Background: Colloid cysts are benign lesions that can obstruct the flow of CSF and in rare cases predispose to sudden death. Treatment strategy for large or symptomatic colloid cysts in the setting of a non-dilated ventricular system is controversial. Historically, the interhemispheric transcallosal corridor has been favored although transcortical/transventricular access for endoscopic resection has also been advocated.
Recent introduction of expandable minimally-invasive tubular retractors has facilitated microscopic minimally-invasive resection of symptomatic colloid cysts, even in the setting of a non-dilated ventricular system. While optimal positioning of this form of retractor to provide maximal visualization of the roof of the third ventricle is likely critical, there is a paucity of information available in the literature.
Methods: The authors performed morphometric analysis to assess visualization provided by (1) Kocher’s point entry and (2) anterolateral site of entry in four cadaveric specimens. Neuronavigation-protocol CT imaging was obtained in each specimen. Ventricular access was obtained using entry sites (1 + 2). Photographs obtained were used to qualitatively assess (A) visualization of roof of the third ventricle and (B) assess for impediments in visualization. Subsequently using 3-D software, a thresholded model of the ventricular system was used to identify an “ideal” point of entry that could visualize the roof of the third ventricle and a simulated cyst wall attachment without the caudate head obscuring the foramen of Monro.
Results: The average angle of the pre-defined trajectories in the coronal plane relative to midline were 24 and 46 degrees for the Kocher’s and anterolateral points respectively. In the sagittal plane, the average angle of the pre-defined trajectories relative to the coronal plane at the origin of the foramen of Monro at the 3rd ventricle were 25 and 60 degrees for Kocher’s and anterolateral points, respectively. Kocher’s point permitted visualization of the foramen of Monro through all 8 entry points. Line of sight to the foramen of Monro was obstructed by the caudate nucleus in 5 out of the 8 (63%) anterolateral entry points. Using 3-D simulation, optimal entry sites for the specimens were found to range between 13 mm and 56 mm anterior to the coronal suture and between 12 mm and 35 mm from midline.
Conclusions: Minimally-invasive tubular retractors allow working angles that facilitate bimanual instrument access to the anterior roof of the third ventricle. The entry points into the lateral ventricle and trajectory of the tubular retractor are consequential for the visualization and mobilization of colloid cyst attachments from the anterior roof of the third ventricle. The caudate head obscured the view of the foramen of Monro in most of the studied anterolateral entry points. Anterolateral points of entry provide greater visualization of the roof of the third ventricle at the expense of partial obstruction of view in non-dilated ventricular systems secondary to the caudate head. In this population we recommend an entry as anterior as possible, coplanar with the mid-pupillary line.
