What is the initial workup for a patient suspected of having Normal Pressure Hydrocephalus (NPH)?

Initial Workup for Normal Pressure Hydrocephalus (NPH)

MRI without IV contrast is the first-line imaging modality for diagnosing Normal Pressure Hydrocephalus (NPH) due to its superior ability to detect characteristic features and rule out alternative diagnoses. 1, 2

Clinical Presentation Assessment

• NPH is characterized by a clinical triad of gait disturbance (typically occurring first), urinary incontinence, and cognitive impairment 3, 4
• Patients typically present late in the disease course due to the slow, gradual onset of symptoms, with gait alterations occurring early and cognitive impairment developing later 1, 2
• The prevalence of NPH is estimated at 3.7% in patients over 65 years of age, making it a potentially reversible cause of dementia 1, 5
• Careful assessment of the patient's ability to perform routine activities of daily living is essential to establish baseline function 1

Imaging Studies

Primary Imaging: MRI Brain Without Contrast

• MRI without IV contrast can identify classic NPH imaging findings including 1, 2:
  • Ventriculomegaly (Evan's index >0.3)
  • Narrowed posterior callosal angle (<90°)
  • Effaced sulci along high convexities
  • Widened sylvian fissures
  • Periventricular white matter changes
  • Cerebral aqueduct flow void (associated with good shunt response)
  • Disproportionately enlarged subarachnoid-space hydrocephalus (DESH) pattern

Alternative Imaging: CT Head Without Contrast

• When MRI is contraindicated or unavailable, CT head without IV contrast can be used to identify 1, 2:
  • Ventriculomegaly
  • Narrowed posterior callosal angle
  • Effaced sulci and widened sylvian fissures
  • Periventricular white matter hypoattenuation (lower sensitivity than MRI)
• Note that CT cannot detect cerebral aqueduct flow void and may miss small obstructing lesions 1

Supplementary Diagnostic Tests

CSF Dynamics Testing

• CSF tap test (CSF-TT): Removal of 30-50 mL of CSF via lumbar puncture with assessment of gait and cognition before and after 3, 6
  • Positive response suggests shunt responsiveness
  • Low sensitivity but high specificity for predicting shunt response
• For inconclusive single tap test results, consider 3, 6:
  • Repeated CSF tap test (RTT)
  • Continuous lumbar external drainage (LED) for 2-3 days

Additional Testing When Diagnosis Remains Uncertain

• DTPA cisternography may show persistent radiotracer activity in lateral ventricles and absence over cerebral convexities on delayed imaging 1
  • Evidence is insufficient to proceed with shunting based on cisternography findings alone
• Intracranial pressure monitoring to detect B-waves (present >50% of recording time suggests good shunt response) 3
• CSF infusion tests to measure CSF outflow resistance 6

Laboratory Evaluation

• Complete blood count, urinalysis, serum electrolytes (including calcium and magnesium) 1
• Blood urea nitrogen, serum creatinine, fasting blood glucose 1
• Thyroid-stimulating hormone to rule out metabolic causes of cognitive decline 1, 5
• Liver function tests 1

Differential Diagnosis Considerations

• NPH can be comorbid with other neurodegenerative diseases (present in 20-57% of NPH patients) 1, 7
• Approximately 75% of patients with NPH severe enough to require treatment also suffer from another neurodegenerative disorder 7
• Consider vascular dementia, Alzheimer's disease, Parkinson's disease, and other causes of gait disturbance and cognitive decline 5, 6

Pitfalls and Caveats

• Relying solely on clinical presentation can lead to misdiagnosis, as NPH can mimic other neurodegenerative disorders 4, 7
• A single negative CSF tap test should not exclude patients from consideration for shunt surgery due to its low sensitivity 3
• Incomplete clinical manifestations occur in 25-50% of NPH cases, making supplementary testing crucial 3
• Failure to diagnose NPH results in missed treatment opportunities, as approximately 80% of cases remain unrecognized and untreated 7