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About CSF Shunts
Article by Graham Flint, Consultant Neurosurgeon
To most people the word shunt might bring to mind thoughts of a relatively minor bump in a car. To others it might conjure up visions of rolling stock being moved on railways. To an electrician it might have yet another meaning. The word itself simply refers to the movement of something from one place to another, although there is a clear implication of shifting a significant load of some sort, rather than just picking up and moving a light object. The term also often implies that whatever is being moved is diverted from its original direction of travel to another. The load involved could be physical, such as railway carriages or something less tangible such as electric current.
In medicine use of the word shunt usually refers to mass movement of fluids from one part of the body to another. Sometimes such movement comes about as a result of anatomical abnormalities and a cardiologist, for example, might talk of a right to left shunt caused by a hole in the heart. On other occasions vascular surgeons will deliberately create an artificial connection between two veins or between a vein and an artery, in order to shunt blood from one of these channels to the other. Neurosurgeons commonly have cause to shunt cerebrospinal fluid (CSF) from one body compartment to another.
We need to remind ourselves, at this point, about the basic anatomy and circulation of CSF. This subject is dealt with in the initial sections of the Ann Conroy Trust's booklet "Syringomyelia & Hindbrain Hernia" but a brief review here will help. CSF is, essentially, little more than a solution of electrolytes, not unlike the contents of a bottle of mineral water, although the chemical composition is a little more complex. CSF is formed in chambers inside the brain, that we call ventricles. Within the ventricles is a structure called the choroid plexus and this produces about half of the total amount of CSF that forms each day. The other half passes into the CSF channels directly from interstitial fluid of the brain. Interstitial fluid occupies the volume between the cells and the blood vessels, that make up the physical substance of the brain.
Once inside the ventricles, CSF flows though this system of chambers, to exit at the base of the brain, into what is known as the cisterna magna. The word cistern refers to a water container and, in lay usage, the term most often refers to the tank of the water used to flush a toilet. Inside the head, the term cistern also refers to collection water (or more accurately of CSF) and the largest cistern in the head is the one at the base, where the skull joins the spinal column. The term magna means large, hence the name "cisterna magna". In patients with hindbrain hernias (Chiari malformations) the cisterna magna is usually occupied not by CSF but by the tonsils of the cerebellum.
From the cisterna magna some of the CSF flows down into the spinal canal but eventually it all passes upwards, initially underneath and then over the surface of the brain, to be re-absorbed, back into the bloodstream, via a large venous channel which lies along the middle of the top of the head.
There are a variety of disorders that affect the CSF circulation. For the most part these consist of abnormal accumulations of CSF. Syringomyelia is one such condition, where we see an abnormal accumulation of CSF inside the spinal cord. The spinal cord is normally solid, all but for a very narrow central canal which is left over from early development, when the cord is a tubular structure. The other, more common example an abnormal accumulation of CSF is hydrocephalus, where the ventricles in the brain are larger than they should be. On other occasions we may encounter abnormal collections of CSF within other parts of the head or the spinal canal, taking the form of benign cysts. Other, somewhat rarer conditions, affecting the CSF circulation including leakages of the fluid, from the head or the spine.
Abnormal accumulations of CSF may be under raised pressure or may be under normal or only intermittently raised pressure. Either way, diverting, or "shunting" some of this fluid, from one body compartment to another, may be of benefit to the individual who is suffering from the effects of syringomyelia, hydrocephalus or some other disorder of CSF circulation. The commonest site from which CSF is drained is the ventricular system of the brain. Usually this is to deal with hydrocephalus but sometimes it might be to lower the CSF pressure "throughout the system", i.e. the ventricles, the CSF cisterns in the head, the CSF channels in the spinal canal and, along with these, any abnormal collections of CSF, such as a syringomyelia cavity. These types of shunts are referred to as ventricular shunts. CSF can, however, be drained from other sites, including the lower part of the spinal canal, below the level where the spinal cord ends. These are lumbar shunts. Other sites of drainage include the basal cisterns inside the head (cisternal shunts), abnormal cystic collections of CSF (cyst shunts) & syringomyelia cavities (syrinx shunts).
