The anatomical aspects of a surgical approach through the floor of the fourth ventricle

Comprehensive Morphometric Analysis of the Rhomboid Fossa: Implications for Safe Entry Zones in Brainstem Surgery

BACKGROUND

The rhomboid fossa (RF) is a crucial anatomical region in brainstem surgery as it contains essential structures such as the reticular formation and cranial nerve nuclei. This study aimed to provide a detailed understanding of the complex microsurgical anatomy of the RF, which is vital for the safe execution of neurosurgical procedures.

METHODS

Morphometric analysis was conducted on 45 adult human brainstems preserved in 10% formalin. Under 20× magnification, 13 linear measurements were performed using a millimeter graph to identify key anatomical landmarks.

RESULTS

The RF measured 34.65 mm in length and 22.61 mm in width. The facial colliculus measured 4.26 mm in length on the left and 4.45 mm on the right, with corresponding widths of 3.77 mm and 3.50 mm. The distance between the sulcus limitans incisures was 9.52 mm, and the distance from the upper border of the medullary striae to obex was 11.53 mm. The proximity of the facial colliculus to the median sulcus was measured at 0.86 mm on the right and 0.96 mm on the left. Additionally, 2 safe entry zones-the suprafacial and infrafacial triangles-were identified, offering pathways to reach dorsal pons lesions through the RF.

CONCLUSIONS

This comprehensive morphometric analysis of the RF enhances the understanding of its intricate anatomy. By describing safe entry zones, the suprafacial and infrafacial triangles, and providing precise measurements of key anatomical features, this study serves as a valuable resource for neurosurgeons in planning and executing brainstem surgeries.

Axial Sections of Brainstem Safe Entry Zones and Clinical Importance of Intrinsic Structures: A Review

Brainstem surgery is more difficult and riskier than surgeries in other parts of the brain due to the high density of critical tracts and cranial nerves nuclei in this region. For this reason, some safe entry zones into the brainstem have been described. The main purpose of this article is to bring on the agenda the significance of the intrinsic structures of the safe entry zones to the brainstem. Having detailed information about anatomic localization of these sensitive structures is important to predict and avoid possible surgical complications. In order to better understand this complex anatomy, we schematically drew the axial sections of the brainstem showing the intrinsic structures at the level of 9 safe entry zones that we used, taking into account basic neuroanatomy books and atlases. Some illustrations are also supported with intraoperative pictures to provide better surgical orientation. The second purpose is to remind surgeons of clinical syndromes that may occur in case of surgical injury to these delicate structures. Advanced techniques such as tractography, neuronavigation, and neuromonitorization should be used in brainstem surgery, but detailed neuroanatomic knowledge about safe entry zones and a meticulous surgery are more important. The axial brainstem sections we have drawn can help young neurosurgeons better understand this complex anatomy.

Identification of the Facial Colliculus in Two-dimensional and Three-dimensional Images

The facial colliculus (FC), an important landmark for planning a surgical approach to brainstem cavernous malformation (BCM), is a microstructure; therefore, it may be difficult to identify on magnetic resonance imaging (MRI). Three-dimensional (3D) images may improve the FC-identification certainty; hence, this study attempted to validate the FC-identification certainty between two-dimensional (2D) and 3D images of patients with a normal brainstem and those with BCM.

In this retrospective study, we included 10 patients with a normal brainstem and 10 patients who underwent surgery for BCM. The region of the FC in 2D and 3D images was independently identified by three neurosurgeons, three times in each case, using the method for continuously distributed test results (0–100). The intra- and inter-rater reliability of the identification certainty were confirmed using the intraclass correlation coefficient (ICC). The FC-identification certainty for 2D and 3D images was compared using the Wilcoxon signed-rank test.

The ICC (1,3) and ICC (3,3) in both groups ranged from 0.88 to 0.99; therefore, the intra- and inter-rater reliability were good. In both groups, the FC- identification certainty was significantly higher for 3D images than for 2D images (normal brainstem group; 82.4 vs. 61.5, P = .0020, BCM group; 40.2 vs. 24.6, P = .0059 for the unaffected side, 29.3 vs. 17.3, P = .0020 for the affected side).

In the normal brainstem and BCM groups, 3D images had better FC-identification certainty. 3D images are effective for the identification of the FC.

Optimal access route for pontine cavernous malformation resection with preservation of abducens and facial nerve function

OBJECTIVE

The aim of this study was to analyze the differences between posterolateral and posteromedial approaches to pontine cavernous malformations (PCMs) in order to verify the hypothesis that a posterolateral approach is more favorable with regard to preservation of abducens and facial nerve function.

