What are the radiological criteria for diagnosing hydrocephalus?

Radiological Criteria for Diagnosing Hydrocephalus

Hydrocephalus is radiologically defined as ventriculomegaly (ventricular enlargement) that is not entirely attributable to cerebral atrophy or congenital enlargement, with specific quantitative thresholds varying by age group and clinical context.

Core Radiological Definition

The fundamental radiological criterion is ventriculomegaly with evidence of impaired CSF circulation, which can be demonstrated through multiple imaging modalities 1.

Quantitative Measurements

Evans Index (EI):

• EI >0.3 is the traditional threshold for ventriculomegaly in adults and older children 1
• Calculated as the maximal width of the frontal horns divided by the maximal internal diameter of the skull at the same level 1
• Important caveat: The plane of measurement significantly affects the value; measurements should ideally be obtained 16 mm parallel to the anterior commissure-posterior commissure (AC-PC) plane for consistency 2
• EI shows only moderate correlation with actual ventricular volume (r: 0.619), and values can vary significantly depending on the CT slice selected 2

Pediatric-Specific Measurements:

• In premature infants, hydrocephalus can be defined as atrium of lateral ventricle >10 mm on horizontal plane ultrasound 1
• Body of lateral ventricle at midthalamus level >10 mm on sagittal ultrasound 1
• Ventricular/biparietal (V/BP) ratio for severity grading: mild (0.26-0.40), moderate (0.40-0.60), severe (0.60-0.90), extreme (0.91-1.0) 1

Asian and Global Populations:

• >12 mm ventricular dilation represents moderate-to-severe ventriculomegaly 1

Imaging Modality Selection

MRI is the preferred first-line imaging modality for hydrocephalus diagnosis, as recommended by the American College of Radiology 3, 4.

MRI Advantages and Key Features:

• Superior detection of:

  • Cerebral aqueduct flow void (associated with shunt responsiveness, cannot be seen on CT) 3, 4
  • Periventricular white matter changes (higher sensitivity than CT) 3
  • Temporal horn enlargement not explained by hippocampal atrophy 3
  • Callosal angle <90° (narrowed posterior callosal angle) 3, 4
  • Corpus callosum thinning and elevation 3

• Classic MRI constellation for NPH specifically:

  • Ventriculomegaly with EI >0.3 3, 4
  • Disproportionately enlarged subarachnoid-space hydrocephalus (DESH) pattern: tight high-convexity sulci, enlarged Sylvian fissures 3
  • Rounded frontal horns 3
  • Aqueductal or fourth ventricle flow void 3, 4

CT as Alternative:

• CT without contrast is acceptable when MRI is contraindicated or unavailable 3
• Can identify ventriculomegaly, narrowed callosal angle, effaced sulci, widened Sylvian fissures 3
• Limitations: Cannot detect aqueduct flow void, lower sensitivity for periventricular changes, may miss small obstructing lesions 3

Ultrasound in Infants:

• Head ultrasound (HUS) is the primary modality for premature infants and neonates 1
• Allows serial monitoring without radiation exposure 1

Supporting Radiological Features

Beyond basic ventriculomegaly, the following features strengthen the diagnosis:

Structural Changes:

• Anterior cortical mantle thickness <20 mm (in infants, though not a reliable outcome predictor) 1
• Ballooning of frontal horns 3
• Widening of temporal horns disproportionate to hippocampal atrophy 3
• Enlarged Sylvian fissures and basal cisterns with narrowed high convexity sulci 3, 5

Clinical Correlation Required:

• Radiological ventriculomegaly alone is insufficient; associated clinical symptoms and signs must be present 1
• In resource-limited settings, surgical treatment may serve as a surrogate diagnostic indicator when imaging is limited 1

Critical Diagnostic Pitfalls

Avoid over-reliance on single measurements:

• Evans Index can vary significantly based on measurement plane 2
• DESH criteria show substantial interobserver variability and low diagnostic accuracy (only 27-50% sensitivity across evaluators) 6
• Requiring both EI >0.3 AND DESH would result in >70% of treatable patients being misdiagnosed 6, 7

Key principle: Ventricular enlargement must be disproportionate to cerebral atrophy and not explained by congenital enlargement 3, 4. The presence of macroscopic CSF flow obstruction should be excluded 4.

Measurement standardization is essential: Use consistent anatomical landmarks and planes to ensure reproducibility, particularly when serial imaging is performed for monitoring 2.