Hydrodynamic properties of hydrocephalus shunts: United Kingdom Shunt Evaluation Laboratory

Toward the "Perfect" Shunt: Historical Vignette, Current Efforts, and Future Directions

As a concept, drainage of excess fluid volume in the cranium has been around for more than 1000 years. Starting with the original decompression-trepanation of Abulcasis to modern programmable shunt systems, to other nonshunt-based treatments such as endoscopic third ventriculostomy and choroid plexus cauterization, we have come far as a field. However, there are still fundamental limitations that shunts have yet to overcome: namely posture-induced over- and underdrainage, the continual need for valve opening pressure especially in pediatric cases, and the failure to reinstall physiologic intracranial pressure dynamics. However, there are groups worldwide, in the clinic, in industry, and in academia, that are trying to ameliorate the current state of the technology within hydrocephalus treatment. This chapter aims to provide a historical overview of hydrocephalus, current challenges in shunt design, what members of the community have done and continue to do to address these challenges, and finally, a definition of the "perfect" shunt is provided and how the authors are working toward it.

Design and Characterization of an Adjustable Passive Flow Regulator and Application to External CSF Drainage

Passive valves that deliver a constant flow rate regardless of inlet pressure changes have numerous applications in research, industry, and medical fields. The present article describes a passive spring valve that can be adjusted manually to deliver the required flow rate. The valve consists of a movable rod with an engraved microchannel. The fluidic resistance of the device varies together with the inlet pressure to regulate the flow rate. A prototype was made and characterized. Flow-rate adjustment up to +/-30% of the nominal flow rate was shown. A simple numerical model of the fluid flow through the device was made to adapt the design to external ventricular drainage of cerebrospinal fluid (CSF). Some insights about the implementation of this solution are also discussed.

Revision rates of flow- versus pressure-regulated ventricular shunt valves for the treatment of hydrocephalus in neonates following germinal matrix haemorrhage-a retrospective review

PURPOSE

Neonates with ventricular shunts inserted for hydrocephalus following germinal matrix haemorrhage (GMH) have high rates of shunt revision. The shunt valve plays a key role in regulating the function of the shunt. In this study, we aim to determine if the choice of flow-regulated or pressure-regulated valve used in the initial implantation of a shunt affects the rate of shunt revision.

METHODS

A retrospective cohort comparison study was performed on 34 neonates with hydrocephalus following GMH who underwent placement of a ventricular shunt at the Queensland Children's Hospital from November 2014 to June 2020. The primary outcome examined was the need for revision or replacement of the ventricular shunt after successful initial placement within 2 years of implantation. The secondary outcome examined was the survival time of the shunt.

RESULTS

16 patients had placement of a flow-regulated valve, and 18 patients had placement of a pressure-regulated valve. 14 (87.5%) patients with flow-regulated valves required replacement during the follow-up period. 2 (18.18%) patients with a fixed pressure regulated underwent revision, while 2 (28.57%) programmable pressure-regulated shunts required revision. Patients that had a flow-regulated valve had a statistically significant higher rate of revision compared to those who had a pressure-regulated valve, (87.5% flow vs 22.22% pressure) with a P-value of < 0.001. Valve obstruction was also more common in patients with flow-regulated valves than pressure-regulated valves (4 vs 0) with a P-value of 0.010. Overall mean median survival time was 22.06 months, shunts with flow-regulated valves had a shorter median survival time of 3.19 months compared with over 24 months for pressure-regulated valves with a P-value of < 0.001.

CONCLUSION

Our study suggests that the initial implantation of flow-regulated valves may carry an increased total rate of shunt revision and valve obstruction within the first 2 years following implantation compared to pressure-regulated valves in patients with hydrocephalus following GMH.

Hydrogel Check-Valves for the Treatment of Hydrocephalic Fluid Retention with Wireless Fully-Passive Sensor for the Intracranial Pressure Measurement

Hydrocephalus (HCP) is a neurological disease resulting from the disruption of the cerebrospinal fluid (CSF) drainage mechanism in the brain. Reliable draining of CSF is necessary to treat hydrocephalus. The current standard of care is an implantable shunt system. However, shunts have a high failure rate caused by mechanical malfunctions, obstructions, infection, blockage, breakage, and over or under drainage. Such shunt failures can be difficult to diagnose due to nonspecific systems and the lack of long-term implantable pressure sensors. Herein, we present the evaluation of a fully realized and passive implantable valve made of hydrogel to restore CSF draining operations within the cranium. The valves are designed to achieve a non-zero cracking pressure and no reverse flow leakage by using hydrogel swelling. The valves were evaluated in a realistic fluidic environment with ex vivo CSF and brain tissue. They display a successful operation across a range of conditions, with negligible reverse flow leakage. Additionally, a novel wireless pressure sensor was incorporated alongside the valve for in situ intracranial pressure measurement. The wireless pressure sensor successfully replicated standard measurements. Those evaluations show the reproducibility of the valve and sensor functions and support the system’s potential as a chronic implant to replace standard shunt systems.

Advances in CSF shunt devices and their assessment for the treatment of hydrocephalus

INTRODUCTION

Hydrocephalus is a neurological disorder caused by excessive accumulation of the cerebrospinal fluid (CSF) in the ventricles of the brain. It can be treated by diverting the extra fluid to different parts of the body using a device called a shunt. This paper reviews different shunt devices that are used for this purpose.

AREAS COVERED

Shunts have high failure rates either due to infection or mechanical failure, therefore there is still ongoing work to address these two main handicaps. They require additional devices for performance assessment. Here, the paper also reviews different approaches for assessing shunt limitations. Moreover, future prospects are also discussed.

EXPERT OPINION

This study shows that shunt devices still remain an important treatment option for hydrocephalus. However, further efforts are required to design more advanced shunts, to eliminate high failure rates in clinical use. Sophisticated sensor systems that can accurately detect and regulate changes in CSF drainage to optimize drainage for individual needs. Moreover, shunt infection problem is still present despite recent improvements such as antibiotic impregnated catheters.

CSF Dynamics for Shunt Prognostication and Revision in Normal Pressure Hydrocephalus

BACKGROUND

Despite the quantitative information derived from testing of the CSF circulation, there is still no consensus on what the best approach could be in defining criteria for shunting and predicting response to CSF diversion in normal pressure hydrocephalus (NPH).

OBJECTIVE

We aimed to review the lessons learned from assessment of CSF dynamics in our center and summarize our findings to date. We have focused on reporting the objective perspective of CSF dynamics testing, without further inferences to individual patient management.

DISCUSSION

No single parameter from the CSF infusion study has so far been able to serve as an unquestionable outcome predictor. Resistance to CSF outflow (Rout) is an important biological marker of CSF circulation. It should not, however, be used as a single predictor for improvement after shunting. Testing of CSF dynamics provides information on hydrodynamic properties of the cerebrospinal compartment: the system which is being modified by a shunt. Our experience of nearly 30 years of studying CSF dynamics in patients requiring shunting and/or shunt revision, combined with all the recent progress made in producing evidence on the clinical utility of CSF dynamics, has led to reconsidering the relationship between CSF circulation testing and clinical improvement.

