Daniel G. Gomez

DIRECT IN VIVO OBSERVATION OF TRANSVENTRICULAR ABSORPTION IN THE HYDROCEPHALIC DOG USING MAGNETIC RESONANCE IMAGING

RATIONALE AND OBJECTIVES.A model of chronic noncommunicating hydrocephalus in canines was developed, and gadolinium- DTPA (Gd-DTPA)-enhanccd magnetic resonance imaging, physiologic and morphologic studies were performed to investigate transventricular absorption of cerebrospinal fluid. METHODS.Chronic hydrocephalus was induced in 12 mongrel dogs by injection of a silastic mixture into the prepontine cisterns. Ventricular pressure was measured during the development of hydrocephalus, and lateral ventriculo-ventricular perfusions with Gd-DTPA were performed under controlled conditions during serial magnetic resonance imaging studies. RESULTS.Hydrocephalus developed over an average of 129 ± 24 days after induction, and the intraventricular pressure increased from an initial level of 14 ± 4 cm H2O to a stabilized plateau of 25 ± 5 cm H2O. Increased signal intensity in the brain matter, as seen on magnetic resonance images of chronic hydrocephalic dogs perfused with Gd-DTPA in the lateral ventricles, was consistent with the presence of the contrast agent in the periventricular extracellular space. This increased signal intensity was not observed in control animals. CONCLUSIONS.These results provide direct evidence of transventricular absorption in chronic hydrocephalus.

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.

Cerebrospinal fluid absorption in the rabbit. Optic pathways.

Fifteen rabbits were intrathecally perfused with horseradish peroxidase at normal cerebrospinal fluid pressures. Horseradish peroxidase was found in the subarachnoid space around the optic nerve; it permeated the pia and penetrated within the nerve, occupying extracellular spaces between myelinated axons and glial cells up to the area of the lamina cribrosa. Horseradish peroxidase also crossed the perineural sclera and the border tissue of Elschnig to spread into the choroid where it was mostly seen within choriocapillaris, venules, and veins crossing through interendothelial spaces. “Bulk” cerebrospinal fluid absorption in the eye appears to occur through venous drainage in the choroid and SAS.

Cerebrospinal Fluid Absorption in the Rabbit: Inner Ear Pathways

Fifteen adult rabbits were perfused intrathecally with horseradish peroxidase (HRP) for 20-30 min under conditions that prevented any increase in cerebrospinal fluid (CSF) pressure. Histologic and ultrastructural examination of the cochlea disclosed HRP deposits along the cochlear and vestibular branches of the auditory nerve and beyond their ganglia, in a) epineural and perineural spaces; b) intraneural spaces reaching the membrane of myelinated axons via nodes of Ranvier; and c) extending beyond the epineurium into area lymphatics. HRP was also found in the basilar membrane, along with deposits in the scalae tympani, vestibuli, media and the spiral ligament. The endolymph also received HRP which followed vestibular nerve fibers and penetrated between sustentacular and hair cells of the cristae ampullaris and both maculae. HRP permeated interendothelial spaces lining the modiolus to reach the scala vestibuli lymphatics close to all the above areas were also permeated by HRP, but the inner tunnel was devoid of the marker.

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.

Evidence for transventricular absorption in the hydrocephalic dog.

Hydrocephalus was induced in 10 mongrel dogs by injection of a silastic mixture into the basal cisterns. The posterior part of the left lateral ventricle was isolated from the remainder of the ventricular system by introducing Pantopaque into both lateral ventricles and the third ventricle with the animals in prone positions. Labeled albumin injected into the posterior part of the left lateral ventricle was shown to enter the blood stream before it was shown in the fourth ventricle or vallecula. This provides qualitative evidence of transventricular absorption.

Radiological studies of cerebrospinal fluid and hydrocephalus

In recent years several new radiological techniques have been used for the evaluation of patients with hydrocephalus and for the study of cerebrospinal fluid formation and absorption. Radiological methods have been developed in dogs to measure the rates of cerebrospinal fluid formation in various parts of the ventricular system. In carefldly controlled conditions the formation of fluid may be demonstrated in patients during pneumoencephalography. Computerized tomography is a safe and painless technique that may largely replace pneumoencephalography and ventriculography for diagnosis of hydrocephalus. Combined with intravenous contrast enhancement it often gives the neurosurgeon sufficient information to treat the patient without the need for angiography or pneumoencephalography. A new safer radiographic contrast agent, Metrizamide, may be introduced into the subarachnoid space for the study of hydrocephalus. This contrast material can then be demonstrated in the cranial cerebrospinal fluid by computerized tomography.