What Does an Evans Index Over 0.3 Mean on an MRI?

Bottom Line: The Evans Index is a measurement of ventricular size, not a diagnosis or a pressure reading. To understand your condition, doctors look at the complete MRI pattern, your specific symptoms, and changes over time, rather than relying on this single number.

An Evans Index over 0.30 is a common threshold that suggests the fluid-filled spaces in your brain (ventricles) are larger than average ^[1]. In medical terms, enlarged ventricles are called ventriculomegaly. An Evans Index of 0.35 is consistent with ventriculomegaly, but this number alone does not confirm a diagnosis of hydrocephalus or determine if you need treatment ^[2]. Ventriculomegaly is simply an imaging description, whereas hydrocephalus is a medical condition involving the abnormal circulation, absorption, or buildup of cerebrospinal fluid (CSF).

What is the Evans Index?

The Evans Index is a ratio used by radiologists to measure the size of the lateral ventricles (the largest fluid spaces in the brain). It is calculated by dividing the maximum width of the frontal horns of the ventricles by the maximum inner diameter of the skull ^[1].

While it is a useful tool for documenting ventricular size, it has limitations. The measurement can vary depending on the image plane, slice thickness, and technique used by the radiologist, so comparisons are most meaningful when scans are reviewed consistently ^[3]. Furthermore, an Evans Index over 0.30 is not a universal boundary for disease; it can also be seen in older adults or individuals with brain atrophy (shrinkage of brain tissue, where ventricles expand to fill the available space) ^[4][2].

Does the Evans Index measure brain pressure?

No, the Evans Index is a measurement of size, not a measurement of pressure or intracranial pressure (ICP).

It is very common to read a radiology report, see a number like 0.35, and worry that it means brain pressure is dangerously high. However, studies show that the Evans Index is not a dependable proxy for cerebrospinal fluid pressure or ICP ^[5]. Your ventricles can be enlarged without currently having high pressure, and the Evans Index alone cannot tell your doctor what the pressure inside your skull is ^[5]. Assessing pressure requires different clinical evaluations or, in some cases, invasive testing ^[6].

How Doctors Look at the Full Picture

In cases like hydrocephalus with stenosis of the aqueduct of Sylvius (where the narrow channel connecting the third and fourth ventricles is narrowed or blocked), your medical team treats the Evans Index as just one piece of a larger puzzle. Treatment decisions should not be based on this number alone ^[7].

Instead, they will evaluate:

• The MRI Pattern: In aqueductal stenosis, doctors look for specific patterns of fluid backup, such as enlarged lateral and third ventricles with a normal-sized fourth ventricle. They also look for signs like transependymal flow (CSF moving through the ventricular lining into nearby brain tissue) or compressed sulci (narrowed spaces on the surface of the brain) ^[8][7]. CSF-flow MRI scans can sometimes be used to evaluate the movement of CSF and assess the blockage, but these do not directly measure pressure.
• Your Symptoms: They will check for symptoms of hydrocephalus. This can include chronic issues like gait (walking) disturbances, cognitive changes, or urinary issues ^[7][9].
• Changes Over Time: Comparing your current ventricular size and MRI features to previous scans is often useful for monitoring the condition, as a stable Evans Index doesn’t always mean the condition isn’t changing ^[10]. Bringing your actual images to your appointment helps your doctor accurately compare them.

If your doctor is considering a treatment like a shunt or endoscopic third ventriculostomy (ETV), they know that the Evans Index alone is not a reliable predictor of who will benefit from surgery ^[11]. They will base their recommendations on your complete imaging pattern, how you feel, and how your symptoms are changing over time ^[9][11].

When to Seek Immediate Care

Because you have a condition that can block the flow of CSF, the significance of enlarged ventricles depends heavily on whether your symptoms are stable, slowly changing, or acute. Seek urgent medical attention if you experience signs of rapid pressure buildup, such as:

• Sudden or rapidly worsening severe headache
• Repeated nausea and vomiting
• Increasing sleepiness, lethargy, or confusion
• New vision changes (like double vision)
• New weakness, seizures, or severe difficulty walking