In normal pressure hydrocephalus (NPH), CSF pressure is paradoxically normal. The mechanism causing ventriculomegaly despite normal CSF pressure is:

• A Increased CSF production by hyperactive choroid plexus
• B Impaired CSF reabsorption at arachnoid granulations causing gradual ventricular enlargement ✓
• C Progressive brain atrophy creating space that CSF fills passively (ex vacuo dilatation)
• D Increased venous pulsatility transmitting into ventricles causing slow erosion

Explanation

In NPH, the primary defect is impaired CSF reabsorption at the arachnoid granulations (from prior SAH, meningitis, or idiopathic fibrosis). Reduced outflow causes a subtle, transient pressure rise insufficient to sustain classic high-pressure hydrocephalus but sufficient over time to enlarge ventricles. By LaPlace's law, once ventricles enlarge, the same small pressure difference can move larger volumes of CSF, maintaining enlargement at normal mean pressure. The clinical triad is 'wet, wobbly, wacky' (incontinence, ataxia, dementia).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.