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GUEST COMMENTARY
Year : 2018  |  Volume : 66  |  Issue : 1  |  Page : 24-35

Cerebrospinal fluid shunts – How they work: The basics


Department of Neurosurgery, Park Clinic, 4 Gorky Terrace, West Bengal, India

Date of Web Publication11-Jan-2018

Correspondence Address:
Sandip Chatterjee
Department of Neurosurgery, Park Clinic, 4 Gorky Terrace, Kolkata - 700 017, West Bengal
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0028-3886.222820

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How to cite this article:
Chatterjee S, Harischandra L. Cerebrospinal fluid shunts – How they work: The basics. Neurol India 2018;66:24-35

How to cite this URL:
Chatterjee S, Harischandra L. Cerebrospinal fluid shunts – How they work: The basics. Neurol India [serial online] 2018 [cited 2019 Jun 26];66:24-35. Available from: http://www.neurologyindia.com/text.asp?2018/66/1/24/222820




The treatment for hydrocephalus can be divided into 3 phases [1] - The first phase includes the Greco-Roman era, Middle ages and Renaissance (up to 19th century AD) when the pathophysiology of hydrocephalus was unknown (medical treatment was considered useless and surgical treatment was hopeless). The second phase was in the 19th and early half of the 20th century (2nd phase), by which time the anatomy and physiology of cerebrospinal fluid (CSF) was somewhat understood and various surgical treatment modalities were attempted, though largely unsuccessfully. With the development of shunt systems from the 1950s started the 3rd phase where surgical treatment had a dominant role. A dream seemed to have been fulfilled in the life of patients with hydrocephalus. Unfortunately, it started an era of nightmares for the operating surgeons!

Technology in shunting has improved so dramatically, and yet, the cure for hydrocephalus is yet to be achieved. There are more than 400 shunt types available [2] but none have proved to be ideal. As the neurosurgical adage goes, “The best shunt is to have NO SHUNT!” It is not possible for a neurosurgeon to know about all the 400 odd types of shunts available but the principles on the basis of which these shunts evolved should be known so that the right shunt for the right patient can be chosen. This article attempts to throw light on the basics of shunt physics and its components and on how to choose the best suitable shunt (if indeed, that is possible!)

Definition of a cerebrospinal fluid shunt

A cerebrospinal fluid (CSF) shunt is a surgically implanted device used for diverting CSF from the cranio-spinal axis to an appropriate drainage site for absorption in a controlled manner. The diversion of the CSF may be from any CSF containing space-ventricle/subdural space/syrinx/thecal sac (commonly lumbar) or an arachnoid cyst -to any distal compartment which can absorb CSF. The preferred distal compartments may be the peritoneum, atrium and pleura, with peritoneum being the most favoured.[3] In this article, the discussion will be centred around the ubiquitous ventricular shunt system.

Evolution of the shunt system

Diversion of CSF was attempted in the late 19th and early 20th century to various intracranial/extracranial compartments. For the purpose of diversion, several conduits were utilised like silver wires (Cushing), rubber tubes (Kausch), veins (Payr) and serosa of the gut (Heile).[1],[4],[5] Payr et al., could be credited as the first person to use a valve (valves in the vein) in the diversion of CSF.[1],[4] These methods were, however, unsuccessful because the synthetic materials utilised were bio-incompatible, thus inducing tissue reactions, and natural materials could not serve the purpose. This paved the way for more physiological treatments like choroid plexectomy and third ventriculostomy (introduced by Dandy in 1918 and 1922, respectively) which were practised with limited success and a high morbidity rate till the mid-20th century.[4],[5] It was following World War II that the two most significant developments took place in the management of hydrocephalus with shunts. Polyethylene and silicon used for insulating spark plugs on bombers during the War were found to be non-reactive with human tissue.[4] In 1947, it was Franc D. Ingraham who first published a report on the neurosurgical use of polyethylene.[6] Observing the bio-compatible nature of silicon, it was introduced into medical practice from 1953 and for shunting from 1956.[4] The advantages of silicon were instantly obvious – it was available extensively, autoclavable, and induced less tissue reaction.[4] The second greatest advancement was the introduction of the artificial valves by Frank Nulsen and Eugene Spitz in 1949 dubbed the “Johnson Foundation” valve using platinum springs and 2 balls.[7] In 1956, Eugene Spitz along with bioengineer, John Holter, combined the biocompatible material, silicon, to produce silicon elastomer valves (Spitz-Holter valve), which completely changed the outlook for hydrocephalus management.[1],[4],[7] Following these two technological advancements, CSF shunting started dominating the treatment of hydrocephalus with improved success rates.

Components of a shunt system

A basic shunt system (from here the discussion will be with reference to the ventricular shunts) consists of 3 parts- proximal catheter; distal catheter; and shunt valve. The shunt accessories like reservoirs and connectors may also be added on to a shunt system. A shunt system without a valve may be called as “valveless” shunt and this is almost removed from contemporary neurosurgical practice (with the exception of ventriculosubgaleal shunts and some syringosubarachnoid shunts). Among the different components, it has been the shunt valve technology which has advanced significantly to minimise the failures and complications associated with shunt placement. All these components may be separate and need to be assembled together or they may come as a one-piece shunt system (e.g., Unishunt® system, Codman, Raynham, MA, USA).

