An External Ventricular Drain (EVD) is a medical device employed in neurocritical care to manage fluid pressure within the brain. This system involves a flexible tube, or catheter, that a neurosurgeon places into the brain’s fluid-filled spaces, known as ventricles. The primary purpose of an EVD is to divert excess cerebrospinal fluid (CSF) away from the brain, thereby helping to relieve elevated intracranial pressure (ICP). This intervention is temporary and plays a supportive role in allowing the brain to recover from various conditions that disrupt its normal fluid balance.
Understanding Cerebrospinal Fluid and Intracranial Pressure
Cerebrospinal fluid (CSF) is a clear, colorless fluid that surrounds and cushions the brain and spinal cord, providing mechanical and immunological protection. This fluid is primarily produced by specialized structures called choroid plexuses, located within the ventricles of the brain. The brain contains four ventricles: two lateral ventricles, a third ventricle, and a fourth ventricle, through which CSF continuously circulates.
From the lateral ventricles, CSF flows into the third ventricle, then to the fourth ventricle, and finally into the subarachnoid space that envelops the brain and spinal cord. This fluid is then reabsorbed into the bloodstream, mainly through arachnoid granulations that protrude into the dural venous sinuses. Approximately 400 to 600 milliliters of CSF are produced daily, with about 125 to 150 milliliters present in the body at any given time, ensuring a constant turnover.
Intracranial pressure (ICP) refers to the pressure within the rigid skull, which contains the brain tissue, blood, and CSF. Maintaining normal ICP, less than 20 millimeters of mercury (mm Hg), is important for brain health. The Monroe-Kellie doctrine states that the total volume of these components inside the skull remains relatively constant.
An increase in the volume of any one component, such as excess CSF, can lead to elevated ICP. Elevated ICP can cause symptoms like severe headaches, nausea, vomiting, altered mental status, and visual disturbances. If left unmanaged, persistently high ICP can compromise blood flow to the brain, potentially leading to brain damage or even brain herniation, where brain tissue shifts due to pressure differences.
Reasons for EVD Placement
EVD placement becomes necessary in various medical conditions that lead to an accumulation of CSF or blood within the brain, resulting in elevated intracranial pressure. EVDs can temporarily relieve this pressure by draining the excess fluid. Common indications for EVD placement include:
- Hydrocephalus: An excessive CSF buildup from overproduction, impaired absorption, or flow blockage. EVDs temporarily relieve this pressure.
- Intraventricular hemorrhage (IVH): Bleeding into brain ventricles that obstructs CSF pathways. EVDs remove blood products and reduce pressure.
- Severe traumatic brain injury (TBI): EVDs monitor ICP and drain CSF to manage brain swelling.
- Central nervous system infections: Such as meningitis or ventriculitis. The drain removes infected CSF and allows direct antibiotic administration.
- Brain tumors or other space-occupying lesions: These can obstruct CSF flow, causing hydrocephalus and increased ICP. EVD placement alleviates this pressure.
The Procedure for EVD Placement
The placement of an external ventricular drain is a neurosurgical procedure performed under sterile conditions, often in an operating room or at the patient’s bedside in emergencies. Patient preparation includes general anesthesia or sedation, antibiotics to prevent infection, hair clipping on a portion of the patient’s scalp, and thorough cleaning of the surgical site.
A small incision (2-3 cm) is made in the scalp, commonly at a location known as Kocher’s point. A surgical drill creates a burr hole through the skull. Then, a small incision is made in the dura mater, the tough outer membrane covering the brain.
A thin, flexible catheter is inserted through this opening into a brain ventricle, usually the lateral ventricle. The surgeon aims the catheter to a depth of 5 to 7 centimeters to ensure proper placement. Once clear cerebrospinal fluid flows back through the catheter, a metal stylet inside the catheter is removed.
The catheter is tunneled a few centimeters under the skin away from the initial incision and secured to the scalp with sutures or surgical staples to prevent dislodgement. The incision is then closed, and the external end of the catheter connects to a closed drainage system outside the body. This system includes a CSF collection bag and a transducer that monitors intracranial pressure.
Post-Placement Management and Potential Risks
After EVD placement, monitoring of intracranial pressure (ICP) and cerebrospinal fluid (CSF) drainage is performed. Healthcare providers regularly check the amount of CSF drained into the collection bag and record its color. The height of the external drainage system is adjusted relative to a specific anatomical landmark, such as the patient’s ear or the foramen of Monro, to control the rate of CSF drainage and maintain the desired ICP.
Adjustments to the drain’s height determine the pressure at which CSF will drain. Neurological assessments are conducted. The duration an EVD remains in place varies, but it is a temporary measure until the underlying condition improves and CSF flow normalizes.
Despite the benefits, EVD placement carries potential risks:
- Infection: Ventriculitis (inflammation of the ventricular lining) is a concern. Strict sterile techniques and antibiotics minimize this risk.
- Hemorrhage: Bleeding can occur during insertion or removal, though significant bleeding is less common.
- Catheter malfunction: This includes occlusion by blood clots or tissue debris, or displacement from its intended position.
- Over-drainage or under-drainage: Over-drainage can cause severe headaches or, rarely, brain ventricle collapse and subdural hematomas. Under-drainage can result in persistent elevated ICP if the system is blocked or set too high.