Cerebrospinal fluid (CSF) is secreted by the choroid plexus of the lateral and fourth ventricles.  The path of CSF flow is as follows: lateral ventricles → interventricular foramen of Monro → the third ventricle → cerebral aqueduct → the fourth ventricle → the foramina of Luschka and Magendie → subarachnoid space.  In the subarachnoid space, CSF is absorbed by arachnoid granulations and then enters the venous sinuses.  The dynamic balance between production and absorption of CSF allows stable intracranial volume (120-150 ml) and pressure (50-180 mm H₂O).

If the normal flow of CSF from the ventriculi to the subarachnoid space is disrupted, non-communicating hydrocephalus occurs.  Ventriculi above the obstruction are enlarged, while those below the obstruction are normal.  Congenital anomalies, such as aqueductal stenosis, Arnold-Chiari or Dandy-Walker malformations, cause non-communicating hydrocephalus.  In all cases, the obstacle to CSF flow exists within the ventriculi.

In communicating hydrocephalus, there is no obstruction to CSF flow from the ventriculi to the subarachnoid space.  Communicating hydrocephalus usually occurs secondary to dysfunction or obliteration of subarachnoid villi.  This dysfunction is usually a sequelae of meningeal infections (including tuberculosis meningitis, as in this patient) or subarachnoid/intraventricular hemorrhage.  In communicating hydrocephalus, all ventriculi are symmetrically enlarged, as in the image above.

(Choice A)  The emissary veins pass through apertures between the intracranial sinuses and veins outside the cranial vault.  They do not participate in absorption of CSF.

(Choice B)  Arachnoid trabeculae pass from the arachnoid through the subarachnoid space to the pia mater.  They are not involved in the metabolism of CSF.

(Choice D)  The choroid plexus is the site of CSF secretion; in communicating hydrocephalus, CSF absorption, not secretion, is impaired.  If present, a choroid plexus papilloma might cause a symmetric ventricular enlargement that appears virtually identical to communicating hydrocephalus.  Communicating hydrocephalus is far more common than choroid plexus papilloma, however, making Choice D a plausible answer, but not the most likely one.

(Choice E and F)  Neither dural septae nor communicating arteries are implicated in the pathogenesis of communicating hydrocephalus.

Educational Objective:
Symmetrical enlargement of the ventriculi is characteristic of communicating hydrocephalus.  Communicating hydrocephalus usually occurs secondary to dysfunction or obliteration of subarachnoid villi.  This dysfunction is usually a sequelae of meningeal infection (including tuberculosis meningitis) or subarachnoid/intraventricular hemorrhage.