Alpha-synuclein in Cerebrospinal Fluid to Differentiate Alzheimer's Disease From Lewy Body Disease.

Inclusion Criteria: * At least 45 years of age * Subject agrees to participate in the study * Subject meets the criteria Dubois et al., 2007. for diagnosis of probable AD. * Subject with a syndromic presentation of mild cognitive impairment or mild dementia * Subject has a study partner who is able to provide accurate information about the patient * Subject affiliated to a social security scheme Exclusion Criteria: * Patient who meets both AD and Lewy Body disease criteria * Contraindications to lumbar puncture * Contraindications to an MRI scan * Subject who is not affiliated to a social security scheme * Subject under guardianship or curatorship * Subject under judicial protection * Subject refuses to participate in the study * Subject has evidence of clinically relevant neurological disorder (fronto-temporal dementia, brain tumor, stroke, …) that led to persistent cognitive disorders * Subject has evidence of clinically relevant psychiatric disorders (schizophrenia, major depression, …) * Pregnant or attempting to become pregnant women * Subject's vision and audition is sufficient for neuropsychological tests's assessment, based on the investigator's judgment.