Specific Imaging Findings

Dilated supratentorial ventricular system (triventricular hydrocephalus), with rounded frontal horns, enlarged temporal horns, bowing of the corpus callosum, and ballooning of the third ventricle are characteristic for normal pressure hydrocephalus (NPH), while the fourth ventricle is of normal size and cerebral aqueduct is patent. The Evans' index (ratio between the maximum widths of the frontal horns and the skull, measured along the inner table of the calvarium) is >0.3. There is disproportion between the ventricular size and the width of the sulci, especially in the mesial temporal regions and adjacent to the falx along the convexities, where the sulci may be effaced. This is known as “gyral crowding” and is best appreciated on coronal images. The sylvian fissures and the basal cisterns are, on the other hand, wide. Evidence of periventricular CSF resorption is frequently seen as periventricular areas that are hypodense on CT, of high T2 signal, and with increased ADC values. Flow-sensitive MR sequences characteristically reveal increased flow within the aqueduct. Parenchymal atrophy and leukoaraiosis can coexist or develop in the late phases. MRS may show lactate within the ventricles.

Pertinent Clinical Information

The original description of this entity included the triad of ataxia, dementia, and incontinence. It is estimated that 5–10% of cases of dementia are caused by idiopathic NPH (iNPH), and this is the only surgically treatable dementia (by CSF shunting). The diagnosis is made in the presence of gait disturbance accompanied by at least one of the other two elements of the triad in a patient over 40 years of age with gradual onset of symptoms, neuroimaging of triventricular hydrocephalus with patent sylvian aqueduct, and CSF opening pressure between 70 and 245 mm H2O. Patients with a history of prior head trauma, intracranial hemorrhage or infection are classified as secondary NPH. Prognostic criteria for a positive response to shunting have not yet been validated; among the most reliable are the evidence of hyperdynamic flow in the aqueduct, large volume CSF tap test, and infusion tests, performed through ventricular catheters.