Shunting

Published on 26/03/2015 by admin

Last modified 26/03/2015

Print this page

This article have been viewed 2249 times

CHAPTER 35 Shunting*

Hydrocephalus is a commonly encountered disorder that occurs either as a primary condition or as the sequela to an intracranial hemorrhage, a space-occupying lesion, or meningitis. For more than a half-century, a cerebrospinal fluid (CSF) shunt has been the mainstay for treatment of hydrocephalus. Although many consider shunting a relatively simple procedure, problems with CSF shunts are common, costly, and sometimes debilitating. Within the first year, shunts fail at extraordinary rates of up to 40% and show nearly a 10% infection rate.¹^–⁴ Thus, the shunt operation has one of the highest associated complication rates in neurosurgery. Furthermore, cases of hydrocephalus can be some of the most complex and challenging clinical scenarios facing a neurosurgeon.⁵^,⁶

The aim of this chapter is to help neurosurgeons choose the type of shunt, valve setting, and shunt location that will offer the highest probability of a good outcome while avoiding complications and revisions. Unfortunately, there are scant class I and class II evidentiary data on which to base guidelines pertaining to shunting methods and materials for adult hydrocephalus patients. Our recommendations are therefore derived from personal experience (more than 6000 outpatient encounters and 700 surgical procedures on adult hydrocephalus patients during a 14-year-period), insight drawn from our clinical studies,⁷^–⁹ and information gleaned from the literature.

Although this chapter is entitled Shunting, neurosurgeons should reflexively consider endoscopic third ventriculostomy an alternative when appropriate.⁷^,¹⁰ The “knee-jerk” response to proceed automatically with a shunt operation, particularly in patients presenting with shunt failure, robs the patient of an opportunity to live shunt free. Clinicians should investigate the etiology and ventricular anatomy in every case of hydrocephalus. In some cases, even patients whose physicians previously said that they had “communicating” hydrocephalus in fact have a ventricular obstruction that clinicians can readily visualize by modern high-resolution magnetic resonance imaging (MRI) technology (such as the CISS [constructive interference in steady state] or FIESTA [fast imaging employing steady-state acquisition] sagittal sequences^7,^11,¹²). Adult patients shunted in early childhood have a particularly high incidence of noncommunicating (intraventricular) hydrocephalus in our experience.

Key Point

In the initial evaluation of a newly diagnosed hydrocephalus patient or a previously shunted one, an essential component of the evaluation is to determine whether an endoscopic third ventriculostomy (or related procedure) is appropriate.

Valve Design and Terminology

Probably the most important component of a shunt system is the valve. Neurosurgeons can choose from more than 125 commercially available valves.¹³ During the past 50 years, the predominant theme in the evolution of valve design has been the goal of preventing CSF overdrainage. This includes the introduction of anti-siphon devices, flow-restricting elements, multistage valves, and adjustable valves. It is important to understand that manufacturers have little or no direct in vivo intracranial pressure or CSF flow data to back up advertised claims, such as “preventing excessive flow while allowing constant physiological drainage” or “regulates flow through the valve at a rate close to that of CSF secretion, therefore minimizing the risks of underdrainage or overdrainage.” Our studies⁸ demonstrate that the in vivo behavior of even the simplest shunt, the ventriculoperitoneal shunt with a standard differential pressure valve, is poorly predicted by the first-order, steady-flow equations that are the basis of the many valve designs.

In our opinion, there is no single valve mechanism, design, or arrangement that is clearly the “best,” nor one that will be adequate for every hydrocephalus patient. There are some valves and valve settings, however, that are poorly suited for adult hydrocephalus and will likely result in a higher complication rate. Hydrocephalus is a heterogeneous disorder, with a wide range of intracranial pressures, ventricular compliance, and CSF profiles across patients. It is somewhat fortunate that many valve designs work satisfactorily, at least in the short term, in the majority of patients. The main challenges arise from problematic patients, such as those suffering from headaches, subdural hematomas, repeated shunt obstructions, slit ventricle syndrome caused by chronic overdrainage, and so on. Shunt management is often a trial-and-error process, one in which knowledge of valve design and function can greatly help in the selection of a better choice should a revision be necessary.

