Intraoperative Neurophysiological Techniques Research Articles

15. Aortic IOM for thoraco-abdominal aneurysm surgery

In this presentation, I will give an overview of why and how intraoperative neurophysiologic techniques help reducing post-operative neurologic deficit in Aortic surgery. I will start by briefly presenting the vascular anatomy of the spinal cord as well as of the main types of Aortic aneurysms, surgical repair procedures (i.e., open TAA repair with distal aortic perfusion and endovascular repair/TEVAR) and their associated neurological risks. I will continue with integrating the intraoperative neurophysiologic techniques within the main methods used for intraoperative neuroprotection that allow reaching the balance between metabolic demand of the spinal cord and its oxygen supply. Next, I will present the main neurophysiologic tests used during these procedures, including their role, and methodology. I will then give examples of how specific characteristics of neurophysiologic changes help with differential diagnosis of intraoperative spinal cord ischemia and its management. I will conclude with thoughts about delayed paraplegia and how intraoperative neurophysiology can help in predicting and preventing its occurrence, by informing the postoperative hemodynamic management in the ICU.

Awake craniotomy using electromagnetic navigation technology without rigid pin fixation

We report our institutional experience using an electromagnetic navigation system, without rigid head fixation, for awake craniotomy patients. The StealthStation® S7 AxiEM™ navigation system (Medtronic, Inc.) was used for this technique. Detailed preoperative clinical and neuropsychological evaluations, patient education and contrast-enhanced MRI (thickness 1.5mm) were performed for each patient. The AxiEM Mobile Emitter was typically placed in a holder, which was mounted to the operating room table, and a non-invasive patient tracker was used as the patient reference device. A monitored conscious sedation technique was used in all awake craniotomy patients, and the AxiEM Navigation Pointer was used for navigation during the procedure. This offers the same accuracy as optical navigation, but without head pin fixation or interference with intraoperative neurophysiological techniques and surgical instruments. The application of the electromagnetic neuronavigation technology without rigid head fixation during an awake craniotomy is accurate, and offers superior patient comfort. It is recommended as an effective adjunctive technique for the conduct of awake surgery.

The mirror neuron system and the strange case of Broca's area.

Mirror neurons, originally described in the monkey premotor area F5, are embedded in a frontoparietal network for action execution and observation. A similar Mirror Neuron System (MNS) exists in humans, including precentral gyrus, inferior parietal lobule, and superior temporal sulcus. Controversial is the inclusion of Broca's area, as homologous to F5, a relevant issue in light of the mirror hypothesis of language evolution, which postulates a key role of Broca's area in action/speech perception/production. We assess "mirror" properties of this area by combining neuroimaging and intraoperative neurophysiological techniques. Our results show that Broca's area is minimally involved in action observation and has no motor output on hand or phonoarticulatory muscles, challenging its inclusion in the MNS. The presence of these functions in premotor BA6 makes this area the likely homologue of F5 suggesting that the MNS may be involved in the representation of articulatory rather than semantic components of speech.

Intraoperative Neurophysiologic Sensorimotor Mapping and Monitoring in Supratentorial Surgery

It has been shown that aggressive removal of gliomas improves survival and the quality of life in both adults and children. Conversely, there is a strong correlation between incomplete resection of an epileptic focus and poor seizure control outcome in epilepsy surgery. Thus, it is no surprise that maximal resection of supratentorial lesions remains a priority in neurologic surgery. In many circumstances, this is difficult to achieve because of the close proximity of functionally eloquent regions. As a consequence, accurate identification of the latter is imperative to reliably identify safe boundaries for resection and to expand them as much as possible, while preserving neurologic function. Along these lines, preservation of sensorimotor function, with significant impact on postoperative outcome and quality of life, remains essential as achieving maximal resection. Although there is a wide range of methods that could be used for functional sensorimotor mapping, intraoperative neurophysiologic techniques are still considered by many to be the "gold standard." This article provides a detailed overview of these techniques, their principles, and several alternative methodologies. Although the overview directly reflects the current practice at our institution, it also shows the temporal evolution of the major motor mapping methods, relating them to all significant contributions made over the years by different experts in the field. I have tried to exemplify the relevant points of these techniques by using as many pictures and clinical examples as possible.

