A study of safe entry zones via the floor of the fourth ventricle for brain-stem lesions. Report of three cases

Evaluation of the hematoma consequences, neurobehavioral profiles, and histopathology in a rat model of pontine hemorrhage

OBJECT

Primary pontine hemorrhage (PPH) represents approximately 7% of all intracerebral hemorrhages (ICHs) and is a clinical condition of which little is known. The aim of this study was to characterize the early brain injury, neurobehavioral outcome, and long-term histopathology in a novel preclinical rat model of PPH.

METHODS

The authors stereotactically infused collagenase (Type VII) into the ventral pontine tegmentum of the rats, in accordance with the most commonly affected clinical region. Measures of cerebrovascular permeability (brain water content, hemoglobin assay, Evans blue, collagen Type IV, ZO-1, and MMP-2 and MMP-9) and neurological deficit were quantified at 24 hours postinfusion (Experiment 1). Functional outcome was measured over a 30-day period using a vertebrobasilar scale (the modified Voetsch score), open field, wire suspension, beam balance, and inclined-plane tests (Experiment 2). Neurocognitive ability was determined at Week 3 using the rotarod (motor learning), T-maze (working memory), and water maze (spatial learning and memory) (Experiment 3), followed by histopathological analysis 1 week later (Experiment 4).

RESULTS

Stereotactic collagenase infusion caused dose-dependent elevations in hematoma volume, brain edema, neurological deficit, and blood-brain barrier rupture, while physiological variables remained stable. Functional outcomes mostly normalized by Week 3, whereas neurocognitive deficits paralleled the cystic cavitary lesion at 30 days. Obstructive hydrocephalus did not develop despite a clinically relevant 30-day mortality rate (approximately 54%).

CONCLUSIONS

These results suggest that the model can mimic several translational aspects of pontine hemorrhage in humans and can be used in the evaluation of potential preclinical therapeutic interventions.

Surgical resection of cavernous malformations of the brainstem: evolution of a minimally invasive technique

OBJECTIVE

The purpose of this study is to provide an institutional retrospective review of surgically treated brainstem cavernous malformations.

METHODS

Between 2005 and 2010, 22 consecutive patients with brainstem cavernous malformations (15 female and 7 male) with a mean age of 43 years underwent surgical treatment. Mean volume of the resected cavernous malformations was 0.65 cm(3). A minimally invasive resection technique was used for these cases, in conjunction with skull base approaches.

RESULTS

The mean follow-up period was 26.6 months (range, 4-68 months). Of the 22 patients, 9% did not have clear evidence of hemorrhage at the time of presentation. Of the remainder, 22% had two or more instances of hemorrhage documented by magnetic resonance imaging. After resection and during follow-up, 54% of patients had an improvement in their modified Rankin scale, whereas 14% were worse compared with their preoperative presentation; 32% were unchanged and 9% of patients were found to have residual cavernoma post-surgery.

CONCLUSION

Our longitudinal experience has guided us to emphasize minimally invasive approaches during resection of the brainstem cavernous malformations, occasionally at the expense of achieving a complete resection, to improve patient outcomes.

Neurophysiologic monitoring of the spinal accessory nerve, hypoglossal nerve, and the spinomedullary region

This review of hypoglossal nerve, spinal accessory nerve, and spinomedullary region intraoperative monitoring details pertinent central and extramedullary anatomy, an updated understanding of proper free-run EMG recording methods and recent developments in stimulation technique and instrumentation. Mapping and monitoring the floor of the fourth ventricle, especially the vagal/hypoglossal trigone region, are emphasized. Although cranial nerve transcranial electrical motor evoked potential recordings can afford appreciation of corticobulbar/corticospinal tract function and secure a more dependable measure of proximate extramedullary somatoefferents, the sometimes difficult implementation and the, as yet, unresolved alert criteria of these recordings demand critical appraisal. Nearby and intimately associated cardiochronotropic and barocontrol neural networks are described; their better understanding is recommended as an important adjunct to "routine" neural monitoring. Finally, an Illustrative case is presented to highlight the many strengths and weaknesses of "state of the art" lower cranial nerve/spinomedullary region monitoring.

Leptomeningeal Dissemination of a Low-Grade Brainstem Glioma without Local Recurrence

It is rare for low-grade gliomas to disseminate to the leptomeninges. However, low-grade gliomas with dissemination to the leptomeninges have been occasionally reported in children, and have generally been associated with local recurrence. A 16-year-old boy sought evaluation for diplopia and gait disturbance. A brain magnetic resonance imaging (MRI) revealed pontine mass, which was proved to be fibrillary astrocytoma on biopsy, later. Radiation therapy (5400 cGy) was given and the patient's symptoms were improved. He was followed-up radiologically for brain lesion. Seven months after diagnosis he complained of back pain and gait disturbance. A brain MRI showed a newly-developed lesion at the left cerebellopontine angle without an interval change in the primary lesion. A spinal MRI demonstrated leptomeningeal dissemination of the entire spine. Radiation therapy (3750 cGy) to the spine, and adjuvant chemotherapy with a carboplatin plus vincristine regimen were administered. However, he had a progressive course with tumoral hemorrhage and expired 13 months after diagnosis. We report an unusual case of a low-grade brainstem glioma with spinal dissemination, but without local recurrence, and a progressive course associated with hemorrhage.

