Hydrocephalus merely denotes an increase in the volume of CSF and thus of the cerebral ventricles (ventriculomegaly).

Although hydrocephalus is typically referred to as either being "obstructive" or "communicating", this can lead to confusion as to the underlying cause of ventriculomegaly as the terms are referring to different aspects of the underlying pathophysiology (namely "why" and "where").

For example, acute subarachnoid hemorrhage confined to the basal cisterns can result in ventriculomegaly by obstructing the normal flow of CSF through the basal cisterns, and by filling the arachnoid granulations. Given that this is mechanistically an obstruction to CSF flow outside of the ventricular system should it be considered communicating or obstructive hydrocephalus? The correct answer is that is actually a communicating obstructive hydrocephalus.

As such a more precise terminology is to divide hydrocephalus into:

  • communicating and non-communicating: addressing "where" the obstruction is located
  • obstructive and non-obstructive: on the grounds of whether or not there is obstruction of CSF pathways in the ventricles or the subarachnoid space
  • This nomenclature leads to the following types of hydrocephalus (see figures 1 and 2):

    • communicating (i.e. CSF can exit the ventricular system)
      • with obstruction to CSF absorption
        • usually referred to merely as communicating hydrocephalus
        • passage of CSF from the ventricular system into the subarachnoid space is unimpeded but at some point between the basal cisterns and the arachnoid granulations, normal flow is impeded
          • subarachnoid hemorrhage (obstruction can be acute when filling the basal cisterns with blood clot, or chronic due to scarring of the subarachnoid space and arachnoid granulations)
          • infective meningitis (both during infection and chronic) 
            • TB meningitis is typically basal filling the basal cisterns
            • bacterial meningitis is typically also over the convexities
          • leptomeningeal carcinomatosis
      • without obstruction to CSF absorption
        • a particular group of conditions with disparate, and often poorly understood, abnormal CSF dynamics, including:
        • additionally, other conditions with large ventricles fall into this group although they are often not thought of as hydrocephalus. These include
    • non-communicating (i.e. CSF cannot exit the ventricular system, and thus there is by definition obstruction to CSF absorption)
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