Clinical Article Anatomy of the cerebral ventricular system for endoscopic neurosurgery: a magnetic resonance study

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1 Acta Neurochir (2003) 145: DOI /s Clinical Article Anatomy of the cerebral ventricular system for endoscopic neurosurgery: a magnetic resonance study F. Duffner 1, H. Schiffbauer 1, D. Glemser 1, M. Skalej 2, and D. Freudenstein 1 1 Department of Neurosurgery, Eberhard-Karls-University and University Hospital T ubingen, Germany 2 Department of Neuroradiology, Eberhard-Karls-University and University Hospital T ubingen, Germany Published online June 4, 2003 # Springer-Verlag 2003 Summary Background. Endoscopy has developed into an integral part of minimally invasive neurosurgery. For further technological innovations, detailed knowledge about the pathological anatomy is essential. The gross anatomy of the cerebral ventricular system has been meticulously investigated with ventriculography and casts. Extensive volumetric measurements based on neuroradiological images have been performed, but only little is known about the surgically relevant linear distances in patients with hydrocephalus. Method. Thirty healthy volunteers and thirty patients suffering from hydrocephalus were scanned with high-resolution 3-D magnetic resonance imaging sequences. The image volumes were sliced identically with the help of Siemens Prominence + software. Individual anatomical measurements of the ventricular system were carried out, mean values and standard deviations were calculated, and different endoscopic approaches were investigated. Findings. In healthy volunteers the measurements confirmed the results obtained from ventriculography and anatomic casts. In hydrocephalic patients the ventricular system was found to be enlarged asymmetrically. The optimal neuroendoscopic approach showed considerable, interindividual variation. Interpretation. This 3-D magnetic resonance imaging study revealed surgically and clinically relevant aspects of the pathologic anatomy of hydrocephalic patients, in comparison to healthy volunteers. Individualized planning of the endoscopic approach appears to be warranted. Finally, the data provided a sound basis for the further development of neuroendoscopes. Keywords: Magnetic resonance imaging; hydrocephalus; neuroendoscopy; neuroanatomy; ventricular size; biomedical engineering. Introduction Endoscopy is an integral part of the spectrum of minimally invasive neurosurgery. Technological innovations, such as the development of small endoscopes and associated instruments, adapted to the nervous system, have improved the safety and efficiency of endoscopic neurosurgery. For further development of specific instruments, exact knowledge of the pathological anatomy is required. Magnetic resonance (MR) imaging allows acquisition of 3-D volume data and measurement of linear distances in any chosen image plane, using recent software. The possibilities offered by the modern imaging methods, new operative approaches and minimally invasive techniques motivated us to revisit the anatomy of the ventricular system and to re-measure the distances in individual volunteers and hydrocephalic patients. The purposes of this study were to confirm, in normal volunteers, the measurements achieved with different previous techniques, including anatomical casts and ventriculography, and to compare these distances to those of patients with hydrocephalus [1, 3, 9 12, 20]. In view of the relevance to neuroendoscopic operations, our goal was to evaluate anatomical peculiarities in patients with hydrocephalus, to derive the maximal diameter and the optimal length of neuroendoscopes, and to examine the usefulness of standardized endoscopic approaches. Patients and methods Patients and volunteers Thirty patients with hydrocephalus, 17 males and 13 females, were included into the study: 22 had occlusive hydrocephalus due to a stenosis of the cerebral aqueduct, and 8 had communicating hydrocephalus. They ranged in age between 20 and 78 years with a mean age of 54.6 years. Thirty healthy volunteers, 15 males and 15 females, ranging in age between 20 and 71 years with a mean age of 33.5 years, were recruited

2 360 F. Duffner et al. for comparison. All research procedures were performed in accordance with the approval of the Ethics Committee of the Medical Faculty at the University of T ubingen. Imaging technique and image analysis The patients and volunteers were scanned on two different clinical MR scanners (Magnetom Expert [1.0 T] or Magnetom Vision [1.5 T]; Siemens, Erlangen, Germany). For each subject 180 contiguous, sagittal 3-D FLASH images (TR 20 ms, TE 5 ms, flip angle 30,matrix ) were acquired with a slice thickness of 1.3 mm. The images were analysed with Prominence + software (Siemens, Erlangen, Germany). This enabled first, visualization of the axial, coronal and sagittal planes through any free chosen point in the image volume, second, definition of oblique planes displaying two structures of interest at a time and, third, measurement of angles and distances within any of these planes. Fig. 1. The lateral ventricles: their length, height, width, and distance from the cortex. (a) Length of the lateral ventricle and of the posterior horn (m. 1 & 2). (b) Height of the frontal horn measured 3 cm rostrally of the coronal suture (m. 3). (c) Thickness of the cortex and the subcortical white matter measured as a distance from the lateral ventricle to the brain surface (m. 4). (d) Distance of the most lateral points of the lateral ventricles on a coronal plane which includes the interventricular foramina (m. 7)

