Hydrocephalus

Hydrocephalus merely denotes is an increase in the volume of CSF and thusactive distension of the cerebral ventricles (ventriculomegaly (ventriculomegaly) of the brain resulting from inadequate passage of cerebrospinal fluid (CSF) from its point of production within the cerebral ventricles to its point of absorption into the systemic circulation: it is caused by a mismatch between CSF production and absorption.⁵

Although raised intracranial pressure often co-exists with hydrocephalus it doesn’t have to be present.

Hydrocephalus is typically referred to as either being "obstructive"either obstructive (non-communicating) or "communicating",non-obstructive (communicating):

Obstructive refers to a barrier to flow between two or more CSF spaces (eg aqueductal stenosis blocking the cerebral aqueduct of sylvius causing dilatation of the 3^rd and lateral ventricles).

Non-obstructive is broken down into over production of CSF (a CSF producing coroid plexus tumour) and under-absorption of CSF (for example sub-arachnoid haemorrhage blocking the absorption pathway).

There has been a recent effort to change the classification system into one which classes under-absorption of CSF (technically an obstruction) as an obstructive hydrocephalus and therefore which does not consider communicating and non-obstructive as the same (likewise with non-communicating and obstructive). Thus:

1.    Non-obstructive hydrocephalus:

• Choroid plexus papilloma
• Normal pressure hydrocephalus (NPH)

2.     Obstructive hydrocephalus:

• All other causes

The intention of this can leadclassification is to confusion as toclear up 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 haemorrhage confined 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 technically correct answer is that is actually a communicating obstructive hydrocephalus.

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

1. communicating

   1.     Communicating and non-communicating: addressingAddressing "where" the obstruction is located

2. obstructive

   2.     Obstructive and non-obstructive: onOn the grounds of whether or not there is obstruction of CSF pathways in the ventricles or the subarachnoid space ^1-3

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

• communicating (i.e. CSF

  Communicating (CSF can exit the ventricular system):

  • withWith obstruction to CSF absorption
    • usuallyUsually referred to merely as communicating hydrocephalus
    • passagePassage 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
    • Ssubarachnoidubarachnoid haemorrhage (obstruction can be acute when filling the basal cisterns with blood clot, or chronic due to scarring of the subarachnoid space and arachnoid granulations)
    • infectiveInfective meningitis (both during infection and chronic) 
    • TB meningitis is is typically basal filling the basal cisterns
    • bacterialBacterial meningitis is typically also over the convexities
  • Lleptomeningealeptomeningeal carcinomatosis

• often merely referred to as obstructive hydrocephalus
• upUp-stream ventricles are dilated and exert mass effect upon adjacent brain (e.g. effacement of sulci)
• numerousNumerous causes including
  • Fforamenoramen of Monro: colloid cyst
  • Aaqueductqueduct of Sylvius: aqueduct stenosis,tectal glioma 
  • fourthFourth ventricle:posterior fossa tumour, or cerebellar infarct or or cerebellar haemorrhage