What is a Shunt?
The most common treatment for hydrocephalus is the surgical placement of a medical device called a shunt. A shunt, in its simplest form, is a flexible tube called a catheter, which is placed into the area of the brain where cerebrospinal fluid (CSF) is produced. This area of the brain is known as the lateral ventricles. The tubing is then passed under the skin to another region of the body, most often the abdominal cavity, or heart, diverting the excess CSF away from the brain, where it can be absorbed naturally by the body. By draining the extra fluid to another location in the body, it is relieving pressure on the brain.
A shunt consists of three major components:
- An inflow or proximal catheter, which drains CSF from the lateral ventricles. This tube leaves the brain through a small hole drilled in the skull and then runs for a short distance under the skin.
- A valve mechanism, which regulates intracranial pressure by controlling fluid flow through the shunt tubing. This device is connected to the proximal catheter and lies between the skin and the skull, usually on top or the back of the head, or just behind the ear. Valves operate within a specific pressure range. There are many types of valves and shunt manufacturers. Your doctor will determine the type of valve based on his/her experience, preference, and your needs.
- An outflow or distal catheter, which runs under the skin and directs CSF from the valve to the abdominal (peritoneal) cavity, heart or another suitable drainage site.
What are the most common shunt systems?
The most common shunt systems are:
- Ventriculoperitoneal (VP) shunts. This type of shunt diverts CSF from the ventricles of the brain into the peritoneal cavity, the space in the abdomen where the digestive organs are located. The tip of the distal catheter rests in this cavity near the loops of the intestine and bowel but not inside them. The CSF shunted to this area is reabsorbed into the bloodstream and is eventually excreted through normal urination.
- Ventriculoatrial (VA) shunts. This type of shunt diverts CSF from the ventricles of the brain into the right atrium of the heart. The distal catheter is placed into a vein in the neck and then gently advanced through the vein into the right atrium of the heart. Here the CSF passes directly into the bloodstream and subsequently excreted through normal urination.
- Ventriculopleural (VPL) shunts. This type of shunt diverts the CSF from the ventricles of the brain into the pleural (chest) cavity. This cavity is a space between the chest wall and the lungs. It is lined by a membrane along the chest wall (parietal pleura membrane) and the lung surface (visceral pleura membrane) with pleural fluid in between. The CSF is added to this fluid and is absorbed and subsequently excreted through normal urination.
- Lumboperitoneal (LP) shunts. This is a unique type of shunt which diverts CSF from an area within the spine, not the brain (not starting in the brain), called the intrathecal space. The CSF is diverted into the peritoneal cavity. The CSF shunted to this area is reabsorbed into the bloodstream and is eventually excreted through normal urination. For more information about the peritoneal cavity, see the VP shunt description above.
Fixed and Adjustable (Programmable) Valves
In a person with hydrocephalus, intracranial pressure (ICP), or pressure within the brain is higher when compared to that of an individual without hydrocephalus. This is typically due to an excess of CSF within the ventricular system of the brain. In order to ensure that the pressure within the brain of a person with hydrocephalus is lowered to the range found in an adult or child without hydrocephalus, a shunt is placed to divert excess CSF and lower intracranial pressure. Most pressure control valves operate on the principles of change in differential pressure (DP)—the difference between the pressure at the proximal catheter tip and the pressure at the distal catheter tip. Neurosurgeons select a DP valve based upon the age of the patient, the size of the ventricles, the amount of pressure that needs to be relieved, and other important clinical factors. There are a variety of valves, but all of them work to control the amount of CSF drained. Valves are either set to a fixed pressure or they can be adjustable, also referred to as programmable, from outside the body.
Fixed pressure valves drain to a defined intracranial pressure. Fixed pressure valves regulate the pressure within the brain using a one-way valve at a predetermined pressure setting. When open, the valve allows CSF to flow away from the brain. Most commercially available fixed pressure valves have three to five possible settings: low, medium or high pressures (and very low and very high). Once implanted, the pressure setting cannot be changed without additional surgery.
Adjustable (programmable) valves regulate the ICP based on a pressure setting, like the fixed pressure valve, but the setting can be adjusted by your doctor using an external adjustment tool applied outside the body if there needs to be a change in how much CSF is draining. This allows your health care professional to non-invasively change or program the valve pressure setting during an office visit. The number of available settings depends on the valve model and manufacturer.
These valves are designed to be adjusted by a strong magnetic field found in the external adjustment tool. Some of these valves may be susceptible to adjustment by strong environmental magnetic fields and care must be taken to keep toys with magnets and other sources of magnetic fields away from the implanted device. Some adjustable valves incorporate mechanisms that cannot be adjusted by magnetic fields other than those produced by the programmer. It’s important to ask your doctor what precautionary measures should be taken.
The following table shows examples of Shunt Valves:
|ProGAV Adjustable Shunt|
|Codman® Certas™ Programmable Valve
|OSV II® Flow Regulating Valve|
|Strata Adjustable Valve|
|Polaris Adjustable Valves
numerous configurations available
Many shunt systems also have a reservoir located under the skin between the inflow (proximal) catheter and the valve. The reservoir serves several important functions. It can be used to remove samples of CSF for testing, your doctor can inject fluid into the system to test the flow and function of the shunt, and it can be used to measure pressure. In addition, the reservoir can be felt through the skin and pumped manually to help keep the proximal catheter open. In general, if one pushes on the reservoir and it does not spring back, then there might be an obstruction in the proximal catheter because the reservoir is not filling with CSF. On the other hand, if the reservoir feels rather stiff and more force is needed to depress it, then the valve and/or distal catheter may be clogged. You should not pump your reservoir unless explicitly instructed to do so by your doctor.
Overdrainage Control Devices
Ideal shunt pressure-flow characteristics must match the patient’s specific needs both when lying down and when standing/sitting. However, in some patients standing or sitting may cause a siphoning effect, which essentially “pulls” fluid out of the lateral ventricles or the lumbar region. This results in over-drainage of CSF from the brain. In order to prevent this from happening, the clinician may decide to add a “siphon control Device” to your shunt system in order to prevent over-drainage.
A variety of siphon and overdrainage regulating devices have been developed to reduce CSF overdrainage. The purpose of a siphon control device is to minimize excessive drainage, which can cause more CSF to drain when the individual is upright.
Shunts save lives and are successful in reducing the pressure in the brain for most people. A shunt allows individuals to live full lives but is not without complications, some life-threatening. An estimated 50% of shunts in the pediatric population fail within two years of placement and repeated neurosurgical operations are often required. The most common shunt complications are malfunction and infection.
This article was produced by the Hydrocephalus Association, copyright 2021. We would like to thank the following individuals for their valuable contributions and expert input: J. P. McAllister, PhD, Daniel J. McCusker M.S., and Marvin Sussman, PhD.
We also would like to credit our industry partners for their continued support:
Codman Specialty Surgical, an Integra LifeSciences Company