3 Anatomy of the cerebral ventricular system 361 To achieve equivalent measurement conditions for all patients and volunteers, the image volume was centred in such a way that on a median sagittal image there was an angle of 15 degrees between a line touching the anterior and posterior commissures and a horizontal line. While looking at a coronal plane through the mamillary bodies, the image volume was then rotated to the right or left in such a way that a line through the midpoint between the mamillary bodies, the third ventricle and the interventricular adhesion was displayed vertically. While looking at an axial plane through the mamillary bodies, the image volume was rotated in such a way that a line cutting through the midpoint between the mamillary bodies and the cerebral aqueduct was displayed vertically. Measurements of the lateral ventricles The total length of the lateral ventricle (measurement 1, Fig. 1a) and the length of the posterior horn (m. 2, Fig. 1a) were measured in an oblique plane, which displayed the most rostral point of the frontal horn Fig. 2. The third ventricle: its width and height (a) Width of the third ventricle measured on a coronal plane which includes the interventricular foramina (m. 10). (b) Height of the third ventricle measured on the median sagittal plane through the midst of the interthalamic adhesion (m. 11) Fig. 3. Measurements of the size of the interventricular foramen. (a) Coronal image of the interventricular foramen, where it was seen best; on this image the smallest diameter was measured. (b) Oblique image for the measurement of the anterior posterior diameter (m. 13)

4 362 F. Duffner et al. and the most occipital point of the posterior horn. The height of the frontal horn was measured 3 cm rostral to the coronal suture (m. 3, Fig. 1b). The distance between the lateral ventricle and the brain surface 2 cm lateral of the midline (m. 4, Fig. 1c) was measured in an oblique plane, including a point 1 cm posterior to the coronal suture and cutting the surface of the corpus callosum perpendicularly. Measurement 5 was the distance between the most rostral points of the frontal horns. In a coronal plane, which included the interventricular foramina, we measured the width of the septum pellucidum 1 cm below the upper surface of the corpus callosum (m. 6), and the distance of the most lateral points of the lateral ventricles (m. 7, Fig. 1d). The maximal width (m. 8) and height (m. 9) of the ventricular system conclude this first set of measurements. Measurements of the third and fourth ventricles, and the cerebral aqueduct In a coronal plane, which included the interventricular foramina, we measured the width (m. 10, Fig. 2a) and height (m. 11, Fig. 2b) of the third ventricle. Measurement 12 was the distance between the centres of the anterior and posterior commissures on a mid-sagittal plane. Figure 3 displays the measurements of the size of the interventricular foramen of Monro (m. 13). The distance (m. 14) between a point in the middle between the interventricular foramina and the entrance of the cerebral aqueduct at the rostro-caudale surface of the posterior commissure was measured. Measurement 15 was the distance between the entrance of the cerebral aqueduct at the rostro-caudal surface of the posterior commissure and the upper surface of the optic chiasm. On a median sagittal plane, the size of the interthalamic adhesion was quantified in anterior posterior and cranio-caudal direction (m. 16). For description of the cerebral aqueduct, its length from the entrance at the rostro-caudal surface of the posterior commissure to the lower surface of the inferior colliculi was measured, as well as its diameter half way between its entrance and caudal end (m. 17). The height and depth of the interpeduncular fossa (m. 18) and the height of the fourth ventricle, as displayed on the median sagittal plane from the lower edge of the caudal colliculus Fig. 4. The fourth ventricle: its height and anterior posterior expansion (m. 19 & 20) to the obex (m. 19, Fig. 4) and the antero-posterior width of the fourth ventricle at the level of the fastigium (m. 20, Fig. 4) were measured. Distances of possible approaches to the cerebello-pontine angle and to the ventricular system First, a left retromastoid approach to the trigeminal nerve in the cerebello-pontine angle was planned (Fig. 5). The trigeminal nerve and the external auditory canal were visualized on an oblique plane. Three cen- Fig. 5. Left retromastoidal approach to the trigeminal nerve. (a) Coronal plane displaying the trigeminal nerve and the external auditory canal used for the definition of an oblique plane. (b) Oblique plane showing the left retromastoidal approach to the trigeminal nerve 3 cm posterior to the anterior surface of the external auditory canal: length of the approach (m. 21)