Proximal catheter

This represents the part of the shunt system which will drain the ventricular compartment (also called the ventricular catheter). The most common cause of shunt obstruction is catheter obstruction [8] and this would explain why so many variations are available in the catheter design. These catheters come with a stainless-steel stylet and a tantalum tip.

Biomaterial

All the catheters are made from silicon rubber (PDMS – polydimethylsiloxane).[9] Silicon is available as a gel/oil/elastic material. In medical practise, it is the silicon elastomer (SILASTIC®) that is utilised. Other bio-compatible materials like pHEMA (polyhydroxyethylmethacrylate), e-PTFE (expanded polytetrafluoroethylele) and PVP (polyvinylpyrollidone) have been used but with limited success, and hence, still silicone is being extensively used.[6] Among them, PVP needs a special mention – silicon was considered hydrophobic, and hence, bacteria could adsorb to its surface following which infections could occur. To avoid this problem, PVP was covalently bonded with silicon which made it hydrophilic and a water coating was formed around the catheter which prevented bacterial colonisation (marketed as BioGlide by Medtronics from 1995).[10] But unfortunately, this catheter had such a slippery surface that it was easily disconnected from the connector and hence was removed from the market from 2010.[6],[11]

Dimensions

The length of a catheter varies from 15cm – 23 cm.[6] The inner diameter of a catheter ranges from 1.0-1.6 mm and the external diameter from 2.1-3.2 mm. The larger the inner diameter of the catheter, the lesser will be the resistance to CSF flow, and hence, the better the drainage up to the valve. Unfortunately, it means the greater will be the size of the cortical tunnel created by the catheter to reach the ventricle.

Shape

Proximal end of the catheter may be straight/flanged/J-shaped [Figure 1]a,[Figure 1]b,[Figure 1]c. Obstruction of the proximal catheter with choroid plexus/brain parenchyma was noted as a common cause for shunt dysfunction by Hakim et al., in 1969.[12] Hakim et al., had introduced the J-shaped catheter (Shepherd -Crook) with the drainage holes along the concave side to avoid direct contact with the choroid plexus [12] [Figure 1]d. Portnoy et al., introduced the flanged catheter with an idea – the flanges could protect the holes as long as the catheter passed via the brain parenchyma, and inside the ventricle, the flanges would keep the catheter tip away from the choroid plexus.[13] [Figure 1]c. This design was found to be successful by Haase and Weeth et al., in 1976 in reducing obstruction,[14] but in those patients who needed a revision, it was associated with a higher risk of re-bleed.[15] Another concept introduced in 1982 was the “floating ventricular catheter” (Nakamura et al.,) in which an air pocket was introduced around the catheter to keep it floating and to help in avoiding contact with the ventricular wall and choroid tissue.[16] Saint-Rose et al., in his analysis of 1,719 patients, found that none of these catheters reduced the incidence of catheter blockage.[15]
Figure 1: Different shapes of proximal catheter. (a) Straight catheter; (b) Right-angled catheter; (c) Flanged catheter; (d) Shepherd-Crook J shaped catheter

Click here to view


Distal end of the catheter may be right-angled or straight. When the proximal catheter comes out of the burr hole, it undergoes an almost right-angled kinking and this may cause resistance to the flow of CSF. To avoid this, the concept of right-angled catheters [Figure 1]b were introduced, which has an inherent bend that lies across the burr hole. These right-angled catheters, however, have a fixed length which needs to be within the ventricle and cannot be adjusted to the length a neurosurgeon would desire.[17] Today's right-angled connectors or right-angled adapters [Figure 2] are also available to avoid kinking across the burr hole but in infants this may lead to erosion of the skin because of their hard nature (especially the right-angled plastic adapters).[7]
Figure 2: Devices to avoid proximal catheter kinking. (a) Right-angled connector; (b) Right-angled adaptor (with the catheter passing through the adapter)

Click here to view


Draining holes

Draining holes of size 0.25-0.5 mm are found near the distal end 1-1.5 cm away from the tip. They are arranged in 3-4 rows either parallel or staggered, and either are of the same size or of variable sizes depending on the manufacturer. To avoid obstruction of these holes, a few manufacturers use grooves on the distal end of the catheter and the holes are encased inside these grooves (PS Medical® Slotted ventricular catheters). Lin et al., (2003) in his study on computational fluid dynamics noted that most of the CSF flow occurs through proximal holes (away from catheter tip) and obstruction occurs when these holes are either outside the ventricle or blocked.[18] Hence they suggested that if uniform flow occurs across all holes, the chance of obstruction is less.[18] This could be achieved by having small proximal holes (away from the catheter tip) and larger distal holes (towards the catheter tip). Another study by Thomale et al., had suggested that more the number of holes, more is the probability of obstruction and so minimising the number of holes (ideally keeping the count to 4-6 holes) helps in improving the outcome.[19] Although from a physical point of view this hydrodynamic theory is attractive, a number of studies have failed to show any merit in this arrangement of holes.[7],[9]

