Subdural haematomas complicating insertion of the low-pressure Novus hydrocephalus valve: a simple method for intra-operative testing of the anti-siphon device.

We previously reported 52 patients with hydrocephalus who were followed up after insertion of low-pressure Novus valves. These valves have a normally open anti-siphon device (ASD) incorporated. There were no cases of subdural haematomas (SDH). Subsequently, three new patients suffered SDH after insertion of these valves. We investigated a simple method for intra-operative testing of the ASD. These new patients had their valves replaced. In the laboratory, flow rates through five valves were recorded as a function of proximal positive pressure and distal negative pressures (siphoning). The flow rates were influenced by both proximal positive and distal negative pressures. The ASD stopped flow at distal negative pressures between -40 and -60 cm H(2)O. Proximal positive pressures increased this threshold. The flow can be measured by counting drops per minute. Three valves removed from patients were functioning as expected, one had unexpectedly slow flow at very high siphoning pressure and one had unexpectedly slow flow rates. In three patients with SDH complicating low-pressure Novus valves, the valves and anti-siphon devices were functioning adequately. Using a simple device, measuring flow rates in drops per minute was reliable and reproducible.