CONCLUSIONS

Despite many open questions and limitations, testing of CSF dynamics provides unique perspectives for the clinician. We have found value in understanding shunt function and potentially shunt response through shunt testing in vivo. In the absence of infusion tests, further methods that provide a clear description of the pre and post-shunting CSF circulation, and potentially cerebral blood flow, should be developed and adapted to the bed-space.

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

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.

Gravitational shunt valves in hydrocephalus to challenge the sequelae of over-drainage

INTRODUCTION

In hydrocephalus treatment, ventriculo-peritoneal shunts (VPS) have become the most relevant therapy for seven decades among other treatment options. Due to the hydrostatic pressure in vertical position, CSF diversion is somehow non-physiological. The integration of gravitational valves in VPS was established to counteract the hydrostatic draining force and to approach a physiological condition of the cerebrospinal diverting system. Numerous clinical studies have shown that gravitational valves are able to reduce secondary complications of VPS treatment. It remains a challenge for the treating neurosurgeon to select the correct valve resistance based on individual anatomies and different etiologies of hydrocephalus as well as varying levels of activity of the patient.

AREAS COVERED

This review covers the development of gravitational shunt valves from historical, theoretical and clinical aspects for pediatric and adult etiologies of hydrocephalus. We discuss the role of gravitational shunt valves in preventing over-drainage issues and present the state-of-the-art literature. Furthermore, ongoing prospective trials are presented.

EXPERT OPINION

Counteracting the hydrostatic force by selecting the correct valve in a VPS system to achieve physiological balance in CSF diversion during vertical and horizontal body changes has become the current standard for hydrocephalus management. Gravitational shunt valves reliably address this need to minimize over-drainage events in the vertical position without affecting the CSF flow in the horizontal position. The results of ongoing prospective studies on the safety and efficacy of adjustable gravitational valves are still pending. Due to the complexity of the CSF flow, lifelong follow-up care for patients with VPS is critical.

Three-Dimensionally Printed Microelectromechanical-System Hydrogel Valve for Communicating Hydrocephalus

Hydrocephalus (HCP) is a chronic neurological brain disorder caused by a malfunction of the cerebrospinal fluid (CSF) drainage mechanism in the brain. The current standard method to treat HCP is a shunt system. Unfortunately, the shunt system suffers from complications including mechanical malfunctions, obstructions, infections, blockage, breakage, overdrainage, and/or underdrainage. Some of these complications may be attributed to the shunts' physically large and lengthy course making them susceptible to external forces, siphoning effects, and risks of infection. Additionally, intracranial catheters artificially traverse the brain and drain the ventricle rather than the subarachnoid space. We report a 3D-printed microelectromechanical system-based implantable valve to improve HCP treatment. This device provides an alternative approach targeting restoration of near-natural CSF dynamics by artificial arachnoid granulations (AGs), natural components for CSF drainage in the brain. The valve, made of hydrogel, aims to regulate the CSF flow between the subarachnoid space and the superior sagittal sinus, in essence, substituting for the obstructed arachnoid granulations. The valve, operating in a fully passive manner, utilizes the hydrogel swelling feature to create nonzero cracking pressure, P_T ≈ 47.4 ± 6.8 mmH₂O, as well as minimize reverse flow leakage, Q_O ≈ 0.7 μL/min on benchtop experiments. The additional measurements performed in realistic experimental setups using a fixed sheep brain also deliver comparable results, P_T ≈ 113.0 ± 9.8 mmH₂O and Q_O ≈ 3.7 μL/min. In automated loop functional tests, the valve maintains functionality for a maximum of 1536 cycles with the P_T variance of 44.5 mmH₂O < P_T < 61.1 mmH₂O and negligible average reverse flow leakage rates of ∼0.3 μL/min.

Enhanced in vitro model of the CSF dynamics

BACKGROUND

Fluid dynamics of the craniospinal system are complex and still not completely understood. In vivo flow and pressure measurements of the cerebrospinal fluid (CSF) are limited. Whereas in silico modeling can be an adequate pathway for parameter studies, in vitro modeling of the craniospinal system is essential for testing and evaluation of therapeutic measures associated with innovative implants relating to, for example, normal pressure hydrocephalus and other fluid disorders. Previously-reported in vitro models focused on the investigation of only one hypothesis of the fluid dynamics rather than developing a modular set-up to allow changes in focus of the investigation. The aim of this study is to present an enhanced and validated in vitro model of the CSF system which enables the future embedding of implants, the validation of in silico models or phase-contrast magnetic resonance imaging (PC-MRI) measurements and a variety of sensitivity analyses regarding pathological behavior, such as reduced CSF compliances, higher resistances or altered blood dynamics.

METHODS

The in vitro model consists of a ventricular system which is connected via the aqueduct to the cranial and spinal subarachnoid spaces. Two compliance chambers are integrated to cushion the arteriovenous blood flow generated by a cam plate unit enabling the modeling of patient specific flow dynamics. The CSF dynamics are monitored using three cranial pressure sensors and a spinal ultrasound flow meter. Measurements of the in vitro spinal flow were compared to cervical flow data recorded with PC-MRI from nine healthy young volunteers, and pressure measurements were compared to the literature values reported for intracranial pressure (ICP) to validate the newly developed in vitro model.

RESULTS

The maximum spinal CSF flow recorded in the in vitro simulation was 133.60 ml/min in the caudal direction and 68.01 ml/min in the cranial direction, whereas the PC-MRI flow data of the subjects showed 122.82 ml/min in the caudal and 77.86 ml/min in the cranial direction. In addition, the mean ICP (in vitro) was 12.68 mmHg and the pressure wave amplitude, 4.86 mmHg, which is in the physiological range.

CONCLUSIONS

The in vitro pressure values were in the physiological range. The amplitudes of the flow results were in good agreement with PC-MRI data of young and healthy volunteers. However, the maximum cranial flow in the in vitro model occurred earlier than in the PC-MRI data, which might be due to a lack of an in vitro dynamic compliance. Implementing dynamic compliances and related sensitivity analyses are major aspects of our ongoing research.

In vitro performance of the fixed and adjustable gravity-assisted unit with and without motion-evidence of motion-induced flow

BACKGROUND

Anti-siphon devices and gravitational-assisted valves have been introduced to counteract the effects of overdrainage after implantation of a shunt system. The study examined the flow performance of two gravitational-assisted valves (shunt assistant - SA and programmable shunt assistant - proSA, Miethke & Co. KG, Potsdam, Germany) in an in vitro shunt laboratory with and without motion.

METHODS

An in vitro laboratory setup was used to model the cerebrospinal fluid (CSF) drainage conditions similar to a ventriculo-peritoneal shunt and to test the SA (resistance of +20 cmH2O in 90°) and proSA (adjustable resistance of 0 to +40 cmH2O in 90°). The differential pressure (DP) through the simulated shunt and tested valve was adjusted between 0 and 60 cmH2O by combinations of different inflow pressures (40, 30, 20, 10, and 0 cmH2O) and the hydrostatic negative outflow pressure (0, -20, and -40 cmH2O) in several differing device positions (0°, 30°, 60°, and 90°). In addition, the two devices were tested under vertical motion with movement frequencies of 2, 3, and 4 Hz.