Antibiotic impregnated catheter (AIC)

In 1982, in a study by Sekhar et al., investigating various tissue types adsorbing to the catheter surface, it was found that not only does the choroid plexus or glial tissue have an adsorbing tendency, but inflammatory and granulomatous tissue also adsorb to the catheter.[20] This could probably be based on the hydrophobic properties of silicon and is the cause of bacteria colonisation.[6] Based on the early experiment by Bayston and Milner to introduce antibiotics along with silicon polymer to reduce infection rates, came up the concept of antibiotic impregnated shunts.[6] There are 3 varieties available now –Bactiseal® (Codman); Ares® (Medtronics) and Silverline® (Spiegelberg).[7]The first two contain rifampicin and clindamycin coating along with silicon and seem to be quite effective in avoiding infections for at least one month (most infections are transmitted during surgery and these antibiotics are useful in preventing them).[21],[22] Silverline® is different in the sense that it is coated with an insoluble silver salt, which is gradually released; this silver has strong anti-bacterial properties against both gram-positive/negative organisms, and hence works for a longer period. A meta-analysis conducted in 2014 regarding infection rates on comparing AICs and non-coated silicon catheters revealed that there is only class III evidence available to suggest that AICs reduce the infection rate.[23] At present, there is a randomised control trial (BASICS – British Antibiotic and Silver Impregnated Catheter for Ventriculoperitoneal Shunts) going on and the results are expected to give a clearer picture about the efficacy of these devices.

Slit tip catheters

In order to improvise the technique of placement of the proximal catheter inside the ventricle, these catheters have been ergonomically modified to have a slit tip through which a fibreoptic endoscope (Neuropen®, Medtronics) can be passed, which acts as a stylet as well as provides vision to place the catheter in a desired location [Figure 3]. These catheter are available as Neuropen ® endoscope – Innervision® catheter (Medtronics) and Bactiseal endoscopic ventricular catheter® (BEVC, Codman). In the experience of the senior author, the introduction of endoscope (the so-called shuntscope) remains essentially a “blind” procedure so far as the site of exact placement of the shunt tip is concerned.
Figure 3: Pictorial representations of slit tip catheters. (a) When the endoscope is not passed; (b) how the slit tip opens up when the endoscope is passed

Click here to view


Distal catheter

The part of shunt system which opens into the draining compartment forms the distal catheter. These are usually 90-120 cm in length, depending on the manufacturer. The internal diameter varies from 0.7-1.3 mm and external diameter from 2.1-2.5 mm. They are also mostly made from silicon polymer. To be visible on an X-ray imaging, they may be coated with barium (barium impregnated). Most shunt catheters are not radio-opaque although this would seem to be a logical innovation to aid in their visualization on X rays.,

Slit valves

Following the introduction of slit valves, it was incorporated into the distal catheters as an additional valve by Pudenz et al.[5],[24] It was suggested by Raimondi and Matsumoto in 1967 that slit valves prevent obstruction of the distal catheter opening by the omentum.[25],[26] Usually the distal tip is closed and slit valves are present proximal to the tip. But in the 1980s, the distal ends were kept open and the slit valves left to act as safety valves. The latter open up when the distal open tip is occluded by viscous material/omentum.[7] A landmark study in 1997 by Cozzens et al., had suggested that it is the presence of distal slit valves that increased the chance of distal catheter block and so catheters must be placed open-ended without slits.[27] This conclusion was also supported by C. Saint-Rose and J. Drake in their shunt book in 1995.[24]

Barium impregnated

Like proximal catheters, distal catheters were also coated with barium to make them opaque on an X-ray and this is also responsible for their white colour.[9] In 2 separate studies by Yamamoto et al., and Boch et al., on long-term shunt complications, it was identified that barium sulphate impregnated shunt systems have a higher rate of calcification followed by breakage, leading to dysfunction.[28],[29] It was proven by Tomes et al., in an experimental study in 2003 that barium impregnated catheters required less strain to fracture when subjected to strength testing.[30] Hence, to avoid this, now the distal catheters are available as barium striped or radiolucent (barium free) and tantalum marked.[7]

Shunt valves

They are the most important components of the shunt system which regulates the outflow of CSF from the ventricles to the draining compartment. They basically control the direction of flow (they provide only one-way directionality from the ventricle to the drain site) and the quantity of flow (based on pressure difference between the two compartments). Shunt valves have gone through more rigorous testing than aircraft engines over the last few decades, and at present, there are nearly 200 types of shunt valves used by 3 generations of neurosurgeons.[7],[31] The next few paragraphs illustrate why the physics of shunt valves has to be understood in great details.