"Kissing" brainstem cavernous angiomas removed using a trans-fourth ventricular floor approach--case report

A 39-year-old woman presented with a rare case of "kissing" brainstem cavernomas formed by separate lesions enlarging with simultaneous recurrent hemorrhages, which was successfully treated by staged resection using a trans-fourth ventricular floor approach. She had a familial history of cerebral cavernous angioma, and presented with a history of four episodes of sudden neurological deterioration. Magnetic resonance (MR) imaging obtained at each neurological event demonstrated two distinct brainstem cavernomas located in the pontine tegmentum and ventral part of the lower pons, both of which enlarged stepwise caused by simultaneous recurrent hemorrhages. Both cavernomas contacted and formed "kissing" lesions. She underwent midline suboccipital craniotomy in the prone position. The cavernoma in the pontine tegmentum was resected through a trans-fourth ventricular floor approach. Although "kissing" formation appeared on preoperative MR imaging, parenchyma was identified at the bottom of the removal cavity of the dorsal lesion, and resection was terminated. MR imaging following the first surgery revealed complete resection of the pontine tegmentum cavernoma and the ventral pontine cavernoma, which was located adjacent to the bottom of the removal cavity and aligned in same direction along the fourth ventricular floor approach. At 10 days after first surgery, she underwent the same procedure with the aid of neuronavigation to resect the ventral pontine cavernoma through the former removal cavity. This approach through the previous removal route, particularly for resection of "kissing" lesions which are difficult to access in the brainstem, is a technically feasible microsurgical procedure.

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 strategies in treating brainstem cavernous malformations

BACKGROUND

Optimal therapy of brainstem cavernous malformations (BSCMs) remains controversial because their biological behavior is unpredictable and surgical removal is challenging.

OBJECTIVE

To analyze our experience with BSCMs and to conduct a review of the literature to identify a rational approach to the management of these lesions.

METHODS

Fifty-five patients harboring 57 BSCMs underwent surgery and 17 patients were treated conservatively during the 10-year period from 1999 to 2008. The operative strategy was to perform complete CM resection and to preserve any associated venous malformation with minimal functional brainstem tissue sacrificed. The National Institutes of Health Strength Scale (NIHSS) was used to assess neurological status.

RESULTS

The average hemorrhagic and rehemorrhagic rates were 4.7% and 32.7% per patient-year, respectively. Total lesional resection was achieved in all operated patients. Their mean NIHSS score was 4.6 after the first episode, 3.5 preoperatively, 3.2 at discharge, and 1.4 after a mean follow-up of 49 months. Complete recovery rates of motor deficits and sensory disturbances from the preoperative state were 70.4% and 51.7%, respectively. Complete recovery rates for cranial nerves III, V, VI, and VII and the lower group were 60%, 63.2%, 25%, 57.1%, and 80%, respectively. For the conservative patients, the mean NIHSS score was 5.9 after the first episode and 1.7 after a mean follow-up of 40 months.

CONCLUSION

NIHSS is optimal for evaluating the natural history and surgical effect of patients harboring BSCMs. Surgical resection remains the primary therapeutic option after careful patient screening and preoperative planning.

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.

The presigmoid approach to anterolateral pontine cavernomas. Clinical article

OBJECT

Anterolateral cavernomas of the pons have been surgically removed via a variety of approaches, commonly retrosigmoid or transventricular. The goal in this study was to evaluate the presigmoid approach as an alternative.

METHODS

Clinical data were reviewed in 9 patients presenting with anterolateral pontine cavernomas between 1999 and 2007.

RESULTS

All patients were treated via a presigmoid approach, which provided a nearly perpendicular trajectory to the anterolateral pons. The brainstem was entered through a "safe zone" between the trigeminal nerve and the facial/vestibulocochlear nerve complex. Complete resection was achieved in all cases. No patient experienced recurrent events during follow-up (1-24 months). The patients' modified Rankin Scale score improved within 1 year of surgery (1.7 ± 0.4) compared with baseline (2.6 ± 0.2; p < 0.05). Only one patient experienced a new deficit (decreased hearing), which was corrected with a hearing aid.

CONCLUSIONS

The presigmoid approach is recommended for the resection of anterolateral pontine cavernomas. With this approach, the need for cerebellar retraction is nearly eliminated. The lateral "presigmoid" entry point creates a trajectory that allows complete resection of even deep lesions at this level, or anterior to the internal acoustic meatus.

