A model of hydrocephalus due to windkessel impairment caused by subarachnoid space obstruction

OBJECTIVE The traditional view that hydrocephalus due to obstruction in the subarachnoid space (SAS) is caused by malabsorption of CSF does not account for many experimental and clinical aspects of the disorder. Flow MRI reveals that nearly all CSF motion is pulsatile, and that hydrocephalus is linked to significant redistribution of pulsatility within the CSF pathways. Pulsatility in the cranium is governed by the cerebral windkessel system that buffers the energy of arterial pulsations via CSF to ensure smooth capillary blood flow. This study proposes a new model of intracranial thermodynamics based on the theory that high impedance to pulsatile energy flow through the CSF pathways causes hydrocephalus. METHODS The authors used a simple current-divider electrical circuit model of the cerebral windkessel system with input voltage derived from arterial blood pressure tracings from 12 normal dogs. SAS obstruction in the model was simulated using CSF flow values corresponding to published data from flow MRI studies of patients with hydrocephalus. RESULTS Modeling of hydrocephalus due to subarachnoid obstruction shows windkessel impairment and redistribution of CSF pulsatility caused by high damping of pulsatility in the SAS. Modeling of ventricular dilation as an active physiological adaptation shows improved windkessel function. Modeling of shunting shows windkessel restoration. The model produces the salient features of hydrocephalus due to obstruction in the SAS without invoking CSF malabsorption. CONCLUSIONS The authors propose that hydrocephalus due to SAS obstruction is impairment of the cerebral windkessel system due to high impedance to pulsatility in the subarachnoid CSF path that redistributes arterial pulsatile energy to the capillaries and jeopardizes capillary integrity. Windkessel theory introduces several new perspectives on hydrocephalus: 1) adaptive ventricular dilation is an active physiological response to windkessel impairment and lowers CSF path impedance to pulsations; 2) increased intracranial pressure (ICP) results from pressure energy accumulation due to windkessel dysfunction; 3) shunting is an accessory windkessel and reduces ICP by draining energy; 4) windkessel theory offers a new taxonomy of hydrocephalus, linking hydrocephalus due to SAS obstruction and hydrocephalus caused by aqueductal obstruction, normal pressure hydrocephalus, and low pressure hydrocephalus as related but distinct disorders of CSF path impedance; and 5) windkessel theory provides an explanation for hydrocephalus without invoking CSF malabsorption. Windkessel theory provides a new understanding of hydrocephalus and indicates new approaches to treatment.