Operative planning aid for optimal endoscopic third ventriculostomy entry points in pediatric cases

Abstract

Object: Endoscopic third ventriculostomy (ETV) uses anatomical spaces of the ventricular system to reach the third ventricle floor and create an alternative pathway for cerebrospinal fluid flow. Optimal ETV trajectories have been previously proposed in the literature, designed to grant access to the third ventricle floor without a displacement of eloquent periventricular structures. However, in hydrocephalus, there is a significant variability to the configuration of the ventricular system, implying that the optimal ETV trajectory and cranial entry point needs to be planned on a case-by-case basis. In the current study, we created a mathematical model, which tailors the optimal ETV entry point to the individual case by incorporating the ventricle dimensions.

Methods: We retrospectively reviewed the imaging of 30 consecutive pediatric patients with varying degrees of ventriculomegaly. Three dimensional radioanatomical models were created using preoperative MRI scans to simulate the optimal ETV trajectory and entry point for each case. The surface location of cranial entry points for individual ETV trajectories was recorded as Cartesian coordinates centered at Bregma. The distance from the Bregma in the coronal plane represented as "x", and the distance from the coronal suture in the sagittal plane represented as "y". The correlation between the ventricle dimensions and the x, y coordinates were tested using linear regression models.

Results: The distance of the optimal ETV entry point from the Bregma in the coronal plane ("x") and from the coronal suture in the sagittal plane ("y") correlated well with the frontal horn ratio (FHR). The coordinates for x and y were fitted along the following linear equations: x = 85.8 FHR-13.3 (r ² = 0.84, p < 0.001) and y = -69.6 FHR + 16.7 (r ² = 0.83, p < 0.001).

Conclusion: The surface location of the optimal cranial ETV entry point correlates well with the ventricle size. We provide the first model that can be used as a surgical planning aid for a case specific ETV entry site with the incorporation of the ventricle size.

Keywords: Endoscopic third ventriculostomy; Optimal ETV trajectory; Pediatric hydrocephalus.

Fig. 1

Surgical anatomy of the third ventriculostomy and the concept of optimal ETV trajectory. A: schematic showing optimal ETV trajectory (interrupted line) aligned along the foramen of Monro and the third ventricle floor. B and C: diagram depicting the operative view and eloquent periventricular structures at the level of the foramen of Monro (B) and the third ventricle floor (C). Panel B: eloquent structures are highlighted as follows, brown: fornix; yellow: caudate nucleus; green: genu of internal capsule; red: thalamus. Panel C: yellow: chiasm; red: basilar artery termination; green: hypothalamus. Interrupted circle: fenestration site

Fig. 2

Radioanatomical analysis of optimal ETV trajectories. Panel A: coronal view of the optimal ETV trajectory (yellow line) connecting the foramen of Monro and the floor of the third ventricle. Arrow indicates the midline. Insert: measurement of ETV entry point (yellow target) distance from Bregma in the coronal plane (“x”). Panel B: sagittal view of the optimal ETV trajectory depicted in “A”. Arrows indicate the coronal suture. Insert: distance of the ETV entry point (“y”) form the coronal suture (yellow arrow). Panel C: surgical view of a three-dimensional model created for the case shown in A and B. Optimal ETV trajectory (yellow bar) and entry point (yellow dot) correspond to case in “A” and “B”. Arrows mark the coronal suture. Inset: “x” and “y” represent entry point distance from the Bregma and coronal suture, respectively. Panel D: frontal horn ratio expressed as the distance between the frontal horns (“a”) and the internal diameter of the skull along the same line (“b”)

Fig. 3

Optimal entry points are plotted in a Cartesian coordinate system with analysis of linear regression. Scatter plot of optimal entry point distances from Bregma in the midline along the coronal plane (A) and from the coronal suture in the sagittal plane (B) plotted against the frontal horn ratio (FHR). Note the linear correlation between the frontal horn ratio and both “x” and “y” variables. Panel C: labeling in the formulas follow Cartesian coordinate system reconstituting the skull surface with “0” indicating Bregma, “x” and “y” axis representing distances in the coronal and sagittal plane, respectively. Note the linear distribution of optimal entry points in a posterolateral direction with an increasing ventricle size. Linear equations with r ² values are inserted in each graph; variable notations follow those in Fig. 2