Brainstem cavernous malformations: anatomical, clinical, and surgical considerations

Symptomatic brainstem cavernous malformations carry a high risk of permanent neurological deficit related to recurrent hemorrhage, which justifies aggressive management. Detailed knowledge of the microscopic and surface anatomy is important for understanding the clinical presentation, predicting possible surgical complications, and formulating an adequate surgical plan. In this article the authors review and illustrate the surgical and microscopic anatomy of the brainstem, provide anatomoclinical correlations, and illustrate a few clinical cases of cavernous malformations in the most common brainstem areas.

Anterior transpetrosal and subtemporal transtentorial approaches for pontine cavernomas

OBJECTIVE

Pontine cavernomas are benign vascular lesions whose surgical treatment is challenging due to their localization. We report our experience in the surgical management of these lesions exclusively using a lateral, subtemporal transtentorial approach in high pontine lesions and an anterior petrosal approach in low pontine lesions.

METHODS

We performed a retrospective study on a series of patients who were operated on for a pontine cavernoma in our neurosurgery department between 1987 and 2007. In the study, we detail the patients' clinical and preoperative radiological data and compare the two surgical techniques we used. Finally, we analyze the postoperative follow-up, the morbidity encountered according to the surgical approach used, and the long-term outcomes.

RESULTS

We enrolled nine patients into the study. Six patients were operated on using an anterior petrosal approach. None of the patients died. Five patients were able to resume their former professional activity after surgery and were clearly improved following surgery. One patient was worse after surgery (hemiplegia and deafness). We used a subtemporal transtentorial approach in three of the patients. None of the patients died. Two of the patients were able to resume their prior professional activities without any sequels, and the third patient's condition worsened following surgery (temporal hematoma).

CONCLUSION

The lateral surgical approach for pontine cavernomas constitutes a reasonable surgical alternative to the transventricular, suboccipital, retromastoid, or transclival approaches. Patient morbidity in both approaches is acceptable, and the long-term outcome is satisfactory with respect to sequels and the resumption of prior professional activity.

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.

Lateral Transpeduncular Approach to Intrinsic Lesions of the Rostral Pons

Objective:

We describe the lateral transpeduncular approach to access lesions in the rostral pons. The surgical indications and technique are discussed in the context of an illustrative case and pertinent anatomic considerations.

Methods:

A 38-year-old man with acute right hemiparesis and bulbar symptoms had a left pontine hemorrhage with an associated cavernous malformation and venous anomaly. There was no pial or ependymal representation of the lesion. To avoid disruption of eloquent structures, the pia was entered in the posterolateral aspect of the middle cerebellar peduncle. Subsequent dissection was guided by stereotactic neuronavigation in a ventromedial trajectory along the course of the pontocerebellar fibers.

Results:

The cavernous malformation was resected completely without procedure-related morbidity. The patient’s preoperative deficits slowly improved to a functionally independent state.

Conclusion:

The lateral transpeduncular approach may be used to access intrinsic lesions of the rostral pons with relatively low morbidity. Stereotactic neuronavigation and intra-operative electrophysiological monitoring are important surgical adjuncts to guide dissection and lesion extirpation. Candidate selection, microsurgical technique, and pragmatic treatment goals remain fundamental to optimal patient outcomes.

Post-operative remnants of brainstem cavernomas: incidence, risk factors and management

INTRODUCTION

The risk of leaving a remnant after surgery for a cavernous malformation in the brainstem is generally not stressed enough, even though such remnants appear to have a high risk of re-bleeding. At least 40% of known cavernoma remnants after surgery have further bleeding episodes. A retrospective analysis of 30 patients with brainstem cavernoma who underwent surgery is presented, focusing on incidence, risk factors and management of post-surgical residuals. The sites were, medulla in three patients, pons-medulla in four, pons in 16, pons-midbrain in four and midbrain in three. All 30 patients came to our clinical observation with at least one episode of acute-onset neurological deficit and all were operated in the sub-acute phase. Only one patient had a worse stable outcome than the pre-surgical state, and 29 did better or were stable. All patients had a brain MRI scan within 72 h after surgery to confirm that complete removal had been achieved. In three, although the surgical cavity and its border appeared clean at the end of surgery, with no lesion remaining, post-operative MRI detected a residuum. These three patients were re-operated, but one had a further bleed prior to excision.

MATERIALS AND METHODS

In our series, the surgical finding of a multi-lobular cavernoma (as opposed to the more frequent finding of a discrete lesion with a thick capsule), with a thin wall and satellite nodules separated by a thin layer of apparently intact white matter, was common (seven patients). This group included the three patients with evidence of residuum on post-operative MRI. In our experience, the surgical finding of a multi-lobular cavernoma carries a higher risk of residuum and post-surgical re-bleeding.

CONCLUSION

Immediate post-operative brain MRI scans are therefore strongly recommended for their detection, especially in this group of patients, and if a residual is detected early re-intervention is less risky than the natural history.

Microsurgical anatomy of the safe entry zones on the anterolateral brainstem related to surgical approaches to cavernous malformations

OBJECTIVE

To study the microanatomy of the brainstem related to the different safe entry zones used to approach intrinsic brainstem lesions.

