Why don't ventricles dilate in pseudotumor cerebri? A circuit model of the cerebral windkessel.

OBJECTIVE: Pseudotumor cerebri is a disorder of intracranial dynamics characterized by elevated intracranial pressure (ICP) and chronic cerebral venous hypertension without structural abnormalities. A perplexing feature of pseudotumor is the absence of the ventriculomegaly found in obstructive hydrocephalus, although both diseases are associated with increased resistance to cerebrospinal fluid (CSF) resorption. Traditionally, the pathophysiology of ventricular dilation and obstructive hydrocephalus has been attributed to the backup of CSF due to impaired absorption, and it is unclear why backup of CSF with resulting ventriculomegaly would not occur in pseudotumor. In this study, the authors used an electrical circuit model to simulate the cerebral windkessel effect and explain the presence of ventriculomegaly in obstructive hydrocephalus but not in pseudotumor cerebri.

METHODS: The cerebral windkessel is a band-stop filter that dampens the arterial blood pressure pulse in the cranium. The authors used a tank circuit with parallel inductance and capacitance to model the windkessel. The authors distinguished the smooth flow of blood and CSF and the pulsatile flow of blood and CSF by using direct current (DC) and alternating current (AC) sources, respectively. The authors measured the dampening notch from ABP to ICP as the band-stop filter of the windkessel.

RESULTS: In obstructive hydrocephalus, loss of CSF pathway volume impaired the flow of AC power in the cranium and caused windkessel impairment, to which ventriculomegaly is an adaptation. In pseudotumor, venous hypertension affected DC power flow in the capillaries but did not affect AC power or the windkessel, therefore obviating the need for adaptive ventriculomegaly.

CONCLUSIONS: In pseudotumor, the CSF spaces are unaffected and the windkessel remains effective. Therefore, ventricles remain normal in size. In hydrocephalus, the windkessel, which depends on the flow of AC power in patent CSF spaces, is impaired, and the ventricles dilate as an adaptive process to restore CSF pathway volume. The windkessel model explains both ventriculomegaly in obstructive hydrocephalus and the lack of ventriculomegaly in pseudotumor. This model provides a novel understanding of the pathophysiology of disorders of CSF dynamics and has significant implications in clinical management.