Diagnosing Normal Pressure Hydrocephalus
Normal pressure hydrocephalus (NPH) is diagnosed through a combination of clinical assessment, brain imaging, and cerebrospinal fluid testing, with MRI or CT showing ventriculomegaly being the cornerstone of diagnosis. 1
Clinical Presentation
The diagnosis of NPH relies on identifying the classic clinical triad:
- Gait disturbance: Usually the first and most prominent symptom
- Cognitive impairment/dementia: Typically subcortical type
- Urinary urgency/incontinence: Often develops later in the disease course
Important clinical features to note:
- Gait is typically wide-based, magnetic, with short shuffling steps
- Cognitive symptoms include slowed processing, attention deficits, and executive dysfunction
- Urinary symptoms progress from urgency to frank incontinence
Neuroimaging Evaluation
Primary Imaging
MRI brain without contrast is the preferred imaging modality 1, 2
- Shows ventriculomegaly out of proportion to cortical atrophy
- Evan's index >0.3 (ratio of maximal width of frontal horns to maximal width of inner skull)
- Callosal angle <90 degrees
- Enlargement of temporal horns
- Evidence of altered brain water content
- Aqueductal or fourth ventricle flow void
CT head without contrast is an acceptable alternative if MRI is contraindicated 1
- Can show ventriculomegaly and transependymal CSF flow
- Coronal reformations help assess hippocampal atrophy
Key Imaging Features
- Ventricular enlargement not entirely attributable to cerebral atrophy
- No macroscopic obstruction of CSF flow
- Rounded frontal horns and marked enlargement of temporal horns and third ventricle 1
CSF Studies
Lumbar puncture with opening pressure measurement:
- Normal or mildly elevated CSF pressure (typically 70-245 mm H₂O)
- CSF composition is typically normal
CSF tap test (CSF-TT):
- Removal of 30-50 mL of CSF with assessment of gait before and after
- Temporary improvement suggests potential shunt responsiveness
- Single tap has low sensitivity but high specificity 3
Extended CSF drainage tests for inconclusive cases:
- Repeated CSF tap test (RTT)
- Continuous lumbar external drainage (LED) for 3-5 days 3
Supplementary Diagnostic Tests
For cases where diagnosis remains uncertain:
Intracranial pressure (ICP) monitoring:
- Presence of B-waves during >50% of recording time suggests NPH 3
Cine MRI:
- Shows increased ventricular flow rate
- Hyperdynamic CSF flow through the aqueduct correlates with better shunt response 4
Radionuclide cisternography:
- May show delayed clearance of radiotracer over cerebral convexities
- Less commonly used now with improved MRI techniques 1
Differential Diagnosis
NPH must be distinguished from other conditions that can present similarly:
- Alzheimer's disease
- Vascular dementia
- Parkinson's disease
- Lewy body dementia
- Other causes of communicating hydrocephalus
Complicating factors:
- Approximately 75% of NPH patients also have another neurodegenerative disorder 5
- NPH accounts for approximately 5% of dementia cases 6
Diagnostic Algorithm
- Initial screening: Identify patients with elements of the clinical triad plus ventriculomegaly
- Brain imaging: MRI without contrast (or CT if MRI contraindicated)
- CSF studies: Lumbar puncture with opening pressure measurement
- Response testing: CSF tap test to assess potential shunt responsiveness
- Advanced testing: For inconclusive cases, consider extended CSF drainage, ICP monitoring, or cine MRI
Common Pitfalls to Avoid
- Failing to consider NPH in elderly patients with gait disturbance and cognitive decline
- Relying solely on imaging without clinical correlation
- Excluding patients from shunt consideration based on a negative single CSF tap test
- Not recognizing co-existing neurodegenerative conditions
- Delaying diagnosis, as early intervention leads to better outcomes 5
Remember that NPH is one of the few potentially reversible causes of dementia, making accurate and timely diagnosis crucial for improving patient outcomes through appropriate treatment.