infection,13,31,34 occlusion, 13,31,34 migration 13,31,34 and overdrainage12,13,21,31,34 are a regular occurrence. In an attempt to minimize the risk of overdrainage, Portnoy designed the anti-siphon device in the late 1970's. The anti-siphon device maintains the differential pressure across the valve close to atmospheric pressure, suppressing the hydrostatic column that forms in the distal catheter when the patient stands.7,16,21,31,37 However, the anti-siphon device will only suppress the flow through the shunt providing that the proximal pressure exceeds the atmospheric pressure, otherwise the presence of the anti-siphon device is negligable. Additionally, since the anti-siphon device functions in response to pressure changes in the hydrostatic column, it will function differently at different heights. As a result, the anatomical orientation of the valve is crucial element of implantation. The anti-siphon device has no provision for controlling flow when the patient is not upright. Consequently overdrainage may occur as a result of REM sleep. Not surprisingly, a high rate of complications due to both under and over- drainage is associated with the use of an anti-siphon device.10,11,14,19,25
In addition, the anti-siphon device introduced the entirely new complications of occlusion due to subgaleal blood, swelling of the overlying scalp tissue, or tissue encapsulation.10,11,14,19,25 Other solutions such as gravity compensating devices and dual-switch mechanisms, have also been tried. Unfortunately these devices are highly position sensitive and increase resistance of flow only when vertical; as a result, it is necessary to align the gravity compensating devices with the long axis of the body. In addition, they add cost, bulk and additional connections, which can increase the likelihood of complications, disconnects and shunt failure.

        Programmable valves have been introduced as a solution to the problems of under and overdrainage. The opening pressure can be increased or decreased by external adjustment. However, the programmable valves do not regulate the flow of CSF but allow only a non-invasive change in the opening pressure of the valve mechanics. The constant resistance flow characteristics of these valves still permit excessive flow rates under certain conditions. Programmable valves have no provision to counteract excess flow that occurs when the differential pressure across the valve exceeds the opening pressure,1,24 that may occur as a result of REM sleep, exertion or changes in posture.8,23 In addition, continual adjustments may need to be made to increase or decrease valve opening pressure. Kamano et al. presented a case report in 1991 in which a 37 year-old man had to undergo four external adjustments in a matter of eight months. Even after making these adjustments, the patient was diagnosed twice with bilateral subdural hematomas and once with dilated ventricles and impaired physical capability similar to his preoperative state.24

       The metallic components of the programmable valves pose an additional problem. The metal and magnetic parts of these valves produce interference during CT or MRI procedures, making examination difficult.24,41 Additionally, the magnet position controlling the pressure has been reported to change spontaneously during examination, indicating that the mechanism is susceptible to the influences of EMI (electromagnetic interference).41 A potential problem that may occur with programmable valves is protein build-up. Excessive protein built-up in the gears may lock-up the gears and possibly prevent reprogramming. The use of programmable valves can result in costly and time-consuming patient management. The valves requires cranial x-ray after each CT, MRI or re-programming to confirm valve setting, thereby increasing the