RESULTS

Both gravity-assisted units effectively counteract the hydrostatic effect in relation to the chosen differential pressure. The setting the proSA resulted in flow reductions in the 90° position according to the chosen resistance of the device. Angulation-related flow changes were similar in the two devices in 30-90° position, however, in the 0-30° position, a higher flow is seen in the proSA. Repeated vertical movement significantly increased flow through both devices. While with the proSA a 2-Hz motion was not able to induce additional flow (0.006 ± 0.05 ml/min), 3- and 4-Hz motion significantly induced higher flow values (3 Hz: +0.56 ± 0.12 ml/min, 4 Hz: +0.54 ± 0.04 ml/min). The flow through the SA was not induced by vertical movements at a low DP of 10 cmH2O at all frequencies, but at DPs of 30 cmH2O and higher, all frequencies significantly induced higher flow values (2 Hz: +0.36 ± 0.14 ml/min, 3 Hz: +0.32 ± 0.08 ml/min, 4 Hz: +0.28 ± 0.09 ml/min).

CONCLUSIONS

In a static setup, both tested valves effectively counteracted the hydrostatic effect according to their adjusted or predefined resistance in vertical position. Motion-induced increased flow was demonstrated for both devices with different patterns of flow depending on applied DP and setting of the respective valve. The documented increased drainage should be considered when selecting appropriate valves and settings in very active patients.

A physical framework for implementing virtual models of intracranial pressure and cerebrospinal fluid dynamics in hydrocephalus shunt testing

OBJECTIVE The surgical placement of a shunt designed to resolve the brain's impaired ability to drain excess CSF is one of the most common treatments for hydrocephalus. The use of a dynamic testing platform is an important part of shunt testing that can faithfully reproduce the physiological environment of the implanted shunts. METHODS A simulation-based framework that serves as a proof of concept for enabling the application of virtual intracranial pressure (ICP) and CSF models to a physical shunt-testing system was engineered. This was achieved by designing hardware and software that enabled the application of dynamic model-driven inlet and outlet pressures to a shunt and the subsequent measurement of the resulting drainage rate. RESULTS A set of common physiological scenarios was simulated, including oscillations in ICP due to respiratory and cardiac cycles, changes in baseline ICP due to changes in patient posture, and transient ICP spikes caused by activities such as exercise, coughing, sneezing, and the Valsalva maneuver. The behavior of the Strata valve under a few of these physiological conditions is also demonstrated. CONCLUSIONS Testing shunts with dynamic ICP and CSF simulations can facilitate the optimization of shunts to be more failure resistant and better suited to patient physiology.

Spectral analysis of intracranial pressure: Is it helpful in the assessment of shunt functioning in-vivo?

OBJECTIVE

Shunt failure is common in hydrocephalic patients. The cerebrospinal fluid (CSF) infusion test enables the assessment of CSF absorption capacity, which is represented by the resistance to CSF outflow (ROUT) However, shunt failure may not only affect the CSF absorption capacity but also the intracranial compliance or compensatory properties. Spectral analysis of the ICP signal obtained during the infusion test may enable the comprehensive assessment of the overall deterioration caused by shunt failure.

MATERIAL AND METHODS

A total of 121 hydrocephalic shunted patients underwent the infusion test with continuous intracranial pressure (ICP) and arterial blood pressure (ABP) recording. The maximum amplitudes of three major frequency bandwidths (0.2-2.6, 2.6-4.0 and 4.0-15 Hz, respectively) were calculated from the ICP. Statistical analyses were conducted to identify factors significantly associated with shunt failure, to construct an index (i.e., the shunt response parameter, SRP) for detecting shunt failure, and to define thresholds for ROUT and SRP.

RESULTS

The ROUT threshold for detecting shunt failure was 7.59 mmHg/ml/min, and this threshold showed an accuracy of 82.64%. All spectral parameters were found to be significantly associated with shunt patency (p<0.05). The SRP exhibited significantly better accuracy than ROUT in detecting shunt failure (91.74%).

CONCLUSION

The hydrodynamic assessment of shunted patients enhanced by spectral analysis during the infusion test detected shunt failure with high accuracy. Although further validation is needed, the SRP exhibited promising results.

A randomized controlled dual-center trial on shunt complications in idiopathic normal-pressure hydrocephalus treated with gradually reduced or “fixed” pressure valve settings

Object

This study was undertaken to investigate whether a gradual reduction of the valve setting (opening pressure) decreases the complication rate in patients with idiopathic normal-pressure hydrocephalus (iNPH) treated with a ventriculoperitoneal (VP) shunt.

Methods

In this prospective double-blinded, randomized, controlled, dual-center study, a VP shunt with an adjustable valve was implanted in 68 patients with iNPH, randomized into two groups. In one group (the 20–4 group) the valve setting was initially set to 20 cm H2O and gradually reduced to 4 cm H2O over the course of the 6-month study period. In the other group (the 12 group), the valve was kept at a medium pressure setting of 12 cm H2O during the whole study period. The time to and type of complications (hematoma, infection, and mechanical problems) as well as overdrainage symptoms were recorded. Symptoms, signs, and outcome were assessed by means of the iNPH scale and the NPH grading scale.

Results

Six patients in the 20–4 group (22%) and 7 patients in the 12 group (23%) experienced a shunt complication; 9 had subdural hematomas, 3 mechanical obstructions, and 1 infection (no significant difference between groups). The frequency of overdrainage symptoms was significantly higher for a valve setting ≤ 12 cm H2O compared with a setting > 12 cm H2O. The 20–4 group had a higher improvement rate (88%) than the 12 group (62%) (p = 0.032). There was no significant relationship between complications and body mass index, the use of an antisiphon device, or the use of anticoagulants.

Conclusions

Gradual lowering of the valve setting to a mean of 7 cm H2O led to the same rate of shunt complications and overdrainage symptoms as a fixed valve setting at a mean of 13 cm H2O but was associated with a significantly better outcome.

In vitro performance and principles of anti-siphoning devices

BACKGROUND

Anti-siphon devices (ASDs) of various working principles were developed to overcome overdrainage-related complications associated with ventriculoperitoneal shunting.

OBJECTIVE

We aimed to provide comparative data on the pressure and flow characteristics of six different types of ASDs (gravity-assisted, membrane-controlled, and flow-regulated) in order to achieve a better understanding of these devices and their potential clinical application.

METHODS

We analyzed three gravity-dependent ASDs (ShuntAssistant [SA], Miethke; Gravity Compensating Accessory [GCA], Integra; SiphonX [SX], Sophysa), two membrane-controlled ASDs (Anti-Siphon Device [IASD], Integra; Delta Chamber [DC], Medtronic), and one flow-regulated ASD (SiphonGuard [SG], Codman). Defined pressure conditions within a simulated shunt system were generated (differential pressure 10-80 cmH2O), and the specific flow and pressure characteristics were measured. In addition, the gravity-dependent ASDs were measured in defined spatial positions (0-90°).

