Neurology

Subtypes of Hydrocephalus and Diagnostic Tests

At a Glance

Hydrocephalus is classified by whether fluid flow is blocked or absorption is impaired. MRI and CT show the brain’s fluid pathways, while tap testing or external lumbar drainage can help predict whether a shunt may improve symptoms in normal pressure hydrocephalus.

Understanding the specific type of hydrocephalus you or your loved one is facing is a critical step in determining the right path forward. Because hydrocephalus can be caused by many different factors—from genetic variants to life events like a head injury—doctors use a combination of classification systems and advanced testing to map out the “plumbing” of the brain.

Two Main Categories: Flow vs. Absorption

The most fundamental distinction doctors make is whether the fluid can move freely between the brain’s internal chambers (the ventricles) and the space around the brain.

Noncommunicating (Obstructive) Hydrocephalus: This occurs when there is a physical blockage somewhere along the narrow pathways that connect the ventricles ^[1]. It is like a dam in a river; the fluid builds up behind the obstruction. Common causes include a narrowing of the aqueduct of Sylvius (aqueductal stenosis), tumors, or cysts that pinch the pathways shut ^[2].
Communicating Hydrocephalus: In this type, the fluid can “communicate” or flow between the ventricles, but it isn’t being absorbed into the bloodstream correctly once it leaves them ^[1]^[3]. It is like a drain that is partially plugged; the fluid moves through the pipes but pools at the exit.

Subtypes and Their Causes

Hydrocephalus is also categorized by when and why it started.

Congenital and Pediatric Subtypes

Congenital hydrocephalus is present at birth and often stems from events during fetal development.

Genetic Variants: Some cases are linked to mutations that affect ependymal cilia—tiny hair-like structures that help move fluid through the brain ^[4]^[5].
Developmental Malformations: Conditions like Dandy-Walker malformation, which affects the development of the cerebellum and the fluid-filled spaces around it, can lead to obstructive hydrocephalus ^[2].

Acquired Hydrocephalus

This type develops after birth due to a specific event or underlying condition.

Post-Hemorrhagic: Bleeding in the brain (often seen in premature infants or from aneurysms) can cause inflammation and scarring that blocks fluid pathways ^[6].
Post-Infectious: Infections like meningitis can lead to “debris” in the fluid or scarring of the absorption sites ^[6].
Trauma or Tumors: Head injuries or growths can physically compress the ventricles or damage the absorption system ^[2]^[7].

Idiopathic Normal Pressure Hydrocephalus (iNPH)

Found primarily in older adults, iNPH is a form of communicating hydrocephalus where the ventricles enlarge but the pressure often remains in the normal range ^[8]. “Idiopathic” means the cause is unknown, though researchers are investigating links to high blood pressure, diabetes, and age-related changes in the brain’s waste-clearance system ^[9]^[5].

Diagnostic Imaging: Looking for Clues

Doctors use scans to see exactly how the fluid is behaving. The choice of scan depends on local protocols and the clinical question.

MRI (Magnetic Resonance Imaging): This is the preferred tool for a detailed look at the brain’s structure.
- Specialized Sequences (e.g., 3D-SPACE): Some centers use this high-resolution technique to clarify fluid pathways and obstruction anatomy ^[10].
- The DESH Sign: Standing for Disproportionately Enlarged Subarachnoid space Hydrocephalus, this pattern is a hallmark of iNPH. It shows enlarged ventricles alongside “crowding” or tightness at the very top of the brain ^[11]^[12]. This helps distinguish iNPH from atrophy, where the brain naturally shrinks with age ^[13].
CT (Computed Tomography): Often chosen in emergencies because it is fast, CT scans can quickly show if the ventricles are enlarged or if there is bleeding, though it doesn’t measure pressure directly ^[3]^[14].

Predictive Tests: Will a Shunt Help?

If a doctor suspects iNPH, they need to evaluate whether placing a shunt (a drainage tube) will actually improve the patient’s symptoms. These tests are prognostic aids used alongside symptoms and imaging, not definitive diagnostic guarantees.

