Hydrocephalus. A Practical Guide to CSF Dynamics and Ventriculoperitoneal Shunts

H ydrocephalus is defined and the mechanisms of CSF hydrodynamics discussed. Supplementary tests used in the investigation of idiopathic normal pressure hydrocephalus are reviewed with a detailed explanation of constant flow CSF infusion tests. The principles governing valve selection are illustrated. Hydrocephalus is the abnormal accumulation of CSF within the cranium due to defective CSF production, flow or absorption. The CSF usually accumulates within the ventricular system, however ‘external hydrocephalus’ with widening of the subarachnoid spaces is described. Hydrocephalus can be due to obstructive causes preventing normal CSF flow through the CSF pathways, or due to abnormal absorption of CSF: communicating hydrocephalus. CSF flow studies frequently show a complex picture with contributions from both mechanisms. Overproduction of CSF that exceeds the absorption capacity of the arachnoid granulations is rare.