However, Weiner, et al., found that the Orbis Sigma Valve (OSV) was also susceptible to the complications of overdrainage. When they, compared the success of standard DP valves with the success of the Orbis Sigma valve (OSV), he found that there was no significant difference between the two types of valves used in the outcome of gait disturbance or cognitive outcome. But, the OSV presented an additional complication of subdural hematomas.22 One explanation for this finding is that the valve mechanism on the OSV can easily be obstructed. When the valve becomes obstructed, pressure builds up proximally to the valve mechanism, which may force the obstruction out of the valve. When this happens, a surge of fluid is suddenly released from the valve, resulting in a significant rapid pressure change, which may be attributed to the occurrence of subdural hematomas.

The initial results of the Diamond-A valve used for the treatment of NPH appear very promising. As indicated by McQuarrie and Boon, a low-pressure valve is most effective for the treatment of hydrocephalus. However, in standard differential pressure valves, a small change in pressure will lead to a huge change in flow, which may result in subdural hematomas in the fragile geriatric brain. The mechanism of the Diamond-A valve lends itself to use in older patients. Resistance of the valve is sufficiently low at low pressures, to provide adequate flow rates for NPH patients. But valve resistance increases progressively, so that small changes in pressure will result in small changes in flow, and the occurrence of a subdural hematoma is avoided. At excessively high pressures, the valve resistance should respond automatically to prevent overdrainage in response to postural changes, exertion, or REM sleep. The Diamond-A provides increased resistance to overdrainage at high pressures, specifically lending itself to use in NPH patients. (Fig. 1)

Figure 1. In Vitro flow rates of the Diamond-A, PS Medical Delta-1,
Heyer-Shulte Low Profile Medium Pressure Valve and
the Cordis Orbis- Sigma Valve at progressively increasing pressures