Hydrostatic pressure

This represents the pressure exerted by a fluid column, depending on its height and density at a given point in the column, with respect to a reference level. It is represented by the formula:



Where ƿ represents the density of the fluid, g the gavitational force (9.8m/s) and, h the height of column of the fluid. With ƿ and g remaining constant for a fluid, it is the height of the fluid column that determines the HSP. The reference level represents the level at which the fluid column equals the atmospheric pressure (0 cm H2O).

In the above figure [Figure 4], there is a fixed quantity of water inside a water container with an opening at the bottom (reference level). When this container is laid flat, the water column has a lesser height (h); therefore, HSP is lower and water flows out at a particular speed (2 drops/min). When this water container is placed vertically, the height of the column increases to H and, therefore, the HSP increases leading to a faster flow rate (4 drops/min).
Figure 4: Hydrostatic pressure (HSP) – what happens when a tank is placed horizontally (Height –h; low flow state) and when a tank is placed vertically (Height – H; High flow state)

Click here to view


In reality, the fluid in this experiment represents the CSF and the container represents the ventricular system and the spinal subarachnoid space (SAS). When person is lying flat, the reference level (0cm H2O) is the surface of the abdomen; and, when the person stands, the reference level moves to the diaphragm. When a person is in a supine posture, there is slow CSF flow within the 2 compartments; whereas, when the person stands up, the CSF inside the ventricle flows rapidly into the spinal SAS creating a negative pressure in the cranium which is physiologically compensated by collapse of the jugular veins.

When a patient has a shunt, flow of CSF occurs through the shunt system and not through the normal pathway. When this patient is lying down, flow across the shunt tube occurs similar to the physiological manner whenever the intracranial pressure increases (ICP). But when the same patient stands up, there is a rapid increase in the HSP (height between the foramen of Monroe and diaphragm is around 30-40cm) and this may lead to a non-physiological high flow rate across the shunt tube. The reason behind deploying valves across the shunt tube is for regulation of this flow.

Opening pressure

The pressure needed to open a valve is defined as the opening pressure (OP) of the valve. Once the pressure decreases below this opening pressure, the valve will automatically close. On similar lines, certain valves (especially slit valves) are defined by their closing pressure (CP) which represents the pressure above which a valve closes automatically. The overall pressure difference across the two compartments (ventricle and abdomen in case of ventriculoperitoneal shunts) is calculated as follows –

Differential pressure (DP) = ICP + HSP – IAP

Where ICP represents the intracranial pressure, HSP the hydrostatic pressure depending on the position of the patient, and IAP the intra-abdominal pressure. Therefore, in the case of valves which operate based on OP:

DP > OP = VALVE OPENS (NORMALLY IN A CLOSED STATE)

In case a valve functions based on closing pressure (CP):

DP > CP = VALVE CLOSES (NORMALLY IN A OPEN STATE)

The OP or CP is the most important defining characteristic of a valve, and those valves which function solely based on pressure difference between the 2 compartments are known as the Differential Pressure Valves (DPV).

Fixed differential pressure valves (DPV)

They represent the first-generation valves that were developed. They function based on differential pressure between the 2 compartments by either opening or closing. They were classified as– Very low; Low; Medium; High; and Very high, based on the differential pressure range around which they function. Unfortunately, different manufacturers have defined their own pressure ranges to describe these valves and there is no uniformity. For example, if a medium pressure valve is defined with an OP of 10cm H2O, this valve opens when the DP between the 2 compartments exceeds 10 cm H2O and remains closed if the DP is <10cm H2O. Those valves that close when the DP exceeds the CP are known as the Inverse DPV (iDPV). Based on their mechanical design, the DPV can be classified into four types: Slit; Mitre; Diaphragm; and, Ball in cone [Figure 5].
Figure 5: Mechanical designs of valves with the direction of CSF flow marked with arrows. (a) Slit valve; (b) Mitre valve

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Slit valves represent the first design that was developed by Spitz-Holter in 1956. As the name suggests, they have a slit in the tube which opens up when the pressure builds inside the catheter, or else they remain closed.

Mitre valves, commonly known as “duck bill” like valves,[7] have 2 leaflets which are closely opposed to each other. When a particular pressure builds up, the CSF pushes the 2 leaflets apart and finds its way out into the distal catheter or the draining compartment.

Diaphragm valves are actually constituted by a membrane closing an opening. When the pressure increases in the proximal end, CSF pushes the membrane away from the opening and drains CSF [Figure 6]a.
Figure 6: Mechanical designs of valves with direction of CSF flow marked with the arrows. (a) Diaphragm valve; (b) Ball-in-cone valve

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Ball-in-cone valves (Ball-spring valve) were first described by Solomon and Hakim in the 1970s.[7] They consist of a ball (a metallic one) held occluding an opening with the support of a spring. When DP crosses the OP, the ball is pushed against the spring and CSF escapes distally [Figure 6]b.

