Normal pressure hydrocephalus, Alzheimer's disease and syringomyelia appear to be completely unrelated diseases, however, they share a reduction in subarachnoid space compliance as part of their pathophysiology. This paper discusses the physiology of pulsatile fluid flow and its relationship to compliance/impedance. Unlike continuous or non-pulsatile flow where the vessel resistance and pressure gradient are the major determinants of the volume of fluid flowing, when the fluid flow in a vessel pulsates then the vessel compliance/impedance becomes important. A reduction in compliance in the craniospinal cavity in each of the three diseases discussed, leads to a limitation of the outflow vessel compliance. Therefore, there is an increase in outflow vessel impedance. The venous blood, CSF and interstitial brain/spinal cord fluid all have significantly pulsatile flow and an increase in the impedance of the fluid outflow in each disease would limit the volume of these fluids as they attempt to cross the subarachnoid space. It is hypothesised that a reduction in the efficiency of the outflow of venous blood, CSF and interstitial brain/spinal cord fluid would lead to the accumulation of CSF in NPH, cord fluid in syringomyelia and delay the excretion of beta amyloid via the interstitial drainage pathways in AD.