The following is a primer on shunt valve design and characteristics with which every neurosurgeon placing shunts should be familiar.

Differential Pressure Valve

The basic building block of most shunt valves is a differential pressure “check valve” mechanism. The basic design of John Holter continues in some form more than half a century after its development.¹⁴ In most current valve designs, it consists of a tiny ball situated on a ring, with a spring pushing the ball downward on the ring. CSF passes through the ring, elevating the ball if the pressure exceeds the pressure exerted by the spring. This creates a one-way flow mechanism because reverse flow will not occur as the ball sits down onto the ring.

Opening of this valve mechanism depends on the differential pressure across the ring. For example, if the spring is exerting downward pressure of 100 mm H₂O, CSF will flow if the difference between the inlet and outlet pressures is greater than 100 mm H₂O, regardless of whether the inlet pressure is positive or negative.

A common misconception is that the valve opening pressure must be lower than the ventricular pressure (as measured at the time of surgery) for CSF to flow down the shunt. Our studies demonstrate that this is clearly an invalid assumption. In a study of patients with normal-pressure hydrocephalus (NPH), intracranial pressure was statistically lower at all head-of-bed elevations compared with preoperative values, even with the valve set at 200 mm H₂O opening pressure. For example, despite a mean preoperative intracranial pressure of 164 ± 64 mm H₂O, the mean postoperative intracranial pressure was 125 ± 69 mm H₂O (P = .04).⁸

Most commercially available CSF shunt valves contain a differential pressure valve mechanism in one form or another. For some, it is the sole valve mechanism, whereas in others, it is the first in-series component of the valve assembly. Examples of ball-spring valves are the Medtronic Strata valve, the Codman Hakim programmable and Precision valves, and the Aesculap proGAV valve. A simpler, less accurate mechanism consists of a valve mechanism derived from two apposing semirigid membranes. These valves, which include the Medtronic, Pudenz, and Codman distal slit valves, are manufactured and then individually tested to determine the approximate opening pressure. They are then segregated into different bins covering a range of pressures. For example, the “medium-pressure valve” bin would contain valves ranging from 50 to 90 mm H₂O opening pressure.

Key Point

There is a basic misconception that the valve opening pressure must be lower than the mean ventricular pressure for the shunt to flow.

Adjustable (“Programmable”) Valves

A “programmable” or adjustable valve is created by adding a mechanism that enables precise changes of the spring tension of a differential pressure valve. There are several competing designs enabling this—all incorporating a magnetic actuation of a rotor. Strictly speaking, these valves are not truly programmable and are better considered as merely adjustable valves. Adjustable valves arose from the realization that fixed-pressure differential pressure valves result in either overdrainage or underdrainage in a significant number of adult patients. The overdrainage side of this argument is supported by data from the Dutch Normal-Pressure Hydrocephalus Study,¹⁵ one of the few prospective, randomized studies performed in adult hydrocephalus. This study demonstrated that subdural hygromas occurred in 71% of patients with low-pressure valve shunts versus 34% of patients randomized to medium-pressure shunts. Given the likelihood that expanding or large subdural hygromas are a risk for subdural hematoma, this is one example that there is clearly a risk of selecting too low of an opening pressure. The analysis of our series of 114 consecutive idiopathic NPH patients, each treated with an initial valve opening pressure of 200 mm H₂O, revealed a subdural hygroma incidence of 4%.⁷ As shown in Figure 35-1, combining the results of the Dutch Normal-Pressure Hydrocephalus Study with our experience suggests a direct relationship between subdural hygromas and valve opening pressure.

FIGURE 35-1 Estimated risk of subdural hygroma formation with idiopathic NPH. The Dutch Normal-Pressure Hydrocephalus Study ¹⁵ documented a subdural hygroma (effusion) incidence of approximately 70% and approximately 30% with low- and medium-pressure differential pressure valves, respectively (data signified with an asterisk). We encountered a 4% incidence among patients with an initial valve setting of 200 mm H₂O. Combining these data sets results in a direct relationship between valve opening pressure and subdural hygroma incidence. The hygroma incidence for other valve designs and arrangements has not been well documented.