METHODS

The authors conducted a retrospective analysis of 135 consecutive patients who underwent microsurgical resection of a PCM. The vascular lesions were first classified in a blinded fashion into 4 categories according to the possible or only reasonable surgical access route. In a second step, the lesions were assessed according to which approach was performed and different patient groups and subgroups were determined. In a third step, the modified Rankin Scale score and the rates of permanent postoperative abducens and facial nerve palsies were assessed.

RESULTS

The largest group in this series comprised 77 patients. Their pontine lesion was eligible for resection from either a posterolateral or posteromedial approach, in contrast to the remaining 3 patient groups in which the lesion location already had dictated a specific surgical approach. Fifty-four of these 77 individuals underwent surgery via a posterolateral approach and 23 via a posteromedial approach. When comparing these 2 patient subgroups, there was a statistically significant difference between postoperative rates of permanent abducens (3.7% vs 21.7%) and facial (1.9% vs 21.7%) nerve palsies. In the entire patient population, the abducens and facial nerve deficit rates were 5.9% and 5.2%, respectively, and the modified Rankin Scale score significantly decreased from 1.6 ± 1.1 preoperatively to 1.0 ± 1.1 at follow-up.

CONCLUSIONS

The authors' results suggest favoring a posterolateral over a posteromedial access route to PCMs in patients in whom a lesion is encountered that can be removed via either surgical approach. In the present series, the authors have found such a constellation in 57% of all patients. This retrospective analysis confirms their hypothesis in a large patient cohort. Additionally, the authors demonstrated that 4 types of PCMs can be distinguished by preoperatively evaluating whether only one reasonable or two alternative surgical approaches are available to access a specific lesion. The rates of postoperative sixth and seventh nerve palsies in this series are substantially lower than those in the majority of other published reports.

Virtual Exploration of Safe Entry Zones in the Brainstem: Comprehensive Definition and Analysis of the Operative Approach

BACKGROUND

A detailed and accurate understanding of the intrinsic brainstem anatomy and the interrelationship between its internal tracts and nuclei and external landmarks is of paramount importance for safe and effective brainstem surgery. Using anatomical models can be an important step in increasing such understanding. In the present study, we have shown the applicability of our developed virtual 3-dimensional (3D) model in depicting the safe entry zones (SEZs) to the brainstem.

METHODS

Accurate 3D virtual models of brainstem elements were created using high-resolution magnetic resonance imaging and computed tomography to depict the brainstem SEZs.

RESULTS

All the described SEZs to different parts of the brainstem were successfully depicted using our 3D virtual models.

CONCLUSIONS

The virtual models provide an immersive experience of brainstem anatomy, allowing users to understand the intricacies of the microdissection that is necessary to appropriately work through the brainstem nuclei and tracts toward a particular target. The models provide an unparalleled learning environment to understand the SEZs into the brainstem that can be used for training and research.

Intraoperative facial motor evoked potential monitoring for pontine cavernous malformation resection

OBJECTIVE

The aim of this study was to predict postoperative facial nerve function during pontine cavernous malformation surgery by monitoring facial motor evoked potentials (FMEPs).

METHODS

From 2008 to 2017, 10 patients with pontine cavernous malformations underwent total resection via the trans-fourth ventricle floor approach with FMEP monitoring. House-Brackmann grades and Karnofsky Performance Scale (KPS) scores were obtained pre- and postoperatively. The surgeries were performed using one of 2 safe entry zones into the brainstem: the suprafacial triangle and infrafacial triangle approaches. Six patients underwent the suprafacial triangle approach, and 4 patients underwent the infrafacial triangle approach. A cranial peg screw electrode was used to deliver electrical stimulation for FMEP by a train of 4 or 5 pulse anodal constant current stimulation. FMEP was recorded from needle electrodes on the ipsilateral facial muscles and monitored throughout surgery by using a threshold-level stimulation method.

RESULTS

FMEPs were recorded and analyzed in 8 patients; they were not recorded in 2 patients who had severe preoperative facial palsy and underwent an infrafacial triangle approach. Warning signs appeared in all patients who underwent the suprafacial triangle approach. However, after temporarily stopping the procedures, FMEP findings during surgery showed recovery of the thresholds. FMEPs in patients who underwent the infrafacial triangle approach were stable during the surgery. House-Brackmann grades were unchanged postoperatively in all patients. Postoperative KPS scores improved in 3 patients, decreased in 1, and remained the same in 6 patients.

CONCLUSIONS

FMEPs can be used to monitor facial nerve function during surgery for pontine cavernous malformations, especially when the suprafacial triangle approach is performed.

