Pulsatile flow: The aqueduct of Sylvius connects the third and fourth ventricles. It is situated in the dorsal midbrain, surrounded by the periaqueductal gray matter. Passage of the CSF through the narrow aqueduct is fast and pulsatile in nature, with a systolic and diastolic to-and-fro displacement. This pulsatile flow has been well studied with dynamic MRI techniques (83) that permit observation of the craniocaudal CSF displacement occurring in the aqueduct, basal cisterns, and cervical subarachnoid space during systole. This to-and-fro displacement is related to the increase in the amount of blood volume in the cranial cavity that occurs during systole.
Monro-Kelly doctrine and volume of flow: According to the Monro-Kelly doctrine, the same volume of CSF is forced through the aqueduct and foramen magnum as the amount of venous blood and CSF leaving the cranial cavity during systole. The exiting CSF is due to an enlargement in the lumbar sac that acts as a buffer to changes in intracranial volume. During diastole the decrease in brain blood volume and recoil of CSF that had been shifted into the lumbar sac during systole reverses this CSF displacement through the foramina and aqueduct (25, 27, 58, 83). The net outward movement is equivalent to the CSF production (0.0067 ml/sec), which occurs predominantly in the lateral and third ventricles (10).
Resistance to flow through aqueduct: Jacobson et al. (49, 50) analyzed the flow dynamics in normal and stenotic aqueducts with a computer model. They modeled the flow through the aqueduct using a cylindrical pathway with similar dimensions and found that only a small pressure drop (less than 1 Pa) was sufficient to move the CSF in the “supratentorial” or upstream compartment across the “aqueduct” or simulated pathway. They also found that in normal circumstances CSF flow in the aqueduct appears to be laminar, even when its direction is reversed during the systolic and diastolic to-and-fro displacement.
Effect of aqueduct diameter on resistance: Jellinger found that the effect of the shape of the aqueduct walls on flow is minimized by this laminar flow that drives the core of flow centrally, away from the wall, thus minimizing the pressure drop due to resistance to flow that the wall could generate (51). A cross-sectional area as narrow as 0.1 mm2 is sufficient for adequate passage of CSF (34, 51).