Hydrocephalus is diagnosed through a combination of physical examination, brain imaging, and in some cases, specialized tests that measure how cerebrospinal fluid (CSF) moves through the brain. The specific approach depends heavily on age. In infants, a rapidly growing head and a bulging soft spot may be the first clues. In adults, an MRI or CT scan showing enlarged ventricles with an Evans Index greater than 0.3 is the standard imaging criterion. For older adults with normal pressure hydrocephalus, diagnosis often requires additional functional testing beyond imaging alone.
How It’s Detected Before Birth
Hydrocephalus can be identified during routine prenatal ultrasound. The key measurement is the width of the fetal brain’s lateral ventricles. A width of 10 mm or greater is considered ventriculomegaly, the medical term for abnormally enlarged ventricles. The Society for Maternal-Fetal Medicine classifies the severity in three tiers: mild (10 to 12 mm), moderate (13 to 15 mm), and severe (greater than 15 mm). The chance of an adverse outcome and the likelihood of finding other abnormalities both increase as the measurement climbs from the mild to moderate range.
When enlargement is spotted, doctors typically perform a more thorough evaluation, which may include a detailed anatomy scan, genetic testing, and infection screening to look for an underlying cause. Not all fetal ventriculomegaly progresses to hydrocephalus requiring treatment, so follow-up imaging is usually scheduled to track whether the ventricles are stable, shrinking, or continuing to grow.
Signs and Diagnosis in Infants
Infants have open fontanelles (the soft spots where skull bones haven’t yet fused), which gives doctors both a warning sign and a diagnostic window. Clinical signs of rising pressure inside the skull include a rapidly increasing head circumference, a bulging fontanelle, widened skull sutures, eyes fixed downward in what’s called “sun setting,” lethargy, vomiting, and episodes of slowed heart rate.
Head circumference is tracked on standardized growth charts at every well-child visit. A head that crosses percentile lines upward, growing faster than expected, is one of the earliest red flags. When hydrocephalus is suspected, cranial ultrasound is typically the first imaging study because it can be done at the bedside, requires no sedation, and works well while the fontanelle is still open. The ultrasound measures ventricle size directly and can show fluid buildup in real time.
If ultrasound findings are concerning or more anatomical detail is needed, an MRI provides a clearer picture of the brain’s structure and can help identify what’s causing the fluid buildup, whether it’s a blockage, a malformation, or something else.
Brain Imaging in Children and Adults
For older children and adults, CT and MRI scans are the primary diagnostic tools. Doctors look for several specific findings on these scans.
The most important measurement is the Evans Index: the ratio of the widest part of the front horns of the lateral ventricles to the maximum inner width of the skull. A normal ratio falls between 0.20 and 0.25. A ratio between 0.25 and 0.30 suggests early or borderline enlargement. A ratio above 0.30 confirms definite ventricular enlargement and is the standard threshold used in diagnostic guidelines worldwide. This single number is the most widely used indicator for assessing ventricle size.
Beyond the Evans Index, doctors look for ventricles that are enlarged out of proportion to any overall brain shrinkage, narrowing of the grooves on the brain’s surface (especially near the top of the head), and changes in the tissue surrounding the ventricles that suggest fluid is being pushed into brain tissue. On MRI, this periventricular signal change appears as a halo of brightness around the ventricles, distinct from the small-vessel changes that are common in aging brains.
Telling Hydrocephalus Apart From Brain Shrinkage
One of the trickiest parts of diagnosis is distinguishing true hydrocephalus from ventricles that are simply larger because surrounding brain tissue has shrunk, a condition called ventriculomegaly ex vacuo. Both look like big ventricles on a scan, but they have very different causes and very different treatments.
Several imaging clues help doctors tell them apart. In hydrocephalus, the spaces at the top of the brain are tight and compressed even as the ventricles expand. In brain shrinkage from conditions like Alzheimer’s disease, those spaces widen along with the ventricles because there’s simply less brain tissue overall. This pattern of enlarged ventricles paired with tight spaces at the top of the brain has been validated as a useful diagnostic sign. Advanced MRI techniques that measure water movement patterns in brain tissue can provide additional information when standard imaging is ambiguous.
Physical Exam and Eye Examination
A neurological exam is part of the workup at any age. One specific test, a fundoscopic examination, involves shining a light through the pupil to view the optic nerve at the back of the eye. When pressure inside the skull is elevated, the optic nerve can appear swollen, a condition called papilledema. Finding this swelling strongly suggests that fluid pressure is too high and supports the diagnosis.
The rest of the neurological exam varies by age. In infants, doctors assess muscle tone, reflexes, and eye movements. In adults, the exam focuses on gait, balance, cognitive function, and bladder control, particularly when normal pressure hydrocephalus is suspected.
Lumbar Puncture and Pressure Measurement
A lumbar puncture (spinal tap) can directly measure the pressure of the cerebrospinal fluid. A needle is inserted into the lower spine, and the opening pressure is recorded. In children, a pressure above 28 cm of water is generally considered elevated. Previous guidelines used a lower cutoff of 20 cm of water, but more recent research based on larger patient groups suggests the 28 cm threshold is more accurate for most children. The mean pressure in a normal pediatric population is about 20 cm of water, and factors like body weight and sedation level can influence the reading.
In adults with suspected normal pressure hydrocephalus, the opening pressure is often within the normal range, which is part of what makes the condition challenging to diagnose. The pressure readings alone won’t confirm it. Instead, the lumbar puncture serves a dual purpose: measuring pressure and performing a “tap test.”
The Tap Test for Normal Pressure Hydrocephalus
Normal pressure hydrocephalus, which primarily affects adults over 60, presents with a distinctive triad of symptoms: difficulty walking, bladder control problems, and trouble with thinking and reasoning. Because these symptoms overlap with many other conditions common in older adults, diagnosis requires more than just imaging.
The tap test involves removing a large volume of spinal fluid during a lumbar puncture, then monitoring whether symptoms improve. Gait is the most reliably measured outcome. Doctors often use a timed walking test, where you stand from a chair, walk a set distance, turn around, walk back, and sit down again. Your time is measured before and after the fluid is removed.
Research has found that a roughly 11% improvement in this timed walking test on the first day after the tap has good diagnostic accuracy, with about 78% sensitivity and 80% specificity for predicting who will benefit from surgical treatment. An international expert group has recently developed a standardized protocol for performing this tap test to make results more consistent across different medical centers.
CSF Flow Studies
Phase-contrast MRI is a specialized, noninvasive technique that measures how cerebrospinal fluid moves through the narrow channels of the brain, particularly the aqueduct connecting the third and fourth ventricles. The scan is synchronized to the heartbeat and captures fluid velocity across multiple frames of the cardiac cycle.
This type of imaging helps doctors determine whether the fluid pathway is blocked (obstructive hydrocephalus) or whether fluid can flow freely but isn’t being absorbed properly (communicating hydrocephalus). The distinction matters because it directly influences which surgical approach is most appropriate. It’s not used in every case but adds valuable information when the cause of hydrocephalus isn’t clear from standard imaging.