The first level of classification of hydrocephalus should be based on the point where flow of CSF is restricted. These potential sites of restriction would include:
- the foramen of Monro
- the aqueduct of Sylvius
- the basal cisterns
- the arachnoid granulations
- theoutflow of venous blood from dural venous sinuses
Hydrocephalus without a point of obstruction or increased resistance to flow would be communicating hydrocephalus.
After the point of obstruction has been determined, the classification should then include the etiology of the condition, chronicity and age of the person.
[ABOVE] Illustration derived from applying engineering principles to the study of ventricular volume regulation. The CSF system is illustrated using a compartmental model, with each compartment having its own pressure and volume related to CSF flow. Circuit Diagram of the CSF pathway as a hydraulic analog of an electrical circuit.
[BELOW] Six compartment model of the CSF pathways.
[BELOW] Artist rendering of the circuit diagram.
Based on their studies, there were no pressure differential anywhere within the system. Pulse wave was transmitted undiminished and instantaneously to all transducers intracranialy. This means that the brain is a viscoelastic substance, acting as a fluid chamber where changes in pressure are transmitted instantaneously and fully to all areas.
Exception to the rule: If one of the lateral ventricles was drained to subatmospheric pressure, a pressure differential of 12mm Hg can be measured. This is seen in “post-shunt ventricular asymmetry” – in children who are shunted, septum pellucidum is drawn toward shunted ventricle and rests on head of caudate nucleus leading to a functional and reversible obstruction of flow.
NPH – likely obstruction between spinal and cortical subarachnoid spaces, dense arachnoidal thickening around brainstem in posterior fossa, blockage results from either SAH or infection, frequently involves area around brainstem selectively, amenable to endoscopic III ventriculostomy
Increased pressure in dural venous sinus results in pseudotumor cerebri and not HCP. Drainage of CSF into venous sinuses requires a gradient between ICP and sagital sinus pressure of 5-7mm Hg. If pressure in sagittal sinus is elevated, ICP must elevated in order for CSF to be absorbed. ICP slowly goes up until CSF can be absorbed (if skull volume is fixed). In cases of large craniectomies, ICP is in communication with atmospheric pressure. ICP cannot go above atmospheric pressure and patient develops hydrocephalus.
Study done in rabbits where SSS was occluded. Rabbits whose skull was intact developed intracranial hypertension without ventriculomegaly. Craniectomized rabbits developed hydrocephalus.
Table below shows the treatment options for each point of obstruction in the CSF pathway.
Rekate, Harold L. “A Consensus On The Classification Of Hydrocephalus: Its Utility In The Assessment Of Abnormalities Of Cerebrospinal Fluid Dynamics”. Child’s Nervous System 27.10 (2011): 1535-1541.