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Overdrainage is a significant clinical problem after shunting for hydrocephalus as confirmed by the UK Shunt Registry.1 Various devices have been developed to reduce the rate of CSF drainage in the upright position which have been assessed by the UK Shunt Evaluation Laboratory.2 The average price of a shunt varies from £175 to £650 in the United Kingdom. Surprisingly, the prices are higher in the developing countries.3 However, some local lower cost constructions are available and are reported to function well.4
The Chhabra shunt is a low cost device, developed and manufactured in India, that incorporates a gravitational siphon preventing mechanism. In the vertical position one, two, or three (depending on performance level) stainless steel weighting balls press on a sapphire ball which closes the CSF flow aperture, increasing the shunt’s opening pressure (figure (A)). In the horizontal position the opening pressure is theoretically equal to zero mm Hg, as the balls fall away. A similar principle is applied in constructions of other “gravitational” shunts—namely, the Cordis horizontal-vertical LP valve, the newly designed dual-switch Miethke valve (Germany) and the Fuji, another low cost valve (Philippines).
We tested a sample of two Chhabra medium pressure shunts (containing two balls) using a 2 week evaluation protocol.2 Our main aim was to investigate the impact of posture (horizontal-vertical) on shunt pressure-flow performance. We also investigated how the fluctuations in proximal pressure, simulating the presence of naturally occurring waves of intraventricular pressure, may alter shunt function. Such waves may occur not only due to heart and respiratory function but also due to body movements during walking, jogging, etc.
The figure (B) shows two typical pressure-flow performance curves recorded in the horizontal and the vertical position. They represent two almost straight parallel lines. Their slopes depict the low hydrodynamic resistance of the shunt (1.3 mm Hg/ml/min) This is much lower than the physiological resistance to CSF outflow, which normally lies within the range 6–10 mm Hg/ml/min.5 The average operating pressure determined for the vertical shunt position was around 7 mm Hg and for the horizontal position it was 0.6 mm Hg. The area between these lines represents the possible operating range in all the intermediate body positions. Therefore, we conclude that the operating pressure of the shunt varies with the body position, as intended.
The main problem with this valve arises when the patient moves upwards and downwards. Such a situation has been simulated by the addition of a pulsating waveform of variable amplitude to the proximal pressure (frequency was controlled from 90 revolutions/min to 5/min). As a result, the lower end of the pressure-flow performance curve was consistently twisted to the left (towards lower pressures, figure (B)). In vivo, variations in intraventricular pressure, produced by either an increased magnitude of the pulse waveform or repeated body movement, may accelerate the drainage rate, in some cases possibly leading to overdrainage. The above phenomenon is probably a common feature of all gravitational valves, which should always be implanted with caution, taking into account the usual risk factors for serious consequences from overdrainage ( thin cerebral mantle and a high brain compliance) and the possible lifestyle of the patient after surgery.
Contrary to other gravitational valves, the Chhabra shunt does not have any valve working with the weighting balls that would prevent the reflux of CSF from the peritoneal cavity to the ventricles. Therefore, CSF reflux is possible, undoubtedly in the upside down and also in the horizontal body position (albeit at a lower rate).
The behaviour of the valve in a strong magnetic field (1.5 T) does not exhibit any alarming variation. However, the artefact on MRI may be considerable.
In conclusion, the intentions of the designers of the Chhabra shunt to make it operate at a higher operating pressure in the vertical than the horizontal body position were confirmed during this evaluation. The shunt has a repeatable pressure-flow performance that does not differ from the performance of more expensive valves manufactured by the big western corporations. However, because the shunt has a very low resistance to flow, overdrainage may be a problem depending on the patient’s life activity. Reflux of CSF from the peritoneal to the ventricular cavity is possible.
We are very grateful to Mr R Hayward and Dr P Mittal for providing us with Chhabra shunts for evaluation. We thank the Department of Health Medical Devices Agency for funding the laboratory.
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