Pericollicular approaches to the rhomboid fossa. Part II. Neurophysiological basis

Intraoperative neurophysiology in pediatric neurosurgery: a historical perspective

INTRODUCTION

Intraoperative neurophysiology (ION) has been established over the past three decades as a valuable discipline to improve the safety of neurosurgical procedures with the main goal of reducing neurological morbidity. Neurosurgeons have substantially contributed to the development of this field not only by implementing the use and refinement of ION in the operating room but also by introducing novel techniques for both mapping and monitoring of neural pathways.

METHODS

This review provides a personal perspective on the evolution of ION in a variety of pediatric neurosurgical procedures: from brain tumor to brainstem surgery, from spinal cord tumor to tethered cord surgery.

RESULTS AND DISCUSSION

The contribution of pediatric neurosurgeons is highlighted showing how our discipline has played a crucial role in promoting ION at the turn of the century. Finally, a view on novel ION techniques and their potential implications for pediatric neurosurgery will provide insights into the future of ION, further supporting the view of a functional, rather than merely anatomical, approach to pediatric neurosurgery.

Brainstem Surgery: Functional Surgical Anatomy with the Use of an Advanced Modern Intraoperative Neurophysiological Procedure

Intraoperative neurophysiology (ION) in brainstem surgery evolved as brainstem surgery advanced.The original idea of brainstem mapping (BSM) is a neurophysiological procedure to locate cranial nerve motor nuclei (CNMN) on the floor of the fourth ventricle. With the introduction of various skull base approaches to the brainstem, BSM is carried out on any surface of the brainstem to expose the safe entry zone to the intrinsic brainstem lesion. It is the modern concept of BSM, a broader definition of BSM. BSM enables to avoid direct damage to the CNMN when approaching the brainstem through the negative mapping region.The corticobulbar tract (CBT) motor evoked potential (MEP) is another ION procedure in brainstem surgery. It enables monitoring of the functional integrity of the whole cranial motor pathway without interrupting surgical procedures. Combined application of both BSM and CBT-MEP monitoring is indispensable for the functional preservation of the CNMN and their supranuclear innervation during the brainstem surgery.In this paper, the neurophysiological aspect of BSM and the CBT-MEP was fully described. Normal anatomical background of the floor of the fourth ventricle and the detail of the CBT anatomy were demonstrated to better understand their clinical usefulness, limitations, and surgical implications derived from ION procedures. Finally, a future perspective in the role of ION procedures in brainstem surgery was presented. The latest magnetic resonance imaging (MRI) technology can allow surgeons to find an "on the image" safe entry zone to the brainstem. However, the role of BSM and the CBT-MEP monitoring in terms of safe brainstem surgery stays unshakable. Special attention was paid for the recent trend of management in diffuse intrinsic pontine gliomas. A new role of BSM during a stereotactic biopsy was discussed.It is the authors' expectation that the paper enhances the clinical application of a contemporary standard of the ION in brainstem surgery and supports safer brainstem surgery more than ever and in the future.

Anterior transpetrosal resection of the lower ventral pontine cavernous malformation: A technical case report with operative video

Background

Surgical treatment of pontine cavernous malformations (CMs) is challenging due to the anatomical difficulties and potential risks involved. We successfully applied an anterior transpetrosal approach (ATPA) to remove a lower ventral pontine CM, and herein we discuss the outline of our procedure accompanied by a surgical video.

Case Description

A 50-year-old woman presenting with progressively worsening diplopia was urgently admitted to our hospital. Preoperative images showed a lower ventral pontine CM compressing the corticospinal tract posteriorly. Considering the location of the CM, we determined that an ATPA was the appropriate approach to achieve a more anterolateral trajectory. We performed extradural anteromedial petrosectomy and penetrated the brainstem from the point just below the anterior inferior cerebellar artery and above the root exit zone of the abducens nerve, which might be located in the somewhat lowest border of actual maneuverability in the ATPA. Maneuverability through this corridor was sufficient without hindering and darkening the high magnification microscopic view, as demonstrated in our surgical video.