METHODS

Ten formalin-fixed and frozen brainstem specimens (20 sides) were analyzed. The white fiber dissection technique was used to study the intrinsic microsurgical anatomy as related to safe entry zones on the brainstem surface. Three anatomic landmarks on the anterolateral brainstem surface were selected: lateral mesencephalic sulcus, peritrigeminal area, and olivary body. Ten other specimens were used to study the axial sections of the inferior olivary nucleus. The clinical application of these anatomic nuances is presented.

RESULTS

The lateral mesencephalic sulcus has a length of 7.4 to 13.3 mm (mean, 9.6 mm) and can be dissected safely in depths up to 4.9 to 11.7 mm (mean, 8.02 mm). In the peritrigeminal area, the distance of the fifth cranial nerve to the pyramidal tract is 3.1 to 5.7 mm (mean, 4.64 mm). The dissection may be performed 9.5 to 13.1 mm (mean, 11.2 mm) deeper, to the nucleus of the fifth cranial nerve. The inferior olivary nucleus provides safe access to lesions located up to 4.7 to 6.9 mm (mean, 5.52 mm) in the anterolateral aspect of the medulla. Clinical results confirm that these entry zones constitute surgical routes through which the brainstem may be safely approached.

CONCLUSION

The white fiber dissection technique is a valuable tool for understanding the three-dimensional disposition of the anatomic structures. The lateral mesencephalic sulcus, the peritrigeminal area, and the inferior olivary nucleus provide surgical spaces and delineate the relatively safe alleys where the brainstem can be approached without injuring important neural structures.

Cavernomes profonds (corps calleux, intraventriculaires, ganglions de la base, insulaires) et du tronc cérébral. Expérience d'une série brésilienne

With a review of the literature, we report our experience with surgical treatment of deep-seated cavernomas (intraventricular, of the corpus callosum, the capsula interna, the insula and the brain stem). Outcome was good in all nine patients after surgery for deep-seated brain cavernomas. There we also 13 cases of the brain stem cavernomas treated surgically. Of them, nine patients were stabilized or improved, one patient worsened, one patient died and two were lost to follow-up. Whatever the location, surgery should only concern symptomatic or hemorrhagic lesions close to the pia-matter or the ependyma as well as those covered by a thin layer of parenchyma. Neuronavigation and microsurgical procedures are essential in the treatment of deep-seated cavernomas.

[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.

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.

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.

Endotracheal tube electrodes to map and monitor activities of the vagus nerve intraoperatively

✓ Difficulty swallowing due to damage of the vagus nerve is one of the most devastating complications of surgery in and around the medulla oblongata; therefore, intraoperative anatomical and functional evaluation of this nerve is crucial. The authors applied endotracheal tube surface electrodes to record electromyography (EMG) activity from vocal cords innervated by the vagus nerve. The vagal nucleus or rootlet was electrically stimulated during surgery and vocalis muscle EMG activities were displayed by auditory and visual signals. This technique was used successfully to identify the vagus motor nerve and evaluate its integrity during surgery. The advantages of this method compared with the use of needle electrodes include safe simple electrode placement and stable recording during surgery. In cases involving a pontine cavernoma pressing the nucleus or a jugular foramen tumor encircling the rootlet, this method would be particularly valuable. Additional studies with a larger number of patients are needed to estimate the significance of this method as a means of functional monitoring to predict clinical function.

Comparison of the transvermian and telovelar approaches to the fourth ventricle

Object. The two most common surgical routes to the fourth ventricle are the transvermian and telovelar approaches. The purpose of this study was to compare the microanatomy and exposures gained through these approaches.

Methods. Ten formalin-fixed specimens were dissected in a stepwise manner to simulate the transvermian and telovelar surgical approaches. Stealth image guidance was used to compare the exposures and working angles obtained using these approaches.

The transvermian and telovelar approaches provided access to the entire rostrocaudal length of the fourth ventricle floor from the aqueduct to the obex. In addition, both approaches provided access to the entire width of the floor of the fourth ventricle. The major difference between the two approaches regarded the exposure of the lateral recess and the foramen of Luschka. The telovelar, but not the transvermian, approach exposed the lateral and superolateral recesses and the foramen of Luschka. The transvermian approach, which offered an incision through at least the lower third of the vermis, afforded a modest increase in the operator's working angle compared with the telovelar approach when accessing the rostral half of the fourth ventricle.

Conclusions. The transvermian approach provides slightly better visualization of the medial part of the superior half of the roof of the fourth ventricle. The telovelar approach, which lacks incision of any part of the cerebellum, provides an additional exposure to the lateral recesses and the foramen of Luschka.

