Hydrostatic and Hydrodynamic Considerations in Shunted Normal Pressure

Symptoms of overdrainage, a frequent complication following shunting procedures especially in patients with normal-pressure hydrocephalus (NPH), are presumed to be caused by siphoning: gravity-enhanced ¯ow of cerebro spinal ¯uid (CSF) through shunts in upright position. A critical reappraisal of siphoning and over-drainage (OD) is presented considering the occurrence of subdural hygromas (SDH) encountered in a group of patients shunted for NPH. The patients participated in the multicentre Dutch NPH study: 101 NPH patients were randomised for treatment with shunts with low or medium-high working pressure. Treatment with shunts with low working pressure resulted in an increased incidence of SDH (iSDH). Current physiological concepts of overdrainage are discussed and criticised. A new theory is presented on the basis of general physiological considerations. The theoretical model pro-posed yields a revised explanation for the origin of extremely low ventricular pressure in the upright position found with overdrainage. The ecacy of siphoning-preventing shunts and subtemporal de-compression is explored in the light of the proposed theory