Laboratory study on "intracranial hypotension" created by pumping the chamber of a hydrocephalus shunt
© Bromby et al; licensee BioMed Central Ltd. 2007
Received: 09 January 2007
Accepted: 26 March 2007
Published: 26 March 2007
It has been reported that pumping a shunt in situ may precipitate a proximal occlusion, and/or lead to ventricular over-drainage, particularly in the context of small ventricles. In the laboratory we measured the effect of pumping the pre-chamber of hydrocephalus shunts on intracranial hypotension.
Materials and methods
A simple physical model of the CSF space in a hydrocephalic patient was constructed with appropriate compliance, CSF production and circulation. This was used to test eleven different hydrocephalus shunts. The lowest pressure obtained, the number of pumps needed to reach this pressure, and the maximum pressure change with a single pump, were recorded.
All models were able to produce negative pressures ranging from -11.5 mmHg (Orbis-Sigma valve) to -233.1 mmHg (Sinu-Shunt). The number of pumps required reaching these levels ranged from 21 (PS Medical LP Reservoir) to 315 (Codman Hakim-Programmable). The maximum pressure change per pump ranged from 0.39 mmHg (Orbis-Sigma valve) to 23.1 (PS Medical LP Reservoir).
Patients, carers and professionals should be warned that 'pumping' a shunt's pre-chamber may cause a large change in intracranial pressure and predispose the patient to ventricular catheter obstruction or other complications.
It has been reported previously that hydrocephalus shunts may cause over-drainage, in particular, during changes in posture. Kajimoto et al  postulated that this over-drainage was due to increased hydrostatic pressure in the ventriculoperitoneal shunt system. This increases the differential pressure acting across a shunt of relatively low hydrodynamic resistance  and may provoke excessive drainage, leading to intracranial hypotension.
The pumping of a shunt's pre-chamber has been used to test shunt patency . However, the specificity and sensitivity of such testing were assessed as not satisfactory. Historically, some patients and their families were encouraged to pump the shunts periodically to avoid blockage of the valve or to relieve headaches. However, such a maneuver may possibly lead to over-drainage.
Low intracranial pressure may result in headache, nausea and vomiting, diplopia, lethargy, paresis of upwards gaze and strabismus, dizziness and hearing disturbances. These symptoms mainly occur when the patient is upright and active . The orthostatic headache is thought to be the result of a downward displacement of the brain. When a person is upright, the brain is kept afloat by the buoyant action of the CSF in conjunction with the anchoring effects of the vascular structures in the cranium; if the buoyant action of the CSF is decreased i.e. when the CSF volume is decreased, the burden on the vascular structures increases resulting in traction and distortion . Diencephalic compression of the brain due to downward dislocation has also been reported to decrease consciousness [6, 7]. As these vascular structures in the cranium are pain-sensitive , orthostatic headache occurs. In terminal conditions, CSF hypovolaemia may result in subdural haemorrhage due to the tearing of the bridging veins as the brain pulls away from the dura [9, 10]. However, there is no clinical report of this happening following shunt pumping.
CSF drainage through some shunt systems, may be accelerated by 'pumping' the shunt pre-chamber. So far, 'pumping' has been evaluated quantitatively with results reported in the form of various conference presentations [11, 12], but not in peer-reviewed journals. We have built a physical model of CSF circulation and compensation, 'shunted' it and investigated eleven shunts in the laboratory, to determine to what extent CSF pressure may be reduced by repetitive pumping.
Materials and methods
Name, manufacturer, sub-type, performance levels, and catalogue numbers of tested shunts.
Micro Valve and standard
3 cmH2O and 20 cmH2O
Hakim Programmable with Siphon-Guard
3 cm H2O and 20 cmH2O
Small and regular
Performance 0.5 and 2.5
Contoured, standard, burr-hole
Unknown, (only one type available)
Integra Neuroscience Implants, France
Unknown, (only one type available)
All shunts were differential pressure valves with the exception of the Orbis-Sigma valve and the Diamond valve, which work by a principle of stabilizing flow over wide range of differential pressures.
The Codman Programmable Valve (with Siphon-Guard), the Medtronic Delta and Strata Valves all had siphon-preventing devices.
