A shunt is an alternative pathway for CSF to bypass an obstruction or a bad absorption of the CSF in natural pathways.
The shunt is positioned to enable the CSF to be drained from the lateral ventricles or lumbar sub-arachnoid spaces into another absorption site which needs to be well supplied with blood. This is where the CSF will be reabsorbed to re-establish the loop production/circulation/absorption. Physicians will mainly choose to shunt towards the abdomen (peritoneal cavity, from the ventricles or lumbar subarachnoid spaces) or the heart (right atrium from the ventricles only). These are two cavities well supplied by blood.
The CSF will pass through a system of small tubes known as catheters. A regulating device, known as a valve, which be more or less sophisticated, will be inserted into the pathway of the catheters to drive and control CSF flow.
This drainage enables the excess of CSF within the brain to be evacuated and, thereby, the pressure within the cranium to be reduced so the ICP decrease and remain in near-normal values.
A CSF shunt system is made of three components:
- A proximal catheter which is inserted proximally into the cerebral ventricles or into the sub-arachnoid lumbar spaces. This catheter is closed at its proximal end and multi-perforated in order to enable the CSF to pass through. Many possible types and configurations are available, the mostly widely used being the straight or right angle ventricular catheters.
- A unidirectional valve or anti-reflux valve. The valve is the main element of a shunt and has two functions. Firstly, the valve will drive and control the flow of CSF in one direction, i.e. it prevents the flow of CSF backwards the ventricles once the fluid has passed through the valve. Secondly, it regulates the amount of pressure inside the cranium to maintain a pressure in the near-normal values. If ICP is too high, the valve will open to let the CSF pass through the shunt.
Reversely, the valve will not open if ICP is not sufficient to activate the mechanism. This avoids excessive CSF drainage. The valve is placed between the proximal and distal catheters. Many types and configurations are available. They are designed to work at different pressures, depending on the patient clinical needs.
- A distal catheter is linked proximally to the valve and inserted distally into the peritoneal cavity or into the entrance to the right atrium of the heart. Again, many types and configurations are available. The distal tip is usually open and multi-perforated in order to facilitate the flow of CSF into the peritoneal or cardiac cavity.
Two other optional components can be used in a shunt:
- A reservoir which may be attached upstream to the valve. When present, the reservoir may be used to assess the patency of the shunt and to access the CSF for injections and/or CSF sampling.
- An anti-siphon device, which may be attached downstream to the valve. It helps fighting the siphon effect. The anti-siphon device theoretically allows sudden increases in the differential pressure between the proximal and distal parts of the shunt to be corrected when subjects move from supine position to standing up. However, the siphon effect is a natural phenomenon that can be absorbed by the body.
Shunts are made from materials which have been shown to be well tolerated by the body, such as silicone, polysulfone or stainless steel.
The entire shunt system is positioned under the skin. No part can be seen from outside the body. You will barely feel the catheters under your skin. The valve, which is more or less 5 mm thick, can be easily found on your head.
In obstructive hydrocephalus, the tip of the proximal catheter must imperatively be placed within the ventricular system and is known as the ventricular or proximal catheter.
In non-obstructive hydrocephalus, the proximal catheter is placed either in the ventricles (ventricular catheter) or in the sub-arachnoid space, at the level of the lumbar spinal column. In this case, it is known as the lumbar catheter.
From these clinical features, the neurosurgeon will decide to insert one of the three CSF shunt systems from a choice of the three following types of implantation:
- Ventriculo-Peritoneal Shunt (V-P): The CSF is shunted from the cerebral ventricles to the peritoneal cavity where it is reabsorbed into the blood through the peritoneum, the membrane which lines the gastro-intestinal organs.
- Ventriculo-Atrial Shunt (V-A): The CSF is shunted from the cerebral ventricles into the right atrium of the heart. The CSF is then shunted directly into the blood circulation
- Lumbo-Peritoneal Shunt (L-P): The CSF is shunted from the lumbar sub-arachnoid spaces to the peritoneal
Insertion of a CSF shunt is a surgical procedure performed in the operating room under general anesthesia. Neurosurgeons will choose the most appropriate operating method and valve for each patient depending on their experience and clinical needs.
Two types of valves are available:
- Monopressure or fixed-pressure valves
- Adjustable pressure valves
Standard valves for hydrocephalus work at constant pressure or resistance. It is the neurosurgeon who chooses the type and pressure of valve which he will insert before implantation, depending on the patient's clinical characteristics and further investigations.
Standard shunt valve manufacturers generally offer three different calibrations of each of their models: either a low pressure model, a medium pressure model or a high pressure model. Although in most cases the choice will be correct at the time the system is implanted, shunting the patient's CSF changes the patient's clinical condition and in a significant number of cases the neurosurgeon has to change the valve, replacing it with a valve which operates at a different pressure. Repeated surgeries are not without danger for the patient. The solution to this problem is to implant a valve which has several different pressure settings covering the entire desired pressure range and which can be adjusted to change the pressure as desired, without re-operating the patient, i.e. non-invasively. This is the concept underlying the adjustable pressure valves.
An adjustable pressure valve will include in the same medical device several operating pressures, i.e. the possibility of modifying the operating pressure by simple external adjustment. The physician will be able to adapt the operating pressure and to adjust the CSF drainage flow according to the patient's clinical requirements, without the need for surgical re-intervention. Depending on clinical and tomodensitometric (CT) MRI findings, it offers the neurosurgeon the facility to quickly relieve under-drainage or over-drainage symptoms by changing the operating pressure.
The operating pressure may be changed with the appropriate adjustment kit. The pressure adjustment is non-invasive and is done percutaneously. Adjustable pressure valves mechanisms contain magnets that allow them to be activated with their dedicated adjustment kit. The neurosurgeon places the magnet of the adjustment kit on the patient's skin over the valve and can change the operating pressure. A patient will be able to keep the same shunt longer even if his/her clinical needs can continue to evolve.
One drawback of this adjustability of valves is that an adjustable valve can be sensitive to magnetic fields, which allows its non-invasive adjustment. In our daily life, we use some products that contain magnets: smartphones, hair dryers, induction plates and children toys amongst others. If these magnets met with the valve, it might accidentally disadjust the operating pressure of the valve. Medical follow-ups usually include an MRI-exam which may misadjust the valve too.
To prevent these unintentional and undesired operating pressure changes, some valves offer locking system of the mechanism which is an unequalled security against the clinical risks associated with those dysadjustments.