A standard way of management in communicating hydrocephalus is to facilitate the cerebrospinal fluid (CSF) drainage by an implantation of a hydrocephalus shunt. As shunting is an almost purely mechanical treatment, which radically affects pressure-volume compensation, ideally the hydrodynamics of patient's own compensation should be examined before a shunt is implanted.
Testing of CSF dynamics, although invasive, may help with the decision about surgery. It also provides basic information for further management of shunted patients, when complications, such as shunt blockage, under- and over-drainage, arise. In such cases, physiological measurement may aid the decision about revision of the shunt.
Almost all authors agree that in hydrocephalus the drainage of CSF is disturbed [1,2]. This may be expressed quantitatively by an elevated resistance to CSF outflow. The limit of an increased resistance is reported to range from 13 (in younger patients) to 18 mmHg mL −1min−1. Normal resistance ranges from 6 to 10 mmHg mL−1min−1 [3,4].
The computerized infusion test  is a modification of the traditional constant-rate infusion as described by Katzman and Hussey . The method requires a fluid infusion to be made into any accessible CSF compartment. A lumbar infusion, even if it has understandable limitations, is less invasive than ventricular infusion. However, the most frequent approach in our centre is an intraventricular infusion into a subcutaneously positioned reservoir, connected to an intraventricular catheter or shunt antechamber. In such cases, two hypodermic needles (25-G) are used: one for the pressure measurement and the second for the infusion.
During the infusion, the computer calculates (for details see http://www.neurosurg.cam.ac.uk/icmplus) and presents mean pressure and pulse amplitude (with time along the x-axis - see also Fig. 1). The resistance to CSF outflow can be estimated using simple arithmetic as the difference between the value of the plateau pressure during infusion and the resting pressure divided by the infusion rate. However, in many cases strong vasogenic waves or an excessive elevation of the pressure above the safe limit of 40 mmHg do not allow the precise measurement of the final pressure plateau. Computerized analysis produces results even in difficult cases when the infusion is terminated prematurely (i.e. without reaching the end-plateau). The algorithm utilizes a time series analysis for volume-pressure curve retrieval, the least-mean-square model fitting and an examination of the relationship between the pulse amplitude and the mean CSF pressure. Apart from resting CSF pressure and the resistance to CSF outflow, the elastance coefficient or pressure-volume index, cerebrospinal compliance, CSF formation rate and the pulse wave amplitude of CSF pressure are estimated.
However, not all patients presenting with abnormal CSF circulation may improve after shunting. As the positive predictive power of infusion study is usually reported as satisfactory, some patients with apparently normal profile of CSF circulation may still get better after surgery . Therefore, infusion test does not offer a definite indication for the management of hydrocephalus. It should always be interpreted in conjunction with other forms of investigations (neuropsychological, brain imaging, gait analysis, CSF tap test or diagnostic drainage, vascular reactivity, biochemical composition of CSF, etc). Still, infusion study can provide information about the functioning of the CSF circulation system, important in the whole management process of a hydrocephalic patient, both as a baseline reference and as a diagnostic tool for a shunt assessment in vivo or efficiency of a third ventriculostomy .
In hydrocephalus, a shunt is used to drain excess CSF to elsewhere in the body according to a pressure difference between inlet (ventricles) and outlet (peritoneal or atrial) compartments. Ideally, the resistance of an open shunt taken together with the natural CSF outflow resistance (usually increased in hydrocephalus) should be close to the normal resistance to CSF outflow. The flow through the shunt should not depend on the body posture or be affected by body temperature, external pressure (within the physiological range for subcutaneous pressure) or the pulsatile component of CSF pressure.
After shunting, the model of CSF space should be supplemented by the branch representing property of the shunt. The most sensitive indicator of the shunt partial blockage is the steady-state level achieved during the test . With known value of the shunt pressure-flow curve (opening pressure and its hydrodynamic resistance where the curve is quasi-linear), the critical threshold may be evaluated for each individual type of shunt:
5 mmHg + shunt opening pressure + infusion rate × hydrodynamic resistance of shunt.
Even in cases when a patient initially benefits from shunting, some complications related to the shunt performance can occur. Also, the patient may present with symptoms (specifically those elderly with normal-pressure hydrocephalus (NPH)) not always resulting from the shunt malfunction but clinically ambiguous.
To illustrate how the infusion study can assist the management of hydrocephalus, the case of a 70-yr-old female presenting with widened ventricles (bi-candate index, BCI = 0.42) and no neurological symptoms with exception of wide gate is discussed. She was first seen in a specialist Hydrocephalus Clinic and than Ommaya reservoir was inserted to perform an infusion study as a day case.
The study (Fig. 1) indicated normal opening pressure with increased resistance to CSF outflow. In pressure recording, vasogenic waves were clearly visible. The pulse amplitude was very much changing proportionally to changes in the mean intracranial pressure (ICP). All these four findings, normal pressure, increased resistance to CSF outflow, vasogenic waves present in the recording and a good response of pulse amplitude to the change in a mean pressure, are specific and suggested NPH.
Following the study a ventriculo-peritoneal shunt with a Strata valve (PS Medical Inc., Goleta, CA, USA) set at 1.5 was inserted (connected to Ommaya). The patient improved after shunting although she still had numbers of falls since the insertion of a shunt. MRI scan was performed and revealed chronic ventriculomegaly. Another infusion study was performed, through Ommaya reservoir (Fig. 2).
This time, the study revealed normal opening pressure, plateau pressure less than the critical level for her shunt and definitely lower than achieved during the first test. The resistance to CSF outflow was also reduced and vasogenic waves were limited.
There was no indication for shunt malfunction. The opening pressure was 7.7 mmHg, with the pulse amplitude present in the recording. The plateau pressure during infusion was 17.1 mmHg and it increased in response to the valve occlusion (around 1:10p.m.). In the sitting position the pressure decreased to around −0.2 mmHg.
One year later she was seen in the Clinic again with worsening of gait and headaches. Infusion test was arranged (Fig. 3).
This time the shunt system proved to underdrain. The opening pressure was 11.5 mmHg (with a pulse waveform present in the signal). During the infusion phase the pressure increased to around 30 mmHg, e.g. over the limit for the valve setting.
In the sitting position the pressure stabilized at around 6 mmHg. Strong vasogenic waves were recorded during the test. Resistance to CSF outflow was increased in comparison to the previous test.
The shunt was revised. During surgery some debris was found in the shunt pre-chamber and partial blockage of the distal tubing was confirmed. Following the revision the patient improved.
Infusion study is a valuable tool in the proper management of hydrocephalus. Baseline test is very helpful to follow the changes in CSF dynamics in time. Apart from an opening pressure and a resistance to CSF outflow, pulse amplitude of ICP and a content of vasogenic waves are useful to gauge CSF dynamics. Little is known about clinical value of elastance, perhaps with its use in third ventriculostomy .
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