Conclusion

This report demonstrates surgical treatment of a lower ventral pontine CM using the ATPA. The surgical video we present provides information that is useful for understanding this technique's maneuverability and working window.

The brainstem and its neurosurgical history

Brainstem is one of the most complex structures of the human body, and has the most complex intracranial anatomy, which makes surgery at this level the most difficult. Due to its hidden position, the brainstem became known later by anatomists, and moreover, brainstem surgery cannot be understood without knowing the evolution of ideas in neuroanatomy, neuropathology, and neuroscience. Starting from the first attempts at identifying brainstem anatomy in prehistory and antiquity, the history of brainstem discoveries and approach may be divided into four periods: macroscopic anatomy, microscopic anatomy and neurophysiology, posterior fossa surgery, and brainstem surgery. From the first trepanning of the posterior fossa and later finger surgery, to the occurrence of safe entry zones, this paper aims to review how neuroanatomy and brainstem surgery were understood historically, and how the surgical technique evolved from Galen of Pergamon up to the twenty-first century.

Intraoperative neurophysiology of the cerebellum: a tabula rasa

PURPOSE

Cerebellar mutism (CM) is a condition that occurs predominantly in children, after posterior fossa surgery (PFS). It is characterized by motor, speech, and behavioral disorders. Despite widespread use of intraoperative neurophysiological monitoring (IONM), little is known about the neurophysiological aspects involved in the pathophysiology of CM. We reviewed the IONM literature to identify working hypotheses aimed to investigate intraoperatively the circuits involved in CM.

METHODS

A systematic review of the literature was conducted using PubMed central database. Papers describing the use of IONM techniques in the cerebellum were selected, thoroughly reviewed, and discussed.

RESULTS AND DISCUSSION

Only two studies reported the use of intraoperative neurophysiology of the cerebellum, suggesting a possible somatotopic motor organization of the cerebellar cortex. In addition, extra-operative studies using transcranial magnetic stimulation showed the possibility to modulate-possibly through the dentato-thalamic-cortical (DTC) pathway-primary motor cortex output using an appropriate cerebellar stimulus. In theory, the preservation of this either inhibitory or facilitatory modulation may predict the preservation of this pathway, while a loss of the effect may indicate an injury to the pathway, and predict a CM. Analogously, in the extra-operative setting, the comparison of pre-operative and post-operative transcranial magnetic stimulation of the cerebellum may predict the onset of CM whenever a pre-existing modulatory effect is lost as a result of surgery.

CONCLUSION

Virtually, no data exist on the intraoperative neurophysiology of the cerebellum. This limited knowledge, nevertheless, offers a unique opportunity to pediatric neurosurgeons to develop and test working hypotheses on the pathophysiology of CM, through the use of IONM.

Telovelar approach for microsurgical resection of fourth ventricular subependymoma arising from rhomboid fossa: operative video and technical nuances

Fourth ventricular tumors have traditionally been removed via transvermian approaches, which can result in potential dysequilibrium and mutism. The telovelar approach is an excellent alternative to widely expose fourth ventricular tumors without transgressing the cerebellar vermis. This is achieved by opening the cerebellomedullary fissure and incising the tela choroidea and inferior medullary velum, which form the lower half of the roof of the fourth ventricle. In this operative video manuscript, the authors demonstrate microsurgical resection of a fourth ventricular subependymoma arising from the rhomboid fossa via the telovelar approach. The key technical nuance in this video is to demonstrate a gentle and safe technique to identify a dissectable plane to peel the tumor off of the rhomboid fossa using a microspreading technique with fine micro-bayonetted forceps. A gross-total resection was achieved, and the patient was neurologically intact.

The video can be found here: https://youtu.be/ZEHHbUGb9zk.

