What is the diagnostic workup for normal pressure hydrocephalus (NPH)?

Diagnostic Workup for Normal Pressure Hydrocephalus

MRI brain without contrast is the preferred initial imaging modality for diagnosing normal pressure hydrocephalus (NPH), followed by CSF testing through a tap test or lumbar drainage to confirm the diagnosis and predict shunt responsiveness. 1

Clinical Presentation

• Classic triad of symptoms:
  • Gait disturbance (typically appears first)
  • Cognitive impairment
  • Urinary incontinence 2, 3
• Only 50-75% of cases present with the complete triad 2
• Gait is characterized by shuffling, bradykinesia, and instability 3

Imaging Studies

First-Line Imaging

• MRI head without IV contrast or CT head without IV contrast are both appropriate initial imaging options 4, 1
• MRI is preferred when available due to superior detection of:
  • Flow voids
  • Subtle obstructing lesions
  • White matter changes 4

Key Imaging Findings

MRI Findings Suggestive of NPH:

• Ventricular enlargement not entirely attributable to cerebral atrophy (Evans index >0.3, defined as maximal width of frontal horns/maximal width of inner skull) 4, 1
• No macroscopic obstruction of CSF flow 4
• At least one of the following:
  • Enlargement of temporal horns
  • Callosal angle <90°
  • Evidence of altered brain water content
  • Aqueductal or fourth ventricle flow void 4, 1, 5
• Additional supportive findings:
  • Rounded frontal horns
  • Corpus callosum thinning and elevation
  • Enlarged Sylvian fissures with narrowing of high convexity sulci 5
  • Prior MRI showing smaller ventricles before symptom onset 4
  • Cine MRI showing increased ventricular flow rate 4, 1

CT Findings:

• Ventriculomegaly out of proportion to sulci
• Possible transependymal CSF flow 4, 1
• Cannot detect aqueductal flow void 4

CSF Testing

After suggestive imaging, CSF testing is crucial to:

1. Confirm diagnosis
2. Predict response to shunting 1, 6

CSF Testing Options:

1. CSF Tap Test (large-volume lumbar puncture):

   • Measure opening pressure (normal or mildly elevated in NPH)
   • Remove 30-50 mL of CSF
   • Assess for clinical improvement in gait and cognition 2, 6
   • Low sensitivity but high positive predictive value (80-100%) 5

2. Repeated CSF Tap Test or Continuous Lumbar External Drainage:

   • Higher sensitivity (50-100%)
   • Drain at least 150 mL/day for 3-5 days 5
   • Best simulates effect of shunt surgery 1, 2

Additional Testing (Less Commonly Used)

• DTPA Cisternography:

  • Shows delayed clearance of radiotracer over cerebral convexities
  • Abnormal reflux into ventricles
  • Insufficient evidence alone to determine shunt responsiveness 4

• Tc-99m HMPAO SPECT/CT Brain:

  • May help identify patients with impaired cerebral blood flow reactivity who might benefit from shunting 4

• Intracranial Pressure (ICP) Monitoring:

  • Presence of B-waves during >50% of recording time suggests good surgical outcome 2

Diagnostic Algorithm

1. Initial Assessment:

   • Identify clinical triad (gait disturbance, cognitive impairment, urinary incontinence)
   • Rule out mimicking conditions

2. Brain Imaging:

   • MRI brain without contrast (preferred) or CT brain without contrast
   • Evaluate for characteristic findings of NPH

3. CSF Testing (if imaging is suggestive):

   • CSF tap test with measurement of opening pressure
   • If tap test negative but high clinical suspicion, proceed to:
     • Repeated tap test or
     • Continuous lumbar external drainage (3-5 days)

4. Evaluate Response to CSF Removal:

   • Objective assessment of gait and cognition before and after CSF removal
   • Positive response strongly predicts shunt success

Common Pitfalls

• Up to 80% of NPH cases remain unrecognized and untreated 1, 7
• Three-quarters of patients with NPH also have another neurodegenerative disorder, complicating diagnosis 7
• Single CSF tap test has low sensitivity and should not be used to exclude patients from surgery if clinical suspicion remains high 2
• MR spectroscopy is not useful for differentiating NPH from other types of dementia or for patient selection for shunting 4

Predictors of Positive Surgical Outcome

• Positive response to CSF removal
• Presence of all three symptoms of the classic triad
• Short duration of symptoms
• Minimal comorbidities 1, 6