William Guerin Bradley MD, Presenter: Nothing to Disclose

Gautam Bahl, Abstract Co-Author: Nothing to Disclose

It has recently been hypothesized that normal pressure hydrocephalus (NPH) is a “two hit” disease: benign external hydrocephalus (BEH) in infancy followed by deep white matter ischemia (DWMI) in late adulthood. The purpose of this study was to determine if a similar “two hit” mechanism could be argued for patients with aqueductal stenosis (AS) presenting in late adulthood.

It has been suggested that BEH/NPH patients have permanently reduced resorption of CSF by the arachnoidal villi and must rely on a parallel CSF drainage pathway through the extracellular space of the brain (which is the dominant pathway in children with aqueductal obstruction due to tectal gliomas). Adult onset AS patients are hypothesized to be similar to BEH/NPH patients except that the increased resistance occurs more proximally. In both diseases, DWMI is suggested to be the “second hit”. Histologically DWMI produces myelin pallor which leads to an increasingly hydrophilic environment and increased resistance to the peripheral egress of CSF via the extracellular space. CSF flow in AS was modeled as an electrical circuit with parallel pathways for CSF drainage via the usual ventricular outflow route and via the extracellular space of the brain. Diffusion imaging and thin slice, sagittal, bright CSF gradient echo acquisitions, eg, CISS or FIESTA, were performed in 6 patients with AS.

The electrical circuit model demonstrates increasing ventricular enlargement with increasing resistance to outflow of CSF via the extracellular space of the brain. Diffusion imaging demonstrates higher apparent diffusion coefficient (ADC) values in the centrum semiovale in patients with AS compared to age-matched controls for a comparable degree of DWMI. This suggests higher water content in the extracellular space of the brain, possibly due to back up of CSF. Bright CSF acquisitions demonstrate an aqueductal membrane.

DWMI is the “second hit” in both NPH and AS due to increasing resistance to egress of CSF through the extracellular space of the brain.

W.G.B.: Research support from GE.

Bradley, W, Bahl, G, Deep White Matter Ischemia Is the Precipitating Event for Aqueductal Stenosis Presenting in Late Adulthood.  Radiological Society of North America 2005 Scientific Assembly and Annual Meeting, November 27 - December 2, 2005 ,Chicago IL. http://archive.rsna.org/2005/4415680.html