Emerging Subspecialties in Neurology: Neurophysiologic intraoperative monitoring

In the last 30 years, neurophysiologic intraoperative monitoring (NIOM), also referred to as surgical or operative neurophysiology, has evolved from a part-time preoccupation of a few neurologists to a subspecialty in neurology with dynamic career opportunities. NIOM uses a variety of neurophysiologic signals to warn surgeons and anesthesiologists when the nervous system is at risk of injury during a surgical procedure. Several studies have shown that these warnings often lead to modification of the surgery and consequent reduction in surgical morbidity.1,–,3 The use of intraoperative neurophysiologic techniques to aid surgery can be traced to Wilder Penfield and Herbert Jasper's use of electrocorticography for resection of epileptogenic cortex in the 1930s.4 However, it was not until the 1970s that neurophysiologic techniques began to be used during surgeries to reduce the risk of injury to the nervous system.5 During the early years, NIOM equipment was “homemade” by neurophysiologists, who served the function of not only interpreting physician but also biomedical engineer and technologist. By the 1980s and 1990s, research documented the clear utility of somatosensory and brainstem auditory evoked potentials (SEP, BAEP), EEG, and EMG in reducing morbidity of many types of surgeries.1,6 Commercial NIOM equipment became available, and academic hospitals started offering NIOM services. Technologists became available with specialized training and certification in NIOM through the American Board of Registration of Electroencephalographic and Evoked Potential Technologists. The technologists were able to help set up the monitoring and run the NIOM equipment. …

Intraoperative neurophysiology of the motor system in children: a tailored approach

Intraoperative neurophysiology has moved giant steps forward over the past 15 years thanks to the advent of techniques aimed to reliably assess the functional integrity of motor areas and pathways. Motor evoked potentials recorded from the muscles and/or the spinal cord (D-wave) after transcranial electrical stimulation allow to preserve the integrity of descending pathways, especially the corticospinal tract (CT), during brain and spinal cord surgery. Mapping techniques allow to identify the motor cortex through direct cortical stimulation and to localize the CT at subcortical levels during brain and brainstem surgery. These techniques are extensively used in adult neurosurgery and, in their principles, can be applied to children. However, especially in younger children, the motor system is still under development, making both mapping and monitoring techniques more challenging. In this paper, we review intraoperative neurophysiological techniques commonly used in adult neurosurgery and discuss their application to pediatric neurosurgery, in the light of preliminary experience from our and other centers. The principles of development and maturation of the motor system, and especially of the CT, are reviewed focusing on clinical studies with transcranial magnetical stimulation.

Intraoperative neurophysiology of the conus medullaris and cauda equina

Intraoperative neurophysiological techniques are becoming routine tools for neurosurgical practice. Procedures affecting the lumbosacral nervous system are frequent in adult and pediatric neurosurgery. This review provides an overview of the techniques utilized in cauda and conus operations. Two basic methodologies of intraoperative neurophysiological testing are utilized during surgery in the lumbosacral spinal canal. Mapping techniques help identify functional neural structures, namely, nerve roots and their respective spinal levels. Monitoring is referred to as the technology to continuously assess the functional integrity of pathways and reflex circuits. For mapping direct electrical stimulation of a structure within the surgical field and recording at a distant site, usually a muscle is the most commonly used setup. Sensory nerve roots or spinal cord areas can be mapped by stimulation of a distant sensory nerve or skin area and recording from a structure within the surgical field. Continuous monitoring of the motor system is done with motor evoked potentials. These are evoked by transcranial electrical stimulation and recorded from lower extremity and sphincter muscles. Presence or absence of muscle responses are the monitored parameters. To monitor the sensory pathways, sensory potentials evoked by tibial, peroneal, or pudendal nerve stimulation and recorded from the dorsal columns with a spinal electrode or as cortical responses from scalp electrodes are used. Amplitudes and latencies of these responses are measured for interpretation. The bulbocavernosus reflex, with stimulation of the pudendal nerve and recording from the external anal sphincter, is used for continuous monitoring of the reflex circuitry. The presence of absence of this response is the pertinent parameter monitored. Stimulation of individual dorsal nerve roots is used to identify those segments that generate spastic activity and which may be cut during selective dorsal rhizotomy. Electromyographic activity can be continuously observed during surgery, and monitoring concepts developed in cranial nerve surgery may be used in the cauda equina as well. A range of intraoperative neurophysiological techniques are available for neurophysiological testing of the neural structures of conus medullaris and cauda equina.

