What imaging modality is recommended for confirming normal pressure hydrocephalus (NPH)?

Imaging for Normal Pressure Hydrocephalus Confirmation

MRI of the head without IV contrast is the optimal imaging modality for confirming normal pressure hydrocephalus (NPH), as it provides superior detection of characteristic findings including ventriculomegaly, flow voids, and white matter changes. 1, 2

Primary Imaging Options

MRI Head Without Contrast (First Choice)

• Superior for NPH diagnosis due to better soft tissue characterization 1, 3
• Key diagnostic findings include:
  • Ventricular enlargement disproportionate to cerebral atrophy (Evans index >0.3)
  • Narrowed posterior callosal angle (<90°)
  • Effaced sulci along high convexities with widened Sylvian fissures ("DESH" pattern)
  • Periventricular white matter changes (better visualized than on CT)
  • Aqueductal or fourth ventricle flow void
  • Corpus callosum thinning and elevation
  • Enlarged temporal horns not fully explained by hippocampal atrophy 1, 2, 4

CT Head Without Contrast (Alternative)

• Acceptable alternative when MRI is contraindicated or unavailable 1, 2
• Can identify:
  • Ventriculomegaly
  • Narrowed posterior callosal angle
  • Effaced sulci along high convexities with widened Sylvian fissures
  • Periventricular white matter hypoattenuation (less sensitive than MRI) 1
• Limitations:
  • Cannot detect cerebral aqueduct flow void
  • May miss small obstructing lesions 1

Advanced/Supplementary Imaging Techniques

Cine MRI (Phase-Contrast MRI)

• Valuable for assessing CSF flow dynamics
• Shows hyperdynamic CSF flow through the aqueduct in shunt-responsive NPH 2, 5
• Elevated aqueductal CSF stroke volume has high positive predictive value for shunt response 5

DTPA Cisternography

• Shows persistent radiotracer in lateral ventricles and absent activity over cerebral convexities 1
• Limited utility: insufficient evidence to determine shunt responsiveness based on cisternography alone 1, 2

FDG-PET/CT Brain

• Not recommended as initial imaging for NPH diagnosis 1, 2
• May help distinguish NPH from other dementias by showing hypometabolism in dorsal striatum with preserved cortical metabolism 1
• Useful for identifying comorbid neurodegenerative diseases 1, 6

Diagnostic Algorithm

1. Initial Imaging: MRI head without contrast (preferred) or CT head without contrast if MRI contraindicated
2. If MRI findings are suggestive of NPH: Proceed to functional assessment with CSF removal (tap test or lumbar drainage)
3. If initial findings are equivocal: Consider advanced imaging:
   • Cine MRI to assess CSF flow dynamics
   • FDG-PET/CT to rule out comorbid neurodegenerative conditions

Important Considerations

• Imaging alone is insufficient for NPH diagnosis; functional assessment with CSF removal is crucial to confirm diagnosis and predict shunt response 2, 4
• Up to 80% of NPH cases remain unrecognized and untreated, highlighting the importance of thorough evaluation 2
• NPH can coexist with other neurodegenerative conditions (approximately 30% have comorbid Alzheimer's pathology) 6
• Positive response to CSF removal, presence of all three classic symptoms, short duration of symptoms, and minimal comorbidities predict good response to shunting 2

Common Pitfalls to Avoid

• Relying solely on ventriculomegaly for diagnosis (can be a normal aging finding)
• Failing to assess for comorbid conditions that may impact treatment response
• Not performing functional assessment with CSF removal despite suggestive imaging
• Overlooking the importance of the complete clinical triad (gait disturbance, cognitive impairment, urinary incontinence) in conjunction with imaging findings