RESULTS

The flow characteristics of the three gravity-assisted ASDs were largely dependent upon differential pressure and on their spatial position. All three devices were able to reduce the siphoning effect, but each to a different extent (flow at inflow pressure: 10 cmH2O, siphoning -20 cmH2O at 0°/90°: SA, 7.1 ± 1.2*/2.3 ±  0.5* ml/min; GCA, 10.5 ± 0.8/3.4 ± 0.4* ml/min; SX, 9.5 ± 1.2*/4.7 ± 1.9* ml/min, compared to control, 11.1 ± 0.4 ml/min [*p < 0.05]). The flow characteristics of the remaining ASDs were primarily dependent upon the inflow pressure effect (flow at 10 cmH2O, siphoning 0 cmH2O/ siphoning -20cmH2O: DC, 2.6 ± 0.1/ 4 ± 0.3* ml/min; IASD, 2.5 ± 0.2/ 0.8 ± 0.4* ml/min; SG, 0.8 ± 0.2*/ 0.2 ± 0.1* ml/min [*p < 0.05 vs. control, respectively]).

CONCLUSION

The tested ASDs were able to control the siphoning effect within a simulated shunt system to differing degrees. Future comparative trials are needed to determine the type of device that is superior for clinical application.

Shunt implantations and peritoneal catheters: Do not cut beyond 20 cm

BACKGROUND

Ventriculoperitoneal shunts are supplied with long peritoneal catheters, most commonly between 80 and 120 cm long. ISO/DIS 7197/2006([15]) shunt manufacturing procedures include peritoneal catheter as an integrate of the total resistance. Cutting pieces of peritoneal catheters upon shunt implantation or revision is a common procedure.

METHODS

We evaluated five shunts assembled with different total pressure resistances and variable peritoneal catheter lengths in order to clarify the changes that occurred in the hydrodynamic profile when peritoneal catheters were cut upon shunt implantation or shunt revision.

RESULTS

Originally, all shunts performed within the operational range. Shunt 1 performed in a lower pressure range at 200 mm cut off peritoneal catheter and as a low-pressure shunt with -300 mm cut off. Shunt 2 was manufactured to run at the higher border pressure range, and it went out of specification with a 300 mm cut off. Shunt 3 was manufactured to run close to the lower border pressure range, and at 100 mm cutoff, it was already borderline in a lower resistive category. Other shunts also responded similarly.

CONCLUSION

The limit to maintain a shunt in its original pressure settings was 20 cm peritoneal catheter cutting length. By cutting longer pieces of peritoneal catheter, one would submit patients to a less-resistive regimen than intended and his reasoning will be compromised. The pediatric population is more prone to suffer from the consequences of cutting catheters. Shunt manufacturers should consider adopting peritoneal catheters according to the age (height) of the patient.

Hydrocephalus shunt technology: 20 years of experience from the Cambridge Shunt Evaluation Laboratory

OBJECT

The Cambridge Shunt Evaluation Laboratory was established 20 years ago. This paper summarizes the findings of that laboratory for the clinician.

METHODS

Twenty-six models of valves have been tested long-term in the shunt laboratory according to the expanded International Organization for Standardization 7197 standard protocol.

RESULTS

The majority of the valves had a nonphysiologically low hydrodynamic resistance (from 1.5 to 3 mm Hg/[ml/min]), which may result in overdrainage related to posture and during nocturnal cerebral vasogenic waves. A long distal catheter increases the resistance of these valves by 100%-200%. Drainage through valves without a siphon-preventing mechanism is very sensitive to body posture, which may result in grossly negative intracranial pressure. Siphon-preventing accessories offer a reasonable resistance to negative outlet pressure; however, accessories with membrane devices may be blocked by raised subcutaneous pressure. In adjustable valves, the settings may be changed by external magnetic fields of intensity above 40 mT (exceptions: ProGAV, Polaris, and Certas). Most of the magnetically adjustable valves produce large distortions on MRI studies.

CONCLUSIONS

The behavior of a valve revealed during testing is of relevance to the surgeon and may not be adequately described in the manufacturer's product information. The results of shunt testing are helpful in many circumstances, such as the initial choice of shunt and the evaluation of the shunt when its dysfunction is suspected.

Miniaturized passive hydrogel check valve for hydrocephalus treatment

Improvements in cerebrospinal fluid (CSF) draining techniques for treatment of hydrocephalus are urgently sought after to substitute for current CSF shunts that are plagued by high failure rates. The passive check valve aims to restore near natural CSF draining operations while mitigating possible failure mechanisms caused by finite leakage or low resilience that frequently constrain practical implementation of miniaturized valves. A simple hydrogel diaphragm structures core passive valve operations and enforce valve sealing properties to substantially lower reverse flow leakage. Experimental measurements demonstrate realization of targeted cracking pressures (PT ≈ 20-110 mmH2O) and operation at -800 <; ΔP <; 600 mmH2O without observable degradation or leakage.

Hydrodynamic properties of the Certas hydrocephalus shunt

OBJECT

Independent testing of hydrocephalus shunts provides information about the quality of CSF drainage after shunt implantation. Moreover, hydrodynamic parameters of a valve assessed in the laboratory create a comparative pattern for testing of shunt performance in vivo. This study sought to assess the hydrodynamic parameters of the Certas valve, a new model of a hydrocephalus shunt.

METHODS

The Certas valve is an adjustable ball-on-spring hydrocephalus valve. It can be adjusted magnetically in vivo in 7 steps, equally distributed within the therapeutic limit for hydrocephalus, and the eighth step at high pressures intended to block CSF drainage. The magnetically adjustable rotor is designed to prevent accidental readjustment of the valve in a magnetic field, including clinical MRI.

RESULTS

The pressure-flow performance curves, as well as the operating, opening, and closing pressures, were stable, fell within the specified limits, and changed according to the adjusted performance levels. The valve at settings 1-7 demonstrated low hydrodynamic resistance of 1.4 mm Hg/ml/min, increasing to 5.1 mm Hg/ml/min after connection of a distal drain provided by the manufacturer. At performance Level 8 the hydrodynamic resistance was greater than 20 mm Hg/ml/min. External programming of the valve proved to be easy and reliable. The valve is safe in 3-T MRI and the performance level of the valve is unlikely to be changed. However, with the valve implanted, distortion of the image is substantial. Integration of the valve with the SiphonGuard limits the drainage rate.

CONCLUSIONS

In the laboratory the Certas valve appears to be a reliable differential-pressure adjustable valve. Laboratory evaluation should be supplemented by results of a clinical audit in the future.

The hydrokinetic parameters of shunts for hydrocephalus might be inadequate

Long-term treatment of hydrocephalus continues to be dismal. Shunting is the neurosurgical procedure more frequently associated with complications, which are mostly related with dysfunctions of the shunting device, rather than to mishaps of the rather simple surgical procedure. Overdrainage and underdrainage are the most common dysfunctions; of them, overdrainage is a conspicuous companion of most devices. Even when literally hundreds of different models have been proposed, developed, and tested, overdrainage has plagued all shunts for the last 60 years. Several investigations have demonstrated that changes in the posture of the subject induce unavoidable and drastic differences of intraventricular hydrokinetic pressure and cerebrospinal fluid (CSF) drainage through the shunt. Of all the parameters that participate in the pathophysiology of hydrocephalus, the only invariable one is cerebrospinal fluid production at a constant rate of approximately 0.35 ml/min. However, this feature has not been considered in the design of currently available shunts. Our experimental and clinical studies have shown that a simple shunt, whose drainage capacity complies with this unique parameter, would prevent most complications of shunting for hydrocephalus.

