Hydrostatic and hydrodynamic considerations in shunted normal pressure hydrocephalus

Symptoms of overdrainage, a frequent complication following shunting proced ures especially in patients with normal-pressure hydrocephalus (NPH), are p resumed to be caused by siphoning: gravity-enhanced flow of cerebro spinal fluid (CSF) through shunts in upright position. A critical reappraisal of s iphoning and overdrainage (OD) is presented considering the occurrence of s ubdural hygromas (SDH) encountered in a group of patients shunted for NPK. The patients participated in the multicentre Dutch NPH study: 101 NPH patie nts were randomised for treatment with shunts with low or medium-high worki ng pressure. Treatment with shunts with law working pressure resulted in an increased incidence of SDH (iSDH). Current physiological concepts of overd rainage are discussed and criticised. A new theory is presented on the basi s of general physiological considerations. The theoretical model proposed y ields a revised explanation for the origin of extremely low ventricular pre ssure in the upright position found with overdrainage. The efficacy of siph oning-preventing shunts and subtemporal decompression is explored in the li ght of the proposed theory.