In medical terms, EVD most commonly stands for external ventricular drain, a small temporary tube placed into the brain to drain excess cerebrospinal fluid (CSF) and relieve dangerous pressure. The acronym can also refer to Ebola virus disease in infectious disease contexts, though the neurosurgical meaning is far more common in hospital settings.
What an External Ventricular Drain Does
An EVD is a thin catheter inserted through the skull into one of the brain’s fluid-filled chambers, called ventricles. These ventricles produce and circulate cerebrospinal fluid, the clear liquid that cushions the brain and spinal cord. When disease or injury blocks that circulation or causes too much fluid to accumulate, pressure inside the skull rises. Left unchecked, elevated intracranial pressure can damage brain tissue and become life-threatening.
The drain works in two ways. First, it diverts excess fluid out of the brain into an external collection bag, which can be adjusted to control how much fluid drains. Second, the system includes a pressure sensor called a transducer that gives the medical team a continuous reading of intracranial pressure. This lets them fine-tune drainage in real time, keeping pressure within a safe range. Normal intracranial pressure falls between about 7 and 15 mmHg, and the drain is typically set to begin releasing fluid if pressure climbs above that upper limit.
Conditions That Require an EVD
The primary reason for placing an EVD is acute hydrocephalus, a sudden buildup of cerebrospinal fluid in the brain. Several serious conditions can trigger it:
- Subarachnoid hemorrhage (SAH): Bleeding around the brain, often from a ruptured aneurysm, frequently causes fluid blockages. A significant percentage of SAH patients develop acute hydrocephalus that requires drainage.
- Traumatic brain injury (TBI): Brain Trauma Foundation guidelines include EVD placement as both a treatment for swelling and a way to monitor pressure in severe head injuries.
- Brain hemorrhage: Bleeding within the brain tissue or into the ventricles themselves can obstruct normal fluid flow, and an EVD becomes necessary when medications alone cannot control the rising pressure.
- Brain tumors: Tumors can physically block the pathways that cerebrospinal fluid travels through, causing rapid fluid buildup.
- Brain infections: Meningitis or other central nervous system infections sometimes require temporary fluid drainage to manage pressure and clear infected fluid.
- Shunt failure: Patients who already have a permanent internal drain (a shunt) may need an emergency EVD if that device stops working.
EVD placement is considered one of the most commonly performed neurosurgical procedures and is often described as life-saving in emergency situations.
How the Drain Is Placed
Neurosurgeons insert the catheter through a small hole drilled in the skull, typically at a location called Kocher’s point. This entry site sits roughly 2 to 3 centimeters from the midline of the head and about 10 to 12 centimeters back from the bridge of the nose, just in front of a natural seam in the skull called the coronal suture. The spot is chosen because it provides a relatively safe, direct path into the ventricle while avoiding critical brain structures.
The procedure can be performed at the bedside in an intensive care unit or in an operating room, depending on the urgency. Once the catheter reaches the ventricle, fluid begins draining, and the system is connected to the external collection apparatus and pressure monitor.
How Pressure Is Managed
Accurate pressure readings depend on precise positioning. The pressure sensor must be aligned with a specific anatomical reference point: the opening of the ear canal, called the external auditory meatus. When a patient is lying on their back, the ear canal approximates the location of the fluid-filled spaces inside the brain. Nurses use leveling tools (sometimes a laser or carpenter’s level) rather than eyeballing it, because even small errors in height throw off the pressure readings.
The drain’s height setting controls how freely fluid flows. A lower setting, like 5 centimeters of water, encourages more drainage because normal brain pressure easily exceeds that threshold. A higher setting, like 20 centimeters of water (roughly equivalent to 15 mmHg), acts more like a safety valve, only allowing fluid to drain if pressure rises above the normal range. The medical team adjusts these settings based on the patient’s condition and the underlying cause of the pressure problem.
Risks of an EVD
Infection is the most significant complication. Because the catheter creates a direct channel from the brain’s interior to the outside world, bacteria can travel along the tubing and cause ventriculitis, an infection of the brain’s fluid spaces. Reported infection rates typically range from 5% to 20% of patients, with most studies landing somewhere around 12% to 13%. Hospitals use sterile insertion techniques and careful handling protocols to minimize this risk, but the danger remains present for as long as the drain stays in place.
Other complications include bleeding during insertion, the catheter ending up in the wrong position, or draining too much fluid too quickly, which can cause its own set of problems.
Weaning Off the Drain
An EVD is always intended to be temporary. Once the underlying condition stabilizes, the medical team tests whether the brain can manage fluid circulation on its own through a process called weaning. This involves gradually raising the drain’s height setting or clamping the tubing so less fluid (and eventually no fluid) drains out. During this trial, the team watches closely for warning signs that the patient still needs the drain.
If the patient develops headaches, nausea, vomiting, confusion, or if intracranial pressure climbs above 20 mmHg for more than five minutes, the wean has failed. The drain gets reopened, and the team tries again later. Worsening hydrocephalus visible on brain imaging can also signal failure, though imaging changes alone are not as reliable a predictor as clinical symptoms.
Research shows that patients who fail weaning due to clinical symptoms like headache and altered mental status are nearly three times more likely to need a permanent shunt, a surgically implanted device called a ventriculoperitoneal (VP) shunt that continuously redirects excess fluid from the brain into the abdominal cavity, where the body absorbs it. If weaning succeeds and the patient tolerates clamping without symptoms, the catheter is removed.
EVD as Ebola Virus Disease
In infectious disease and public health contexts, EVD stands for Ebola virus disease. This is a severe and often fatal illness caused by the Ebola virus, characterized by fever, headache, muscle pain, weakness, vomiting, diarrhea, and in some cases internal and external bleeding. The CDC considers recent travel to affected regions within 21 days of symptom onset a key screening criterion. While this meaning of EVD appears frequently in global health literature, the neurosurgical meaning is what most patients and families encounter in a hospital setting.