Pressure was recorded using a Gaeltec Luer Lock transducer with an accuracy of pressure measurement better than +/- 5 mm Hg over the range -250 to 250 mm Hg. A pressure waveform calibrator was used to simulate the ICP pulse pressure with amplitude of 1 mmHg and a rate of 90 beats per minute.
The number of pumps taken to reach the asymptote was measured from the 'switching point' where the model changed from the high compliance of the plateau of the pressure-volume curve to that of the first slope to the point where an asymptote was reached (Fig. 3).
The maximum pressure change in a single pump (ΔPmax/pump) was measured at the point of the maximum pressure change over time on the recorded curve, i.e. where the curve like in Fig. 3 had minimal first derivative. One sample of each kind of valve was tested. The test was repeated five times for each valve.
The One-Way Analysis of Variance (ANOVA, Tukey test) was used where the normality test was passed. Where this was not passed, the Kruskal-Wallis One-Way ANOVA on Ranks (Dunn's Method) was used.
Two models with a higher compliance were built. The Orbis-Sigma valve and the Sinu-Shunt were retested using these models. The asymptotes achieved were not significantly different on each model with the same shunt. The time to reach the asymptote was not significantly different when testing the Orbis-Sigma valve but was significantly longer in the Sinu-Shunt (P < 0.001 One-way ANOVA Tukey test).
It can always be disputed whether in-vitro testing is clinically relevant. We used a model that increased the volume of CSF in a system that had a total volume of 270 ml. This volume is not uncommon in hydrocephalus with gross ventricular dilatation. The model has a relatively low compliance and the width of the horizontal section of pressure-volume curve was chosen somewhat arbitrarily. There is little data regarding this width, although some reports give values around 5 ml . Therefore, we think that a comparison of valves in terms 'how many pumps are needed to reach steeper section of P/V curve' or 'how many pumps are safe' may be misleading. The most important message, repeated after other studies [11, 12], is that pumping the chamber of any valve has the potential to reduce proximal CSF pressure significantly, to the extent that clinically relevant over-drainage would be possible.
Patients and families should be advised against pumping, particularly when the ventricles are small. It is possible for a physician to test the valve using a single compression: however such tests are unreliable [3, 11, 12]. The majority of shunts were able to produce an intracranial pressure of less than -30 to -35 cmH2O (-22 to -26 mmHg). This falls within the range that can produce orthostatic headaches . It is at this pressure that the vasodilatation compensation for CSF depletion is no longer sufficient, and thus the symptoms of CSF hypovolaemia become apparent .
There is a range of differences between the shunts, even between different models from the same manufacturer. In terms of the recorded negative pressures, the Orbis-Sigma was the best performer, and the Sinu-Shunt the worst. The number of strokes required to reach the asymptotes showed that the LP Shunt was the fastest, probably due to its large pumping reservoir. With Codman Programmable valves, the use of the Siphon-Guard, programmed to a higher performance level, increased the number of strokes required to reach the asymptote but did not affect the position of the asymptote.
Testing the two valves on the higher compliance models, demonstrated the validity of the model in terms of modeling the CSF space and compliance in the hydrocephalic patient. This was shown in both the worst-case scenario (low compliance) and in other hydrocephalic situations since the asymptotes remained unchanged. In patients with a higher compliance than the model, it may be expected that they reach the same asymptote but require a longer period of pumping to get there.
Flow control valves (Orbis-Sigma and Diamond Valve) minimize the degree of intracranial hypotension during pumping. The presence or absence of a siphon-control device has no effect on intracranial hypotension caused by pumping.
Pumping of shunt pre-chambers may cause gross intracranial hypotension in a relatively short time. The number of pumps and time needed for producing possibly detrimentally low levels of ICP depends on the shunt type.
The UK Shunt Evaluation Laboratory was established by the UK Department of Health, but from 1997 is maintained partially by UK Shunt Registry and R&D grants from shunt manufacturers: Medtronic, Codman, Aesculap, Sinu-Shunt, Spiegelberg, Sophysa. All the reports and publications are independent and based solely on independent test conducted in the Laboratory. M. C. is on unpaid leave from Warsaw University of Technology, Poland.
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