Intrinsic Well-Demarcated Midline Brainstem Lesion Successfully Resected through a Midline Pontine Splitting Approach

INTRODUCTION

Surgical approaches to intrinsic pontine lesions are technically difficult and prone to complications. The surgical approach to the brainstem through midline pontine splitting is regarded as safe since there are no crossing vital fibers in the midline between the abducens nuclei at the facial colliculi in the pons and the oculomotor nucleus in the midbrain, although its actual utilization has not been reported previously.

CASE PRESENTATION

A 6-year-old boy presented with a large intrinsic cystic lesion in the pons. We successfully achieved gross total removal via the median sulcus of the fourth ventricle. The fixation in adduction and limitation of abduction were newly observed in the left eye after surgery.

DISCUSSION

The advantage of the surgical approach through the median sulcus is the longer line of dissection in an axial direction and the gain of a wider operative view. On the other hand, the disadvantage of this approach is the limited orientation and view toward lateral side and a possible impairment of the medial longitudinal fasciculi and paramedian pontine reticular formation, which are located lateral to the midline sulcus bilaterally and are easily affected via the median sulcus of the fourth ventricular floor. Ongoing developments in intraoperative neuro-monitoring and navigation systems are expected to enhance this promising approach, resulting in a safer and less complicated procedure in the future.

CONCLUSION

The surgical approach through midline pontine splitting is suitable for midline and deep locations of relatively large pontine lesions that necessitate a wider surgical window.

Usefulness of the anterior transpetrosal approach for pontine cavernous malformations

Surgical resection of pontine cavernous malformation remains a particularly formidable challenge. We report the surgical outcome of eight cases with pontine cavernous malformations operated using the anterior transpetrosal approach. All cases presented with neurological deficits caused by hemorrhage before surgery. Gross-total removal was achieved in all cases without any postoperative complication such as worsening of facial nerve palsy, ocular movement disorder, or hemiplegia. A small incision of the pons with multidirectional dissection is the most important factor for minimizing postoperative neurological deficits, so resection of a pontine cavernous malformation via this approach can be an alternative better option.

The video can be found here: https://youtu.be/2Q2CUhBbo28.

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.

Mapping the Internal Anatomy of the Lateral Brainstem: Anatomical Study with Application to Far Lateral Approaches to Intrinsic Brainstem Tumors

INTRODUCTION

Intramedullary brainstem tumors present a special challenge to the neurosurgeon. Unfortunately, there is no ideal part of the brainstem to incise for approaches to such pathology. Therefore, the present study was performed to identify what incisions on the lateral brainstem would result in the least amount of damage to eloquent tracts and nuclei. Case illustrations are also discussed.

MATERIALS AND METHODS

Eight human brainstems were evaluated. Based on dissections and the use of standard atlases of brainstem anatomy, the most important deeper brainstem structures were mapped to the surface of the lateral brainstem.

RESULTS

With these data, we defined superior acute and inferior obtuse corridors for surgical entrance into the lateral brainstem that would minimize injury to deeper tracts and nuclei, the damage to which would result in significant morbidity.

CONCLUSIONS

To our knowledge, a superficial map of the lateral brainstem for identifying deeper lying and clinically significant nuclei and tracts has not previously been available. Such data might decrease patient morbidity following biopsy or tumor removal or aspiration of brainstem hemorrhage. Additionally, this information can be coupled with the previous literature on approaches into the fourth ventricular floor for more complex, multidimensional lesions.

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.

Brainstem cavernoma hemorrhage during pregnancy in a 15-year-old: description of a unique neurosurgical approach

Cavernous haemangiomas, or cavernous malformations, have been reported during pregnancy, most of which have been either supratentorial or spinal lesions. We encountered a 15-year old pregnant patient with a rapidly progressive and haemorrhagic brainstem cavernous haemangioma. The case presented here describes the history and findings of this patient, as well as the less-commonly utilized technique we used to access the floor of the fourth ventricle via occipital craniotomy for complete macroscopic resection of this lesion, resulting in the gradual return of most of her neurological deficits.

