HYDRODYNAMIC PROPERTIES OF HYDROCEPHALUS SHUNTS - UNITED-KINGDOM SHUNT EVALUATION LABORATORY

Background-Although about 80% of properly diagnosed patients with hydr ocephalus improve after implantation of any model of shunt, the remain ing 20% may develop further complications because of inadequate shunt performance. Therefore, hydrocephalus shunts require careful independe nt laboratory evaluation. Method-Computer supported shunt testing, bas ed on the new International Standard Organisation directives, characte rises various aspects of pressure-flow performance of shunts such as v ariability with time, susceptibility to reflux, siphoning, temperature related behaviour, external pressure, the influence of a strong magne tic field (for example, MRI), presence of pulsation in differential pr essure, particles in drained fluid, etc. Results-Seven different model s of valves, representing most common constructions, have been tested so far. Most contemporary valves have a hydrodynamic resistance which is too low. This may result in overdrainage both related to posture an d during nocturnal cerebral vasogenic waves. A long distal catheter in creases the resistance of these valves by 100%-200%. Most shunts are v ery sensitive to the presence of air bubbles and small particles in dr ained fluid. Few shunt models offer reasonable resistance to negative outlet pressure, preventing complications related to overdrainage. Val ves with an antisiphon device may be blocked by raised subcutaneous pr essure. All programmable valves are susceptible to overdrainage in an upright position. Conclusion-The behaviour of a valve during such test ing is of immediate relevance to the surgeon and may not be adequately described in the manufacturer's product information.