Risks in using siphon-reducing devices in adult patients with normal-pressure hydrocephalus: bench test investigations with Delta valves.

To elucidate the reason for malfunction of Delta valves in patients with normal-pressure hydrocephalus, the authors applied a new concept of the bench test and simulated intracranial pressure (ICP) to measure the resultant flow with a complete shunt system. Subcutaneous pressure on the valve, valve implantation site, and postural hydrostatic differential-pressure changes were simulated in this bench test designed for adult patients with normal-pressure hydrocephalus. Subcutaneous pressure ranged within 6.4 +/- 0.09 cm H2O (mean +/- standard deviation) on the 7th day after the implantation of the valve in rats. A linear correlation between valve closing pressures and the external pressure was observed: gradual increase of the latter resulted in an approximately equal rise in the closing pressure. Closing pressure ranged within the physiological variation of ICP (10 +/- 5 cm H2O) in the supine position. In the erect position it was higher than the physiological variation (+5 to -5 cm H2O) when the valve was placed at the level of the foramen of Monro and lower when placed at the level of the clavicle. This observation indicates that the subcutaneous pressure around the Delta valve significantly affects its closing pressure. An excessive reduction of flow or a functional obstruction occurs when the patient with a Delta valve implanted at the foramen of Monro assumes an erect posture. The authors conclude that Delta valves are not recommended for implantation at the level of the foramen of Monro in patients with normal-pressure hydrocephalus because of the risk of underdrainage. The mastoid process or clavicle may be the alternative levels for its implantation to prevent underdrainage; however, the possibility of shunt malfunction should be kept in mind because subcutaneous pressure is variable among patients and that might affect the performance characteristics of these valves.