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 cavernomas: selection of approaches and microsurgical techniques

This study reviewed surgical experience with brainstem cavernomas in an attempt to define optimal surgical approaches and risks associated with surgical management. Clinical courses were retrospectively reviewed for 36 consecutive patients (12 men, 24 women; mean age, 42 years) who underwent microsurgical resection of brainstem cavernomas between 1996 and 2006. Medical records, surgical records, and neuroimaging examinations were evaluated. All 36 patients presented with > or =1 hemorrhage from the cavernomas and preoperatively displayed some neurological symptoms. Surgical approach was midline suboccipital for 16 pontine and/or medullary cavernomas under the floor of the fourth ventricle, retrosigmoid for 10 lateral mesencephalic, pontine, and/or medullary cavernomas, occipital transtentorial for 2 thalamomesencephalic and 3 mesencephalic cavernomas, combined petrosal for 2 pontine cavernomas, and other for 3 cavernomas. Complete resection according to postoperative magnetic resonance imaging was achieved in 33 of 36 patients. No mortality was encountered in this study. New neurological deficit occurred in the early postoperative period for 18 patients, but was transient in 15 of these. Neurological state as of final follow-up was improved in 16 patients (44%), unchanged in 17 (47%), and worsened in 3 (8%) compared with preoperatively. In conclusion, symptomatic brainstem cavernomas should be considered for surgical treatment. Careful selection of the optimal operative approach and a meticulous microsurgical technique are mandatory.

[Surgical anatomy of the brain stem]

We reviewed the literature on brain stem anatomy, to identify possible and non-hazardous entry zones with a minimum of functional risks. Using the reticular formation defined as a median structure in a coronal plane, we determined six anatomic zones, 3 ventral, 3 dorsal (mesencephalic, pontic, medulla-oblongata). Considering the functional structures surrounding each zone, the possible penetration points are described. There are ventral, one for the mesencephale, one for the pons, one for the medulla oblongata; and dorsal, one for the mesencephale, two for the floor of the 4th ventricle, one for the medulla oblongata.

Cavernous Malformations of the Brainstem: Three-Dimensional-Constructive Interference in Steady-State Magnetic Resonance Imaging for Improvement of Surgical Approach and Clinical Results

OBJECTIVE

The indications for resection of cavernous malformations (CMs) of the brainstem include neurological deficits, (recurrent) hemorrhage, and surgically accessible location. In particular, knowledge of the thickness of the parenchymal layer and of the CM's spatial relation to nuclei, tracts, cranial nerves, and vessels is critical for planning the surgical approach. We reviewed the operative treatment of 13 patients with 14 brainstem CMs, with special regard to refined three-dimensional (3D)-constructive interference in steady-state (CISS) magnetic resonance imaging (MRI).

METHODS

Patients were evaluated neurologically and by conventional spin-echo/fast spin-echo and 3D-CISS MRI. Surgery was performed with the use of microsurgical techniques and neurophysiological monitoring.

RESULTS

Eleven CMs were located in the pons/pontomedullary region; 10 of the 11 were operated on via the lateral suboccipital approach. Three CMs were located near the floor of the fourth ventricle and operated on via the median suboccipital approach, with total removal of all CMs. Results were excellent or good in 10 patients; one patient transiently required tracheostomy, and two patients developed new hemipareses/ataxia with subsequent improvement. Not only did 3D-CISS sequences allow improved judgment of the thickness of the parenchymal layer over the lesion compared with spin-echo/fast spin-echo MRI, but 3D-CISS imaging also proved particularly superior in demonstrating the spatial relation of the lesion to fairly "safe" entry zones (e.g., between the trigeminal nerve and the VIIth and VIIIth nerve groups) by displaying the cranial nerves and vessels within the cerebellopontine cistern more precisely.

CONCLUSION

Surgical treatment of brainstem CMs is recommended in symptomatic patients. Especially in patients with lesions situated ventrolaterally, the 3D-CISS sequence seems to be a valuable method for identifying the CM's relation to safe entry zones, thereby facilitating the surgical approach.

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.

[Optimization of microsurgical operation technique to insert auditory brainstem implants, taking into account the results of a morphometric study]

STATE OF THE ART

The surgical placement of auditory brainstem implants to stimulate the cochlear nuclear region in patients with acquired bilateral retrocochlear deafness allows limited restitution of hearing. However, there have been few studies on the topographical relations in the target region, particularly the floor of the IVth ventricle.

TOPIC OF THE STUDY

Is it possible to obtain more precise anatomical data in order to improve the surgical approaches and techniques for the placement of auditory brainstem implants? AIMS. To obtain a more precise topo- anatomical orientation in the target region for microsurgical lateral and midline approaches or a stereotactic operative strategy.

METHODS AND RESULTS

Landmarks for the placement of an auditory brainstem implant via the IVth ventricle were examined and measured in a series of formalin-fixed human brainstems ( n=28). These data, and knowledge of their variability, allow a more precise surgical lateral approach. It is essential to precisely localise the target region, as it can only be partly discerned under the microscope during an operation. For this reason, to date its precise localisation has been determined only electrophysiologically.

