Hydrodynamic properties of hydrocephalus shunts: United Kingdom Shunt Evaluation Laboratory.

BACKGROUND Although about 80% of properly diagnosed patients with hydrocephalus improve after implantation of any model of shunt, the remaining 20% may develop further complications because of inadequate shunt performance. Therefore, hydrocephalus shunts require careful independent laboratory evaluation. METHOD Computer supported shunt testing, based on the new International Standard Organisation directives, characterises various aspects of pressure-flow performance of shunts such as variability with time, susceptibility to reflux, siphoning, temperature related behaviour, external pressure, the influence of a strong magnetic field (for example, MRI), presence of pulsation in differential pressure, particles in drained fluid, etc. RESULTS Seven different models 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 and during nocturnal cerebral vasogenic waves. A long distal catheter increases the resistance of these valves by 100%-200%. Most shunts are very sensitive to the presence of air bubbles and small particles in drained fluid. Few shunt models offer reasonable resistance to negative outlet pressure, preventing complications related to overdrainage. Valves with an antisiphon device may be blocked by raised subcutaneous pressure. All programmable valves are susceptible to overdrainage in an upright position. CONCLUSION The behaviour of a valve during such testing is of immediate relevance to the surgeon and may not be adequately described in the manufacturer's product information.