Deep brain stimulation for the treatment of various chronic pain syndromes

Electrical intracerebral stimulation (also referred to as deep brain stimulation [DBS]) is a tool for the treatment of chronic pain states that do not respond to less invasive or conservative treatment options. Careful patient selection, accurate target localization, and identification with intraoperative neurophysiological techniques and blinded test evaluation are the key requirements for success and good long-term results. The authors present their experience with DBS for the treatment of various chronic pain syndromes. In this study 56 patients with different forms of neuropathic and mixed nociceptive/neuropathic pain syndromes were treated with DBS according to a rigorous protocol. The postoperative follow-up duration ranged from 1 to 8 years, with a mean of 3.5 years. Electrodes were implanted in the somatosensory thalamus and the periventricular gray region. Before implantation of the stimulation device, a double-blinded evaluation was carefully performed to test the effect of each electrode on its own as well as combined stimulation with different parameter settings. The best long-term results were attained in patients with chronic low-back and leg pain, for example, in so-called failed-back surgery syndrome. Patients with neuropathic pain of peripheral origin (such as complex regional pain syndrome Type II) also responded well to DBS. Disappointing results were documented in patients with central pain syndromes, such as pain due to spinal cord injury and poststroke pain. Possible reasons for the therapeutic failures are discussed; these include central reorganization and neuroplastic changes of the pain-transmitting pathways and pain modulation centers after brain and spinal cord lesions. The authors found that, in carefully selected patients with chronic pain syndromes, DBS can be helpful and can add to the quality of life.

Symptomatic epilepsy in children with poroencephalic cysts secondary to perinatal middle cerebral artery occlusion

Perinatal cerebral artery occlusion is responsible for ischemic cerebral infarction leading to brain cavitation and gliosis; the territory of the middle cerebral artery is most frequently involved. The resulting poroencephalic cysts are frequently associated with hemiplegia and epilepsy; that can be managed medically in most cases, only 6-7% of them being refractory to medical treatment. This particular subset of congenitally hemiplegic children will be possible candidates for electrophysiological investigation and eventually for resective surgery. Whatever the kind of surgical treatment, surgery should be performed as soon as possible to optimize functional brain reorganization. Twelve children with poroencephalic cysts and refractory epilepsy were studied and operated on at the Divisions of Child Neurology and Pediatric Neurosurgery, the Catholic University Medical School, Rome. The hemiparesis ranged from mild to moderate; the developmental delay was of mild degree in three cases, moderate in four cases and severe in the remaining five. Behavioral disorders were observed in patients with mental retardation; two of them also manifested autistic features. All the children presented with a severe epileptic syndrome (starting almost invariably during the first year of life); six patients presented with a West syndrome followed by symptomatic partial epilepsy; the other six presented with partial epilepsy, followed in two cases by continuous spike-waves during sleep. The electroencephalograph (EEG) recordings disclosed focal unilateral interictal epileptiform abnormalities that usually corresponded to the side of the cystic lesion; however, paroxysmal activity often spread synchronously over the contralateral hemisphere. The selection of candidates for surgical treatment was based on neuroimaging and video-EEG monitoring; in particular, we did not use invasive intraoperative neurophysiologic techniques. The convergence of neuroimaging and neurophysiologic findings guided us in performing a limited cortical excision corresponding to the malacic cortex (cyst "membrane"). All the patients underwent excision of the cyst wall. Careful attention was paid not to enter the body of the lateral ventricle to avoid ventriculo-subarachnoid fistulas, eventually responsible for subdural hygroma or cerebrospinal fluid leak. There was one surgery-related death secondary to disseminated intravascular coagulation, following an otherwise uneventful surgical procedure. An elevated systemic blood pressure, secondary to repeated adrenocorticotropic hormone therapy, can represent a possible concurrent factor for this event. No major complications were recorded among the remaining 11 children. Seizure control was excellent in all the 11 survivors in the early postoperative period. Two children presented a relapse of seizures, after an initial improvement, respectively 3 and 4 years after the operation. These two children underwent subsequently a functional hemispherectomy. Overall, seizure outcome was excellent in all the cases. Seven patients (including the two who underwent functional hemispherectomy) are seizure-free (Engel's class Ia), and in one of them antiepileptic therapy has been weaned. In the remaining five children, seizures are sporadic and definitely improved (Engel's class II). An improvement of developmental delay, in particular of cognitive competence, was registered in 8 out of the 11 patients. Two of the four severely retarded children, who also presented behavioral abnormalities, did not show any cognitive improvement, whereas some mild improvement of their basal abilities was demonstrated in the other two. All the remaining children, even though maintaining a moderate retardation, definitely improved their abilities; in particular, one of them reached an almost borderline level. The three patients with unchanged neurodevelopmental delay presented also persistent seizures. On the other hand, two children with persistent seizures presented neurodevelopmental improvement. Simple surgical excision of the cyst "membrane" of epileptogenic poroencephalic cysts can represent an excellent means to control epilepsy in affected children. However, postoperative seizure persistence and late recurrences, although rare, do not allow to exclude that hemispherectomy or partial resections (based on electrocorticography findings) might represent the good answer at least in some cases.