Analysis of safe entry zones into the brainstem

Intra-axial brainstem surgeries are challenging. Many experience-based "safe entry zones (SEZs)" into brainstem lesions have been proposed in the existing literature. The evidence for each one seems limited. English-language publications were retrieved using PubMed/MEDLINE. Studies that focused only on cadaveric anatomy were also included, but the clinical case number was treated as zero. The clinical evidence level was defined as "case report" when the surgical case number was ≤ 5, "limited evidence" when there were more than 5 but less than 25 cases, and "credible evidence" when a publication presented more than 25 cases. Twenty-five out of 32 publications were included, and 21 different SEZs were found for the brainstem: six SEZs were located in the midbrain, 9 SEZs in the pons, and 6 SEZs in the medulla. Case report evidence was found for 10 SEZs, and limited evidence for 7 SEZs. Four SEZs were determined to be backed by credible evidence. The proposed SEZs came from initial cadaveric anatomy studies, followed by some published clinical experience. Only a few SEZs have elevated clinical evidence. The choice of the right approach into the brainstem remains a challenge in each case.

3D microsurgical anatomy of the cortico-spinal tract and lemniscal pathway based on fiber microdissection and demonstration with tractography

OBJECTIVE

To demonstrate tridimensionally the anatomy of the cortico-spinal tract and the medial lemniscus, based on fiber microdissection and diffusion tensor tractography (DTT).

MATERIAL AND METHODS

Ten brain hemispheres and brain-stem human specimens were dissected and studied under the operating microscope with microsurgical instruments by applying the fiber microdissection technique. Brain magnetic resonance imaging was obtained from 15 healthy subjects using diffusion-weighted images, in order to reproduce the cortico-spinal tract and the lemniscal pathway on DTT images.

RESULTS

The main bundles of the cortico-spinal tract and medial lemniscus were demonstrated and delineated throughout most of their trajectories, noticing their gross anatomical relation to one another and with other white matter tracts and gray matter nuclei the surround them, specially in the brain-stem; together with their corresponding representation on DTT images.

CONCLUSIONS

Using the fiber microdissection technique we were able to distinguish the disposition, architecture and general topography of the cortico-spinal tract and medial lemniscus. This knowledge has provided a unique and profound anatomical perspective, supporting the correct representation and interpretation of DTT images. This information should be incorporated in the clinical scenario in order to assist surgeons in the detailed and critic analysis of lesions located inside the brain-stem, and therefore, improve the surgical indications and planning, including the preoperative selection of optimal surgical strategies and possible corridors to enter the brainstem, to achieve safer and more precise microsurgical technique.

Human Connectome-Based Tractographic Atlas of the Brainstem Connections and Surgical Approaches

BACKGROUND

The brainstem is one of the most challenging areas for the neurosurgeon because of the limited space between gray matter nuclei and white matter pathways. Diffusion tensor imaging-based tractography has been used to study the brainstem structure, but the angular and spatial resolution could be improved further with advanced diffusion magnetic resonance imaging (MRI).

OBJECTIVE

To construct a high-angular/spatial resolution, wide-population-based, comprehensive tractography atlas that presents an anatomical review of the surgical approaches to the brainstem.

METHODS

We applied advanced diffusion MRI fiber tractography to a population-based atlas constructed with data from a total of 488 subjects from the Human Connectome Project-488. Five formalin-fixed brains were studied for surgical landmarks. Luxol Fast Blue-stained histological sections were used to validate the results of tractography.

RESULTS

We acquired the tractography of the major brainstem pathways and validated them with histological analysis. The pathways included the cerebellar peduncles, corticospinal tract, corticopontine tracts, medial lemniscus, lateral lemniscus, spinothalamic tract, rubrospinal tract, central tegmental tract, medial longitudinal fasciculus, and dorsal longitudinal fasciculus. Then, the reconstructed 3-dimensional brainstem structure was sectioned at the level of classic surgical approaches, namely supracollicular, infracollicular, lateral mesencephalic, perioculomotor, peritrigeminal, anterolateral (to the medulla), and retro-olivary approaches.