5 Anatomy of the cerebral ventricular system 363 Fig. 6. This figure demonstrates how the position of an optimally located, individualized burr hole for third ventriculostomy, in a first step, in anterior posterior direction in relation to the coronal suture (a), and in a second step, in relation to the midline in lateral direction (b) is determined. The line on the median sagittal plane in Fig. 6a indicates the position of an oblique coronal plane. This oblique plane is chosen to include a point on the floor of the third ventricle in the middle between mamillary body and infundibular recessus, and is then tilted in anterior posterior direction to include the longest possible connection between lateral ventricle and third ventricle (b). The diagonal line on image B shows the final approach, which, in this specific case, included a burr hole posterior to the coronal suture Fig. 7. Position of an optimally located burr hole (m. 25 & 26) in order to reach the cerebral aqueduct through one of the interventricular foramen. (a) Median sagittal plane for the definition of an oblique axial plane which includes the entrance of the cerebral aqueduct at the rostro-caudale surface of the posterior commissure. (b) Oblique axial plane displaying as a diagonal line (m. 27) the longest possible interventricular passage through the frontal horn of the lateral ventricle and the interventricular foramen timetres posterior to the anterior surface of the external auditory canal a point of entrance was defined, and distance 21 was measured between this point and the trigeminal nerve using a line touching the cerebellum. Second, an optimal burr hole position for third ventriculostomy was defined as displayed on Fig. 6(a, b) and as explained in the relating figure legend. The distances from the centre of the burr hole to the

6 364 F. Duffner et al. coronal suture (m. 22), to the midline (m. 23) and to the floor of the third ventricle (m. 24) were calculated. Third, the location of an optimally located burr hole for an individualized endoscopic approach to the cerebral aqueduct through one of the interventricular foramen was determined. The position of an oblique axial plane was defined on the median sagittal plane (Fig. 7a). It was chosen to include the entrance of the cerebral aqueduct at the rostrocaudale surface of the posterior commissure, and the longest possible interventricular passage through the frontal horn of the lateral ventricle and the interventricular foramen. The position of the burr hole in relation to the nasion was measured on the sagittal plane (m. 25, Fig. 7a) and in relation to the midline on the oblique axial plane (m. 26, Fig. 7b). Finally, the distance between the burr hole and the entrance of the cerebral aqueduct at the rostro-caudale surface of the posterior commissure was measured (m. 27, Fig. 7b). Results Ventricular sizes The measured linear distances are summarized in Table 1: mean, minimal and maximal values and standard deviations are given for both groups. Measurements 1 9 relate to the lateral ventricles and their relationship to the surrounding brain. The third ventricle and the cerebral aqueduct were subject of measurements Results for the fourth ventricle are given in measurements The results were subjected to t-testing. All cases revealed a statistically significant difference between the patient and the control groups (p< 0.005). Of specific interest is the comparison of the overall sizes of the lateral ventricles and the third ventricle. The height of the lateral ventricles of the hydrocephalic patients (range: mm; mean: 40.2 mm) was 2.25 times that of the volunteers (range mm; mean: 17.9 mm). The distance between the most lateral points of the lateral ventricles on a coronal plane through the interventricular foramina was for the hydrocephalic patients (range: mm; mean: 57.7 mm) 2.08 times that of the volunteers (range: mm; mean: 28.6 mm). The average height of the third ventricle in hydrocephalic patients (16 mm) was only 0.86 times of that for the volunteers (18.6 mm), but its width was 4.39 times larger in the patients (14.5 mm) than in the volunteers (3.3 mm). Corresponding with the latter finding, the mean distance between anterior and posterior commissures was 1.19 times longer in patients (31.6 mm) than in volunteers (26.6 mm). In hydrocephalic patients, the mean size of the interventricular foramina was about 20 times the size in normal individuals. In 24 out of 30 patients the size of the foramen of Monro was larger than 5 mm5mm. In summary, assuming that the ventricles of the patients with hydrocephalus had been of the same size as in the volunteers before they changed, the lateral ventricles of the hydrocephalic patients had enlarged more in height than in width, whereas the opposite was found for the third ventricle. For bilateral structures, all measurements were carried out separately for the right and the left side. In measurements 1 and 2, the left lateral ventricle was found to be significantly (p<0.05) larger than the right, and correspondingly the distance between the lateral ventricle and the brain surface (m. 4) was found to be larger on the right than on the left side. Thickness of the cerebral cortex The brain thickness, measured as the distance between the ventricular system and the cortical surface (m. 4), varied between 34.5 mm and 45.8 mm in volunteers. It was reduced to between 5.4 mm to 34.6 mm in hydrocephalic patients. The brain tissue appeared to be compressed on average by 37 percent. Some hydrocephalic patients even had a residual brain thickness of less than 15 mm, a critical threshold for neuropsychologic functioning in adults. Anterior posterior commissure line In the volunteers, the distance between the anterior (AC) and posterior (PC) commissures (m. 12) varied between 23.8 mm and 30.5 mm (mean 26.6 mm), and in the group of hydrocephalic patients between 23.3 mm and 49.9 mm (mean 31.6 mm). These findings are 1.5 to 2 mm larger than those measured on ventriculograms [17] because we measured the distance between the midpoints of the anterior and posterior commissure, and not from the inside surface. Interestingly, the mean value of the AC PC distance of the hydrocephalic patients is substantially higher than that of the volunteers. Approaches to the floor of the third ventricle, to the cerebral aqueduct, and to the trigeminal nerve The position of an optimally located burr hole for endoscopic third ventriculostomy was found to be very different in different patients. In an anterior posterior direction, it varied between 16.1 mm in front of and 46.5 mm behind the coronal suture, the mean value being 8.2 mm posterior to the coronal suture. The distance to the midline varied between 18.1 mm and 44.4 mm, with a mean value of 30 mm. The distance from the burr hole to the floor of the third ventricle was between 77.2 mm and mm, and a mean of 92.8 mm. In all patients it was possible to define a straight line from the burr hole to the floor of the third ventricle without