Role of intraoperative neurophysiology in primary surgery for obstetrical brachial plexus palsy (OBPP)

Management of conducting neuroma-in-continuity in primary surgery for obstetrical brachial plexus palsy (OBPP) is still discussed controversially. We present our experience with intraoperative neurophysiological recordings in the management of lesions in continuity in OBPP. A series of ten children with lesions in continuity of the upper brachial plexus is presented. Due to recordable compound nerve action potentials (CNAPs) and muscle response to motor stimulation across the neuroma, five children underwent external neurolysis of neuroma only (neurolysis group). Due to lack of recordable CNAPs or muscle response, resection of neuroma and interpositional nerve grafting were performed in another five children (resection and grafting group). Functional recovery after at least 30 months of follow-up was assessed. There was a marked difference in functional recovery between the neurolysis and the resection and grafting group. Especially, recovery of shoulder function was disappointing after external neurolysis of conducting neuroma-in-continuity. At the end of follow-up, results of shoulder and elbow function after resection of neuroma followed by interpositional nerve grafting were better without exception. Intraoperative neurophysiological recordings face certain difficulties when used in small children with OBPP. Due to overoptimistic assessment of prognosis after intraoperative CNAP recordings and motor stimulation, the functional results after neurolysis of conducting neuroma-in-continuity are disappointing. Resection of neuroma-in-continuity, conducting or not, offers the best opportunity for maximal functional recovery of the compromised upper limb in OBPP. The role of intraoperative neurophysiological techniques should be confined to the diagnosis of root avulsions.

Schwannoma of the Cranial Nerves—When To Operate?

Despite the evolution of cranial base approaches and the improvement of intraoperative neurophysiological techniques, the management of intracranial schwannomas (both for the more common vestibular schwannomas and for less frequently treated schwannomas of other cranial nerves) remains challenging and the results of surgery remain better for smaller tumors. The time of treatment should be individualized by a well-experienced team. In fact, advances of skull base techniques and early diagnosis by means of MRI allowed the change of the goal of schwannoma surgery from life preservation to function preservation-recovery. The ideal treatment of intracranial schwannomas is total tumor excision with preservation of neurological function. Usually three groups of patients are considered at major surgical risk and the time and type of treatment is under discussion: elderly patients (more than 65 years), patients with medical problems, and patients with a previous partial tumor resection. In our experience the patient's general medical condition and life expectancy, more than his or her chronological age, are the factors to be considered when surgery is contemplated. In our opinion age is not a limit for patients in which schwannoma resection can be safely performed. Our policy is to offer conservative treatment with close radiological follow-up for elderly patients without severe neurological symptoms and without evidence of tumor growth on serial MRI scans. We adapt for radiosurgery patients in whom general medical problems or extremely advanced age contraindicate operative treatment.

Intraoperative neurophysiological monitoring in pediatric neurosurgery: why, when, how?

This review is primarily based on peer-reviewed scientific publications and on the authors' experience in the field of intraoperative neurophysiology. The purpose is a critical analysis of the role of intraoperative neurophysiological monitoring (INM) during various neurosurgical procedures, emphasizing the aspects that mainly concern the pediatric population. Original papers related to the field of intraoperative neurophysiology were collected using medline. INM consists in monitoring (continuous "on-line" assessment of the functional integrity of neural pathways) and mapping (functional identification and preservation of anatomically ambiguous nervous tissue) techniques. We attempted to delineate indications for intraoperative neurophysiological techniques according to their feasibility and reliability (specificity and sensitivity). In compiling this review, controversies about indications, methodologies and the usefulness of some INM techniques have surfaced. These discrepancies are often due to lack of familiarity with new techniques in groups from around the globe. Accordingly, internationally accepted guidelines for INM are still far from being established. Nevertheless, the studies reviewed provide sufficient evidence to enable us to make the following recommendations. (1) INM is mandatory whenever neurological complications are expected on the basis of a known pathophysiological mechanism. INM becomes optional when its role is limited to predicting postoperative outcome or it is used for purely research purposes. (2) INM should always be performed when any of the following are involved: supratentorial lesions in the central region and language-related cortex; brain stem tumors; intramedullary spinal cord tumors; conus-cauda equina tumors; rhizotomy for relief of spasticity; spina bifida with tethered cord. (3) Monitoring of motor evoked potentials (MEPs) is now a feasible and reliable technique that can be used under general anesthesia. MEP monitoring is the most appropriate technique to assess the functional integrity of descending motor pathways in the brain, the brain stem and, especially, the spinal cord. (4) Somatosensory evoked potential (SEP) monitoring is of value in assessment of the functional integrity of sensory pathways leading from the peripheral nerve, through the dorsal column and to the sensory cortex. SEPs cannot provide reliable information on the functional integrity of the motor system (for which MEPs should be used). (5) Monitoring of brain stem auditory evoked potentials remains a standard technique during surgery in the brain stem, the cerebellopontine angle, and the posterior fossa. (6) Mapping techniques (such as the phase reversal and the direct cortical/subcortical stimulation techniques) are invaluable and strongly recommended for brain surgery in eloquent cortex or along subcortical motor pathways. (7) Mapping of the motor nuclei of the VIIth, IXth-Xth and XIIth cranial nerves on the floor of the fourth ventricle is of great value in identification of "safe entry zones" into the brain stem. Techniques for mapping cranial nerves in the cerebellopontine angle and cauda equina have also been standardized. Other techniques, although safe and feasible, still lack a strong validation in terms of prognostic value and correlation with the postoperative neurological outcome. These techniques include monitoring of the bulbocavernosus reflex, monitoring of the corticobulbar tracts, and mapping of the dorsal columns. These techniques, however, are expected to open up new perspectives in the near future.