CONCLUSION

The advanced diffusion MRI fiber tracking is a powerful tool to explore the brainstem neuroanatomy and to achieve a better understanding of surgical approaches.

ABBREVIATIONS

CN, cranial nerveCPT, corticopontine tractCST, corticospinal tractCTT, central tegmental tractDLF, dorsal longitudinal fasciculusHCP, Human Connectome ProjectML, medial lemniscusMLF, medial longitudinal fasciculusRST, rubrospinal tractSTT, spinothalamic tract.

Three-dimensional microsurgical anatomy and the safe entry zones of the brainstem

BACKGROUND

There have been no studies of the structure and safe surgical entry zones of the brainstem based on fiber dissection studies combined with 3-dimensional (3-D) photography.

OBJECTIVE

To examine the 3-D internal architecture and relationships of the proposed safe entry zones into the midbrain, pons, and medulla.

METHODS

Fifteen formalin and alcohol-fixed human brainstems were dissected by using fiber dissection techniques, ×6 to ×40 magnification, and 3-D photography to define the anatomy and the safe entry zones. The entry zones evaluated were the perioculomotor, lateral mesencephalic sulcus, and supra- and infracollicular areas in the midbrain; the peritrigeminal zone, supra- and infrafacial approaches, acoustic area, and median sulcus above the facial colliculus in the pons; and the anterolateral, postolivary, and dorsal medullary sulci in the medulla.

RESULTS

The safest approach for lesions located below the surface is usually the shortest and most direct route. Previous studies have often focused on surface structures. In this study, the deeper structures that may be at risk in each of the proposed safe entry zones plus the borders of each entry zone were defined. This study includes an examination of the relationships of the cerebellar peduncles, long tracts, intra-axial segments of the cranial nerves, and important nuclei of the brainstem to the proposed safe entry zones.

CONCLUSION

Fiber dissection technique in combination with the 3-D photography is a useful addition to the goal of making entry into the brainstem more accurate and safe.

Surgical approaches for brainstem tumors in pediatric patients

PURPOSE

To analyze the pathways to brainstem tumors in childhood, as well as safe entry zones.

METHOD

We conducted a retrospective study of 207 patients less than 18 years old who underwent brainstem tumor resection by the first author (Cavalheiro, S.) at the Neurosurgical Service and Pediatric Oncology Institute of the São Paulo Federal University from 1991 to 2011.

RESULTS

Brainstem tumors corresponded to 9.1 % of all pediatric tumors operated in that same period. Eleven previously described "safe entry zones" were used. We describe a new safe zone located in the superior ventral pons, which we named supratrigeminal approach. The operative mortality seen in the first 2 months after surgery was 1.9 % (four patients), and the morbidity rate was 21.2 %.

CONCLUSIONS

Anatomic knowledge of intrinsic and extrinsic brainstem structures, in association with a refined neurosurgical technique assisted by intraoperative monitoring, and surgical planning based on magnetic resonance imaging (MRI) and tractography have allowed for wide resection of brainstem lesions with low mortality and acceptable morbidity rates.

Brainstem mapping

The complex neuroanatomy of the brainstem and the additional distortion incurred by intrinsic lesions have previously discouraged aggressive neurosurgical procedures. Safe access to the brainstem and complete lesionectomy has been thwarted by considerable perioperative risk. Brainstem mapping has established itself as one of the means by which modern neurosurgery can improve surgical outcome while decreasing morbidity. It involves the use of neurophysiologic techniques for the identification of critical structures, such as the cranial motor nuclei and their nerves, and the corticobulbar and corticospinal tracts at different stages of the operation. Familiarity with these techniques can map a safe corridor toward a brainstem lesion and guide the surgeon during the resection. By means of reviewing the available literature, we discuss the anatomic, pathophysiologic, technical, and interpretational aspects of brainstem mapping and elaborate on its indications, limitations, and future directions.

