Some examples suffice to describe the relationship between the site of obstruction and the configuration of the subsequent hydrocephalus. Obstruction may cause expansion of one lateral ventricle or a portion of it. Tumors within the third ventricle (e.g., colloid cyst) block the egress of CSF from both foramina of Monro, leading to dilatation of both lateral ventricles. The aqueduct of Sylvius, narrow to begin with, may be occluded by a number of developmental or acquired lesions (e.g., congenital atresia or forking, perinatally acquired gliosis, ependymitis, hemorrhage, or tumor), and lead to dilatation of the third and both lateral ventricles. If the obstruction is in the fourth ventricle, the dilatation includes the aqueduct. Other sites of obstruction of the CSF in the ventricular pathways are at the foramina of Luschka and Magendie (e.g., congenital failure of opening of the foramina, Dandy-Walker syndrome). More common is a blockage to flow of CSF in the subarachnoid space around the brainstem caused by inflammatory or fibrosing meningitis. This form of obstruction results in enlargement of the entire ventricular system, including the fourth ventricle.
Another potential site of obstruction is in the subarachnoid spaces over the cerebral convexities. A matter of considerable practical as well as theoretical interest is whether a meningeal obstruction at the site of the arachnoidal villi, or a blockage of the venous sinuses into which the CSF is absorbed, can result in hydrocephalus. Russell, in her extensive neuropathologic material and review of the literature, could not find a well-documented example of either of these suggested etiologies, and the same is true of the pathologic material collected by our colleague R.D. Adams. Moreover, experiments in animals in which all the draining veins had been occluded, tension hydrocephalus with enlarging lateral ventricles was produced in only a few cases. More often, the site of obstruction in meningitis or subarachnoid hemorrhage is in the basal cisterns. Yet Gilles and Davidson have stated that hydrocephalus in children may be the result of a congenital absence, or deficient number of arachnoidal villi, and Rosman and Shands have reported an instance that they attributed to increased intracranial venous pressure. Our hesitation in accepting such examples stems from the difficulty that the pathologist has in judging the patency of the basilar subarachnoid space. This is much more reliably visualized by radiologic than by neuropathologic means. Theoretically, if the obstruction is near (or in) the superior sagittal sinus, the CSF should accumulate under pressure outside as well as inside the brain, so that there is no gradient from the ventricles across the mantle of the substance of the cerebral hemispheres and the ventricles should not enlarge at all, or only slightly, and only after a prolonged period.
The infrequently encountered imaging picture of enlarged subarachnoid spaces over and between the cerebral hemispheres, coupled with modest enlargement of the lateral ventricles has been referred to as external hydrocephalus. Although such a condition does exist, many of the cases so designated have proved to be examples of subdural hygromas or arachnoid cysts. The condition is now seen after surgical hemicraniectomy, if the bone is not replaced.
Processes such as subarachnoid hemorrhage, or cerebral hemorrhage or brain abscess that rupture into the ventricles and rapidly expand the volume of CSF produce the most dramatic forms of acute hydrocephalus. An obstruction of the CSF pathways in these conditions may also be found within the ventricular system or at the basal meninges. The corresponding clinical syndrome of acute hydrocephalus is described below.