Flow dynamics in lumboperitoneal shunts and their implications in vivo

Object

Lumboperitoneal shunting is the standard treatment for pseudotumour cerebri or idiopathic intracranial hypertension. Complications are common, particularly the problem of overdrainage leading to low pressure symptoms. The authors designed a simple experiment using catheters of different lengths that drained at different pressure heads and with different vertical drops to study the flow characteristics in these shunts and determine the optimal catheter placement and length that would reduce the occurrence of low pressure headaches.

Methods

The flow rates through catheters of 3 different lengths (60, 83, and 100 cm) with the same internal radius, at 3 different pressure heads (15, 25, and 35 cm H2O to simulate 3 different placements in the lumbar theca), and 3 different vertical drops (10, 20, and 30 cm to simulate the possible effect of siphoning) were measured and the results analyzed.

Results

Application of Poiseuille's law and Bernoulli's principle to the experimental design shows that the volume of flow is directly proportional to the sum of the pressure head and the vertical drop and inversely proportional to the length of the catheter. The flow rate through the standard catheter lengths over the course of 24 hours can be abnormally high. An attempt to predict the optimal catheter length was made.

Conclusions

Although the catheter position in the theca and abdomen cannot be altered significantly and the internal radius of the tube cannot be reduced further without increasing the risk of blockage, the length of the tube can be increased to combat overdrainage. The authors suggest that currently available catheters are too short.

Investigation of the hydrodynamic properties of a new MRI-resistant programmable hydrocephalus shunt

Background

The Polaris valve is a newly released hydrocephalus shunt that is designed to drain cerebrospinal fluid (CSF) from the brain ventricles or lumbar CSF space. The aim of this study was to bench test the properties of the Polaris shunt, independently of the manufacturer.

Methods

The Polaris Valve is a ball-on-spring valve, which can be adjusted magnetically in vivo. A special mechanism is incorporated to prevent accidental re-adjustment by an external magnetic field. The performance and hydrodynamic properties of the valve were evaluated in the UK Shunt Evaluation Laboratory, Cambridge, UK.

Results

The three shunts tested showed good mechanical durability over the 3-month period of testing, and a stable hydrodynamic performance over 45 days. The pressure-flow performance curves, operating, opening and closing pressures were stable. The drainage rate of the shunt increased when a negative outlet pressure (siphoning) was applied. The hydrodynamic parameters fell within the limits specified by the manufacturer and changed according to the five programmed performance levels. Hydrodynamic resistance was dependant on operating pressure, changing from low values of 1.6 mmHg/ml/min at the lowest level to 11.2 mmHg/ml/min at the highest performance level. External programming proved to be easy and reliable. Even very strong magnetic fields (3 Tesla) were not able to change the programming of the valve. However, distortion of magnetic resonance images was present.

Conclusion

The Polaris Valve is a reliable, adjustable valve. Unlike other adjustable valves (except the Miethke ProGAV valve), the Polaris cannot be accidentally re-adjusted by an external magnetic field.

Intra-abdominal pressure: the neglected variable in selecting the ventriculoperitoneal shunt for treating hydrocephalus

OBJECTIVE

In the selection of a ventriculoperitoneal cerebrospinal fluid shunt, the intra-abdominal pressure (IAP) is traditionally neglected as a result of the idea that its value is close to 0 mmHg. Our aim was to explore the relationship between body mass index (BMI) and IAP with the goal of providing clinically relevant data that could help neurosurgeons to estimate IAP and select the appropriate shunt for patients with hydrocephalus and especially those with normal-pressure hydrocephalus syndrome.

METHODS

Sixty patients requiring the placement of a ventriculoperitoneal shunt were included in the study. We determined weight, BMI, and IAP. IAP was measured through an intraperitoneal catheter during the shunt surgery. To determine whether a linear relationship existed between quantitative variables, linear regression analysis was used.

RESULTS

BMI was 28.1 +/- 4.8 kg/m2. Eighteen patients (30%) had normal weight, 21 (35%) were moderately overweight, and 21 (35%) were obese. IAP was related to patient BMI. A significant positive linear correlation was identified between BMI and IAP (r = 0.52; P = 0.018) with a slope of 0.31 (P < 0.001) and an intercept of -5.5.

CONCLUSION

In our study, we determined that IAP had a strong positive linear relationship with BMI. This correlation was independent of sex. An IAP of 0 mmHg can, therefore, only be assumed for patients with a normal BMI who are recumbent. In obese or overweight patients, neurosurgeons should take IAP into account when selecting both the most adequate differential pressure valve to be implanted and in which distal cavity to place the distal catheter to avoid shunt underdrainage induced by high IAP.

Assessment of cerebrospinal fluid outflow resistance

The brain and the spinal cord are contained in a cavity and are surrounded by cerebrospinal fluid (CSF), which provides physical support for the brain and a cushion against external pressure. Hydrocephalus is a disease, associated with disturbances in the CSF dynamics, which can be surgically treated by inserting a shunt or third ventriculostomy. This review describes the physiological background, modeling and mathematics, and the investigational methods for determining the CSF dynamic properties, with specific focus on the CSF outflow resistance, R out. A model of the cerebrospinal fluid dynamic system, with a pressure-independent R out, a pressure-dependent compliance and a constant formation rate of CSF is widely accepted. Using mathematical expressions calculated from the model, along with active infusion of artificial CSF and observation of corresponding change in ICP allows measurements of CSF dynamics. Distinction between normal pressure hydrocephalus and differential diagnoses, prediction of clinical response to shunting and the possibility of assessment of shunt function in vivo are the three most important applications of infusion studies in clinical practice.

Programmable shunt-related suicide attempt

The treatment of hydrocephalus has benefited recently from the use of programmable shunt valves. These devices can be adjusted using magnets to regulate how much spinal fluid is drained. However, it is unclear to what extent other environmental magnetic sources can affect programmable valves. We present the case of a man who attempted suicide by successfully turning his adjustable valve to a near-maximal setting using a hand-held electromagnet, and discuss other reported cases in order to better understand the effects of environmental magnets on programmable shunt valves.