Continuous Laryngoscopic Vocal Cord Monitoring for Vascular Malformation Surgery in the Medulla Oblongata: Technical Note

OBJECTIVE

Resection of lesions located in the medulla oblongata may result in significant morbidity. The most lethal complications are swallowing disturbances, which can lead to aspiration pneumonia. To prevent this problem, the lower cranial nerves can be mapped with recording needles placed in the posterior pharyngeal wall and the tongue. However, mapping alone is not sufficient to preserve the lower cranial nerves and swallowing functions. To overcome this problem, we attempted to devise a method to intraoperatively monitor vocal cord movements with a laryngoscope. We used this method, in addition to other types of brainstem mapping, in three cases.

METHODS

Recording needles were inserted into the posterior pharyngeal wall and the tongue, to record the responses of Cranial Nerves IX and XII. A laryngoscope was inserted orally, for direct observation of vocal cord movements, and was maintained until the end of the operation. The floor of the fourth ventricle was stimulated with a monopolar stimulator. Somatosensory evoked potentials, auditory evoked potentials, and motor evoked potentials were simultaneously monitored.

RESULTS

We were able to confirm synchronized vocal cord adduction with stimulation of the expected vagal trigonum location and to monitor rhythmic vocal cord movements during spontaneous respiration. In all three cases, we removed the lesions without postoperative complications.

CONCLUSION

In addition to intraoperative vocal cord monitoring with a laryngoscope, we could safely determine the optimal location for the first incision in the floor of the fourth ventricle. Potentially lethal postoperative complications can be avoided with brainstem mapping and vocal cord monitoring.

Gross total surgical removal of malignant glioma from the medulla oblongata: report of two adult cases with reference to surgical anatomy

Surgery was performed on the medulla oblongata of two adult patients with malignant glioma. Gross total resection of the tumors, located laterally or medially in the upper half of the medulla respectively, was achieved. The patient with the medially located tumor experienced significant postoperative neurological deterioration including sleep apnea. The other patient with the laterally located tumor showed symptomatic improvement without respiratory complications. The patient with an anaplastic astrocytoma survived approximately 4 years and the patient with a glioblastoma multiforme approximately 2 years. Although the upper half of the medulla is more critical than the lower half, a lateral approach to the upper half of the medulla appears to be relatively safer than a medial approach. Some cases of focal malignant gliomas in the medulla may be amenable to gross total resection in order to achieve improved outcome. Surgery can be undertaken when a tumor is unilateral and its margin appears relatively clear on magnetic resonance images.

Surgical management of brain-stem cavernous malformations: report of 137 cases

BACKGROUND

With the improvement in neuroimaging and microsurgical techniques, brain stem cavernous malformations are no longer considered inoperable. Surgical indications for brainstem cavernoma are evolving, with better understanding of its natural history and decreasing surgical complications.

METHODS

During 1986 through 1998, a series of 137 patients (4 patients each with two brain stem lesions, total number of lesions, 141) with brain stem cavernous malformations were treated microsurgically at Beijing Neurosurgery Institute. The age distribution, lesion location, and clinical presentations were analyzed. The bleeding rate, surgical indications and microsurgical techniques were also discussed.

RESULTS

In our series, 92 of 137 cases (67.2%) suffered more than one hemorrhage. Female patients had a higher risk of recurrent hemorrhage than that of male patients. Unlike cavernomas malformations from other locations, repeated hemorrhages from brain stem malformations are much more common and usually lead to new neurologic deficits. Among all 137 surgically treated patients, there was no operative mortality. Ninety-nine patients (72.3%) either improved or remained clinically stable postoperatively. The size of the cavernoma/hematoma does not necessarily correlate with the surgical result. While the acute hematoma can facilitate the surgical dissection, longer clinical history with multiple hemorrhages often makes total surgical resection difficult, partially because of the firmer capsule that may not shrink or collapse after hematoma is released. Pathologically those capsules were associated with more hyaline degeneration, fibrous proliferation and even calcifications. During the follow-up period between 0.5 to 11 years in 129 cases, 115 patients (89.2%) have been working, studying, or doing house work. Three patients (2.3%) suffered recurrent hemorrhages.

CONCLUSIONS

Surgical indications of brain stem cavernoma include (1) progressive neurologic deficits; (2) overt acute or subacute hemorrhage on MRI either inside or outside cavernous malformations with mass effect; (3) cavernoma/hematoma reaching brainstem surface (<2 mm brain tissue between cavernoma /hematoma and pial surface). Grave clinical presentations like coma, respiratory, or cardiac instability are not surgical contraindications. Emergent surgical evacuation may lead to satisfactory outcome. Repeated hemorrhages will worsen the pre-existing neurologic deficits and possibly make the surgical dissections more difficult. Patients with minimum, stable neurologic deficits and lesion/hematoma that has not reached the brain stem surface should be followed conservatively.

Preoperative Evaluation of Neural Tracts by Use of Three-dimensional Anisotropy Contrast Imaging in a Patient with Brainstem Cavernous Angioma: Technical Case Report

OBJECTIVE AND IMPORTANCE

We describe a case of brainstem cavernous angioma in which the neural tracts were evaluated before surgery by three-dimensional anisotropy contrast (3-DAC) magnetic resonance imaging.

