Progressive gliosis: In a stenotic aqueduct, to maintain adequate flow of CSF, the pressure gradient and velocity of CSF increase, leading to an increase in the wall shear stresses, and consequent gliosis with further narrowing may develop (49).
CSF flow effect on aqueduct: The flow pattern in the aqueduct may become turbulent because of changes in aqueductal size and/or shape (for example, forking without stenosis).
Ventricular Dilation Due to Pulsations
Rebounding pulse waves create shear stresses: Mise et al. (73) highlighted the importance of pulsations in causing ventricular dilation. They postulated that during systole in patients with aqueductal stenosis the CSF pulse is not transmitted to the infratentorial space but increases the wall shear stresses on the supratentorial ventricles, leading to periventricular edema, ischemia, tissue damage, and ventricular dilation.
Result is large, stiff ventricles: The global effect of many pulse waves, slowly remolding the brain, could explain the ventricular enlargement in chronic hydrocephalus (41). It might also explain why, when compared with other forms of hydrocephalus, obstructive hydrocephalus is characterized by a marked reduction in ventricular compliance (30, 41).
Change Over Time
Signs of long-standing hydrocephalus with late diagnosis: Aqueductal stenosis is not a steady condition; signs of congenital hydrocephalus (such as macrocrania and radiological findings of chronic intracranial hypertension), may be observed even if the cases are diagnosed in late childhood or adulthood.
Progression of “arrested hydrocephalus”: Oi et al. (80) described “arrested hydrocephalus” in which the hydrocephalic condition may be compensated. The most compensatory mechanisms are probably reestablishment of the normal CSF circulation through a partially patent aqueduct, utilization of an alternative CSF pathway, and changes in CSF production (79, 80). Further impairment of the compensatory mechanisms may change an arrested hydrocephalus into a progressive hydrocephalus (80).
Events causing progression: Lapras and co-workers (65) hypothesized that occlusion of the aqueduct can be worsened by head traumas, subarachnoid hemorrhages, or viral infections. They also suggested a "vicious" circle caused by functional mechanisms that could make a partial stenosis complete: accumulation of fluid in the supratentorial ventricular system may cause distortion of the brainstem and kinking of the aqueduct, worsening the stenosis.