration for postoperative motor deficit resulting from subcortical postoperative MR changes in the corticospinal tract. In contrast, MEP deterioration points to motor cortex lesion. Thus, even MEP deterioration should be considered a warning sign if surgery close to the motor cortex is performed.

Intra-operative subcortical electrical stimulation: A comparison of two methods

OBJECTIVE For intra-operative subcortical electrical stimulation of the corticospinal tract, two techniques - originally described for cortical stimulation - have evolved: the 50-Hz-stimulation first described by Penfield in 1937 and the high-frequency multipulse train stimulation technique first described by Taniguchi in 1993. Motor thresholds of both methods in combination with a bipolar and monopolar stimulation technique and their reliability for eliciting motor evoked potentials (MEPs) were studied. METHODS Data were obtained in 20 patients (50±17 years; 10 females) undergoing tumour resection under general anaesthesia. Both 50-Hz-stimulation of 1-s duration and a multipulse stimulation (5 pulses interstimulus interval 4 ms, 0.5-Hz repetition rate) were applied with a bipolar probe (1.5-mm ball tip, 8-mm interelectrode distance) and a monopolar probe (1.5-mm-diameter tip). MEPs were recorded in muscles contralateral to the stimulated hemisphere. Comparison of different stimulation modalities was performed at the site where monopolar multipulse stimulation technique elicited MEPs with the lowest stimulation intensity (constant current monophasic cathodal stimulation, individual pulse width 0.5 ms, max. 25 mA). RESULTS MEPs were elicited by monopolar multipulse stimulation with an intensity of 8±3.9 mA (21/21 stimulation sites); monopolar 50-Hz stimulation with 12±5.4 mA (18/21 stimulation sites); bipolar multipulse stimulation with 14±8.1 mA (12/21 stimulation sites) and bipolar 50-Hz stimulation with 15±6.3 mA (11/21 stimulation sites). CONCLUSIONS Stimulation intensities for eliciting MEPs are significantly lowest for the monopolar multipulse stimulation (p<0.025). Monopolar compared to bipolar stimulation resulted in eliciting MEPs in a higher number of tested patients (Fishers p<0.0001). SIGNIFICANCE Subcortical stimulation with a monopolar probe and a multipulse stimulation is most efficient for the purpose of identifying the corticospinal tract. This is explained by the more radiant electric field properties of the monopolar probe compared to the bipolar probe.

Intraoperative microelectrode recording for the delineation of subthalamic nucleus topography in Parkinson's disease

BACKGROUND The subthalamic nucleus (STN) as an effective target for deep brain stimulation (DBS) in advanced Parkinsons disease is functionally divided into the dorsolateral sensorimotor and the ventromedial limbic and associative parts. To implant electrodes for DBS close to the sensorimotor region is considered crucial for optimal motor benefit and for avoidance of potential cognitive and behavioral side effects. OBJECTIVE The aim of this study was to determine whether the functional segregation of the STN is associated with distinct and region-specific neuronal activity patterns and action potential properties obtained by intraoperative microelectrode recordings. METHODS In 12 Parkinsons disease patients, stepwise intraoperative microelectrode recordings were performed using five concentrically configured electrodes starting 10 mm above the calculated target point until the dorsal border of the substantia nigra. RESULTS Based on autocorrelogram analysis of a total of 329 single units, we found a higher occurrence of oscillatory (P < 0.01) and bursty (P = 0.058) spike pattern in the dorsal versus the ventral STN. In contrast the ventral region was characterized by irregular firing neurons (P < 0.01). There were no significant differences in firing frequency, coefficient of variance, asymmetry index as well as spike form, duration, and amplitude. CONCLUSIONS Among all parameters analyzed in the study, spike pattern is the only convenient electrophysiologic parameter for the differentiation of STN subregions in patients with Parkinsons disease. The autocorrelogram-based analysis of spike activity seems to be of certain value for the delineation of the dorsolateral STN and might therefore facilitate the precise electrode implantation for DBS.

Laser Doppler imaging for intraoperative human brain mapping

The identification and accurate location of centers of brain activity are vital both in neuro-surgery and brain research. This study aimed to provide a non-invasive, non-contact, accurate, rapid and user-friendly means of producing functional images intraoperatively. To this end a full field Laser Doppler imager was developed and integrated within the surgical microscope and perfusion images of the cortical surface were acquired during awake surgery whilst the patient performed a predetermined task. The regions of brain activity showed a clear signal (10-20% with respect to the baseline) related to the stimulation protocol which lead to intraoperative functional brain maps of strong statistical significance and which correlate well with the preoperative fMRI and intraoperative cortical electro-stimulation. These initial results achieved with a prototype device and wavelet based regressor analysis (the hemodynamic response function being derived from MRI applications) demonstrate the feasibility of LDI as an appropriate technique for intraoperative functional brain imaging.

Intraoperative Neurophysiological Monitoring in an Open Low-field Magnetic Resonance Imaging System: Clinical Experience and Technical Considerations

OBJECTIVE The intraoperative combination of an open magnetic resonance imaging (MRI) system with neurophysiological localization and continuous monitoring techniques allows for the best available anatomic and physiological orientation as well as real-time functional monitoring. Methodological aspects and technical adaptations for this combination of methods and the experience in 29 patients with tumors in the central region are reported. METHODS MRI-compatible platinum/iridium electrodes for intraoperative neuromonitoring were attached to the patients head. All other electrodes located outside the magnet were stainless steel needle-electrodes for recording of motor evoked potentials and for stimulating somatosensory evoked potentials. Intraoperative MRI was performed using a 0.15-T intraoperative magnetic resonance scanner (PoleStar N20; Medtronic Surgical Navigation Technologies, Louisville, KY). RESULTS The calculated and measured values of the maximum induced magnetic field (2 × 10−6 T), induced voltage (0.1 V), and force (0.01 N) by the static or changing magnetic field within all attached electrodes were negligible and proved the methods safety. In 29 patients, platinum/iridium electrodes with low susceptibility showed no interference with the imaging quality. Furthermore, neurophysiological monitoring could be performed with unaffected recording quality. Side effects (e.g., thermal induction) were not observed. CONCLUSION Neurophysiological monitoring for evoked potentials and direct cortical stimulation can be performed with standard quality within a low-field intraoperative MRI system. Electrodes fixed to the head should be of low magnetic susceptibility to guarantee optimal imaging quality. The combined use of an open ultra low-field MRI system and intraoperative monitoring allows for resection control and continuous functional monitoring.