What is the preferred imaging modality for diagnosing Normal Pressure Hydrocephalus (NPH), Magnetic Resonance Imaging (MRI)?

MRI for NPH Diagnosis

MRI without IV contrast is the preferred imaging modality for diagnosing Normal Pressure Hydrocephalus (NPH). 1

Rationale for MRI as First-Line Imaging

• MRI without IV contrast can identify classic imaging findings of NPH including ventriculomegaly, narrowed posterior callosal angle, effaced sulci along high convexities, widened sylvian fissures, and periventricular white matter changes 1
• MRI can detect cerebral aqueduct flow void, which is associated with good response to shunt surgery and cannot be visualized on CT 1
• MRI has higher sensitivity for detecting periventricular white matter changes compared to CT 1
• MRI can demonstrate specific NPH features such as enlargement of temporal horns, callosal angle <90°, and altered brain water content 1

Key MRI Features for NPH Diagnosis

• Ventriculomegaly not entirely attributable to cerebral atrophy (Evan's index >0.3, defined as maximal width of frontal horns/maximal width of inner skull) 1
• Disproportionately Enlarged Subarachnoid-space Hydrocephalus (DESH) pattern, characterized by:
  • Tight high-convexity and medial subarachnoid spaces 2, 3
  • Enlarged Sylvian fissures 2, 3
  • Ventriculomegaly 2, 3
• Rounded frontal horns with marked enlargement of temporal horns and third ventricle 1
• Absence of or only mild cortical atrophy 1
• Aqueductal or fourth ventricle flow void 1, 4

CT as Alternative When MRI Contraindicated

• CT head without IV contrast can be used when MRI is unavailable or contraindicated 1
• CT can identify ventriculomegaly, narrowed posterior callosal angle, effaced sulci, and widened sylvian fissures 1
• However, CT cannot detect cerebral aqueduct flow void and may miss small obstructing lesions 1
• CT has lower sensitivity for periventricular white matter changes compared to MRI 1

Advanced MRI Techniques

• Cine MRI showing increased ventricular flow rate can help identify shunt-responsive NPH patients 1
• Phase-contrast MRI to measure aqueductal CSF stroke volume has shown excellent positive predictive value for shunt responsiveness 4

Other Imaging Modalities (Not First-Line)

• DTPA cisternography shows persistent radiotracer activity in lateral ventricles and absence over cerebral convexities on delayed imaging, but evidence is insufficient to proceed with shunting based on cisternography findings alone 1
• FDG-PET/CT may help distinguish NPH from other dementias but does not reliably demonstrate classic NPH imaging features 1
• MR spectroscopy cannot reliably distinguish between NPH and other dementias and does not correlate with CSF diversion outcomes 1

Clinical Pearls and Pitfalls

• NPH is a potentially reversible cause of dementia, accounting for approximately 5% of dementia cases, making accurate diagnosis crucial 5, 6
• The clinical triad (gait disturbance, cognitive impairment, urinary incontinence) may be incomplete or atypical, highlighting the importance of imaging 2
• NPH can be comorbid with other neurodegenerative conditions, particularly Alzheimer's disease (in about 30% of cases), which may affect shunt response 5
• Patients typically present late due to the slow, gradual onset of symptoms, with gait alterations occurring early and cognitive impairment developing later 1