One error that can be made is to think that the human ventricular system (the well known four cavities within the brain) is static and that the ventricles do not modify over time.
At birth, or even within the womb, as can be seen on prenatal ultrasound, the ventricles are rather small. Within the first year of life, there is quite a lot of change in the brain and so in the shape and size of the ventricles.
With increasing age the ventricles become larger because of the unfortunate decrease in brain cells we all go through. These changes are easily picked up on CT scan or MRI scans. This also means that the actual size of the ventricular system has very little importance except in the extreme case where the ventricles are so large that there is hardly any brain left.
During each heart beat, a shock wave is produced that changes the shape and size of the ventricles. With a particular sequence on MRI scanning, this movement can be picked up and studied. This movement is also the object of a combined research project between the engineering department and myself.
All this is to say that we are dealing with a dynamic system where there is constant modification of shape and volume.
What are slit ventricles?
Slit ventricles are small ventricles, sometimes so small that they are barely visible on CT scan or MRI. Slit ventricles can occur after severe head injury or viral infection of the brain. In both conditions, the brain becomes so swollen that the fluid is pushed out of the ventricles.
More of interest to this discussion is the appearance of slit ventricles after cerebro-spinal fluid diversion - shunts for example. One of the criteria for a happily functioning shunt is decompression of the ventricular system when compared to the previous CT scan or MRI. Inherent to any shunt system is the change in pressure in the skull and brain. In some patients this can lead to a siphon effect, much as when you siphon petrol from a car. What happens next all depends on the resilience of the brain. At a very young age, the brain is very watery and will easily change shape. The ventricles will usually become smaller, sometimes up to a point that they become slit-like. Unfortunately, this effect is unpredictable and very little can be done about it.
It is important to know that not all small or slit ventricles cause symptoms. What proportion of patients with small ventricles develop slit ventricle syndrome is not really known. We know that the patients at risk of developing slit ventricle syndrome are those who were shunted at an early age and where a low drainage pressure shunt system was used. This does not imply inadequate treatment. It just means the effect of a shunt can be unpredictable.
What is slit ventricle syndrome?
Slit ventricle syndrome is a constellation of symptoms which, for example, can appear in patients with a functioning shunt and in whom the brain has lost part of its elasticity. The symptoms consist of headaches, vomiting, drowsiness etc. Infuriatingly enough, these symptoms resemble those of shunt malformation.
The appearance of the symptoms is very cyclical, often with the regularity of a clock for example, the patient is well for three weeks, then violently ill and sleepy for 24 hours and then well again. There is more often than not no cause for the symptoms, although a minor viral illness can kick off the symptoms. Often the patient is taken to hospital and imaging is done which shows the ventricles to be small, or even unchanged, compared to previous imaging.
To increase the frustration, the symptoms disappear spontaneously. Until the next time, that is.
What actually happens?
We have been able to confirm by MRI scans on several patients the 'gut feeling' many have had about slit ventricle syndrome.
In ordinary circumstances, the shunt drains the cerebro-spinal fluid (CSF) from the ventricles to whichever cavity of the body. This causes, in certain patients, the ventricles to collapse. The ventricle closes on the ventricular catheter that drains the ventricles and blocks off the outflow of CSF. That's when the symptoms appear. The pressure in the brain rises very quickly and the patient becomes ill. Because the brain has lost some of its elasticity due to the initial disease which caused the hydrocephalus, the ventricles do not blow up rapidly and the symptoms can persist. After a while, the ventricles get slightly bigger and the normal CSF drainage resumes. All these changes, in shape and size, may not be easy to detect and often the illness goes unexplained.
What can be done?
This is a difficult question. The crucial point is to make sure that the shunt is working properly. This means you must make sure that you are not dealing with intermittent shunt blockage due to malfunction of the system. Often this can be done by measuring the pressure in the skull (ICP=intra cranial pressure monitoring) and with extra imaging (CT scan or MRI). Once you are satisfied with the functioning of the shunt, there are several options available. In our department, we tend to proceed with volume expansion procedures.
This means that, since the ventricles are small, you can give the ventricle, and so the catheter inside the brain, some extra space by removing a scale of bone on the side of the skull. This is called a subtemporal decompression. This simple procedure carries very little risk and resolves the problem.
Other volume expansion procedures consist of moving larger portions of the skull to make the total volume bigger, for example, forehead advancement. This type of procedure is more elaborate and is suitable for very young children. Tampering with the shunt system is usually a bad idea if at least you are satisfied with its functioning.
Moves like changing to a higher pressure valve or adding an anti-syphon device at a later stage can sometimes cause a fatal rise in intra-cranial pressure, particularly in those patients who have had a shunt or who have suffered from slit ventricle syndrome for a long time.
I am often asked what the role of endoscopy is in slit ventricle syndrome. We have treated a number of patients with slit ventricle syndrome with endoscopic third ventriculostomy. This was only possible because these patients presented with shunt failure and large ventricles. The important message is that slit ventricle syndrome does not preclude endoscopic treatment but the ventricles must be large enough to allow access.