The anti-siphon device
An anti-siphon device is a pressure regulating device,
it responds to atmospheric pressure which is transmitted through the skin,
it is 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 overdrainage due to transient, normal rises in
pressure 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.