7 Anatomy of the cerebral ventricular system 365 Table 1. Results of anatomic measurements in millimeter Measured distance Volunteers Volunteers Volunteers Hydrocephalic Patients Patients mean values range standard patients range standard (minimal maximal) deviation mean values (minimal maximal) deviation 1. Total length of the lateral ventricle Length of the posterior horn Height of lateral ventricle Middle of lateral ventricle cortex Frontal horn frontal horn Septum pellucidum Lateral ventricle lateral ventricle Maximal width of ventricular system Maximal height of ventricular system Width of third ventricle Height of third ventricle AC PC Diameter of interventricular foramen ( ) ( ) 0.5= ( ) ( ) 4.3= Interventricular foramen cerebral aqueduct Optic chiasm cerebral aqueduct Interthalamic adhesion ( ) ( ) 3.5= ( ) ( ) 2.0= Length of cerebral aqueduct diameter ( ) ( ) 1.2= ( ) ( ) 2.2= Interpeduncular fossa ( ) ( ) 0.9= ( ) ( ) 1.1= Height of fourth ventricle Width of fourth ventricle

8 366 F. Duffner et al. Table 2. Results of measurements of potential endoscopic approaches Measurement Mean Minimum Maximum Left retromastoidal approach to trigeminal nerve: 21. Distance from left retromastoidal burr hole to trigeminal nerve Third ventriculostomy: 22. Location of burr hole occipital to coronal suture þ Location of burr hole lateral to midline Distance from burr hole to floor of third ventricle Frontal approach to cerebral aqueduct through lateral ventricle: 25. Distance of burr hole from nasion Location of burr hole lateral to midline Distance from burr hole to cerebral aqueduct touching the fornix or the interthalamic adhesion. Such a line could not be defined in any of the volunteers. The position of an optimally located burr hole for a frontal endoscopic approach through one of the interventricular foramen to the entrance of the cerebral aqueduct also showed large inter-individual differences. It had to be placed between 20.0 mm and 70.0 mm above the nasion, with a mean value of 48.8, and between 11.6 mm and 32.2 mm lateral of the midline, with a mean value of 21.4 mm. The distance between the burr hole and the entrance of the cerebral aqueduct was found to vary between 75.3 mm and mm. The mean value was mm. It also was found that in 19 out of 28 patients (67.9%) it was possible to pass a straight endoscope through the frontal horn of the lateral ventricle, the interventricular foramen and the third ventricle to the cerebral aqueduct without hitting any relevant structure, such as the interthalamic adhesion. To reach the trigeminal nerve from a left retromastoidal burr hole (m. 21) the length of a neuroendoscope and of related instruments is required to cover a maximal distance of 60.4 mm. The measured linear distances for approaches to the floor of the third ventricle, to the cerebral aqueduct, and to the trigeminal nerve are summarized in Table 2. Discussion Anatomical aspects To the best of our knowledge, this is the first publication to present linear distance measurement results of healthy volunteers and hydrocephalic adult patients achieved with this 3-D magnetic resonance imaging method. We analysed the data with special respect to image-guided neurosurgical techniques used for access to the ventricular system in neuroendoscopic operations. For the group of healthy volunteers, our results were astoundingly similar to those achieved by measurements on casts and ventriculograms [1, 3, 9 12, 20]. This is specifically true for the total length, width and height of the lateral ventricles, and their distances to the cortex and the midline. The values for the third ventricle were also confirmed, too. We were also able to substantiate that the size of the left lateral ventricle was significantly larger than that of the right. Schwartz and co-authors reported on changes in lateral and third ventricular volumes by over 30% after third ventriculostomy in 16 patients [19]. Thus, the differences between the groups of hydrocephalic patients and healthy volunteers in our study can also be attributed mainly to the disease process. However, a confounding factor affecting all measurements is the significant difference in age between the two study groups. The distance between the anterior and posterior commissures is important in deep brain stimulation in patients with Parkinson s disease. Our study showed a similar mean value, but also a significant variance even in healthy volunteers, similar to previous findings [4, 11]. This again questions the use of target points based solely on ventriculography and standardised distances from the AC PC line. Neuroendoscopic aspects Endoscopic third ventriculostomy is an appropriate, state-of-the-art treatment for non-communicating hydrocephalus. The success rate in experienced hands is about 75% [6], with the remaining 25% still requiring subsequent shunting.