The CSF Tap Test

Also known as a large-volume lumbar puncture, this involves removing 30 to 50 mL of spinal fluid through a needle in the lower back ^[15].

What to expect: Before and after the fluid is removed, physical therapists will carefully measure your walking speed, balance, and memory ^[15]^[16].
What it means: If you show a significant improvement—such as walking faster or thinking more clearly—it is a strong sign that a permanent shunt will be effective ^[17]^[18]. However, a “negative” test doesn’t rule out a benefit; some people still improve with surgery despite a negative test ^[19]^[20].

External Lumbar Drainage (ELD)

If the tap test is inconclusive, a doctor may recommend ELD. This involves a 3-to-5-day hospital stay where a small catheter is placed in the lower back to drain fluid continuously ^[15]^[21]. Because it mimics the constant drainage of a shunt over several days, it may improve sensitivity for predicting who will benefit from surgery ^[22]^[23]. However, ELD is more invasive and carries risks such as infection, over-drainage, severe headaches, and mobility-related complications.

Common questions in this guide

How are obstructive and communicating hydrocephalus different?

In obstructive, or noncommunicating, hydrocephalus, a physical blockage stops cerebrospinal fluid from moving through one of the brain’s narrow pathways. In communicating hydrocephalus, fluid can move between the ventricles but is not absorbed properly afterward. This distinction helps guide the evaluation and choice of procedure.

What can an MRI or CT scan show in hydrocephalus?

MRI gives a detailed view of the ventricles, fluid pathways, and surrounding brain structures, and specialized MRI sequences may clarify an obstruction. CT is faster and is often used in emergencies to look for enlarged ventricles or bleeding. Neither scan directly measures pressure.

What does a DESH pattern mean on an MRI?

A DESH pattern means the ventricles are enlarged while the fluid spaces at the top of the brain look crowded. It can support a diagnosis of idiopathic normal pressure hydrocephalus and help distinguish it from age-related brain shrinkage, called atrophy. Doctors interpret it together with symptoms and other findings.

What happens during a cerebrospinal fluid tap test?

During a tap test, a clinician removes about 30 to 50 milliliters of spinal fluid through a lumbar puncture. Walking speed, balance, and memory are assessed before and after the procedure. Noticeable improvement can suggest that a shunt may help, but the test is not a guarantee.

Does a negative tap test mean a shunt will not help?

No. Some people improve after shunt surgery even when their tap test does not show clear improvement. The result is considered with symptoms, imaging, and other tests, such as external lumbar drainage when appropriate.

Why would a doctor recommend an external lumbar drain?

An external lumbar drain may be used when a tap test is inconclusive. A small catheter drains spinal fluid continuously for about three to five days in the hospital, which may provide more time to observe changes and predict shunt benefit. It is more invasive and can cause infection, over-drainage, severe headaches, or mobility-related complications.

What can cause hydrocephalus in children or adults?

Hydrocephalus may be present at birth because of genetic variants or developmental conditions. It can also develop after bleeding, meningitis or another infection, a head injury, or a tumor. In idiopathic normal pressure hydrocephalus, the cause is unknown, although it is mainly seen in older adults.

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Key takeaways

Obstructive (noncommunicating) hydrocephalus results from a blockage in a fluid pathway, while communicating hydrocephalus involves impaired absorption after fluid leaves the ventricles.
Congenital hydrocephalus may be linked to genetic or developmental conditions, while acquired hydrocephalus can follow bleeding, infection, head injury, or tumors.
MRI provides detailed views of fluid pathways, and CT offers a rapid way to check for enlarged ventricles or bleeding in emergencies.
A CSF tap test removes 30 to 50 mL of spinal fluid and compares walking, balance, and memory before and after removal to estimate possible shunt benefit.
External lumbar drainage over three to five days may clarify an inconclusive tap test but is more invasive and carries risks including infection and over-drainage.

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This page explains hydrocephalus subtypes and diagnostic tests for educational purposes only and does not replace medical advice. A neurologist or neurosurgeon can interpret your imaging and test results and discuss whether a shunt is appropriate.