Subdural haematomas complicating insertion of the low-pressure Novus hydrocephalus valve: a simple method for intra-operative testing of the anti-siphon device

We previously reported 52 patients with hydrocephalus who were followed up after insertion of low-pressure Novus valves. These valves have a normally open anti-siphon device (ASD) incorporated. There were no cases of subdural haematomas (SDH). Subsequently, three new patients suffered SDH after insertion of these valves. We investigated a simple method for intra-operative testing of the ASD. These new patients had their valves replaced. In the laboratory, flow rates through five valves were recorded as a function of proximal positive pressure and distal negative pressures (siphoning). The flow rates were influenced by both proximal positive and distal negative pressures. The ASD stopped flow at distal negative pressures between -40 and -60 cm H(2)O. Proximal positive pressures increased this threshold. The flow can be measured by counting drops per minute. Three valves removed from patients were functioning as expected, one had unexpectedly slow flow at very high siphoning pressure and one had unexpectedly slow flow rates. In three patients with SDH complicating low-pressure Novus valves, the valves and anti-siphon devices were functioning adequately. Using a simple device, measuring flow rates in drops per minute was reliable and reproducible.

Assessment of cerebrospinal fluid outflow conductance using constant-pressure infusion—a method with real time estimation of reliability

The outflow conductance (C(out)) of the cerebrospinal fluid (CSF) system is a parameter considered to be predictive in selection for hydrocephalus surgery. C(out) can be determined through an infusion test. A new apparatus for performing infusion tests in a standardized and automated way was developed. The objective was to evaluate repetitiveness as well as to propose and evaluate a method for real time estimation of the reliability of individual C(out) investigations. Repeated investigations were performed on an experimental model simulating the CSF system, and on 14 patients with hydrocephalus. DeltaC(out), calculated as the 95% confidence interval of C(out), was introduced as an estimate of the reliability of individual C(out) investigations. On the model, no significant difference was found between DeltaC(out) and the actual C(out) variation in repeated investigations (p = 0.135). The correlation between the first and the second patient investigation was high (R = 0.99, p < 0.05), although there was a significant difference between the investigations (p < 0.05). The standard deviation of difference was 2.60 microl (s kPa)(-1). The repetitiveness of C(out) with the new apparatus was high, and DeltaC(out) reflected the reliability of each investigation. This feature has to be taken into account in every individual case, before making a decision or performing research based on measurements of C(out) in the future.

Surgical Management of Idiopathic Normal-pressure Hydrocephalus

OBJECTIVE:

To develop evidence-based guidelines for surgical management of idiopathic normal-pressure hydrocephalus (INPH). Compared with the diagnostic phase, the surgical management of INPH has received less scientific attention. The quality of much of the literature concerning the surgical management has been limited by many factors. These include retrospective analysis, small patient numbers, analysis of a mixed NPH population, and sometimes a lack of detail as to what type of shunt system was used. Many earlier studies predated our current understanding of the hydrodynamics of cerebrospinal fluid shunts, and therefore, the conclusions drawn may no longer be valid.

METHODS:

A MEDLINE and PubMed search from 1966 to the present was conducted using the following key terms: normal-pressure hydrocephalus and idiopathic adult-onset hydrocephalus. Only English-language literature in peer-reviewed journals was reviewed. The search was further limited to articles that described the method of treatment and outcome selectively for INPH patients. Finally, only studies that included 20 or more INPH patients were considered with respect to formulating the recommendations in these Guidelines (27 articles).

RESULTS:

For practical reasons, it is important to identify probable shunt responders diagnosed with INPH. If the patient is an acceptable candidate for anesthesia, then an INPH-specific risk-benefit analysis should be determined. In general, patients exhibiting negligible symptoms may not be suitable candidates for surgical management, given the known risks and complications associated with shunting INPH. The choice of valve type and setting should be based on empirical reasoning and a basic understanding of shunt hydrodynamics. The most conservative choice is a valve incorporating an antisiphon device, with the understanding that underdrainage (despite a low opening pressure) may occur in a small percentage of patients because of the antisiphon device. On the basis of retrospective studies, the use of an adjustable valve seems to be beneficial in the management of INPH.

CONCLUSION:

The treatment of INPH should not be considered lightly, given the seriousness of the potential complications. Within these limitations and the available evidence, guidelines for surgical management were developed.

Ventriculoperitoneal shunt of continuous flow vs valvular shunt for treatment of hydrocephalus in adults

BACKGROUND

Shunting for hydrocephalus is the neurosurgical procedure most frequently associated with long-term complications. We developed an alternative to valvular shunts based on a simple shunt procedure whose functioning depends on a peritoneal catheter with a highly precise cross-sectional internal diameter of 0.51 mm. Preliminary studies have shown that the shunt of continuous flow (SCF) is superior to valvular shunts for the treatment of hydrocephalus in adults. Here, we show the long-term performance of the SCF in adult patients with hydrocephalus secondary to a comprehensive variety of neurological disorders.

METHODS

In a 5-year period, ventriculoperitoneal shunting was performed on 307 patients with hydrocephalus; 114 of them were treated with the SCF and 193 controls were treated with a conventional valvular shunt. Patients were followed from 1 to 5 years after surgery; endpoint observation was considered at surgical reintervention because of shunt failure.

RESULTS

At the end of the observation period (44 +/- 17 months), the failure rate of the shunting device was 14% for the SCF and 46% for controls (P < .0002). Shunt endurance was 88% in patients with SCF and 60% in controls. Along the study, signs of overdrainage developed in 40% of patients treated with valvular shunt, but they were not observed in patients with SCF.

CONCLUSIONS

The design of the SCF was calculated according to the mean rates of cerebrospinal fluid production; it takes simultaneous advantage of the intraventricular pressure and the siphon effect and complies with the principle of uninterrupted flow, maintaining a fair equilibrium that prevents under- and overdrainage. The SCF is a simple, inexpensive, and effective treatment for hydrocephalus in adults.

Features of the Sinushunt and its influence on the cerebrospinal fluid system

OBJECTIVES

A new cerebrospinal fluid (CSF) shunt system, Sinushunt, has recently been introduced. CSF is shunted from the ventricles to the transverse sinus. The Sinushunt is not a classical differential pressure shunt; instead, it opens as soon as there is a positive pressure over the shunt and the flow is dependent on the resistance of the system, which is high compared with traditional CSF shunts. The objective of this study was to characterise the features of the Sinushunt and to evaluate its influence on the CSF system.

METHODS

Five brand new Sinushunts with distal catheters were tested. An automated, computerised experimental apparatus based on regulation of pressure, built into an incubator at 37 degrees C, was used. Opening pressure, resistance, and anti-reflux properties were determined.

RESULTS

The mean (SD) opening pressure was highly dependent on the pressure in the sinus: P(open) = 1.3 (0.6) mm Hg with Psinus = 0.0 mm Hg, and Popen = 7.5 (0.6) mm Hg for Psinus = 6.5 mm Hg. The mean (SD) resistance of the shunts was 7.9 (0.3) mm Hg/ml/min and not clinically significantly affected by the sinus pressure. In one shunt there was reflux, and in another two shunts there was a very small, but similar, tendency.

CONCLUSIONS

This study confirms that the resistance of the Sinushunt is comparable to the physiological values in humans. However, the optimal post-operative resistance for different hydrocephalus types is unknown, and randomised clinical trials are needed to confirm improved outcome and reduced complication rate for the Sinushunt compared with traditional low resistance ventriculoperitoneal shunts. A weakness of the anti-reflux system of the Sinushunt must be suspected and has to be further investigated.