It is usual to name shunts not just in terms of where the CSF is taken from but also in terms of where it is diverted to. Although not a standard term, I find it useful to speak of "receptacles", to refer to the compartments of the body that can receive diverted CSF. The most commonly used receptacle is the abdominal or "peritoneal" cavity. Shunts diverting CSF into the abdomen are referred to as ventriculo-peritoneal, lumbo-peritoneal, cisterno-peritoneal, cysto-peritoneal and syringo-peritoneal, according to where the CSF is diverted from. Sometimes there may be reasons why the peritoneum cannot be used as a receptacle and it may then be reasonable to divert CSF into the right side of the heart or, more specifically the atrium of the right side of the heart. This receptacle is usually used to receive CSF from the ventricles, rather than other sites and we then speak of ventriculo-atrial shunts. Sometimes we make use of the thin cavity between the lungs and the inside of the chest wall. This is known as the pleural cavity. It is an anatomically convenient site for drainage of CSF from a syrinx and we then talk of syringo-pleural shunts. Other parts of the CSF pathways themselves may be used, in some circumstances, to receive diverted CSF. An example is a syringo-subarachnoid shunt, which drains the fluid contents of a syringomyelia cavity into somewhere else within the spinal canal. In some cases of hydrocephalus we divert CSF from the ventricles of the brain into other parts of the CSF channels within the head. In the past other receptacles have been used to receive CSF, including the gall bladder but such structures are rarely used these days.
The majority of CSF diversion operations involve the use of lengths of tubing, referred to as catheters. These are made of silicone elastomer, which is a very durable & flexible plastic and one which is inert inside the body. In other words it does not generally lead to any adverse reactions to its presence. Increasingly, catheters are used which have antibiotic substances embedded within them, to reduce the risk of them becoming infected.
Most shunts need to have the flow of CSF through them regulated, to reduce the chances of over-drainage, which can produce problems of its own. Structures called valves are used to achieve this. There are many different designs of valve but most of them belong to what is called the differential pressure type. This means that they open and allow CSF to flow through when the pressure on one side of the valve exceeds a set level. In many valves this pressure is fixed at one level but increasingly we are using valves where the pressure can subsequently be adjusted, from outside the body, usually using a magnetic adjusting device (see diagram). Another type of valve is called a switchable valve, which adjusts to changes in body posture, so that over-drainage does not occur in the upright position. Yet another modification is the antisyphon device, which is another means of preventing over-drainage in the upright position. Within each of these different categories of valve there are several designs available, from different manufacturers. Each company will claim specific benefits for their product but, in truth, it has proven impossible to demonstrate that one type is clearly superior to another. The wide range available does, at least, give the neurosurgeon the chance to tailor the choice of valve to an individual patient.
CSF shunt operations entail the implantation of man-made apparatus into a biological system. There is inevitably going to be a failure rate. In the long term it is doubtful if any shunt system continues working for more that 10 years and many cease functioning before then. This does not mean, however, that all shunts will need replacing sooner or later. In many cases an individual adjusts to the new CSF dynamics that have developed by the time the shunt ceases functioning. The individual may well, however, remain vulnerable to "decompensating". In other words, having coped for some time, the individual then suddenly develops symptoms again, from their no-longer controlled hydrocephalus, syringomyelia or other CSF disorder.
Leaving long-term failures aside, shunts also have an incidence of complications in the shorter term. The most common problems are caused by over-drainage, under-drainage (or blockage) and infection. Shunt revision operations are, therefore, relatively common procedures in neurosurgical practice. Of all the shunts that are first inserted perhaps 3 out of 4 function well from the outset but up to a quarter will not perform as expected and require revision.
There is, of course, a lot more that could be said about CSF shunts and much of this relates to the complexities of CSF physiology but this, in turn, reflects the fascination of the enigmatic condition that we call syringomyelia, not to mention other disorders of CSF circulation.