Normal pressure hydrocephalus (NPH) is a neurological condition characterized by an abnormal accumulation of cerebrospinal fluid (CSF) in the brain’s ventricles, leading to their enlargement without a corresponding increase in CSF pressure. On a CT scan, the signs of NPH are primarily related to the appearance and size of the brain’s ventricular system and the surrounding brain structures.
The most prominent CT scan sign of NPH is **ventriculomegaly**, which means the lateral ventricles are enlarged. This enlargement is disproportionate to the degree of cortical atrophy, which helps differentiate NPH from other causes of ventricular enlargement such as brain shrinkage due to aging or neurodegenerative diseases. The ventricles appear dilated, especially the lateral and third ventricles, but the fourth ventricle is usually not as enlarged, which can help distinguish NPH from obstructive hydrocephalus.
One key measurement used on CT is the **Evans index**, which is the ratio of the maximum width of the frontal horns of the lateral ventricles to the maximum internal diameter of the skull at the same level. An Evans index greater than 0.3 is suggestive of ventriculomegaly consistent with NPH. However, this index alone is not definitive and must be interpreted alongside other imaging features and clinical symptoms.
Another important sign is the **callosal angle**, which is measured on coronal images through the posterior commissure. In NPH, this angle is typically reduced, often less than 90 degrees, reflecting the shape changes in the ventricles due to CSF accumulation. A smaller callosal angle supports the diagnosis of NPH as it indicates active ventricular dilatation rather than passive enlargement from brain atrophy.
The **sulcal pattern** on the cortical surface is also informative. In NPH, the sulci over the high convexities of the cerebral hemispheres tend to be compressed or effaced, meaning the spaces between the brain folds appear narrowed or obliterated. This contrasts with the Sylvian fissures, which may appear disproportionately enlarged or dilated. This combination is sometimes called the “disproportionately enlarged subarachnoid space hydrocephalus” (DESH) pattern and is a hallmark of NPH on imaging.
Additional CT features include:
– **Periventricular hypodensities** or white matter changes around the ventricles, which may represent transependymal CSF flow or edema due to increased fluid pressure at the ventricular walls.
– **Rounding of the temporal horns** of the lateral ventricles, which become more prominent as the ventricles enlarge.
– **Effacement of the cortical sulci** at the vertex, indicating compression of the subarachnoid spaces.
– **Stretching or thinning of the corpus callosum**, which may appear elevated or flattened due to ventricular enlargement.
– The **third ventricular floor may appear bulging or distended**, reflecting increased CSF volume.
Unlike obstructive hydrocephalus, the basal cisterns and fourth ventricle are usually not compressed in NPH, which helps differentiate it from other types of hydrocephalus.
It is important to note that CT findings must be correlated with clinical symptoms, which classically include gait disturbance, urinary incontinence, and cognitive impairment. The imaging signs alone do not confirm NPH, as some patients may have ventriculomegaly without symptoms.
In summary, the CT scan signs of normal pressure hydrocephalus include:
– Enlarged lateral and third ventricles (ventriculomegaly) with an Evans index > 0.3.
– Reduced callosal angle (< 90 degrees) on coronal views.
- Effaced or compressed sulci over the cerebral convexities with enlarged Sylvian fissures (DESH pattern).
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