Hydrocephalus

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

Although hydrocephalus is often divided into obstructive and communicating typestypically 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 (names why and where). For example, acute subarachnoid haemorrhage 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 in that example it would be communicating obstructive hydrocephalus. 

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

1. communicating and non-communicating: addressing where the obstruction is located
2. obstructive and non-obstructive (on: on the grounds of whether or not there is obstuctionobstruction of CSF pathways in the ventricles or in the subarachnoid space) as well as into communicating and non-communicating (addressing where the obstruction is located) ^1-3.

This nomenclature leads to the following types of hydrocephalus (see figures 1 & 2):

• obstructive hydrocephalus (passage ofcommunicating (ie CSF withincan exit the ventricular system or in the subarachnoid space is impaired at some point)
  • communicating obstructive hydrocephaluswith obstruction to CSF absorption
    • often merelyusually referred to merely as communicating hydrocephalus
    • passage of CSF from the ventricular system and into the subarachnoid space is unimpeded but absorption of CFS via at some point between the basal cisterns and the arachnoid granulations, normal flow is impairedimpeded. 
      • subarachnoid haemorrhage (obstruction can be acute to to filling of the basal cisterns with blood clot, or chronic due to scarring of the subarachnoid space and arachnoid granulations)
      • most commonly seen as a result of subarachnoid haemorrhage or infective meningitis (both during infection and chronic) 
        • TB meningitis is typically basal filling the basal cisterns
        • bacterial meningitis is typically also over the convexities
      • extra-axialleptomeningeal carcinomatosis
  • without obstruction to CSF spaces (e.g. Sylvian fissuresabsorption
    • a special group of conditions with disparate, and sulci) are also distended
    often poorly understood, abnormal CSF dynamics, including:
    • normal pressure hydrocephalus (NPH)
    • choroid plexus papillomas (
    • part of the associated hydrocephalus is a specific subtypethought to be due to over production of communicatingCSF ¹. An obstructive component in larger masses is often also present)
  • additionally, other conditions with large ventricles fall into this group although they are often not thought of as hydrocephalus, which remains poorly understood. . These include:
    • hydrocephalus ex-vacuo and colpocephaly (ventricles are enlarged due to loss of adjacent brain parenchyma)

It should be noted that in the above examples CSF production is either normal or reduced as a result of decreased absorption of CSF. Increase in production of CSF rarely plays a role, with the partial exception is choroid plexus papillomas in which part of the associated hydrocephalus is thought to be due to over production of CSF ¹.