A medical shunt is a device used to divert the flow of a bodily fluid from one area to another, creating an alternate pathway when a natural system fails to drain properly. In neurological medicine, a shunt specifically manages the flow of Cerebrospinal Fluid (CSF) to treat a condition called hydrocephalus. The entire system is implanted beneath the skin, offering a long-term solution by mechanically regulating and draining the excess fluid that builds up inside the brain.
Why Shunts Are Necessary
The condition hydrocephalus, meaning “water on the brain,” occurs when there is an imbalance in the production, circulation, or absorption of CSF. Cerebrospinal Fluid is produced primarily within the brain’s ventricles, the four interconnected cavities deep inside the brain. This fluid cushions the brain and spinal cord, removes waste products, and delivers nutrients to the central nervous system.
The fluid normally circulates through the ventricular system before being absorbed back into the bloodstream. The body produces about 500 milliliters of CSF daily, maintaining a delicate pressure equilibrium inside the skull. If the flow is obstructed or absorption is impaired, the excess volume accumulates in the ventricles.
This buildup causes the ventricles to swell, leading to an increase in Intracranial Pressure (ICP). Unmanaged high pressure can compress and damage surrounding brain tissue, leading to neurological symptoms and permanent injury. A shunt becomes necessary to bypass the obstruction or enhance absorption, creating an artificial drain to relieve the dangerous pressure inside the skull.
Anatomy of a Shunt System
A typical neurosurgical shunt system consists of three primary, interconnected components made of flexible silicone tubing. The first is the proximal catheter, a flexible tube placed directly into one of the brain’s fluid-filled ventricles to collect excess Cerebrospinal Fluid.
The proximal catheter connects to the pressure valve and reservoir mechanism, usually positioned under the skin behind the ear or on the top of the head. The valve regulates CSF flow, ensuring drainage only occurs when pressure exceeds a certain threshold. The reservoir allows a physician to sample the fluid or test the shunt’s function using a needle through the skin.
The final element is the distal catheter, which connects to the valve and is threaded beneath the skin down the neck and torso to the final drainage site. This tube carries the regulated amount of CSF away from the brain. The entire assembly remains completely under the skin, preventing infection and allowing for normal daily activity.
Regulating Fluid Flow and Diversion
The valve mechanism directly manages the outflow of Cerebrospinal Fluid. Devices are either fixed-pressure or programmable, both operating based on a pressure differential. A fixed-pressure valve is preset to open and allow drainage only when the pressure difference reaches a specific, unchangeable level.
Programmable valves allow a physician to non-invasively adjust the pressure setting after implantation. This adjustment is performed using an external magnetic device that changes the valve’s internal resistance. This feature is useful for patients whose pressure requirements change over time, offering fine-tuning without additional surgery.
Once the fluid passes the valve, the distal catheter directs it to a location where it can be safely absorbed back into the bloodstream. The most common destination is the peritoneal cavity in the abdomen, known as a ventriculoperitoneal (VP) shunt. The peritoneum has a large surface area and excellent capacity to absorb the fluid.
Alternative diversion routes are used when the abdomen is not suitable. These include a ventriculoatrial (VA) shunt, which directs fluid into a vein in the neck and ultimately the right atrium of the heart. Another option is the lumboperitoneal (LP) shunt, which drains CSF from the lower spine into the peritoneal cavity.
Identifying Shunt Complications
Shunts are effective treatments, but they can fail or experience complications, requiring prompt medical attention and a shunt revision. The two main problems are malfunction and infection. Malfunction occurs due to a blockage in the catheter tubing, often from tissue or protein buildup, or from the tubing breaking due to wear and tear.
Malfunction also includes improper flow, specifically underdrainage or overdrainage. Underdrainage means the shunt is not removing enough fluid, causing symptoms of high intracranial pressure to return. Overdrainage occurs when the shunt removes fluid too quickly, which can cause the ventricles to collapse and lead to headaches or a collection of blood outside the brain.
Infection is a serious complication, as the shunt is a foreign object implanted in the body. Symptoms of a malfunctioning or infected shunt can include:
- Severe headache, nausea, and vomiting.
- Fever or lethargy.
- Tenderness and swelling along the shunt’s tract under the skin.
In infants, a bulge at the fontanelle or an unusually rapid increase in head size can also indicate a problem. Any sign of complication necessitates an immediate medical evaluation to prevent serious neurological damage.