5 Cerebral Aqueduct and Fourth Ventricle Anatomy
The anatomy of the cerebral aqueduct (CA) and the fourth ventricle has been studied mainly through postmortem laboratory specimen dissections and histologic techniques. Whereas many very detailed transverse histologic sections are available for the fourth ventricle, it is not possible to find the same good quality of sections for the CA. Although microsurgery brightly illuminates much of the fourth ventricle, it rarely illuminates both the extremities of the CA, and its view within the aqueduct′s lumen is totally hampered. Even some areas of the inferior triangle of the fourth ventricle are not easily and safely reached through a microsurgical approach. Neuroradiologic examinations such as computed tomography (CT) and especially magnetic resonance imaging (MRI) have added precious details about the structural features of these regions from a clinical perspective.1 In recent years, technical advances in neuroendoscopy have made it possible to further explore regions of the ventricular system that were previously considered unreachable and untouchable. In selected cases, neuroendoscopy can provide a direct, in vivo description of the inner aspects of both CA and the fourth ventricle.2,3,4
5.2 Cerebral Aqueduct
The CA (also known as aqueductus mesencephali, mesencephalic duct, or aqueduct of Sylvius) is a narrow, cerebrospinal fluid–filled channel connecting the third ventricle with the fourth ventricle. It lies between the tectum and the tegmentum of the mid-brain and is surrounded by the periaqueductal gray matter. The CA often has a curved shape that is concave ventrally.5 The bend in the middle or lower part of its course is called the genu of the aqueduct. Similar to other parts of the ventricular system, the CA develops from the central canal of the neural tube, originating from its mesencephalic portion.
Although first illustrated by Leonardo da Vinci and first recognized as a channel by Arantius in 1587, the CA was named after the anatomist François Sylvius de la Böe (1614–1672). Berengarius Carpensis first described in 1521 the cephalad entry to the aqueduct as a small foramen leading to a “deep vacuity.” Andreas Vesalius accurately described the aqueduct in his 1543 Fabrica. Its function was unveiled during the nineteenth century, when cerebrospinal fluid (CSF) circulation was theorized.6,7
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).
Abbreviation: PICAs, posterior inferior cerebellar arteries.
Source: Data from Longatti et al 2008.3