CLINICAL PRESENTATION

A 64-year-old man presented with a cavernous angioma located intrinsically in the brainstem and manifesting as gait ataxia. 3-DAC imaging demonstrated that the lesion was located outside the left inferior cerebellar peduncle and inside the middle cerebellar peduncle.

INTERVENTION

The intact brain surface was incised, and the lesion was removed successfully on the basis of the preoperative 3-DAC images. The patient exhibited temporary exacerbation of his gait ataxia, but the symptom improved 3 months after surgery. Postoperative 3-DAC imaging demonstrated resection of the lesion and preservation of the left inferior and middle cerebellar peduncles.

CONCLUSION

3-DAC imaging may provide essential information about the neural tracts for the planning of brainstem surgery.

Direct approach to the ventrolateral medulla for cavernous malformation--case report

A 49-year-old man presented with symptomatic cavernous malformation in the ventrolateral portion of the medulla oblongata manifesting as left-sided numbness and gait disturbance. Neurological examination disclosed sensory disturbance on the left, cerebellar ataxia, nystagmus, dysphagia, and right hypoglossal nerve paresis. Magnetic resonance imaging revealed a cavernous malformation with hemorrhage occupying the right paramedian field of the medulla oblongata. The patient underwent complete removal of the lesion through vertical incision of the bulging surface of the ventrolateral medulla, anatomically coinciding with the inferior olive. The neurological deficits improved without additional postoperative deficits. This unusual microsurgical approach through a ventrolateral medullary incision permits direct resection of a subpial intrinsic lesion, even on the ventral medulla.

Arm-down Concorde position: a technical note

BACKGROUND

When the surgeon's approach is from behind the patient's shoulder via the infratentorial supracerebellar approach in the prone position (Concorde position), the patient's shoulder nearest the surgeon occasionally interferes with the visual route and surgical manipulation. To avoid this difficulty the author developed a modified Concorde position.

METHODS

In the prone position, the patient's arm at the surgeon's side hangs down over the head end of the operating table, with elbow flexion supported by an arm-holder.

CONCLUSIONS

This arm-down Concorde position provides good access for the surgeon in muscular- or broad-shouldered, short-necked, or obese patients.

Anterior transpetrosal approach for pontine cavernous angioma--case report

A 58-year-old male patient presented with headache and unsteady gait. Magnetic resonance imaging revealed hemorrhage from a pontine cavernous angioma. The patient experienced stepwise aggravation of symptoms due to repeated hemorrhages. We decided to surgically remove the pontine cavernous angioma through an anterior transpetrosal approach, since the angioma and hematoma were located near the ventrolateral surface of the pons. The brain stem was incised at a site caudal to the trigeminal nerve and the hematoma and angioma were totally removed. No additional neurological deficits were observed following surgery. Brain stem cavernous angiomas are usually removed via a trans-fourth ventricle or lateral suboccipital approach. However, these approaches may not be appropriate if the angioma is located ventrally to the pons. We propose that the anterior transpetrosal approach is the method of choice for ventrally located pontine cavernous angioma.

Presigmoid approach for cavernous angioma in the pons--technical note

Surgical treatment of brainstem lesions has been encouraged after the development of magnetic resonance imaging. However, direct approaches to intra-axial lesions in the brainstem still carry a high risk of morbidity because the neuronal structures can be injured along the entry routes. We present two patients whose pontine cavernous angiomas were removed via incision of the lateral aspect of the pons with presigmoid approach. The first case, a 41-year-old woman, presented with paresis of the cranial nerves VI, VII, and VIII, and left hemiparesis progressing over 2 weeks caused by a cavernous angioma ventrally located in the lower pons. The second case, a 50-year-old woman, developed dizziness over 2 months due to a large cavernous angioma in the center of the pons. These lesions were totally removed through the presigmoid approach and no additional neurological deficits were observed. An image-guided navigation system was used for the craniotomy and removal of the lesion in the second patient. The presigmoid approach provides a safe route to intra-axial lesions in the pons. A technique for presigmoid craniotomy with one-piece bone flap under the image-guided navigation is also described.

Metastatic brainstem tumor manifesting as hearing disturbance--case report

A 53-year-old male, who had undergone a left upper lung lobectomy for cancer 2 years previously, presented with metastatic brainstem tumor manifesting as hearing disturbance. At first an otorhinolaryngologist treated him for senile sensorineural hearing disturbance. However, he suffered gait ataxia and was referred to our department. On admission, neurological examination found mild cerebellar ataxia on the left and gait unsteadiness. Neurootological analysis revealed central-type sensorineural hearing disturbance on the left both in the pure tone audiogram and speech discrimination test. Neuroimaging studies revealed a ring-like enhanced mass centered in the ventral left middle cerebellar peduncle, partly extending to the inferior cerebellar peduncle. Peritumoral edema extending to the ipsilateral cochlear nucleus was recognized. He underwent surgery via a left lateral suboccipital transcondylar approach. The histological diagnosis was adenocarcinoma identical with the primary lung cancer. Intra-axial brainstem metastatic lesion can be a cause of hearing disturbance, so should be included in the differential diagnosis for a patient complaining of hearing disturbance, especially with a past history of cancer.