It would seem obvious from the discussion so far that an ideal DPV would be the one which allows CSF flow in “open” state and completely blocks CSF flow in “closed” state. Most DPV in reality, however, are like a door as the valve opens gradually (the door opens) and the CSF starts flowing even before the valve (the door) reaches its fully open state. Richard et al., have beautifully demonstrated the opening/closing characteristic of the different types of DPV once the OP is exceeded by DP.[32] According to the evaluation, the “ball-in-cone” valve is closest to the ideal valve, offering maximum flow as soon as the DP exceeds the OP; “slit” valves show a gradual increase in the flow rate as the DP increases over the OP; and, “diaphragm” valves have a flow rate that ranges between the rates exhibited by the above two valves.[9],[32]

A problem related to the functioning of the fixed DPV valve is the phenomenon of hysteresis.

Hysteresis is an important phenomenon observed with the fixed DPV, especially when the valve is constructed from silicon.[7],[32],[33] This actually represents an aging process where the valves gradually wear off and allows CSF to leak even in a closed state, and also allows unregulated flow across the valve not corresponding to the OP that has been assigned to it.[7],[32],[33] This phenomenon was mostly observed with mitre/diaphragm (membrane)/slit valves and not with ball-in-cone valve. Hence the ball-in-cone valve is considered to be the “best mechanical valve” as well as the “most widely” used and the” most successful” valve [Figure 7].[9],[34]
Figure 7: The ideal and real pressure curves

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Another problem common to all DPV has been the phenomenon of overdrainage. When a person stands up from a recumbent position; as well as, during the night when nocturnal cerebral vasogenic waves are active, there is a sudden increase in the DP which results in a non-physiological high flow across the valve irrespective of the type of valve used.[9],[34] This is because of the low hydrodynamic resistance offered by the valve to CSF flow (they are only pressure regulated).[34] This was very often noticed in the 1960s and 70s, which led to the development of second generation valves, the hydrostatic valves and programmable valves.[35],[36]

Hydrostatic valve (HSV)

As the name suggests, these were valves that were designed to avoid over-drainage of CSF following a sudden change in hydrostatic pressure. They are classified into 3 categories based on the principle they follow:

  1. Suction controlled devices
  2. Flow reducing devices
  3. Gravitational valves.


All the 3 groups function to reduce CSF flow when a person comes to a standing position but are based on different mechanisms. Another term that needs to be introduced here is “siphoning.” When a person gets up from a recumbent posture to a standing posture, the fluid level (CSF) in the cranial cavity is at a higher level than the abdominal cavity causing the CSF to flow rapidly into the abdomen till the amount of fluid is equal in both the compartments. This phenomenon is known as “siphoning” [Figure 8].[37],[38],[39] It is because of this siphoning of CSF that over-drainage symptoms may occur in a patient who has undergone a shunt procedure. In order to avoid this “siphoning,” a device was created in 1973 by Portnoy and Heyer Schulte known as the anti-siphon device.[35] The earlier anti-siphon devices that were created were extra attachments to the existing DPVs, but later on, the newer ones were hybrid models where the anti-siphon device was incorporated into the shunt system.
Figure 8: The anti-siphon device: The black arrow represents the direction of CSF flow from the cranium. Black sheet represents the flexible membrane valve. Red marking represents another membrane dividing the reservoir into 2 halves. (a) In recumbent position of the patient with DP <OP, there is no flow across the valve; (b) In recumbent position of the patient with DP >OP, flow occurs across the two membranes; (c) In vertical position of the patient, the proximal part of the reservoir (above the red membrane) exerts a PUSHING pressure (PP; single black arrow) to open the black membrane valve, while distal part of the reservoir (below the red membrane) exerts a negative SUCKING pressure (SP; double black arrows) which prevents the black membrane from opening. When PP >SP, CSF flow occurs. When PP

Click here to view


Suction controlled devices

These devices work on the principle that the suction effect due to the upright position of the patient may be used to pull on a membrane, which in turn occludes the pathway for CSF drainage. The greater the suction effect, more effective the occlusion. In a normal DPV, the positive pushing pressure from above (ICP) in addition to the negative pulling/sucking pressure from below (the siphon effect) help to drain the CSF into the collecting compartment (the two pressures complement each other). In the suction controlled devices, the positive pressure from above (ICP) tries to drain CSF through the pathway whereas the suction effect from below (negative pressure) tries to occlude the same pathway for CSF (the two pressures counter each other). These devices are available today in the form of anti-siphon device (ASD® Integra) and Delta® chamber (Medtronics).

One might assume at this stage that we have ready, the design of an ideal shunt with an antisiphon device built in. However, there are two important drawbacks of this system:[9]

  1. The valve has to be positioned in a perfect position so that the positive pressure from above (ICP) and the negative pressure from below (siphon) are able to balance each other and avoid over-drainage. If the device is positioned too low then (ICP > siphon), overdrainage may occur. If the device is positioned very high (siphon > ICP), underdrainage may occur
  2. This entire valve system is placed in the subcutaneous plane and is susceptible to external subcutaneous pressure (leading to scarring or to hematoma formation) which may alter the effective functioning of the valve.


