Hydrocephalus secondary to blockage of the aqueduct of Sylvius or the fourth ventricle outlets usually develops within a few weeks. Scarring of the arachnoid granulations or basal subarachnoid cisterns results in hydrocephalus after a few months. Scarring of the subarachnoid space and parenchymal changes are responsible. Meningitis causes NPH by scarring the subarachnoid cisterns, blocking flow of CSF.Ependymitis leads to aqueductal and scar based narrowing. Carcinomatosis of the meninges also blocks extraventricular circulation.
Many patients have none of the above predisposing conditions listed above.
The pathophysiology of INPH is still undecided. There is a frequent co- ocurrence of INPH and vascular subcortical encephalopathy. Frequent and severe periventricular lesions and DWMIs were noted in INPH but no direct causal relationship is supported. Krauss postulated a parenchymal compliance problem. Vascular lesions reduce elastic properties of cerebral white matter and alter tissue strength.