What is slit ventricle syndrome (SVS)?

I. Understanding our ventricular system

brain parenchyma

One error that we make in considering the ventricular system of the human brain is that it is static and that the ventricles do not modify over time. This is not true and evidence of this can be seen even before birth by viewing a prenatal ultrasound where they are relatively small. What is the ventricular system? Simply put, it is a communicating network of cavities filled with cerebrospinal fluid (CSF) located within the brain parenchyma (pictured at right). It is made up of two (2) lateral ventricles, the third ventricle (the site used for an endoscopic third ventriculostomy or ETV), the cerebral aqueduct, and the fourth ventricle. Additionally, the choroid plexuses - responsible for the production of CSF - are located within the ventricular system.

The the downside of this dynamic ability is the fact that the ventricles tend to grow larger as we age because of a decrease in the number of brain cells. The good news about this increase in size (except in extreme cases) is that the actual size of the ventricular system appears to have little affect on the function of the brain.

One interesting fact that I wasn't aware of until I began reading for this blog is the fact that each time our heart beats a shock wave is produced that changes both the shape and size of the ventricles. This dynamic change can be monitored using magnetic resonance imaging (MRI) which pinpoints a specific sequence.

II. What are slit ventricles

Simply put, they are small ventricles (often so small that they are barely visible on either a computerized-axial tomography (CT) scan (pictured at left) or an MRI. They can occur following a severe head injury or a viral infection of the brain. In either case, the brain becomes so swollen that fluid is literally pushed from them.

Another phenomena that is of interest to the medical community is the fact that they are often seen following cerebrospinal fluid (CSF) diversion such as accomplished by a implant of a shunt. Interestingly that condition (decompression of the ventricular system) is one of the benchmarks of a properly functioning shunt system and is confirmed by comparing a post-implant CT or MRI with one done prior to the implant. In some cases, this decrease in the size of the ventricular system results in a "siphoning" effect. How the brain physically reacts to this siphoning is largely dependent on the patient's age. In young children, the brain is still very watery and easily changes shape. Unfortunately this effect is very unpredictable and and very little can be done to affect it one way or the other.

It is important to understand that small / slit ventricles do not always cause symptoms -- some patients are asymptomatic (showing none of the usual symptoms) and go for long periods never knowing they have a problem. Typical symptoms associated with small / slit ventricles include: 1) Headache; 2) Lethargy that can vary in intensity from mild to debilitating; 3) Nausea and vomiting. These symptoms can be intermittent and the headache is often relieved by laying down.

III. What is slit ventricle syndrome

Slit ventricle syndrome (SVS) is a grouping of symptoms which, for example, can occur in a patient in a patient with a functional shunt, but whose brain has lost some of its elasticity. As I mentioned above, they might experience headache, vomiting, and drowsiness / lethargy which, interestingly enough, are the same symptoms experienced when a person with hydrocephalus is having a shunt malfunction. One thing that makes SVS symptoms slightly different is the fact that they are VERY cyclical in nature often with like clockwork. As an example, the patient can be perfectly fine for three (3) weeks and then become violently ill, sleep for 24-hours, and then be "normal" again. Generally, there is no cause for the symptoms experienced, although a minor viral infection has been known to act as a triggering mechanism.

Under normal circumstances, the shunt drains the CSF from the ventricles to another part of the body - such as the peritoneal cavity - for reabsorption. In certain patients, however, this causes the ventricle(s) to collapse. In a person who is shunted this cause the ventricle to collapse on the ventricular catheter blocking it off and preventing the outflow of CSF and causes the appearance of symptoms. Due to the fact that the brain has lost some of it's elasticity (due to the original condition that caused the hydrocephalus) the ventricles are slow to return to their normal size causing symptoms to persist.

IV. How is SVS treated

Once symptoms have manifested themselves, the most important thing to determine is whether the shunt is functioning properly and not having intermittent blockages due to a malfunction within the system. In many cases, this can be as easy as measuring the intracranial pressure (ICP) within the skull. Once the functionality of the shunt is assessed, the next step is to consider a volume expansion procedure such as a subtemporal decompression. This procedure involves removing a small section of bone from the skull which allows the ventricle - and the catheter - to expand and relieves the excess pressure.