Subtemporal transtentorial resection of cavernous malformations involving the pyramidal tract in the upper pons and mesencephalon

BACKGROUND

Lateral approaches to the brain stem for the resection of the cavernous malformations are preferred in order to avoid the structures within the floor of the fourth ventricle. The entry behind the pyramidal tract (PT) is usually carried out through the posterolateral surface of the brain stem. The more straightforward lateral approach below the temporal lobe is used rarely because of potential risks.

METHODS

The outcome after resection of the cavernomas involving the PT in the mesencephalon and the upper pons via the subtemporal transtentorial approach in nine patients was analysed. Mapping of the PT by direct electrical stimulation was used in the last four patients.

RESULTS

The subtemporal transtentorial approach enabled adequate exposure of the lateral and anterolateral surface of the midbrain and the upper pons. No adverse events from the elevation of the temporal lobe were encountered. Direct electrical stimulation using a bipolar electrode with the parameters of 100 Hz, 1 ms, and 3-9 mA evoked motor responses in three of four patients. It allowed placing the incision in the lateral surface of the midbrain behind the PT or between the fibres of the upper and the lower extremity. No worsening of the PT functions was observed in the series.

CONCLUSIONS

The subtemporal transtentorial approach enables adequate exposure of the lateral and the anterolateral surface of the mesencephalon and upper pons, allowing neurophysiological mapping of the PT and thus avoiding its damage during removal of the cavernoma.

Inter- and intrapatient variability of facial nerve response areas in the floor of the fourth ventricle

BACKGROUND

Surgical exposure of intrinsic brainstem lesions through the floor of the 4th ventricle requires precise identification of facial nerve (CN VII) fibers to avoid damage.

OBJECTIVE

To assess the shape, size, and variability of the area where the facial nerve can be stimulated electrophysiologically on the surface of the rhomboid fossa.

METHODS

Over a period of 18 months, 20 patients were operated on for various brainstem and/or cerebellar lesions. Facial nerve fibers were stimulated to yield compound muscle action potentials (CMAP) in the target muscles. Using the sites of CMAP yield, a detailed functional map of the rhomboid fossa was constructed for each patient.

RESULTS

Lesions resected included 14 gliomas, 5 cavernomas, and 1 epidermoid cyst. Of 40 response areas mapped, 19 reached the median sulcus. The distance from the obex to the caudal border of the response area ranged from 8 to 27 mm (median, 17 mm). The rostrocaudal length of the response area ranged from 2 to 15 mm (median, 5 mm).

CONCLUSION

Facial nerve response areas showed large variability in size and position, even in patients with significant distance between the facial colliculus and underlying pathological lesion. Lesions located close to the facial colliculus markedly distorted the response area. This is the first documentation of variability in the CN VII response area in the rhomboid fossa. Knowledge of this remarkable variability may facilitate the assessment of safe entry zones to the brainstem and may contribute to improved outcome following neurosurgical interventions within this sensitive area of the brain.

Surgical management of brainstem cavernous malformations

Bleeding from brainstem cavernomas may cause severe deficits due to the absence of non-eloquent nervous tissue and the presence of several ascending and descending white matter tracts and nerve nuclei. Surgical removal of these lesions presents a challenge to the most surgeons. The authors present their experience with the surgical treatment of 43 patients with brainstem cavernomas. Important aspects of microsurgical anatomy are reviewed. The surgical management, with special focus on new intraoperative technologies as well as controversies on indications and timing of surgery are presented. According to several published studies the outcome of brainstem cavernomas treated conservatively is poor. In our experience, surgical resection remains the treatment of choice if there was previous hemorrhage and the lesion reaches the surface of brainstem. These procedures should be performed by experienced neurosurgeons in referral centers employing all the currently available technology.