Transoral transclival removal of anteriorly placed cavernous malformations of the brainstem

BACKGROUND

The natural history of brain stem cavernous malformations is unfavorable because of their high hemorrhage rate and resulting neurological deterioration among patients. However, direct surgery of intrinsic and anteriorly situated cavernomas is hazardous and leads to a bad postoperative outcome because of trauma to lateral and dorsally situated eloquent areas of the brain stem.

METHODS

We review the cases of two patients with symptomatic cavernous malformations of the anterior brain stem and describe the usefulness of a transoral-transclival approach. A 23-year-old man developed progressive hemihypaesthesia and paraesthesia, hemiparesis with gait ataxia, dysarthria, dysphonia, and dysphagia. A 38-year-old woman suffered from an acute onset of vertigo with nausea and vomiting, diplopia, and paraesthesia of the left hand and foot. In both patients, computed tomography demonstrated the presence of brain stem hemorrhage, because of cavernous malformation. Magnetic resonance imaging showed a close proximity of the lesions to the pia mater only on the ventral surface of the brain stem.

RESULTS

In both patients, the cavernomas could be safely approached and completely resected via a transoral transclival route. Three months after surgery, neurological examination revealed marked neurological improvement. The 23-year-old patient showed slight gait ataxia, no hemiparesis, no cranial nerve palsies; the 38-year-old woman demonstrated no neurological symptoms except for minimal motor dysfunction of the left hand. In both cases, under perioperative prophylactic antibiotics, no meningitis was observed. The patients could subsequently return to their previous employment.

CONCLUSION

The transoral transclival approach for ventrally situated brain stem cavernomas allows a largely atraumatic resection of the lesion.

Isocentric head fixation for microscopic neurosurgery

The significance of isocentering in head fixation during micro-neurosurgery is described. The approach angle to the lesion could be changeable without adjusting the focus and placement of the operating microscope by rotating the head holder when the lesion is fixed at the isocenter of the rotatable.

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.

Relationship between the clinical manifestations, computed tomographic findings and the outcome in 80 patients with primary pontine hemorrhage

The relationship between the clinical manifestations, computed tomographic (CT) findings and the outcome in 80 patients with primary pontine hemorrhage (PPH) was analyzed to clarify factors predicting the outcome. Patients were 58 males and 22 females (60. 5+/-12.9 years old) with PPH. Correlations between the clinical manifestations, CT findings and the outcome was assessed with multivariate regression analysis. The initial level of consciousness and the transaxial size of the hematoma on CT were strongly related to the outcome. In patients with small hematoma (with the transaxial size </=20 mm), pupillary abnormalities of onset and acute hydrocephalus were related to the outcome of PPH.

Cavernous angiomas of the brain stem. Intra-axial anatomical pitfalls and surgical strategies

BACKGROUND

We review the surgical anatomy of the brain stem in relation to the surgical approaches adopted for treatment of cavernomas and identify possible "safe entry zones" on the anterior face of the brainstem.

METHODS

Twelve symptomatic patients with cavernoma or telangectasia of the brain stem were surgically treated. The brain stem was divided into the following anatomical areas: ventral medulla, dorsal medulla, dorsal pons, ventral pons, ventral mesencephalon, and dorsal mesencephalon, so that the surgical approach could be "individualized" according to the position of the cavernoma, the nerve fasciculi and nuclei.

RESULTS

On the anterior surface of the brain stem a medullar paramedian oblique access to the anterolateral sulcus and a paramedian sagittal pons access seem to avoid the main nerve fasciculi and nuclei.

CONCLUSIONS

Although the parenchymal window produced by the cavernoma is the most important parameter for the choice of approach, fairly safe entry zones may be identified even on the anterior surface of the medulla and pons.

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.

Transcollicular approach to intrinsic tectal lesions

OBJECTIVE

We used a paramedian, infratentorial-supracerebellar, transcollicular approach to resect 11 intrinsic tectal lesions, including 8 tumors and 3 hematomas, in 11 patients. The route of access to the lesions was designed to minimize the anatomic and functional damage to the surrounding structures.

METHODS

Access was through one superior colliculus in each of seven patients, through one inferior colliculus in each of two patients, and through the superior and inferior colliculi of one side in each of two patients.

RESULTS

Of the eight tumors, three were totally resected, four were nearly totally resected, and one was partially resected. The preoperative ocular symptoms did not change in six of these eight patients and worsened in two, and the neurological deficits, except ocular symptoms, improved in two. All three hematomas were completely removed, along with abnormal blood vessels in the wall of the hematoma cavity; all three of these patients experienced neurological improvement.