Flow reducing devices

These devices were developed with an idea that if flow can be controlled then siphoning can be avoided. They were believed to maintain a constant flow rate and were broadly divided into 2 types, either a binary flow control or a continuous flow control. In the binary flow control device, CSF takes two pathways depending on the differential pressure, with the two pathways offering either a low flow resistance or a high flow resistance, respectively.[9] In the continuous flow control system, as the DP increases, the resistance to flow increases linearly till an “emergency state,” where DP is very high and rapid increase in flow occurs to reduce DP. They are available as Siphonguard® [Codman; [Figure 9]; Orbis-Sigma valve ® [OSV – Integra; introduced by Saint Rose in 1989; [Figure 10] and CRx Diamond ®valve (Vyogo Neuro).[39] Siphonguard® from Codman represents a binary flow control device and the other two are continuous flow control devices.[9]
Figure 9: Pictorial representation of the Siphonguard®. Black arrows represent the direction of CSF flow from the proximal to distal end. The rectangular chamber with the ball-spring valve represents the low-resistance flow system and the spiral tube represents the high flow resistance system. (a) When the patient is recumbent, flow occurs via the low-flow resistance pathway. (b) When the patient is vertical, or when the flow exceeds 1ml/min, the ball-in-cone valve blocks the flow and all flow is diverted through the high-flow resistance pathway

Click here to view
Figure 10: Pictorial representation of the Orbis Sigma valve functioning. Black arrow represents the direction of CSF flow from proximal to distal end. There is a flexible silicon membrane attached at both ends (represented by the violet-coloured membrane in the diagram; the dotted lines represent how the membrane moves as ICP increases) with a specially designed red ruby pin in the middle, and sapphire plates at mobile ends of the silicon membrane (represented by the black lines). Left sided pictures represent the section across the valve. Right sided pictures represent the cross-section at the level of the ruby pin to indicate resistance offered to flow. (a) No flow state, when ICP is not high, the membrane remains closed. (b and c) Open state, when ICP increases to the range of 8-35 cm H2O, the specially designed ruby pin helps to maintain CSF flow across the valve at a constant rate of 20-30ml/h. (d) Emergency state, when ICP > 35cm H2O, the valve fully opens to let out more CSF

Click here to view


The major issues with these devices are:[9]

  1. They work based on the differential pressure and are not based on posture though they have been termed flow-control devices
  2. Siphonguard® lacks a safety mechanism for the emergency state (that is, the presence of a very high ICP) and can be risky for a patient, unlike the OSV system
  3. In OSV®, the diameter of the flow chamber during the high resistance period is 15-20um. This can be blocked by monocytes present in the CSF leading to dysfunction of the valve on a long-term basis.


Gravitational valve

This concept of using metal balls to regulate flow across an opening was introduced by Hakim et al., in 1974.[9],[39] It was first used for a lumbo-peritoneal shunt and then implemented in ventricular shunts. These devices function basically like the DPVs but they are capable of increasing the OP as the inclination of the patient increases (thus, their mechanism of action is posture dependent). They work on a very simple concept in which 2-4 metal balls exist in a valve. When a patient is lying supine, they act as a simple DPV with only one metal ball acting in the ball-in-cone mechanism [Figure 11]; the other metallic balls having no role in the pressure regulation to control the CSF flow]. As the posture changes and the inclination increases, the other metal balls, one by one, fall over the conical opening, thereby adding on to the opening pressure needed to maintain the CSF egress through the valve. Basically, the weight of the metal balls counters the increasing hydrostatic pressure. They may also be divided into the binary switcher valves (Dualswitch Valve ®–Miethke) and the analogous valves (Shuntassistant® – Aesculap; Gravity –Compensating Accessory ®– Integra) depending on whether they act solely in the supine (low OP) or standing (high OP) positions, respectively, or continuously change OP as inclination increases.[9]
Figure 11: Pictorial representation of functioning of a gravitational valve. Black arrow represents the direction of CSF flow. There are two separate chambers communicating with each other across a U bend – one side with a simple ball - spring valve and the other side with 2-4 metal balls guarding a conical opening. (a) When the patient is recumbent, CSF flows freely through both the chambers. (b) When the patient's position is vertical, CSF flows as usual through the ball-spring valve but the weight of the metal balls blocks the conical opening in the other chamber. For CSF to cross this chamber, therefore, adequate pressure has to be generated by the CSF flowing across the valve to lift the metallic balls, or else, flow will not occur across this half of the chamber

Click here to view


These valves are not without problems of their own:[9]

During the placement of these valves, the longitudinal axis of the gravitational valve should be in parallel alignment with the spinal column (a 10-20 degree variation is acceptable) so that the device can act based upon the changes in the inclination. If >20 degree misalignment with spinal column occurs, there are higher chances of over/under-drainage of CSF through the valve.[40],[41]

