5.2.1 Normal Anatomy

The shape of the aditus aquaeducti (also known as adytum of the aqueduct, aditus ad aquaeductum cerebri, or superior opening of the aqueduct) is that of a triangle with the base positioned dorsally, and whose base corresponds with the posterior commissure in the posteroinferior recess of the third ventricle (Fig. 5.1a, Animation 5.1). Two small, rounded bulges are visible in the aditus, outlining its anterior contour and determining the triangular shape. They are called rubral eminences, as anatomists assume they are caused by the protrusion of the red (rubral) nuclei into the aqueduct lumen. Caudally to the aditus, the protruding superior colliculus determines the first constriction, which is followed by the ampulla, an internal dilation of the lumen (Fig. 5.1b,c). The ampulla corresponds to the crucial tegmental sulcus. A second constriction is visible corresponding to the inferior colliculus (Fig. 5.1b–d). Ventrally, the trace of a sulcus separates the aditus, flattens in the ampulla, and continues into the sulcus medianus of the floor of the fourth ventricle. The longitudinal medial fascicles lay at the bottom of the sulcus. Beyond the second constriction, the lumen widens and opens abruptly into the fourth ventricle (Fig. 5.1d). Although the CA was initially thought to be subdivided into three parts, later observations distinguished five parts: the adytum, the anterior part with the first constriction (the narrowest part), the ampulla, the genu or second constriction, and the posterior part (Video 5.1 and Video 5.2).6,8 The two constrictions determine the size of the aqueduct and consequently the feasibility of endoscopic navigation. During the fetal period, the anterior (first) constriction ranges from 0.4 to 0.8 mm, and the posterior (second) ranges from 0.5 to 1 mm.9 After a progressive reduction in diameter from a fetal age of 2 months until birth, the aqueduct′s diameter steadily increases with age.8 In adults, the aqueduct′s diameter ranges from 0.5 to 2.84 mm, and its length is ~ 14 to 15 mm (Table 5.1).1,9 However, the aqueduct shows the property of compliance, as suggested by cine phase-contrast magnetic resonance imaging, but also by variations in its diameter during a single cardiac cycle, hydrocephalus, and intraventricular hemorrhages.10,11,12,13 Whereas only the two extremities of the aqueduct can be inspected during micro-surgical procedures, neuroendoscopy usually allows for inspection of all anatomical landmarks (Video 5.1 and Video 5.2), although the diameter of the flexible endoscope is larger than the diameter of a normal CA (Table 5.2).

Source: Data from Quester and Schroeder 1999,19 Bogucki et al 1997,20 Lang et al 1991,21 Matsushima et al 1982,22 and Matys et al 2013.1

Abbreviation: PICAs, posterior inferior cerebellar arteries.

Source: Data from Longatti et al 2008.3