An external ventricular drain (ventriculostomy) is placed to reduce the volume of cerebrospinal fluid within the brain, thereby lowering intracranial pressure. An EVD is typically utilized in the setting of trauma but may be inserted for a variety of other reasons: to drain infected spinal fluid, to reduce protein/blood product with the spinal fluid after brain tumor resection, to promote brain relaxation during surgery, in the face of acute hydrocephalus from aneurysmal subarachnoid hemorrhage. Most often, EVD’s are placed in an emergent setting.
- This procedure is usually performed at the bedside under IV sedation with local anesthesia. An EVD may be placed in the OR however as an adjunct to a larger surgical procedure.
- An incision is marked in the right frontal region (typically) according to standard landmarks.
- The head is then prepped, draped and the previously marked incision is made.
- A “twist drill” is then utilized to gain intracranial access. The diameter of the twist twill hole is approximately 3-4 mm.
- The dura is then opened with a blunt stylet or blade.
- The EVD catheter is then passed into the ventricle (fluid cavity of the brain) utilizing standard technique and trajectory.
- Cerebrospinal fluid flow is confirmed. Often times, a fiberoptic-based pressure transducer is inserted through the center of the catheter into the ventricle. This allows for instantaneous pressure readings (and trend tracking).
- The incision is closed, and the EVD catheter is affixed to a closed drainage system.
- The EVD will be set to drain and varying pressures depending upon the indication for its placement (in the context of the patient’s response both clinically and radiographically).
WHAT YOU WANT TO KNOW
“Is the EVD that tube coming out of my mother’s head? The one with all the bloody fluid in it?”
Yes. Ventriculostomies are often inserted to drain blood-tinged fluid from the fluid cavities of the brain (ventricles). The is typically seen after an aneurysm has ruptured but may also be the case after a head injury. The catheter is not a source to pain to the patient. Remember the brain doesn't "feel" pain.
“What does the number on the monitor (to which the catheter is connected) mean?”
That’s the intracranial pressure or “ICP.” Normal ICP is < 20 mmHg. If ICP is within the normal range, typically cerebral perfusion or blood flow to the brain is being maintained. Remember, the nerve cells (neurons) are extremely sensitive to changes in blood flow given their high metabolic rates. Even short periods of diminshed flow can have significant consequences (due to neuronal ischemia and death).
“For how long will the drain be in place?”
Variable. Dr. Osborn and his colleagues will make every attempt to remove the EVD as soon as possible due to minimize the risk of infection. A “weaning trial” will be performed during which the EVD height will be raised. Some patients will require a shunt in order to provide a route of long-term spinal fluid diversion (from the head) should the EVD fail to be weaned.
“What is the difference between an EVD and an ICP monitor? I often hear these words used by the surgeons.”
An EVD can be used to monitor intracranial pressure (ICP) as well as drain cerebrospinal fluid (CSF). It serves two functions therefore and therefore falls under the global heading of “ICP monitor.” So-called “parenchymal” monitors, placed within the brain substance (and not the ventricles) do not drain cerebrospinal fluid (and therefore have only a solitary purpose: pressure measurement).
“What are the risks associated with an EVD?”
The major risk of an EVD is infection. Quoted rates vary. Most patients with EVD catheters are on IV antibiotics (for other critical illnesses) and this serves a prophylactic effect. That said, there is no consensus regarding use of antibiotics for “catheter protection.” Other potential risks include hemorrhage, malfunction or catheter occlusion (necessitating replacement of the EVD) and malposition (which sometimes warrants repositioning). Overall, the complication rate associated with an EVD is low.