CONCLUSION

Exact target localisation improves safety. From an anatomical point of view the midline approach gives the chance to enlarge the indication spectrum for an implant. The anatomical data obtained here could also be integrated into a stereotactic surgical strategy.

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

OBJECT

The authors describe their technique of electrophysiological mapping to assist pericollicular approaches into the rhomboid fossa.

METHODS

Surgical approaches to the rhomboid fossa can be optimized by direct electrical stimulation of superficially located nuclei and fibers. Electrophysiological mapping allows identification of facial nerve fibers, nuclei of the abducent and hypoglossal nerves, motor nucleus of the trigeminal nerve, and the ambiguous nucleus. Stimulation at the surface of the rhomboid fossa performed using the threshold technique allows localization above the area that is located closest to the surface. Simultaneous bilateral electromyographic (EMG) recordings from cranial motor nerves obtained during stimulation document the selectivity of evoked EMG responses. With respect to stimulation parameters and based on morphometric measurements, the site of stimulation can be assumed to be the postsynaptic fibers at the axonal cone. Strict limitation to 10 Hz with a maximum stimulation intensity not exceeding 2 mA can be considered safe. Direct side effects of electrical stimulation were not observed.

CONCLUSIONS

Electrical stimulation based on morphometric data obtained on superficial brainstem anatomy defines two safe paramedian supra- and infracollicular approaches to the rhomboid fossa and is particularly helpful in treating intrinsic brainstem lesions that displace normal anatomical structures.

Topographic anatomy of the cochlear nuclear region at the floor of the fourth ventricle in humans

OBJECT

The development of appropriate methods to stimulate the dorsal and ventral cochlear nucleus by means of an auditory brainstem implant in patients with acquired bilateral anacusis requires a detailed topoanatomical knowledge both of the location and extension of the nuclear surface in the fourth ventricle and lateral recess and of its variability. The goal of this study was to provide that information. Anatomically, it is possible to use a midline surgical approach to the fourth ventricle rather than the translabyrinthine and suboccipital routes of access used hitherto. This is especially useful if severe scarring, which occurs as a result of tumor removal in the cerebellopontine angle, make the orientation and placement of an auditory brainstem implant via a lateral surgical approach difficult. There have been only a few studies, involving single cases and small series of patients, in which the focus was the exact extension of the cochlear nuclei, whose microsurgically relevant position in relation to the surface structures is not known in detail.

METHODS

Landmarks that are important for the placement of an auditory brainstem implant through the fourth ventricle were examined and measured in a large series of 28 formalin-fixed human brainstems. In all cases, these examinations were supplemented by addition of a histological section series. For the first time values of unfixed fresh brainstem tissue were determined. Anatomical features are discussed with regard to their possible neurosurgical relevance, taking into account inter- and intraindividual variability.

CONCLUSIONS

The midline approach would provide an opportunity to stimulate the whole area of the dorsal as well as the ventral cochlear nucleus with an auditory brainstem implant.

A brainstem stereotactic atlas in a three-dimensional magnetic resonance imaging navigation system: first experiences with atlas-to-patient registration

OBJECT

The authors describe a computer-resident digital representation of a stereotactic atlas of the human brainstem, its semiautomated registration to sagittal fast low-angle shot three-dimensional (3-D) magnetic resonance (MR) imaging data sets in 27 healthy volunteers and 24 neurosurgical patients, and an analysis of the subsequent transforms needed to refine the initial registration.

METHODS

Contour drawings from the atlas, which offer the 70th percentile of variation of anatomical structures, were interpolated into an isotropic 3-D representation. Initial atlas-to-patient registration was based on the fastigium/ventricular floor plane reference system. The quality of the fit was evaluated using superimposition of the atlas and MR images. If necessary, the atlas was tailored to the individual anatomy by using additional transforms. On average, the atlas had to be stretched by 2 to 6% in the three directions of space. Scale factors varied over a broad range from -8 to +19% and the benefit of visual interactive control of the atlas-to-patient registration was evident. Analysis of distances within the pons measured in the midsagittal MR imaging slices and the required scale factors revealed significant correlations that may be used to reduce the amount of user interaction in the coregistration substantially. In 70.6% of the cases, the atlas had to be shifted in a cranial direction along the brainstem axis (in 25.5% of cases 3-4 mm, in 45.1% of cases 1-2 mm). This was due to a more caudal position of the fastigium cerebelli on the MR images compared with the atlas.

CONCLUSIONS

This observation, in conjunction with the variability of the height of the fourth ventricle in our MR imaging data (range 6.1-15.2 mm, mean 10.1 mm, standard deviation 1.8 mm) calls into question the role of the fastigium cerebelli as an anatomical landmark for localization within the brainstem.