Anti-siphon devices pressure regulating devices, they respond to the atmospheric pressure which is transmitted through the skin. They are only active only when there is a negative pressure exerted due to the vertical position of a child, and the resultant siphoning. Anti-siphoning devices were developed to be added to the standard shunt just below the shunt's reservoir.
When the pressure within the fluid column of the shunt drops to below atmospheric pressure (as is the case with siphoning), atmospheric pressure transmitted through the skin pushes a diaphragm down to close the fluid passage way and block further flow of CSF through the system until such time as the pressure within the ventricles climbs back to above atmospheric pressure. When this occurs, the diaphragm is pushed back open.
A variation, on the anti-siphon device, is the Pudenz-Schulte Delta valved shunt which has a valve on it that resists drainage through the system when pressures within the system drop into the negative range. A siphon valve in that it does not need to perceive atmospheric pressure to work thus encasement of the valve by scar tissue does not cause its malfunction as opposed to the anti-siphon valve where this has been reported.
The Cordis's Orbis-Sigma valve, offers a variable resistance to flow as a function of variations in pressure within the ventricles. As pressure increases within the ventricles, the resistance to flow increases until pressures reach an abnormally high range where to valve drops its resistance to flow to zero to allow decompression of the dangerously high state. This design is meant to avoid over-drainage due to transient, normal rises inpressure within the head such as happen when an individual coughs or sneezes.
The Codman Medos adjustable shunt has a pressure differential valve whose resistance can be altered using a magnet field transmitted through the skin. Thus, the valve settings can be changed during a routine office visit avoiding any surgery. Reports from Europe have stated that some patients have benefited from having this shunt. Clearly, siphoning occurs with this system so it is unclear to what degree the feature of valve adjustment will be of benefit.