Intraoperative neurophysiological monitoring and mapping during brain stem Surgery: A modern approach

Although magnetic resonance imaging technology has contributed significantly to the surgical treatment of a variety of brain stem pathologies, it remains an inadequate tool in the prevention of intraoperatively induced brain stem neurological injury. We introduce a new approach to brain stem intraoperative neurophysiological monitoring that promises to diminish intraoperative morbidity to the motor system by the proper identification of exposed brain stem structures, before initial incision or during tumor removal. The approach comprises three intraoperative neurophysiological techniques: The first technique localizes the motor cranial nerve nuclei on the surgically exposed floor of the fourth ventricle. The second technique localizes the corticospinal tract within the cerebral peduncle. The third technique tests the functional integrity of the corticospinal tract through continuous monitoring and eliciting of the D waves (descending activity) from the electrically activated corticospinal tracts.

Presurgical evaluation: current role of invasive EEG.

Our purpose was to review the current role of invasive and semi-invasive EEG in the presurgical evaluation of candidates for epilepsy surgery. The use of stereotactically implanted intracranial depth (stereo-EEG), subdural strip and grid, and foramen ovale electrodes, as well as intraoperative electrocorticography and electrical brain stimulation ("functional mapping") at the Epilepsy Center University Hospital Zurich, from 1984 to 1998, is analyzed. Advantages and disadvantages of the various intracranial EEG techniques are critically discussed. Out of 422 selective amygdalohippocampectomies performed in Zurich, 54% had non-invasive, 32% had semi-invasive, and 14% had invasive presurgical EEG evaluation. Because patients currently referred to our center increasingly present with a complex history of disease, i.e., constitute so-called "difficult cases", there is trend to combine several invasive and semi-invasive, pre- and intraoperative neurophysiological techniques.

Comparison between monopolar and bipolar electrical stimulation of the motor cortex.

Intra-operative neurophysiological techniques allow reliable identification of the sensorimotor region and make their anatomical and functional preservation feasible. Monopolar cortical stimulation has recently been described as a new mapping technique. In the present study this method was compared to the "traditional" technique of bipolar stimulation. Functional mapping of the motor cortex was performed in 35 patients during surgery in the central region. The central sulcus (CS) was identified by somatosensory evoked potential (SEP) phase reversal. Cortical motor mapping was first performed by monopolar anodal stimulation with a train of 500 Hz (7-10 pulses) followed by bipolar stimulation (pulses at 60 Hz with max. 4 sec train duration). Surgery was performed under general anaesthesia without muscle relaxants. Of 280 motor responses elicited by bipolar cortical stimulation, 54.23% [152] were located in the primary motor cortex (PMC), 37.85% 106[ outside the motor strip in the secondary motor cortex (SMC), and 8% 22[ posterior to the CS. Of 175 motor responses elicited by monopolar cortical stimulation. 68.57% 120[ were located in the SMC, 23.42% 41[ in the SMC and 8% 14[ posterior to the CS. Contrary to the general clinical view, there is considerable overlapping of primary motor units over a cortical area much broader than the "classical" narrow motor strip along the CS. Bipolar cortical stimulation is more sensitive than monopolar for mapping motor function in the premotor frontal cortex. Both methods are equally sensitive for mapping the primary motor cortex.