Why CSF Pressure in Normal Pressure Hydrocephalus is Not Elevated
In normal pressure hydrocephalus (NPH), the CSF pressure is not consistently elevated because measurements are typically taken as single-point readings during lumbar puncture, which capture only a snapshot in time rather than the intermittent pressure elevations that likely occur throughout the day and night. 1, 2
The Paradox of "Normal" Pressure
The term "normal pressure hydrocephalus" is somewhat misleading, as it reflects a measurement artifact rather than true physiological reality:
Single-point measurements miss intermittent elevations: When CSF pressure is measured via lumbar puncture in the lateral decubitus position, it typically shows normal or only mildly elevated values (normal range 12-20 cm CSF or approximately 90-180 mm H₂O). 3, 1 However, this represents only a momentary assessment.
The mean basal intracranial pressure is normal or mildly elevated in NPH patients, distinguishing it from acute hydrocephalus where sustained high pressures are present. 1
Pressure dynamics are abnormal despite "normal" readings: The fundamental problem in NPH is not sustained high pressure but rather impaired CSF dynamics, including reduced CSF absorption at the arachnoid villi and altered compliance of the ventricular system. 4, 2
Pathophysiological Mechanisms
The underlying mechanisms explain why pressure readings appear normal:
Impaired CSF reabsorption without obstruction: NPH is a communicating hydrocephalus where CSF can flow freely through the ventricular system, but absorption is compromised. 4, 2 This differs from obstructive hydrocephalus where blockage causes sustained pressure elevation.
Compensatory mechanisms: Over time, the brain adapts to chronic ventricular enlargement through tissue compression and altered compliance, allowing pressure to normalize even as ventricles remain enlarged. 1, 2
Intermittent pressure waves: Continuous monitoring studies suggest that NPH patients experience transient pressure elevations (B-waves and plateau waves) that are not captured during brief lumbar puncture measurements. 2
Clinical Implications
Understanding this pressure paradox is critical for diagnosis and management:
Ventricular enlargement disproportionate to atrophy is the key diagnostic feature, not elevated pressure. The Evans index >0.3 on imaging is more reliable than pressure measurements. 4
Supplemental testing is essential: Because single pressure readings are unreliable, additional tests like CSF tap test (removing 30-50 mL of CSF and assessing clinical improvement) or continuous external lumbar drainage (150 mL/day for 3-5 days) better predict shunt responsiveness, with sensitivity of 50-100% and positive predictive value of 80-100%. 4
70-90% of appropriately selected NPH patients improve with ventriculoperitoneal shunt placement, despite having "normal" baseline pressures, confirming that the pressure measurement does not reflect the true pathophysiology. 1, 5
Common Pitfalls
Do not exclude NPH based solely on normal opening pressure: The diagnosis requires clinical triad (gait disturbance, cognitive impairment, urinary incontinence) plus characteristic imaging findings, regardless of measured CSF pressure. 1, 4
Distinguish from other causes of hydrocephalus: In cryptococcal or coccidioidal meningitis, elevated pressure (≥250 mm H₂O) is common and requires urgent intervention, representing a completely different pathophysiology than idiopathic NPH. 6
Recognize that 75% of NPH patients have coexisting neurodegenerative disease, making clinical assessment challenging and emphasizing the need for objective testing beyond pressure measurement. 1