What is the pathophysiology of normal pressure hydrocephalus (NPH)?

Updated: Oct 19, 2018
  • Author: Michael J Schneck, MD, MBA; Chief Editor: Selim R Benbadis, MD  more...
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Answer

NPH differs from other causes of adult hydrocephalus. An increased subarachnoid space volume does not accompany increased ventricular volume. Clinical symptoms result from distortion of the central portion of the corona radiata by the distended ventricles. This may also lead to interstitial edema of the white matter and impaired blood flow, as suggested in nuclear imaging studies. The periventricular white matter anatomically includes the sacral motor fibers that innervate the legs and the bladder, thus explaining the abnormal gait and incontinence. Compression of the brainstem structures (ie, pedunculopontine nucleus) could also be responsible for gait dysfunction, particularly the freezing of gait that has been well described. Dementia results from distortion of the periventricular limbic system.

The term normal pressure hydrocephalus was based on the finding that all 3 patients reported by Hakim and Adams showed low CSF pressures at lumbar puncture, namely 150, 180, and 160 mm H2 O. However, an isolated CSF pressure measurement by lumbar puncture clearly yields a poor estimation of the real intracranial pressure (ICP) in patients with NPH.

Hakim first described the mechanism by which a normal or high-normal CSF pressure exerts its effects. Using the equation, Force = Pressure X Area, increased CSF pressure over an enlarged ependymal surface applies considerably more force against the brain than the same pressure in normal-sized ventricles. Normal pressure hydrocephalus may begin with a transient high-pressure hydrocephalus with subsequent ventricular enlargement. With further enlargement of the ventricles, CSF pressure returns to normal; thus the term NPH, at least in view of the initial pathophysiologic events, is a misnomer. Intermittent intracranial hypertension has been noted in some patients.

Some authors prefer the term extraventricular obstructive hydrocephalus. They believe that the initial event is diminished CSF absorption at the arachnoid villi. This obstruction to CSF flow leads to transient high-pressure hydrocephalus with subsequent ventricular enlargement. As the ventricles enlarge, CSF pressure returns to normal.


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