How to Choose a Shunt for Patients with Normal Pressure Hydrocephalus: A Short Guide to Selecting the Best Shunt Assembly

Abstract

Most patients with hydrocephalus are still managed with the implantation of a cerebrospinal fluid (CSF) shunt in which the CSF flow is regulated by a differential-pressure valve (DPV). Our aim in this review is to discuss some basic concepts in fluid mechanics that are frequently ignored but that should be understood by neurosurgeons to enable them to choose the most adequate shunt for each patient. We will present data, some of which is not provided by manufacturers, which may help neurosurgeons in selecting the most appropriate shunt. To do so, we focused on the management of patients with idiopathic "normal-pressure hydrocephalus" (iNPH), as one of the most challenging scenarios, in which the combination of optimal technology, patient characteristics, and knowledge of fluid mechanics can significantly modify the surgical results. For a better understanding of the available hardware and its evolution over time, we will have a second look at the design of the first DPV and the reasons why additional devices were incorporated to control for shunt overdrainage and its related complications. We try to persuade the reader that a clear understanding of the physical concepts of the CSF and shunt dynamics is key to understand the pathophysiology of iNPH and to improve its treatment.

Keywords: antisiphon device; differential-pressure valves; fluid mechanics; hydrocephalus; normal-pressure hydrocephalus; shunt overdrainage.

Figure 7

(Left): Schematic drawing of a spring-loaded ball-in-cone mechanical valve. Arrow shows the direction of the flow. (Right): Figure 2 of the patent filed by S. Hakim [25] with a detailed drawing of the valve that the patent describes as “a pair of spring operated ball-type check valves made of metal or other relatively inert and non-toxic material not affected by temperatures in the surgical sterilization range. The twin valves are arranged in series to face in the same direction (outlet of one adjacent to the inlet of the other)” [25].

Figure 9

Estimated in vitro flow through silicon tubes of 100 cm constant length and with an inner diameter (ID) varying from 0.4 to 1.2 mm at different differential pressures (DPs) (0–60 cmH₂O; 0–44 mmHg). The reader can replicate these data by using the online calculator provided by vCalc™ (http://bit.ly/2Y9FEss) and considering that the viscosity of water at a temperature of 37 °C is 0.69 mPa-s [48]. A tube with an ID of 1.2 mm has a theoretical R_shunt of 1.72 mmHg/mL/min, of 2.43 mmHg/mL/min when the ID is decreased to 1.1, and of 139 mmHg/mL/min when the ID is 0.40 mm. At a DP of 60 cmH₂O, the maximum flow for each tube will increase from 19 mL/h for a tube with an ID of 0.4 to 1539 mL/h for a tube with an ID of 1.2 mm. The reader needs to be aware that theoretical estimates in R_shunt reported here can differ from some published results on bench tests. Theoretical estimates may differ from results on tubes conducted in test rigs because of differences in the catheter length, presence of air bubbles, experimental temperature, composition of the infused fluid, and characteristics of the flow (pulsatile vs. steady).

Figure 1

(Left). Reproduced with permission from A. Aschoff. Three different Holter valve designs are shown. The first valve on top is the oldest model (circa 1970), more recent designs in the middle and on the bottom. This valve was discontinued and no longer available. (Right). Reproduced from Figure 2 and Figure 3 of the patent filed 2 October 1956 by Holter and Spitz [12]. Both figures are enlarged sections showing the inlet (FIG. 2) and outlet valves (FIG. 3) of the device. Labels 22 and 38 show the inlet and outlet tubes with lateral slits “three-sixteenth inch in length” (0.80 cm), which functions as the actual valve.

Figure 2

(Top). In this figure, the pressures defining the perfusion pressure of the valve (PP_valve) are shown. P₁ is the pressure at the inlet of the valve and P₂ at the outlet. P₁ is the sum of the intraventricular pressure (IVP) and the gravitational-induced hydrostatic pressure (HP). P₂ is the sum of the closing pressure (CP) of the valve and the intraperitonal pressure (IPP) if the distal catheter is placed in the peritoneum or the intra-atrial pressure (IAP) if the distal tip is placed within the right atrium. (Bottom). When the shunted patient is recumbent, the hydrostatic difference between the tips of the ventricular and intraperitoneal catheters is negligible and ~0 mmHg; therefore, the HP in the equation that calculates the PP_valve can be removed.

Figure 3

This figure shows the changes in PP_valve when a shunted patient is standing. In this scenario, the IVP is ≤0 mmHg and the gravitational-induced HP is equal to the height between the tips of the ventricular and distal catheters. In a 173 cm height adult, this difference can be equal to 70 cm, which will generate a gravitational HP of 51.2 mmHg. If the patient has an implanted differential-pressure valve (DPV) with an opening pressure (OP) of 5 cmH₂O (3.7 mmHg) without any gravitational or antisiphon device, and assuming an IPP of 0–3 mmHg, these conditions will generate a PP_valve in a standing patient of ~44 mmHg and therefore the valve will be permanently open when patient is sitting or standing.

Figure 4

Figure reproduced with permission from the Natus Neurosurgery and Neurocritical CareProduct and Accessory Catalog, 2019 (https://natus.com, accessed on 23 June 2020). In this graph, the flow–pressure data for the Contour-Flex™ family of valves are shown. Contour-Flex™ are typical silicone diaphragm valves with a low–medium- and high-pressure ranges. Flow rates in the x-axis represent a variable flow rate from 1 to 50 mL/h while in the y-axis the pressure generated at each flow rate is shown. Colored bands define the variability of the pressures obtained for each flow rate.

Figure 5

(A). Schematic drawing of a silicone slit valve. Slit valves are essentially silicon tubes that are closed at the end and have slits in their sides. On the left, a closed tube with a lateral slit is shown. On the right, the slit opens and allows CSF flow when the pressure inside the tube exceeds the predefined OP setting defined by the manufacturer. Arrows indicate the direction of CSF flow. (B). On the left, an opener miter valve is shown. The two silicon leaflets open when CSF inside the shunt exceeds the predefined OP. (B). On the right, a drawing of a diaphragm valve is shown. When CSF in the circuit exceeds the OP of the valve, the diaphragm moves down and allows the fluid to pass through. Arrows indicate the direction of the flow.

Figure 6

(Left). The Chabbra^TM “slit n’ spring” valve (Surgiwear Limited, Shahjahanpur, India) is a design based in the slit valve. In this case, the silicone tube within the flushing reservoir in which the slit seat is protected by a stainless-steel spring. The red arrow indicates the direction of the flow. (Right). Radiograph of a Codman–Hakim^TM Precision low-medium DPV (Integra LifeSciences Corporation). This DPV is a conventional mechanical spring-loaded ball-in-cone valve. The three dots indicate the OP of the valve and the short arrowhead the direction of the flow.

Figure 8

This 13-year-old girl had surgery for a Chiari 1 malformation with hydrocephalus. She had a Hakim programmable valve implanted with an OP of 70 mmH₂O. The patient was scanned in a 1.5 T MRI for control (A,B). In (A), the ventricular size was normal. Arrows show the magnetic artifact produced in the different MR1 sequences that was moderate in T2-weighted images and very strong in the diffusion-weighted sequences (B). The patient’s valve was accidentally reprogrammed by the doctor on call to an OP of 150 mmH₂O immediately after the MRI. She was admitted three days later because of headache and drowsiness. The CT scan on admission (C) showed a significant increase in the ventricular size. The OP of the valve was readjusted to 70 mmH₂O. The patient improved and the new CT scan at discharge is shown in (D).