OVERDRAINAGE AND SHUNT TECHNOLOGY - A CRITICAL COMPARISON OF PROGRAMMABLE, HYDROSTATIC AND VARIABLE RESISTANCE VALVES AND FLOW-REDUCING DEVICES

When vertical body position is simulated, conventional differential pr essure valves show an absolutely unphysiological flow, which is 2-170 times the normal liquor production rate. Although this is compensated in part by the resistance of the silicon tubes, which may produce up t o 94% of the resistance of the complete shunt system, a negative intra cranial pressure (ICP) of up to 30-44 cmH(2)O is an unavoidable conseq uence, which can be followed by subdural hematomas, slit ventricles, a nd other well-known complications. Modern shunt technology offers prog rammable, hydrostatic, and ''flow-controlled'' valves and anti-siphon devices; we have tested 13 different designs from 7 manufacturers (56 specimens), using the ''Heidelberg Valve Test Inventory'' with 16 subt ests. ''Programmable'' valves reduce, but cannot exclude, unphysiologi cal flow rates: even in the highest position and in combination with a standard catheter typical programmable Medos-Hakim valves allow a flo w of 93-232 ml/h, Sophy SU-8-valves 86-168 ml/h with 30 cmH(2)O. The e ffect of hydrostatic valves (Hakim-Lumbar, Chhabra) can be inactivated by movements of daily life. The weight of the metal balls in most val ves was too low for adequate flow reduction. Anti-siphon devices are h ighly dependent on external, i.e. subcutaneous, pressure which has unp redictable influences on shunt function, and clinically is sometimes f ollowed by shunt insufficiency. Two new Orbis-Sigma valves showed rela tively physiological flow rates even when the vertical position (30 cm H(2)O) was simulated. One showed an insufficient flow (5.7 ml/h), and one was primarily obstructed. These have by far the smallest outlet of all valves. Additionally, the ruby pin tends to stick. Therefore, a h igh susceptibility to obliterations and blockade is unavoidable. Encou raging results obtained in pediatric patients contrast with disappoint ing experiences in some German and Swedish hospitals, which suggests t hat our laboratory findings are confirmed by clinical results. The con cept of strict flow limitation seems to be inadaequate for adult patie nts, who need a relatively high flow during (nocturnal) ICP crises. Th e problem of shunt overdrainage remains unsolved.