Operative management of brainstem cavernous malformations

Brainstem cavernous malformations (CMs) are complex lesions associated with hemorrhage and neurological deficit. In this review, the authors describe the anatomical nuances relating to the operative techniques for these challenging lesions. The resection of brainstem CMs in properly selected patients has been demonstrated to reduce the risk of rehemorrhage and can be achieved relatively safely in experienced hands.

Microroll retractor for surgical resection of brainstem cavernomas

BACKGROUND

Safe and complete surgical resection of brainstem cavernoma is difficult without injury surrounding normal structures because the lesions are deep and intra-axial. In this article, the authors describe the "microroll retractor" technique for brainstem cavernoma surgery.

METHODS

The microroll retractor is made of expanded polytetrafluoroethylene sheet. The sheet is cut in several sizes of square piece and transformed into a tubular shape. We insert this roll retractor via minimal brain incision and keep surgical corridor without usual spatulas during lesionectomy.

RESULTS

This technique was adopted for 7 patients with brainstem cavernomas. In all patients, we achieved total excision; improvement was recorded in 6 patients, and no change was recorded in 1 patient.

CONCLUSIONS

The microroll retractor is extremely simple but provides enough surgical corridors for safe total resection of brainstem cavernomas.

The importance of brainstem mapping in brainstem surgical anatomy before the fourth ventricle and implication for intraoperative neurophysiological mapping

Brain stem mapping (BSM) is an intraoperative neurophysiological procedure to localize cranial motor nuclei on the floor of the fourth ventricle. BSM enables neurosurgeon to understand functional anatomy on the distorted floor of the fourth ventricle, thus, it is emerging as an indispensable tool for challenging brain stem surgery. The authors described the detail of BSM with the special emphasis on its clinical application for the brain stem lesion. Surgical implications based on the result of brains stem mapping would be also informative before planning a brain stem surgery through the floor of fourth ventricle. Despite the recent advancement of MRI to depict the lesion in the brain stem, BSM remains as the only way to provide surgical anatomy in the operative field. BSM could guide a neurosurgeon to the inside of brain stem while preventing direct damage to the cranial motor nuclei on the floor of the fourth ventricle. It is expected that understanding its advantage and limitations would help neurosurgeon to perform safer surgery to the brain stem lesion.

Brainstem Cavernomas: Long-term Results of Microsurgical Resection in 52 Patients

OBJECTIVE:

To review the natural history and the long-term results of microsurgical resection of brainstem cavernous angiomas operated on in one institution.

METHODS:

A retrospective analysis was conducted of the preoperative and postoperative course in 52 consecutive patients who underwent microsurgical resection of a brainstem cavernoma between 1990 and 2002. The role of sex, age, cavernoma location, size, multiple bleedings, relationships to the pial-ependymal surface, surgical approach, and preoperative magnetic resonance imaging appearance were evaluated as prognostic factors possibly influencing outcome. Discrete data were compared by use of the χ2 test and Fisher's exact test as appropriate.

RESULTS:

The risk of hemorrhage was 3.8% per patient per year. The rebleeding rate was 34.7%. Nineteen of 29 patients who experienced new neurological deficits after surgery improved over time to their preoperative condition or better. Permanent morbidity was observed in 10 (19%) of 52 patients (follow-up: 1.5–10.5 yr; mean, 4.7 yr; median, 4.3 yr; standard deviation, 0.2 yr). The final Karnofsky Performance Scale score for these 10 patients was 90 in 2 patients, 80 in 2, 70 in 2, 60 in 2, 50 in 1, and 30 in 1. The mortality rate was 1.9%. The incidence of permanent new neurological deficits was lower in the 20 patients whose lesion could be removed through an anterolateral pontine approach (5 versus 29%; P = 0.035).

CONCLUSION:

Surgical resection is recommended for superficial lesions and for lesions that can be reached through the anterolateral pontine surface. Surgery is also recommended for symptomatic cavernomas with a satellite subacute hematoma.