Programmable valves

They form the other group of second generation valves that have evolved to avoid the complications associated with fixed DPV.[7],[42] Basically, they are adjustable DPV s in which non-invasively, the DP can be altered. They are in no way related to computer programming and the name 'programmable valve' was given just for marketing of the valves.[9] The most important advantage of the adjustable DPV is the ability to alter the OP non-invasively as per the response of the patient based upon the anticipated CSF over- or underdrainage [Figure 12]. The concept of controlling the opening pressure of a valve from outside was initiated by Portnoy and Solomon Hakim in 1973,[7],[9] following which the first programmable valve designed for clinical use was introduced in 1989 (PAVS, Pressure Adjustable Valve- SOPHY® – Sophysa France).[43],[44] The currently available adjustable DPV are Codman-Hakim programmable valve® (CHPV); Strata® NSC (Medtronics); and, Sophysa Polaris valve® (SPV) (Sophysa).[42]
Figure 12: Pictorial representation of function of a programmable valve. Arrow represents the direction of CSF flow from the proximal to distal end. The two small black arrows represent thin silicon valves holding the ruby ball. At the proximal end, there is a ruby ball which lies on a thin silicon leaflet (represented by the violet membrane in the figure). This is attached to a rotor (represented by the green rectangle in the figure) which can be turned with the help of a magnet (the rotor may be adjusted based upon 5 pressure level markings). If the rotor moves clockwise, it will be easier for the ruby ball to push the silicon leaflet (lower OP). If the rotor moves anti-clockwise, it will require more effort to push the silicon leaflet to drain CSF (high OP)

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The CHPV system[42] can be adjusted between 3cm H2O and 20cm H2O in 18 steps (in 1cm H2O adjustments). To identify the existing pressure setting, however, an X-ray imaging would be needed.

The Strata NSC (no siphon control; Medtronics) system[42] comes with 5 different pressure level (PL) settings which can be identified and altered using an adjustment kit (Varius® system) [PL0.5 – 1.5-3.5cm H2O; PL1.0 – 3.5-5.5cm H2O; PL1.5 – 9-11cm H2O; PL2.0 – 14.5-16.5cm H2O and PL 2.5 – 20-22cm H2O].

The SPV (Sophysa) system[42] comes with 5 different pressure level settings, which can be identified and altered with the help of an adjustment tool kit. Depending on the maximum pressure settings, the devices are available in the following forms: standard SPV (3, 7, 11, 15 and maximum 20cm H2O); SPV-140 (maximum 14cm H2O); SPV-300 (maximum 30cm H2O) and SPV-400 (maximum 40cm H2O).

The most important issue to be remembered about the adjustable DPV devices is that once adjusted to a particular pressure level, they act like fixed DPV only, with an opening pressure to control CSF flow. Hence, there still is a high chance for under- or overdrainage. Therefore, the use of the adjustable DPV does not unequivocally ensure that under or overdrainage will not occur; however, should under- or overdrainage be clinically observed, then the OP of these devices can be suitably altered externally using a magnetic device, thereby reducing the need for another surgery to change the valve.

Hybrid valves (3rd and 4th generation valves)

Combination valve therapy developed after the hydrostatic valves evolved. In these devices, fixed DPV were combined with hydrostatic valves to avoid the siphon effect.[39] Combining programmable DPV with hydrostatic valves represent the third generation of valve technology (the hybrid shunt system).[42],[45] They are considered to be effective in regulating the OP as well as to keep a check on the siphoning when posture changes. Examples include the Codman-Hakim programmable valve with Siphonguard® (CHPV-SG®, Codman); Strata II® (Strata programmable valve with Delta chamber, Medtronics) and ProGAV® (Programmable DPV with Gravity Activity Valve, Aesculap).[9],[39],[42]

The fourth generation valves represent the most modern valves available for clinical use today which are programmable/adjustable antisiphon valves.[42] Their opening pressure is altered continuously depending on the posture of the patient. ProSA® (Programmable Shunt Assistant, Aesculap) is the currently available fourth generation valve.[9],[39],[42]

Shunt Accessories

This includes shunt reservoirs and connectors.

Shunt reservoirs

They are also known as shunt chambers, which are CSF collecting systems along the shunt catheter.[9] They are used to collect CSF samples (to facilitate a shunt tap), to measure the CSF pressure (ICP) and also to inject drugs into the ventricular system. They are made of silicon which can seal off following a puncture. The base may be made of silicon/plastic/titanium/ceramic. They are available in 2 types – the burr hole type or the inline type (the cylindrical type) [Figure 13] and [Figure 14]. Sometimes, medical personal can test shunt function by flushing the chamber (normally in 1 flush, 0.1-0.3 ml of CSF can be drained).[9] This is, however, not a reliable method to test shunt function unless there is a valve to prevent backflow into the ventricle.
Figure 13: Inline reservoir

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Figure 14: Burr-hole type reservoir

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Connectors

They are used in all shunt systems, except the one-piece shunts, to connect different parts of the shunt system [Figure 15] and [Figure 16].[9] They may be manufactured using titanium (which provides a good mechanical stability but causes increased artefacts on magnetic resonance imaging [MRI]) or plastic (these have a poor mechanical stability but cause no MRI artefacts).
Figure 15: Straight connector

