Around 10% to 30% of shunt revisions may be attributed to posture related overdrainage. Of the various siphon preventing devices available at present, two construction types are the most prominent: those using a gravitational mechanism and those using a subcutaneous membrane. Gravitational devices such as Elekta-Cordis Horizontal-Vertical Valve, Chhabra Valve, Fuji Valve, or Miethke Dual-Switch Valve are widely used.1 Their main drawback is susceptibility to malfunction when the shunt becomes displaced from its vertical axis after implantation and unpredictable operation during persistent bodily movements. The membrane devices: the Anti-Siphon Device (ASD, Heyer Schulte) or Siphon Control Device (SCD, Medtronik PS Medical) have generally proved clinically effective,2 3although in some cases these devices may obstruct the CSF drainage when the subcutaneous pressure increases or the scar tissue isolates the device from atmospheric pressure. The flow regulating Orbis-Sigma Valve (Elekta-Cordis) may also reduce clinical complications related to overdrainage in the upright body position.4 It prevents excessive CSF drainage by instantaneously increasing its hydrodynamic resistance when the drainage rate rises.

The new Codman …