Pathophysiology
Clinical meaning
Normal pressure hydrocephalus (NPH) is a communicating hydrocephalus characterized by ventricular enlargement with normal or intermittently elevated cerebrospinal fluid (CSF) pressure, producing the classic triad of gait disturbance, cognitive decline, and urinary incontinence. Understanding CSF dynamics is essential for diagnosing NPH and predicting treatment response. CSF is produced primarily by the choroid plexus in the lateral, third, and fourth ventricles at a rate of approximately 500 mL/day (20 mL/hour), with a total CSF volume of 120-150 mL turning over 3-4 times daily. CSF flows from the lateral ventricles through the foramina of Monro into the third ventricle, through the cerebral aqueduct into the fourth ventricle, and exits via the foramina of Luschka and Magendie into the subarachnoid space. Absorption occurs primarily at the arachnoid granulations (villi) that protrude into the dural venous sinuses, where CSF is reabsorbed into the venous circulation through pressure-dependent one-way valves. In NPH, CSF absorption is impaired at the arachnoid granulations due to fibrosis, scarring, or age-related degeneration. In secondary NPH, the cause is identifiable: subarachnoid hemorrhage, meningitis, or head trauma causes inflammatory debris to obstruct...