9 Anatomy of the cerebral ventricular system 367 Our findings suggest that for endoscopic third ventriculostomy in patients with non-communicating hydrocephalus, a rigid neurosurgical endoscope should not exceed a diameter of 5 mm and should be longer than 12 cm to allow access to the floor of the third ventricle. The same endoscope would be suitable for a frontal approach through the lateral ventricle, Monro s foramen and the third ventricle to the entrance of the cerebral aqueduct. The interthalamic adhesion was much smaller in hydrocephalic patients than in normal individuals, and therefore, did not hinder access through Monro s foramen to the floor of the third ventricle in patients with hydrocephalus. In none of our patients was it a problem to define a straight line from an optimally placed burr hole to the intended place for third ventriculostomy. As we found that such a straight line could not be defined in any of the volunteers, neuroendoscopic interventions on patients with non-enlarged ventricles, but, e.g. small intraventricular tumors, might prove difficult. In a retrospective study, Kanner et al. reported on 31 endoscopic third ventriculostomies [7]. For 17 patients with normal anatomic findings, they concluded that an optimal burr hole should be positioned 3 cm lateral to the midline and 1 cm anterior to the coronal suture. Our data suggest that the interindividual anatomical variability in hydrocephalic patients requiring this type of intervention is considerable. The mean location of the optimal burr hole was 8.2 mm posterior to the coronal suture, which was almost 2 cm posterior to the finding of Kanner. However, the mean distance to the midline was the same. Therefore, in each patient MR imaging is advisable as a base for positioning of an optimal burr hole. For preoperative three-dimensional planning and intraoperative orientation, we and other investigators use a neuronavigation system [15, 18]. It assists achievement of the optimal trajectory for the rigid endoscope through the interventricular foramen, avoiding damage to the fornix and makes it easier to localize structures inferior to the floor of the third ventricle [14]. Threedimensional MR image data sets can be used also for virtual reality simulation of a procedure [5, 8] and for the design of instruments for minimally invasive neurosurgery [16]. The combination of neuroendoscopes with intraoperative MR imaging can be expected to lead to further possibilities in minimally invasive neurosurgery, widening its indications. Neuroendoscopic surgery may include approaches that take even more advantage of the cisterns and sulci, the natural spaces surrounding the brain. Vascular decompression of cranial nerves in the cerebello-pontine angle and surgery for tumors of the pineal gland are two examples. We measured the distance from a left retromastoidal burr hole to the trigeminal nerve according to the idea of endoscopic vascular decompression for trigeminal neuralgia as described by Oppel and Mulch [13]. Conclusions This three-dimensional high-resolution MR imaging study revealed surgically and clinically relevant aspects of the pathologic anatomy of hydrocephalus. In a majority of our hydrocephalic patients, a safe neuroendoscopic approach through the interventricular foramen to the floor of the third ventricle was achievable for third ventriculostomy using an individualized approach. The diameter of a neurosurgical endoscope for this type of intervention should not exceed 5 mm, and its length should be at least 12 cm. The data reported should improve the development of instrumentation for endoscopic neurosurgery. 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