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The recently updated Japanese guidelines draw attention to a specific MRI pattern of disproportionately enlarged subarachnoid space hydrocephalus (DESH), believed to be pathognomonic of idiopathic normal pressure hydrocephalus (iNPH). This chapter discusses why establishing the diagnosis of NPH remains a challenge fifty years after its classic description. The original diagnosis of NPH relied upon the presence of mild dementia, gait, and urinary difficulties (Hakim's triad) seen in association with ventriculomegaly on pneumo-encephalogram. More sensitive cognitive evaluation of iNPH patients requires specific tests for the assessment of subcortical frontal lobe deficits such as the Rey Auditory Verbal Learning Test, Stroop test, Grooved Pegboard, Trail Making A and B Test, and digit span test. This diagnostic test provides information about cerebrospinal fluid (CSF) dynamics and predicts outcome. It consists in either removal of CSF accompanied by pre and post functional evaluation, or an infusion (bolus or continuous) test.
Structural and functional brain imaging have helped to elucidate the neural pathways involved in hydrocephalic cognitive impairment. In addition, studies of brain metabolism and blood flow, molecular imaging, and cerebrospinal fluid (CSF) physiology have provided novel windows into the pathogenesis of dementia in idiopathic normal pressure hydrocephalus (iNPH). A number of pathophysiologic mechanisms have been identified that are potentially relevant to the pathogenesis of the cognitive symptoms of iNPH, namely, mechanical distortion, pressure effects, and cerebrovascular compromise. A possible synthesis of these mechanisms would be that an imbalance of CSF production and clearance leads to progressive ventricular enlargement. The profile of cognitive impairments in iNPH is recognizably that of a subcortical pathological process. Deficiencies in attention, working memory, set shifting, response inhibition and other aspects of executive functioning are commonly observed in iNPH and can be seen early in the disease course.