Reversal of normal pressure hydrocephalus symptoms by subdural collections

The pathogenesis of symptoms in normal pressure hydrocephalus (NPH) is unclear. One theory is that in the presence of enlarged lateral ventricles, the ventricular fluid exerts increased force on the ventricular walls, the cerebrospinal fluid pressure remaining normal. This is in accordance with Pascal's principle of F = PxA. It has not been possible to obtain direct evidence for this view.

The opportunity to gain insight into the matter arose when it was observed that two patients with symptomatic NPH were relieved of their symptoms when they developed bilateral subdural collections, one after head trauma, the other after ventriculo-peritoneal shunting for NPH.

In each case, concomitant with the subdural collections and the reversal of symptoms, the ventricular system became smaller. In the first patient, symptoms returned when the subdural collections were resorbed. In the second patient, symptoms returned when the subdural collections became excessive.

Although documentation was less thorough than desired, it was concluded that relief of symptoms was related to the decrease in the ventricular size rather than lowering of the intraventricular pressure, thus providing evidence for operation of the principle F = PxA as the mechanism of symptoms in NPH.