The cerebral aqueduct compliance: a simple morphometric model

Background and objectives: This work aimed to identify different configurations of the adytum of the cerebral aqueduct suggesting its safe neuroendoscopic navigation. This concept is intimately connected to the physiological aqueductal dilatability or compliance, which is relatively ignored in the literature. A better knowledge of the extent of physiological aqueductal dilatability might better define the ideal diameter and safer features of dedicated flexible endoscopes. Methods: The study includes 45 patients operated on using a flexible scope with a 3.9-mm diameter, where the structural elements of the adytum of the cerebral aqueduct are clearly visible. Patients were grouped according to the pathology (colloid cyst/normal anatomy, intraventricular hemorrhage, tetraventricular obstructive hydrocephalus, normal pressure hydrocephalus, and distal membranous aqueductal stenosis). A simple geometrical scheme was applied to the endoscopic anatomy of the aqueductal adytum in relation to the posterior commissure to measure its pathologic deformations. Eventual damages to the aqueduct walls caused by the endoscope were also reported. Results: Proceeding from normal anatomy to hydrocephalic condition, the ratio between the commissure and the aqueductal access area progressively decreases, while the vertex angle increases. Interestingly, the entity of the ependymal damages due to the passage of the endoscope correlates with such measures. Conclusion: The cerebral aqueduct, excluding atrophic processes, is provided with a certain degree of dilatability, which we estimate to be around a diameter of 4 mm. This represents the maximum size for a flexible neuroendoscope for a safe aqueductal neuronavigation. The schematic model of the aqueductal adytum as a triangle defines 3 different aqueductal patterns and can be helpful when an intraoperative decision on whether to navigate the aqueduct must be taken.