CONCLUSION

We conclude that the paramedian, infratentorial-supracerebellar, transcollicular approach permits safe removal of intrinsic tectal lesions. Resection of the superior or inferior colliculus or both on one side seems to be neurologically well tolerated.

Cavernous malformations of the brainstem: experience with 100 patients

OBJECT

In this study the authors review surgical experience with cavernous malformations of the brainstem (CMBs) in an attempt to define more clearly the natural history, indications, and risks of surgical management of these lesions.

METHODS

The authors retrospectively reviewed the cases of 100 patients (38 males and 62 females; mean age 37 years) harboring 103 lesions at treated a single institution between 1984 and 1997. Clinical histories, radiographs, pathology records, and operative reports were evaluated. The brainstem lesions were distributed as follows: pons in 39 patients, medulla in 16, midbrain in 16, pontomesencephalic junction in 15, pontomedullary junction in 10, midbrain-hypothalamus/thalamus region in two patients, and more than two brainstem levels in five. The retrospective annual hemorrhage rate was most conservatively estimated at 5% per lesion per year. Standard skull base approaches were used to resect lesions in 86 of the 100 patients. Intraoperatively, all 86 patients were found to have a venous anomaly in association with the CMB. Follow up was available in 98% (84 of 86) of the surgical patients. Of these, 73 (87%) were the same or better after surgical intervention, eight (10%) were worse, and three (4%) died. Two surgical patients were lost to follow-up review. Incidences of permanent or severe morbidity occurred in 10 (12%) of the surgically treated patients. The average postoperative Glasgow Outcome Scale score for surgically treated patients was 4.5, with a mean follow-up period of 35 months.

CONCLUSIONS

The natural history of CMBs is worse than that of cavernous malformations in other locations. These CMBs can be resected using skull base approaches, which should be considered in patients with symptomatic hemorrhage who harbor lesions that approach the pial surface. Venous anomalies are always associated with CMBs and must be preserved.

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

In 1993 Kyoshima et al. introduced safe entry zones in the region of the 4th ventricle floor: infrafacial triangle and suprafacial triangle. Is it possible to demarcate these zones precisely in every case intra-operatively? A postmortem study of 40 brainstems of patients who had died of non-brain disease was performed to evaluate the degree of individual morphological and morphometrical variability of the 4th ventricle floor. The purpose of this study was to find constant landmarks and distances within the rhomboid fossa region which would help a neurosurgeon to determine safe approach zones through the 4th ventricle floor to brainstem lesions. Several anatomical landmarks-median sulcus, obex, vestibular area, vagal triangle, hypoglossal triangle-were found to be sufficiently visible in all examined brainstems. However, the facial colliculus which is a border structure between the infrafacial and suprafacial safe approach zone was poorly visible in about 37% of the analyzed material. The striae medullares were not found to be good orientation structures as they were not visible in 30% of the material and exhibited individual variability of a high degree in relation to their number and arrangement. In the morphometrical study analyzed measurements were taken by utilizing the digital image analyzer MULTISCAN. Based on the results obtained the authors suggest new borders of the infrafacial safe approach zone and morphometrical directions to determine the suprafacial safe approach zone in cases when the facial colliculus is not clearly visible or invisible intra-operatively.

Pericollicular surgical approaches to the rhomboid fossa. Part I. Anatomical basis

A safe paramedian approach to the rhomboid fossa for surgical treatment of intrinsic brainstem lesions is based on detailed knowledge of the morphometric anatomy of superficially located motor structures. The morphometry of the rhomboid fossa is described in this report on the basis of histological studies conducted in six human brainstem specimens, with special emphasis on the colliculus facialis and the trigona nervi hypoglossi and vagi. Morphometric data include analysis of shrinkage factors in each specimen. The colliculus is a landmark for the nervus facialis, oculomotor nuclei, and the paramedian pontine reticular formation. In the surgeon's view from the posterior approach, the colliculus covers an area of 5.7 mm in the mediolateral and 6.8 mm in the craniocaudal direction and is located 0.6 mm lateral to the median sulcus. The fibers of the nervus facialis come as close as 0.2 mm to the surface of the fourth ventricle. The colliculus is located 15.7 mm above the obex. The trigona nervi hypoglossi and vagi cover a rectangular area measuring 3.1 by 6.5 mm and serve as a landmark for lower cranial nerve nuclei. These nuclei are located 0.3 mm lateral to the midline. An area with a maximum extension of 0.9 cm between the colliculus and trigona can be used for an infracollicular paramedian approach. The same applies to a supracollicular approach cranial to the colliculus and caudal to the fibers of the nervus trochlearis within the medullary velum, with a craniocaudal extension of 4 mm. Superficial motor nuclei and fibers can be identified by neurophysiological mapping, which helps to define safe surgical corridors into the rhomboid fossa, thus reducing functional morbidity caused by the operative approach in intrinsic pontine and pontomedullary lesions.