(From Bergsneider M, Miller C, Vespa PM, Hu X. Surgical management of adult hydrocephalus. Neurosurgery. 2008;62:SHC643-660.)

Another justification for the routine use of adjustable valves is based on the range of “final” valve opening pressures when these valves are used. In our retrospective evaluation of 114 consecutive NPH patients surgically treated with a CSF shunt, the histogram distribution of the final valve opening pressure revealed a roughly gaussian distribution, with most patients in the range of 120 to 140 mm H₂O (Fig. 35-2).⁷ This finding closely agrees with that of other large NPH studies.¹⁶ With the wide distribution of final valve pressures shown in Figure 35-2 (from <40 to >200 mm H₂O), it is difficult to fathom how a fixed-pressure valve could adequately serve this population unless there is a way of selecting the appropriate valve pressure preoperatively. Although some have suggested algorithms to do so,¹⁶^,¹⁷ none has been independently evaluated or validated.

FIGURE 35-2 Range of valve opening pressures in the treatment of idiopathic NPH. Histogram of final differential valve opening pressure values shows a gaussian distribution centered at approximately 140 mm H₂O. The wide range of valve opening pressures required indicates that no single valve opening pressure is appropriate for the treatment of idiopathic NPH.

(From Bergsneider M, Miller C, Vespa PM, Hu X. Surgical management of adult hydrocephalus. Neurosurgery. 2008;62:SHC643-660.)

Some neurosurgeons remain reluctant to use adjustable valves on a routine basis (or at all). On their side are the results of a prospective, randomized trial comparing the Codman Hakim adjustable valve and a standard differential pressure valve that failed to demonstrate a difference in shunt failure rates.⁴ This study, however, was primarily a pediatric study and, in our opinion, not conclusive with regard to adult hydrocephalus. Arguments that these valves are unreliable, or malfunction more frequently than fixed valves do, are not supported by any clinical study (or our clinical experience with the implantation of more than 400 of these devices). There is a fear that in certain patients, particularly in patients with chronic headache or with particular psychosocial issues, the clinician will be plagued with continued requests for valve adjustments. In our experience, this has not materialized to any significant degree. Perhaps the biggest drawback is cost. Currently, adjustable valves are two to three times more costly compared with fixed-pressure valves, and there is no clinical study comparing cost-effectiveness. A direct comparison of cost utilization would have to factor in the morbidity associated with repeated operations and associated operative risks when fixed-pressure valves are used.

Another drawback of adjustable valves has been MRI compatibility. Because the rotors harbor permanent magnets, there is an inherent susceptibility to large magnetic fields, especially MRI scanners. To date, two manufacturers (Sophysa Polaris and Aesculap proGAV) have designed a locking mechanism that in theory prevents resetting of the valve when the patient is brought in and out of the MRI scanner. The first-generation adjustable valves (Sophysa Sophy, Codman Hakim, and Medtronic Strata valves) are all susceptible to high magnetic fields, and therefore the valve setting must be verified after an MRI scan. In our practice, we specifically use valves with locking mechanisms in patients in whom it is anticipated that future MRI studies are required (such as any patient with a brain tumor).

Commercially available adjustable valves have different opening pressure ranges. Because of physical limitations and spring properties, the maximum and minimum valve opening pressures are constrained. The best example is the Sophysa Polaris valves, which come in the following ranges: 10-140, 30-200, 50-300, and 80-400 mm H₂O (SPVA-140, SPVA, SPVA-300, and SPVA-400, respectively). The Codman Hakim and Medtronic Strata valves are available in only one range of pressure settings. There are no evidence-based guidelines for selection of the most appropriate valve pressure range for any given patient. In our practice, we have some adult patients who require a pressure setting of 10 mm H₂O and others who do best at 400 mm H₂O. See our recommendations on valve pressure selection later.

