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PubMed This is a summary of 20 peer-reviewed journal articles Updated
Neurology

Is it Hydrocephalus or Brain Shrinkage?

At a Glance

Enlarged brain ventricles do not by themselves prove hydrocephalus. Doctors combine MRI patterns, symptoms, and often a tap test to distinguish idiopathic normal pressure hydrocephalus, which may improve with a shunt, from brain shrinkage, which usually does not; both can coexist.

When a brain scan shows enlarged ventricles (the fluid-filled chambers in the brain), it is a sign that something is out of balance. However, “big ventricles” do not always mean you have hydrocephalus. Doctors must distinguish between conditions like idiopathic Normal Pressure Hydrocephalus (iNPH) and Hydrocephalus ex vacuo [1][2].

The distinction is critical: iNPH is a condition where altered fluid dynamics affect the brain and may improve with surgery, while ex vacuo is a symptom of brain tissue loss that cannot be reversed by a shunt [3][4]. Importantly, these two conditions are not mutually exclusive; atrophy and iNPH can coexist in the same patient. A finding of atrophy alone does not rule out coexisting, potentially shunt-responsive iNPH.

The Mechanical Context

Think of the brain and its fluid compartments as sharing space inside the rigid skull.

  • In iNPH (True Hydrocephalus): The fluid system is malfunctioning. Complex changes in fluid production, absorption, and brain compliance cause the ventricles to “inflate” and push outward against the brain tissue [1][5].
  • In Hydrocephalus ex vacuo: The primary problem is that the brain tissue itself has shrunk (atrophy) due to aging or conditions like Alzheimer’s or vascular dementia [6][1]. As the brain tissue disappears, the cerebrospinal fluid simply occupies the space that was lost. The ventricles look large because the brain around them is smaller [2].

Why Shunting Doesn’t Work for Pure “Ex Vacuo”

A shunt is a surgical tube designed to drain excess fluid and relieve pressure or stretch on the brain. In iNPH, this can “deflate” the ventricles and help the brain function better [7].

However, in pure ex vacuo, there is no excess pressure to relieve. The fluid is just occupying the empty space [4]. Placing a shunt in someone with ex vacuo will not bring back the lost brain cells and will not improve symptoms like memory loss or walking difficulty. Instead, it exposes the patient to surgical risks, such as infection or bleeding, without any likely benefit [7][8].

Decoding Your Scan: iNPH vs. Atrophy Clues

Radiologists look for specific patterns on an MRI to find clues. These are not definitive diagnostic features, but supportive signs that must be interpreted with clinical context [6][9].

Feature Suggestive of iNPH Suggestive of Ex Vacuo (Atrophy)
Callosal Angle Small (steep/sharp). The ventricles push upward, making the angle between the two halves of the brain narrow (usually below 90°) [10][9]. Large (flat/wide). As the brain shrinks, the space between the halves opens up [6][11].
The DESH Sign Present. Fluid is crowded at the top of the brain but pooled in the side fissures (Sylvian fissures) [12][13]. Absent. Fluid is evenly distributed; the grooves on the surface of the brain (sulci) look wide and deep everywhere [6][14].
Periventricular Changes “Blurring” or lucency around the edges of the ventricles may reflect transependymal CSF flow [15]. White matter changes are very common with age and small-vessel disease; they are nonspecific and not a direct pressure gauge.
Ventricle Shape Often described as rounded or “ballooned” [12]. May maintain a more angular or irregular shape [16].

Discussing the Radiology Report

When you review your or your loved one’s neurology or radiology report, you may see a mix of findings. A complete report doesn’t have to list every measurement, but it is worth discussing these aspects with your specialist:

  • Ventricular Morphology: Does the report describe the ventricles as disproportionately enlarged compared to generalized atrophy? [12]
  • Callosal Angle: Is a specific angle or measurement mentioned? (A sharp angle supports iNPH) [9]
  • DESH Pattern: Does the report mention “tight high-convexity sulci” or the “DESH sign”? [13]
  • Parenchymal Changes: Does the report note “generalized atrophy” or “focal tissue loss”? (These suggest ex vacuo) [6]
  • Evans Index: This is a ratio of ventricle width to skull width. While an index over 0.30 shows the ventricles are large, it does not tell you why [17][18]. It must be interpreted alongside the other signs above.
  • Obstruction Status: Does the report confirm the pathways (like the aqueduct) are open? [12]

The presence of the DESH sign can support iNPH, but no single sign establishes it or guarantees a shunt response, and its absence does not completely rule it out [19][20]. Because these scans are complex, most doctors will use a tap test (removing fluid to see if symptoms improve) to aid the diagnosis before suggesting surgery [7].

Common questions in this guide

Does having enlarged brain ventricles mean I have hydrocephalus?
Not necessarily. Enlarged ventricles can occur with idiopathic normal pressure hydrocephalus or with hydrocephalus ex vacuo, in which brain tissue loss leaves more room for cerebrospinal fluid. Doctors consider the MRI pattern, symptoms, and sometimes a tap test rather than relying on ventricle size alone.
How is iNPH different from hydrocephalus ex vacuo?
In idiopathic normal pressure hydrocephalus, abnormal cerebrospinal fluid dynamics may affect brain function and symptoms may improve after shunt surgery. In hydrocephalus ex vacuo, brain tissue has shrunk and fluid fills the space that was lost, so a shunt usually does not help. The two conditions can occur in the same person.
What MRI findings can support a diagnosis of iNPH?
A small callosal angle, a DESH pattern, and rounded or ballooned ventricles can support iNPH. These are clues rather than definitive tests, and their meaning depends on the person’s symptoms and other scan findings. No single MRI sign guarantees that a shunt will work.
What does an Evans index above 0.30 tell me?
An Evans index above 0.30 indicates that the ventricles are enlarged compared with the skull. It does not show whether the enlargement is caused by iNPH, brain tissue loss, or another process. Doctors interpret it with the rest of the scan and the clinical history.
Can a shunt help if my scan shows brain atrophy?
A shunt is unlikely to help when enlarged ventricles are caused only by brain tissue loss, and surgery can cause complications such as infection or bleeding. However, atrophy does not rule out coexisting iNPH, which may respond to a shunt. A specialist may use symptoms, imaging, and a tap test to assess the potential benefit.
What is a tap test for suspected hydrocephalus?
A tap test removes a small amount of cerebrospinal fluid, usually through a lumbar puncture, and checks whether walking, thinking, or other symptoms improve. It can help estimate whether shunt surgery might help, but the result is interpreted with the examination and imaging rather than used alone.

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References

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This page explains imaging clues for distinguishing iNPH from hydrocephalus ex vacuo for informational purposes only and does not constitute medical advice. A neurologist or neurosurgeon should interpret your scan, symptoms, and treatment options.

Guide contents Hydrocephalus
Research & Literature
02 Subtypes of Hydrocephalus and Diagnostic Tests 04 Standard Treatment Options