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Figure 16: Right-angled connector

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Selection of shunt

One-piece versus three-piece shunt system

The one-piece shunt systems were introduced to counter the incidence of shunt disconnection. They proved to be equally effective in treating hydrocephalus by Raimondi et al., in an analysis of 357 patients.[46] Haase et al., from Denmark in 1987 had suggested a long-term follow-up of the one-piece shunt systems. They had suggested that the one-piece shunt systems were effective in providing an effective CSF diversion though they could be complicated by proximal catheter obstruction and slit ventricles.[47] Also, the one-piece shunts are associated with a higher incidence of intracranial shunt migrations.[48] The senior author has described how one-piece shunts packaged in a coiled manner have a greater tendency to migrate [due to their elastic recoil, [Figure 17].[49] They are available in the market as the Unishunt® system (Codman).
Figure 17: The antibiotic impregnated shunt is being installed

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Proximal catheters

This part has already been discussed extensively by the author in the section under 'proximal catheter.' The amount of barium impregnated in the catheter does not play a role in their intraventricular placement as they do not get calcified. BioGlide® catheters have been withdrawn from the market.[50] The exact role of antibiotic impregnated catheters will be better known after the results of the BASICS trial have been published [Figure 18].
Figure 18: The various degrees of “coils” that are introduced while packaging different shunt systems.

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Distal catheters

The senior author generally prefers placement of the distal catheter in the peritoneum unless there is a contraindication. It is also his practice in infants less than 6 months of age to shorten the length of the distal catheter, although one needs to emphasize that this practice is based on anecdotal experience of distal catheter problems rather than based on any trial. Maset et al., from Brazil in 2014 had suggested that removing up to 20 cm of the distal catheter length would not be associated with alterations in the pressure dynamics of the CSF shunt system.[51] Open-ended catheters survive better than the closed-ended ones with slit valves.[15],[27] Avoiding fully barium-coated distal catheters could reduce the incidence of breakage of shunts on a long-term basis.[28],[29],[30]

Valves

A randomised control trial conducted at multiple centres across North America and Europe comparing DPV with hydrostatic devices (OSV, Delta valve) by Drake et al., in 1998 (Shunt Design Trial – Class I) concluded that none of the valves were superior to each other with regard to shunt survival, both on a short-term and a long-term follow-up basis.[52],[53],[54] A Cochrane review in 2013 also suggested that at present, there is insufficient evidence available to suggest the superiority of the flow-regulated devices over the simple DPV.[55]

Pollack et al., had conducted another randomised control trial in 1999 comparing fixed/simple DPVs with programmable valves.[56] From his analysis of 377 patients, it was identified that there was no significant difference in the incidence of shunt survival between the 2 groups, though this study was not done with the aim to assess the shunt survival (Class II).[56],[57] From the same group, it was also noted that the use of programmable valves reduced the need for surgical intervention in patients who develop a subdural hematoma as a post-shunt sequel, by manipulating the pressure non-invasively.[58]

A meta-analysis in 2014 had tried to analyse all the available studies comparing different types of shunt valves and concluded that there is at present insufficient evidence to suggest the superiority of one valve hardware design over the other. This meta-analysis also recommended that, at present, there is insufficient evidence to suggest the preferential usage of programmable valves over the simple DPVs.[57] Irrespective of these trials, at present, most neurosurgeons prefer programmable valves, at least for conditions like normal pressure hydrocephalus where the intracranial pressure settings are most unpredictable.[59]

The most important factor predicting shunt survival has been the OP which had been initially chosen.[59] With respect to children with hydrocephalus, the senior author prefers to use low-pressure (LP) valves as long as the anterior fontanelle is open; and, medium pressure (MP) valves, once the fontanelle closes. There have been two large retrospective series from Miranda et al., (103 children) and Robinson et al., (158 children) which provided contradicting reports that MP shunts have been associated with more obstruction; and, that LP shunts are associated with more failures, respectively.[60],[61] A small randomized control study by Sinha et al., had not suggested any role of MP/LP status on shunt survival in children.[62] With respect to the adults (especially those suffering from normal pressure hydrocephalus), the ideal OP to start with is yet to be identified but Miyake et al., had suggested a QRT (Quality Reference Table) which would help to decide this issue based on the height and weight of the patient as well as the starting OP existing at the time of placement of the shunt.[63] Also, the same author had later proposed that if according to the QRT, the OP falls in the range of 15-20cm H2O, it would be better to place a hydrostatic device to avoid over-drainage.[42] Bergsneider et al., had suggested using a programmable valve with the highest setting and then, depending on the response of the patient, to gradually reduce the pressure by 3cm H2O gradations until adequate response is obtained.[64]

The question, what an IDEAL shunt actually is, still remains unanswered due to the lack of adequate, good quality, comparative studies but from the current literature it can be presumed that the IDEAL shunt would be the one which is most suited for a patient as well as most familiar for the neurosurgeon performing the procedure.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15], [Figure 16], [Figure 17], [Figure 18]



 

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