cost and complication of patient management, not to mention the additional radiation exposure that the patient will experience.

    Complications from under and overdrainage have been directly linked to the hydrodynamic characteristics of the shunt. These complications include shunt dependency,32,36 slit ventricle syndrome,32,36 subdural hematoma,14,36 craniosynostosis,36 and orthostatic intracranial hypotension.36 As a result of the complications, new methods of improving shunting have been sought. It has been said that the ideal valve should be flow-controlled. "Such a valve would have to continuously determine the CSF formation rate and the rate of outflow through natural channels and regulate flow through the valve so as to remove the excess fluid."30 Christian Sainte-Rose, designer of the Orbis-Sigma, was the first to introduce the concept of flow regulation by varying valve resistance. The aim of this development was to provide CSF drainage at or below the CSF secretion rate within a physiological ICP range.33

The initial clinical results obtained with the Orbis-Sigma were very promising. Complications due to ventricular obstruction were observed in only 2.3% of the patients. Overdrainage complications, such as subdural hematomas and slit ventricles syndrome, were encountered in only 0.8% of the patients.6 Other physicians have also shown positive results when implanting the Orbis-Sigma in adults or older children.3,4,35

However, there were some hesitations to the use of the Orbis-Sigma. Many neurosurgeons prefer not to implant the OSV in babies because they believe that the pressure required to create physiologic flow is excessively high for infants whose skull sutures have not yet closed.13 In 1995 Serlo reported "the Orbis-Sigma Valve is less suitable for newborns…[T]he pressure gradient between the ventricles and the abdominal cavity is small and insufficient flow may occur with these valves."35 He tested five custom made Orbis-Sigma Valves with extra low opening pressures below 30 mm H2O but still found that two of the valves had to be explanted due to underdrainage.35

In addition, the Orbis-Sigma is reportedly susceptible to particulate obstruction and, as a result, valve malfunction.12,39 Aschoff et al. reported that the OSV has the smallest outlet of any valve currently on the market. "The tiny distance to the surrounding ruby disc may tend to cause friction between pin and disc, resulting in marked hysteresis during increasing and decreasing flow."2 Increased susceptibility of valve obstruction is certainly an inherent risk in the Orbis-Sigma due to the tiny opening of the valve. Aschoff et al. summarizes by saying that "[B]oth the design and the delicate mechanism involve problems. The significant decrease in overdrainage may be outweighed by an increased rate of shunt insufficience."2

Nevertheless, as a result of many trials it may be concluded that flow regulation has demonstrated itself to be an effective treatment alternative to the traditional constant resistance differential pressure mechanisms in the treatment of hydrocephalus.3,4,5,35 However, the limitations of the Orbis-Sigma require some refinement in order to make this concept truly universal. In fact, the manufacturer of the Orbis-Sigma has recently released the OSV 2, which has been designed to address some of the limitations of the initial device. Clinical studies have begun to assess the effectiveness of these changes.

In an attempt to refine the physiologic31 flow-regulating concept and to avoid these limitations, a new flow-regulating device, the Diamond Valve, has been developed