Diagnosing Normal Pressure Hydrocephalus
Normal pressure hydrocephalus (NPH) should be diagnosed based on the clinical triad of gait disturbance, cognitive impairment, and urinary incontinence (with gait typically affected first and most severely), combined with MRI findings of ventriculomegaly and confirmation via CSF tap test, with MRI without IV contrast being the preferred initial imaging modality. 1, 2, 3
Clinical Presentation
The classic triad consists of:
- Gait disturbance - appears first and is most prominent 1, 3
- Cognitive impairment - develops later in the disease course 1, 3
- Urinary incontinence - may be absent in early presentations 1, 3
Important caveat: The full triad is not present in all patients (25-50% have atypical or incomplete manifestations) and is not a prerequisite for pursuing treatment 4, 3. Symptoms develop gradually, with patients often presenting late in the disease course 1.
Diagnostic Imaging Criteria
First-Line: MRI Without IV Contrast
MRI without IV contrast is the preferred imaging modality because it can detect all critical NPH features with higher sensitivity than CT 1, 2.
Key MRI findings that establish NPH diagnosis:
- Ventriculomegaly with Evans index >0.3 (not entirely attributable to cerebral atrophy) 2, 5
- Narrowed posterior callosal angle (between 40° and 90°) 1, 2, 5
- Disproportionately enlarged subarachnoid-space hydrocephalus (DESH) pattern: tight high-convexity and medial subarachnoid spaces with enlarged Sylvian fissures 2, 5
- Rounded frontal horns with marked enlargement of temporal horns 2, 5
- Cerebral aqueduct flow void - strongly associated with good shunt response and cannot be seen on CT 2, 5
- Periventricular white matter hyperintensities - detected with higher sensitivity on MRI than CT 1, 2
- Corpus callosum thinning and elevation 5
Alternative: CT Head Without IV Contrast
CT without IV contrast is acceptable only when MRI is unavailable or contraindicated 1, 2. CT can identify ventriculomegaly, narrowed callosal angle, effaced sulci, and widened Sylvian fissures, but has critical limitations: it cannot detect aqueduct flow void and has lower sensitivity for periventricular white matter changes 1, 2.
Confirmatory Testing: CSF Tap Test
The CSF tap test is the only procedure that can temporarily simulate the effect of definitive shunt surgery and is essential for confirming shunt-responsiveness 4, 5, 3.
CSF Tap Test Protocol:
- Single tap has low sensitivity and cannot exclude patients from surgery if negative 4
- Repeated CSF tap test (RTT) or continuous lumbar external drainage (LED) of 150 mL/day for 3-5 days provides high sensitivity (50-100%) and high positive predictive value (80-100%) 4, 5
- Both objective and subjective improvements in symptoms (particularly gait) identify patients likely to benefit from shunt surgery 3
Critical point: CSF pressure is typically normal or only mildly elevated in NPH, and normal pressure should not exclude the diagnosis 4, 6.
Additional Diagnostic Considerations
Intracranial Pressure Monitoring
The most reliable prediction of shunt response occurs when RTT or LED proves positive AND B-waves are present during >50% of ICP recording time 4.
Other Imaging Modalities (Not First-Line)
- DTPA cisternography shows persistent radiotracer in lateral ventricles on delayed imaging, but evidence is insufficient to proceed with shunting based on cisternography alone 1, 2
- FDG-PET/CT may help distinguish NPH from other dementias but does not reliably demonstrate classic NPH features 1, 2
- Cine MRI showing increased ventricular flow rate has high positive predictive value for shunt responsiveness 2
Differential Diagnosis Challenges
75% of patients with NPH severe enough to require treatment also have another neurodegenerative disorder, making diagnosis complex 6. NPH overlaps significantly with:
- Alzheimer disease
- Vascular dementia
- Progressive supranuclear palsy
- Dementia with Lewy bodies 3
Comorbid cervical stenosis is common and may require separate surgical intervention 3.
Treatment Implications
Ventriculoperitoneal shunt placement is the treatment of choice, leading to clinical improvement in 70-90% of appropriately selected patients 6. Early diagnosis and treatment are critical, as untreated NPH leads to nursing care dependence in the vast majority of patients 6. NPH represents one of the few reversible causes of dementia, accounting for approximately 5% of dementia cases 7.