Shunting to the cranial venous sinus using the SinuShunt

INTRODUCTION

A new shunting principle taking advantage of the knowledge of normal CSF dynamics has been developed.

EXPERIENCE AND OUTCOME

The shunt has been used in more than 150 patients. The final version has shown a stable function in 45 patients. The physiological shunting principle has an expected and immediate clinical effect. We have not seen any over-drainage and any symptoms or signs of thrombosis or occlusion of the sinus. The ventricular system decreases only slightly. The shunt has been used in children and adults and in all types of hydrocephalus. The shunt can be implanted using local anaesthesia. The implantation in the transverse sinus has proven to be simple and safe.

Transcranial color-coded Doppler ultrasonography for evaluation of children with hydrocephalus

OBJECT

Hydrocephalus is a common disease process. Transcranial color-coded Doppler (TCCD) ultrasonography is an accepted noninvasive method with which to quantify intracranial blood flow in adults and children. The authors studied the applications of TCCD ultrasonography and the alterations of the flow velocity of the cerebral arteries in children with hydrocephalus.

METHODS

One hundred thirty-five children were divided into three groups: Group 1 comprised 40 infants with asymptomatic hydrocephalus who had well-functioning ventriculoperitoneal (VP) shunts; Group 2 comprised 10 children with symptomatic hydrocephalus who had malfunctioning shunts that were replaced; and Group 3 was a control group of 85 healthy infants. All patients underwent sequential measurements of cerebral blood flow (CBF) velocities (systolic and diastolic velocities) and resistivity index (RI). One group of patients underwent functional tests (compression of the anterior fontanelle and CO2 vasoreactivity) to determine hemodynamic changes in cerebral circulation. A significant statistical change in RI measurements, end diastolic CBF velocity, and percentage of change in RI was shown in patients with malfunctioning shunts, and in infants with a well-functioning VP shunt vasomotor reactivity was severely reduced.

CONCLUSIONS

Transcranial color-coded Doppler ultrasonography can be used to perform follow-up assessments of normal and malfunctioning shunts in children with hydrocephalus; the functional tests are a noninvasive tool for evaluating the cerebral compliance and the cerebral autoregulation in infants with hydrocephalus. The autoregulatory capacity may partly or completely be lost in cases of long-term shunt-treated hydrocephalus, and loss of cerebral vasoreactivity may be responsible for long-term deficits commonly observed in children, which help explain some of symptoms related to slit ventricles.

Dynamic shunt testing applying short lasting pressure waves--inertia of shunt systems

Laboratory shunt testing often comprises only static pressure flow and flow pressure tests. We applied shorter acting pressure waves using a computerised shunt testing rig to investigate shunt behaviour under conditions that might occur in the clinical situation, e.g. during nocturnal vasogenic pressure waves or shorter rises in ICP at movements or exercise. Additionally the influence of a human-like compliance situation compared to a fixed pressure/volume relationship was investigated. Shunts behaved very differently than seen in static tests and demonstrated a marked inertia the shorter pressure waves acted. Although some valves opened at higher pressure, all valves showed marked hysteresis and none did close--if at all--at the specified pressure level. This behaviour might be a cause of shunt overdrainage not related to siphoning. The simulation of a human-like variable pressure-volume relationship with higher compliance at lower pressures had a positive effect on shunt function by decreasing the amount of drained volume. We therefore suggest to include dynamic pressure wave testing if hydrodynamic properties of shunts are to be evaluated.

Shunting to the sagittal sinus

OBJECTIVES

To develop a shunt that drains CSF from the ventricles to the sagittal sinus under normal-physiological conditions. This shunting principle will not lead to any over-drainage, and a large proportion of the known shunt-complications will be avoided.

METHODS

On the basis of the normal values for ICP, resistance to outflow and the production rate of CSF we have developed a shunt that drains CSF to the sagittal sinus and restores normal condition for the CSF dynamics. The shunt consists of two unidirectional valves, a pre-chamber, a resistance tube made of titanium, and a titanium tube leading CSF into the sagittal sinus. The shunt has been tested in 18 patients. Observation time ranged from 2 to 430 days, mean time 54 days.

RESULTS

The first results from the use of the new shunt are very promising. It has an immediate effect on the clinical symptoms, it restores CSF dynamics (investigated with the shunt inserted) and the size of the ventricles is only gradually diminished. Slit ventricles have not yet been observed. In all patients the symptoms of hydrocephalus were relieved. No occlusion or thrombosis of the sagittal sinus have been observed. This is in agreement with the reports in the literature of shunting to the sagittal sinus, where 99 cases have been presented with an observation period of up to 6 years. The shunt has proven easy and safe to implant.

CONCLUSIONS

Shunting to the sagittal sinus has proven easy and safe with regard to short term results. By using a dedicated shunt that drains at normal physiological parameters for the CSF dynamics any over-drainage is avoided, and it may be expected that the complication rate will be substantially smaller than with existing shunting systems.

Treatment of hydrocephalus in adults by placement of an open ventricular shunt

OBJECT

Ventricular shunt placement is the neurosurgical procedure most frequently associated with complications. Over the years, it has been a growing concern that the performance of most shunting devices does not conform to physiological parameters. An open ventriculoperitoneal (VP) bypass with a peritoneal catheter for which the cross-sectional internal diameter was 0.51 mm as a distinctive element for flow resistance was evaluated for use in the treatment of adult patients with hydrocephalus.

METHODS

During a 2-year period, open shunts were surgically implanted in 54 adults with hydrocephalus; conventional shunts were implanted in 80 matched controls. Periodic evaluations were performed using neuroimaging studies and measures of clinical status. All patients were followed from 12 to 36 months. 18.5 +/- 4 months for patients with the open shunt and 19.1 +/- 8.1 months for controls (mean +/- standard deviation). The device continued to function in 50 patients with the open shunt (93%) and in 49 controls (61%: p < 0.001). The Evans index in patients with the open shunt was 0.33 +/- 0.09 throughout the follow up. No cases of infection, overdrainage, or slit ventricles were observed: the index in controls was 0.28 +/- 0.08; 60% of them developed slit ventricles. During the follow-up period occlusion occurred in four patients with the open shunt (7%) and in 31 controls (39%: p < 0.001).

CONCLUSIONS

The daily cerebrospinal fluid (CSF) drainage through the open VP shunt is close to 500 ml of uninterrupted flow propelled by the hydrokinetic force generated by the combination of ventricular pressure and siphoning effect. It complies with hydrokinetic parameters imposed by a bypass connection between the ventricular and peritoneal cavities as well as with the physiological archetype of continuous flow and drainage according to CSF production. The open shunt is simple, inexpensive, and an effective treatment for hydrocephalus in adults.

Cerebrospinal fluid hydrodynamics after placement of a shunt with an antisiphon device: a long-term study

Object. Few studies have been performed to investigate the cerebrospinal fluid (CSF) hydrodynamic profile in patients with idiopathic adult hydrocephalus syndrome (IAHS) before and after shunt implantation. The authors compared the in vivo CSF hydrodynamic properties, including the degree of gravity-induced CSF flow, of a shunt with an antisiphon device with a standard shunt.

