<qa>  What is the total duration of the study?The COGNIShunt System is a proprietary device designed to increase the flow of cerebrospinal fluid CSF and improve clearance of putative neurotoxins from the CSF that are believed to contribute to the progression of Alzheimers disease symptoms. This clinical study is designed to determine if this device will stop or slow the progression of Alzheimers disease. The pivotal study is a prospective, randomized double-blinded, placebo-controlled trial to evaluate the effect of flow-regulated ventriculoperitoneal CSF drainage with the COGNIShunt system on cognitive and clinical function in approximately 250 participants with Alzheimers Disease NINDSADRDA criteria. Study participants will be permitted to continue anti-dementia drug therapy if their drug regime has been stable for 3 months prior to entry. This is a two-part study. In Part I, participants will be randomized to receive either a functioning COGNIShunt System testintervention group or an occluded shunt controlplacebo group. The duration of Part I is nine months, to be followed by an extension phase of an additional 9 months, constituting Part II. During Part II, subjects with occluded shunts have the opportunity to receive a functioning COGNIShunt, so that all study participants may have open devices during Part II. The total duration of the study is 18 months. Visits to the site include: for screening and baseline may be done in one or two visits; surgery; and a visit the 1st, 3rd, 6th, 9th, 12th, 15th, and 18th month after surgery. 