Both the Aesculap proGAV and Codman Hakim adjustable valve have multiple, smaller discrete settings (from 0 to 200 mm H₂O or 30 to 200 mm H₂O, respectively). Both the Medtronic Strata and Sophysa Polaris valves have only five settings, thereby necessitating a larger jump between steps. We are not aware of any clinical study demonstrating an advantage of smaller steps, although changes as small as 10 mm H₂O can result in clinical responses.¹⁶ Our current management algorithm typically involves making valve adjustments of 30 mm H₂O; only in uncommon scenarios are smaller adjustments apparently beneficial.

Siphon-Control and Anti-Siphon Devices

We refer to these collectively as anti-siphon devices (ASDs), although there are mechanical and marketing differences between them. ASDs are add-on devices, meaning that they are used in conjunction with (immediately distal to) a differential pressure valve mechanism. These devices have been used clinically for more than 30 years.¹⁸

In general, the device is based on a membrane that is mechanically coupled to the subcutaneous tissue overlying it.¹⁸ The pressure differential between the internal valve lumen and the atmosphere, transmitted across the skin and ASD membrane, determines the flow-pressure characteristics of the ASD device. When the intraluminal pressure becomes significantly negative (relative to atmospheric pressure), the membrane is drawn inward—interacting with other fixed components of the ASD and thereby creating an increased pressure gradient. The original ASD was a separate component (Heyer-Schulte) that had to be inserted into the shunt. The Heyer-Schulte ASD fell into disfavor because of a variety of reasons, typically underdrainage, and has been largely supplanted by a more advanced design marketed by Medtronic.¹⁹ The Medtronic Delta chamber is found in the Delta valve (fixed-pressure apposing membrane differential pressure valve with integral Delta chamber) and Strata valve (adjustable ball-ring differential pressure valve with integral Delta chamber).

ASDs were developed on the basis of the premise that “siphoning” is the etiology of shunt-related CSF overdrainage. Shunt overdrainage has existed since the inception of the shunt.^13,²⁰^–²² This phenomenon, better termed gravity-dependent drainage, occurs as the result of gravity-driven CSF flow down the distal catheter when the patient is in the upright position. Early studies²⁰ documented significantly negative intracranial pressures in shunted patients in the upright position. At the time, it was natural to assume that overdrainage complications (such as subdural hematomas) were due to this gravity-dependent drainage.

Our intracranial pressure studies in idiopathic NPH patients,⁸ as well as those of others,¹⁵ suggest that gravity-dependent drainage is likely to play a lesser role in the etiology of overdrainage complications. As any person assumes an upright position, intracranial pressure decreases whether they have a shunt or not. As a matter of fact, in the standing position, most people have a slightly subatmospheric intracranial pressure. When you place a shunt with a differential pressure valve, the curve of intracranial pressure versus head-of-bed elevation in shunted patients nearly parallels that of the pre-shunt state (Fig. 35-3).⁸ In other words, a shunt with a differential pressure valve essentially lowers the intracranial pressure nearly equally across the head-of-bed angulation range. The degree of intracranial pressure reduction is largely a function of the valve opening pressure.

FIGURE 35-3 Intracranial pressure (ICP, mean ± standard deviation) versus head-of-bed (HOB) elevation curves through the full range of differential pressure opening pressures (200, 170, 140, and so on) measured in idiopathic NPH patients treated with a ventriculoperitoneal shunt.⁸ The pre-shunt baseline curve (gray line, filled gray square) was obtained from the same group of patients. Note that the preoperative and postoperative curves roughly parallel one another, demonstrating the limited role of “siphoning” as the cause of overdrainage in idiopathic NPH patients.

Key Point

In vivo intracranial pressure data suggest that the etiology of shunt “overdrainage” with differential pressure valves is in many cases, if not most, a result of too low of a valve opening pressure selection, rather than so-called siphoning.

There is little clinical evidence to support the contention that ASDs prevent overdrainage. A large, prospective, randomized study comparing a standard differential pressure valve, the Medtronic Delta valve, and the Orbis-Sigma valve found no statistical difference in the rate of ventricular reduction, the final ventricle size, or the incidence of clinical shunt failure.¹^,²³ A follow-up single-armed prospective study³ to the prospective, randomized trial comparing the Codman Hakim adjustable valve with a standard differential pressure valve⁴ similarly revealed that the programmable Strata valve also failed to show any benefit in pediatric patients.

