D. Gordon Potts

Arachnoid granulations: radiology and anatomy.

The arachnoid impressions in the skull vault were studied on skull radiographs of patients of various ages. The relationships of the superior superficial cerebral veins and the lacunae laterales tu the arachnoid granulations were demonstrated in the venous phases of carotid angiograms. The cortical veins either coursed into the bony arachnoid impressions or around the margins of the impressions to enter the superior sagittal sinus. Anatomical specimens of the dura, granulations and superior sagittal sinus were studied by injection and dissection techniques to show the relationship of the superior superficial cerebral veins, the lacunae laterales, and the arachnoid granulations.

Perfusion studies of the cerebrospinal fluid absorptive pathways in the dog.

Abstract Perfusion studies with Hypaque 75%, barium gelatin, and barium sulfate demonstrated the absorptive and filtrative functions of the arachnoid villi in dogs during life and immediately after death. Perfusion with artificial cerebrospinal fluid in sacrificed dogs at approximately the rate of CSF formation during life showed a pressure gradient between the cisterna magna and sagittal sinus of 6.8 cm water as compared to 5.8 cm in live dogs. These findings suggest that absorptive pathways function relatively normally for 1 to 2 hr. after death and that the absorptive pathways function as a passive filtration system.

Computed tomography: demonstration of contrast medium within cystic tumors.

Two cases demonstrated that intravenously injected contrast medium entered cystic lesions. It appears that both vascular opacification and transgression of the blood-brain barrier play major roles in contrast enhancement.

Effects of pressure on the arachnoid villus.

Monkey arachnoid villi fixed without controlling the pressures in the subarachnoid space or the venous sinus were irregularly distended. The endothelial cells showed a large number of clear vesicular spaces. In addition there were many greatly enlarged intercellular spaces. Villi fixed with a pressure difference of 10 cm of water between the subarachnoid space and the superior sagittal sinus were more regularly distended. The endothelial cells were stretched and separated by shortened and intermittently widened intercellular spaces. The degree of widening varied greatly. Most of the widenings were minor but a few were relatively large. Many intercellular spaces were permeable to horseradish peroxidase.

The arachnoid granulations of the newborn human: An ultrastructural study

Portions of the superior sagittal sinus and lacunae laterales containing arachnoid villi and granulations from 8 full‐term newborn babies were studied by transmission electron microscopy. Arachnoid proliferations from 3 subjects were distended and fixed in vitro by applying a differential pressure of 8 cm H2O to the subarachnoid aspect of the tissues. The remaining cases were fixed in a collapsed state. Distended arachnoid proliferations showed morphologic characteristics associated with similar functional structures in experimental animals: shortened and enlarged interendothelial spaces; micropinocytotic activity and a system of endothelial‐lined tubules. All this is taken as evidence that arachnoid proliferations in newborn babies could already be engaged in cerebrospinal fluid absorption.

Computed Tomography: Evaluation of the Posterior Third Ventricle

The normal computed tomography (CT) appearances of the posterior third ventricle and related structures are presented. Seventy-six patients with lesions directly involving this ventricle were studied by CT, and results of other neuroradiological procedures evaluated. Primary tumors, metastases, hematomas, infarcts, and cysts may be reliably distinguished by CT, particularly if contrast enhancement is utilized. Size and density resolution limit the direct visualization of the aqueduct of Sylvius; stenosis however may be inferred. Small infarcts may be difficult to demonstrate by the use of CT scans without contrast enhancement, yet be shown by isotope scans.

Changes in the Ventricular Size During Fractional Pneumoencephalography

Measurements of 40 lateral ventricles, 49 third ventricles, and 46 fourth ventricles were made at different stages during the introduction of air via the lumbar route. Results showed that ventricular air filling during pneumoencephalography is accompanied by ventricular enlargement. The incremental increases were most marked as the initial fractions were introduced and less marked as the later fractions were introduced. It is concluded that the “true” resting size of the ventricles prior to the first fraction of injected air is smaller than the ventricular size commonly recorded at pneumoencephalography. These observations are discussed in relation to basic physical principles involved in ventricular air filling.

The Rate of Cerebrospinal Fluid Formation Proximal and Distal to Aqueductal Obstruction in the Dog

The authors describe a stereotactic technique for producing aqueductal obstruction in dogs. Cerebrospinal fluid was collected proximal and distal to the occluded aqueduct for three hours. The choroid plexuses from all four ventricles were weighed. Each lateral ventricle plexus comprised approximately 18% of the total; the plexus of the third ventricle was 25.3%, and that of the fourth ventricle was 38.2%. The rate of fluid formation was not related to the weight of the choroid plexus, suggesting either (a) significant extrachoroidal sources of cerebrospinal fluid or (b) a functional difference between the choroid plexus of the fourth ventricle and those proximal to the aqueduct.

Arachnoid Villi and Granulations

Although as early as 1900 CUSHING believed that wide, valve-like passages linked the meningeal spaces to the adjacent veins, the work of WEED persuaded him that these passages did not exist (1,2). The concept that the arachnoid villus is a blind diverticulum of arachnoid that projects into a venous channel, and that it serves as a membrane through which cerebrospinal fluid passes into the blood stream was then widely accepted, WELCH and FRIEDMAN (3) reported in 1960 that the arachnoid villus of the monkey forms a labyrinth of small tubules that establish open connections between the subarachnoid space and the venous blood and they felt that these tubules had a valve-like function. This would imply a pressure-dependent passive mechanism for the passage of cerebrospinal fluid through the arachnoid villus.

Pneumoencephalographic Evidence of Increased Transventricular Absorption of Cerebrospinal Fluid

The rate of accumulation of cerebrospinal fluid in both lateral ventricles was calculated during pneumoencephalography in 20 patients. In patients with conditions known to be associated with increased transventricular absorption, the rates of accumulation of fluid were slower than in patients in whom increased transventricular absorption was unlikely. These findings suggest that this procedure can be used to distinguish atrophy from communicating hydrocephalus. One patient with pseudotumor cerebri also showed a slow rate of accumulation of fluid, suggesting increased transventricular absorption.