First mass-produced valve used to treat hydrocephalus in 1956.[5] Spitz-Holter Uses slits in silicone to avoid mechanical failure.[6][7] Anti-siphon Prevents over drainage by preventing the siphon effect. The device closes when the pressure within the valve becomes negative relative to the ambient pressure. Prevents overdrainage that might occur when a patient sits, stands or rapidly changes posture.[8] Sigma The Sigma valve operates on a flow-control mechanism as opposed to the pressure-control system of other valves.