For some hydrocephalus patients, the presence of an ASD is detrimental.²⁴^–²⁹ This so-called low-pressure hydrocephalus syndrome, of which the incidence has not been quantified but is presumed to be less than 5%, occurs both in childhood and in adults. Given the low incidence of low-pressure hydrocephalus, we do not think that this “risk” constitutes a contraindication to the general use of products with ASDs. For clinicians who routinely use ASD devices, however, it is important that they become familiar with the signs and symptoms of low-pressure hydrocephalus.⁷^,²⁴ In our experience as well as that of others,³⁰ the addition of an ASD can be effective in patients with clinically symptomatic overdrainage.

Key Point

A patient who fails to improve clinically (or deteriorates), a patient who remains with significant ventriculomegaly that did not change, a patient who has low measured intracranial pressure, or a patient with an ASD should not be written off as a nonresponder. The diagnosis of low-pressure hydrocephalus should be considered.²⁴

Flow Restriction Devices

Another approach taken to counteract shunt overdrainage is the incorporation of a CSF flow restriction mechanism. The premise is that shunt overdrainage occurs as a result of an excessive rate of CSF drainage. It follows that by limiting the maximum CSF flow rate, overdrainage should be averted.

There are several different design approaches to achieve flow restriction. The Integra Orbis-Sigma II valve was designed to directly address flow restriction by use of a multistage needle-valve design. Depending on the differential pressure, a needle is raised or lowered through a small orifice. The diameter of the needle at any given point will determine the cross-sectional area through which the CSF can flow. The manufacturer claims that in stage I, it functions as a low-pressure differential pressure valve to minimize underdrainage complications. When conditions “favor postural or vasogenic overdrainage,” the needle moves to stage II, and the valve functions as a flow regulator to maintain flow within “physiological limits.” Last, the manufacturer claims that if intracranial pressure elevates abruptly, the valve opens widely to function as a “safety valve,” allowing rapid CSF flow.

There is scarce in vivo clinical evidence, however, to support these manufacturer claims. A large, prospective, randomized study comparing a standard differential pressure valve, the Medtronic Delta valve, and the Orbis-Sigma valve (the original design, predating the Orbis-Sigma II) found no statistical difference in the rate of ventricular reduction, the final ventricle size, or the incidence of clinical shunt failure.¹^,²³ In a retrospective study comparing the Orbis-Sigma valve with a standard differential pressure valve in NPH, Weiner and colleagues³¹ found no significant difference in the time to initial malfunction (shunt survival) between the Orbis-Sigma valve and the differential pressure valve shunts. There were three subdural hematomas and one infection in the Orbis-Sigma valve group compared with no complications in the differential pressure valve group (P = .11). Nearly 90% of all patients experienced improvement in gait after shunting, regardless of the valve system that was used.

Remarkably, there exist some in vivo data of measured CSF flow rate through shunts. Miyake and associates ¹⁷ created an externalized loop connected to an indwelling ventriculoperitoneal shunt and measured CSF flow rates in patients with NPH. They assessed the Codman adjustable (differential pressure valve) and Orbis-Sigma valves. They demonstrated that shunt flow differed across patients, but in general, flow increased as the adjustable valve setting was lowered regardless of whether the patient was recumbent or sitting. At higher opening pressures of the adjustable valve (140 to 200 mm H₂O) in the recumbent position, the flow was intermittent, whereas at the lowest setting of 30 mm H₂O, the flow rate was 100 to 200 µL/min. In the sitting position, the shunt flow rates were higher, ranging from 200 and 600 µL/min. For the Orbis-Sigma valve, the flow rates were very similar to the adjustable valve set at 200 mm H₂O in both the recumbent and sitting positions. This actual in vivo flow data would appear to contradict the Orbis-Sigma manufacturer’s concept that in stage I, it functions as a low-pressure differential pressure valve. There are no in vivo data available either to confirm or to refute the manufacturer’s claims regarding stage II and stage III activity.

Buy Membership for Neurosurgery Category to continue reading. Learn more here