A variety of valve designs are currently available (Table 24.2). An array of eponymous names, valve types and manufacturers can lead to great confusion. The table attempts to provide a simple classification of some of the valve designs in common usage but it is by no means exhaustive.

Differential pressure valves are pressure-regulating devices. Pressure regulators maintain a fixed-pressure differential, regardless of flow; this is in contrast to flow regulators that maintain constant flow regardless of pressure. Four types of differential pressure valve are commonly encountered: slit valves, mitre valves, ball and spring and diaphragm valves [11].

For most fixed valves, a low, medium or high-pressure alternative is available. The pressure setting defines the opening or, more commonly, the closing pressure of the valve (the physical characteristics of the valve are the reason for the differences in these two values).

In order to overcome the limitations imposed by fixed-resistance valves, some manufacturers have developed valves whose operating pressure can be varied once the shunt has been implanted. Such devices include the Medos programmable valve (Codman Medos, Le Cocle, Switzerland) and the Sophysa adjustable valve (Sophysa, Orsay, France). An externally applied magnetic field is used to alter the position of an internal rotor and thus vary the pressure setting. The setting is then verified on plain radiograph (Medos) or a compass held over the device (Sophysa).

The pressure gradient across the valve is the difference between the intraventricular pressure and the intra-abdominal pressure in the supine position. In the upright position, the added hydrostatic pressure will increase the differential pressure and so increase the CSF flow through the valve. One of the criticisms of differential pressure valves is that they are subject to this siphoning phenomenon in the upright position.

A number of modifications of shunt design have been devised in order to overcome the problem of siphoning. Anti-siphon or siphon

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