Methods. Twelve patients with IAHS underwent insertion of shunts with Delta valves. Clinical testing, magnetic resonance imaging, and CSF hydrodynamic investigations were conducted with intracranial pressure (ICP), gravity effect, and pressure—flow curve of the shunt estimated at baseline and at 3 and 12 months postoperatively. No shunt was revised.

Despite postoperative clinical improvement in all patients who received Delta valves, the mean ICP was only moderately reduced (mean decrease at 3 months 0.3 kPa [p = 0.02], at 12 months 0.2 kPa [not significant]). Patients with the greatest increase in ICP preoperatively had the most pronounced decrease postoperatively. The hydrostatic effect of the Delta valves was significantly lower than with the Hakim shunts (0.1–0.2 kPa compared with 0.6 kPa). The increased conductance (that is, lowered resistance) was up to 14 times higher with the Delta valves compared with preoperative levels.

Conclusions. The function of a CSF shunt may be more complicated than previously thought; the subcutaneous pressure acting on the antisiphon device can modify the shunt characteristics. A compensatory increase in CSF production may counteract the increased outflow through the shunt. The improved CSF outflow conductance may increase the intracranial compliance and thereby dampen a pathological ICP waveform.

Laboratory evaluation of the phoenix CRx diamond valve

OBJECTIVE

To assess the long-term hydrodynamic properties of a new cerebrospinal fluid flow-regulating hydrocephalus shunt called the CRx Diamond valve (Phoenix Biomedical Corp., Valley Forge, PA).

METHODS

Three samples of a Diamond valve were tested in the United Kingdom Shunt Evaluation Laboratory during a 40-day period. Tests were performed for long-term pressure-flow performance, overdrainage, susceptibility to ambient temperature changes, external pressure, reflux, presence of small particles in the reagent, mechanical durability, and magnetic resonance imaging compatibility.

RESULTS

Tests demonstrated that the Diamond valve stabilized flow within the range of 0.36 to 0.62 ml/min when pressure varied from 14 to 23 mm Hg. Hydrodynamic resistance demonstrated pressure-dependent variability from 20 to 78 mm Hg/(ml/min). The time drift of hydrodynamic parameters was significant (P < 0.001). The valve was insensitive to changes in temperature, external pressure, rapid fluctuations of differential pressure, small particles in fluid, and reflux.

CONCLUSION

The Diamond valve demonstrated the intended variable resistance, which increased as the pressure increased. This property may help it limit overdrainage related to body posture as well as nocturnal vasogenic waves. Flow through the valve stabilizes within a wide range, which may contribute to the prevention of excessive pressure buildup after implantation. However, shunt placement should be avoided in patients who present with normal baseline intracranial pressure but an increased incidence of high vasogenic intracranial pressure waves.

Hydrodynamic properties of hydrocephalus shunts

Hydrodynamic properties of hydrocephalus shunts are not always properly characterized by the manufacturer. Therefore, the choice of the shunt should be made, by matching performance of the shunt to the disturbed profile of CSF circulation of a given patient. The aim of the present shunt evaluation study is to evaluates all types of shunts presently in use in the U.K. and make this information available to neurosurgeons. Ten most common models of valves have been tested to date: Medtronik PS Medical: Delta Valve, Flow Control Valves and Lumbo-Peritoneal Shunt, Heyer-Schulte: In-line, Low Profile and Pudenz Flushing Valve, Codman: Medos-Programmable, Hakim-Precision, Sophy Programmable Valve, Cordis Orbis-Sigma. Our results show the majority of valves have low hydrodynamic resistance (exception: PS Lumboperitoneal, Orbis-Sigma), which increase by 100-200% after connection of a long distal catheter. A few shunts with siphon-preventing mechanism (Delta, Hayer-Schulte Low Profile, Pudenz-Flushing) offer reasonable resistance to negative outlet pressures, however, these valves may be blocked by raised subcutaneous pressure. All programmable valves are susceptible to siphoning. Programmed settings may be changed by external magnetic field.

Dual-switch valve: clinical performance of a new hydrocephalus valve

The Dual-Switch valve (DSV) is the first construction on the market which changes between two different valve-chambers in parallel depending on the posture of the patient. In the lying position the valve acts like a conventional differential pressure valve, in the vertical position the high-pressure chamber only opens, when the pressure exceeds the hydrostatic pressure difference between the formanen of Monro and the peritoneal cavity. The new device has been implanted in 32 adult patients with hydrocephalus of different etiology. The clinical results are excellent to good accompanied by a remarkable slight reduction of the ventricular size. Apart from one case with a nonsymptomatic transient hygroma, we saw no valve related complications like overdrainage, underdrainage or dysfunction. Contrary to conventional differential-pressure valves, adjustable devices and other hydrostatic constructions like the Anti-Siphon-device (ASD) or Deltavalve, the DSV reliably controls the IVP independently of the posture of the patient, the CSF viscosity or the subcutaneous pressure. In contrast to the Orbis-Sigma-valve (OSV) or the Diamond-valve, the DSV does not control the flow but the physiological IVP avoiding the increased risk of mechanical failure. The results of this study give strong evidence that the shunt-therapy of adult hydrocephalic patients can be significantly improved by the DSV.

Posture-related overdrainage: comparison of the performance of 10 hydrocephalus shunts in vitro

OBJECTIVE

Approximately 10 to 30% of shunt revisions may be attributed to posture-related overdrainage. The susceptibility of various hydrocephalus shunts to overdrainage of cerebrospinal fluid requires independent laboratory evaluation.

METHODS

Shunts were tested in vitro by using precise computer-controlled equipment that was able to evaluate pressure-flow performance curves under various conditions. Hydrodynamic resistance and opening, closing, and operational pressures were evaluated for at least 28 days with normal (atmospheric) and decreased (-23 mm Hg, based on the International Standard Organization/Draft International Standard 7197 standard, which simulates conditions in upright body positions) outlet pressures.

RESULTS

Ten different models of valves have been tested to date (Medtronic PS Medical Delta valve, flow-control valve, and lumboperitoneal shunt, Heyer-Schulte in-line, low-profile, and Pudenz flushing valves, Codman-Medos programmable and nonprogrammable valves, Sophy programmable valve, and Cordis Orbis-Sigma valve). The majority of these valves produced significantly negative (less than -10 mm Hg) average intracranial pressures in vertical body positions. In conjunction with nonphysiologically low hydrodynamic resistance (with the exception of the Orbis-Sigma valve, Medtronic PS Medical lumboperitoneal shunt, and Heyer-Schulte in-line valve), this may result in overdrainage related to body posture. The clinically reported rate of complications related to overdrainage is probably reduced by the long distal catheter, which increases the resistance of these valves by 100 to 200%. A few shunts (the Delta valve, low-profile valve, and Pudenz flushing valve with anti-siphon devices) offer reasonable resistance to negative outlet pressure, preventing complications related to overdrainage, but all valves with siphon-preventing devices may be blocked by increased subcutaneous pressure.

CONCLUSION

Shunts without mechanisms preventing very low intracranial pressure in vertical body positions should be identified and avoided for patients likely to develop complications related to cerebrospinal fluid overdrainage.