What are the characteristic findings of Normal Pressure Hydrocephalus (NPH) on a Computed Tomography (CT) scan?

Characteristic Findings of Normal Pressure Hydrocephalus (NPH) on CT Scan

The characteristic CT findings of Normal Pressure Hydrocephalus (NPH) include ventriculomegaly out of proportion to sulcal atrophy, with an Evan's index >0.3, rounded frontal horns, enlarged temporal horns, and evidence of transependymal CSF flow. 1, 2

Primary CT Findings in NPH

• Ventriculomegaly not entirely attributable to cerebral atrophy or congenital enlargement, with an Evan's index (maximal width of frontal horns/maximal width of inner skull) greater than 0.3 1, 3
• Rounded frontal horns with marked enlargement of the temporal horns and third ventricle 1, 2
• Absence of or only mild cortical atrophy, creating a disproportionate appearance between ventricle size and sulci 1, 2
• Evidence of transependymal CSF flow 1
• Effaced sulci along high convexities with widened sylvian fissures (DESH pattern) 3
• Narrowed posterior callosal angle 3

Limitations of CT for NPH Diagnosis

• CT has lower sensitivity for detecting periventricular white matter changes compared to MRI 3
• CT cannot detect cerebral aqueduct flow void, which is associated with good response to shunt surgery 3
• CT may miss small obstructing lesions along the ventricular system 1
• Histopathologically verified cases of vascular dementia with normal CT studies have been reported, suggesting similar limitations may exist for NPH 1

MRI as Superior Alternative

• MRI without IV contrast is the preferred imaging modality for diagnosing NPH according to the American College of Radiology 2, 3
• MRI can better visualize key NPH features including:
  • Callosal angle of <90° 1, 3
  • Evidence of altered brain water content 1
  • Aqueductal or fourth ventricle flow void 1, 3
  • Increased CSF flow through the cerebral aqueduct (hyperdynamic aqueductal CSF) 1, 4
  • Periventricular white matter changes with higher sensitivity 3

Clinical Context of Imaging Findings

• CT findings should be interpreted in the context of the classic clinical triad of NPH: gait disturbance (typically occurring first), cognitive impairment, and urinary incontinence 5, 6
• The diagnosis of NPH is made more confidently when both imaging findings and clinical symptoms are present 7
• CT head without IV contrast is an appropriate first-line imaging test when MRI is contraindicated or unavailable 1, 3

Common Pitfalls in NPH Diagnosis

• Relying solely on CT findings without MRI confirmation when possible can lead to misdiagnosis 2
• Approximately 75% of NPH patients also have another neurodegenerative disorder, complicating the radiological picture 8
• Misdiagnosing NPH as another neurodegenerative disorder due to overlapping imaging findings 2
• Failing to recognize that NPH is potentially reversible with CSF diversion (shunting) 5

For definitive diagnosis and treatment planning, patients with positive CT findings suggestive of NPH should be referred for MRI evaluation when possible and neurosurgical consultation for potential ventriculoperitoneal shunt placement 2, 5.