The aim of this study is to investigate whether there will be any efficacy difference of Memantine treatment in different cognitive severity AD patients stratified with BPSD

Planed to enroll a total of 222 patients with mild, moderate or severe AD treated with Memantine in 2017-2018 from 15 centers.

Inclusion Criteria:

1. Probable AD based on the diagnosis criteria of NIA-AA guideline (the U.S. National Institute on Aging and Alzheimer's Association guideline, McKhann et al, 2011[8];
2. Male or female, 50 years and older ;
3. All inpatients or outpatients recruited into this study must meet MMSE score ≤24 points, patients with MMSE score as 21-24 points will be enrolled into mild group, patients with MMSE score as 10-20 points will be enrolled into moderate group, patients with MMSE score less than 10 points will be enrolled into severe group.
4. Have a reliable caregiver who can accompany the patient to the site to complete the required study procedures;
5. Provide the informed consent.

Exclusion Criteria:

1. Patients with disturbance of consciousness of any causes;
2. Patients with severe aphasia or physical disability who were unable to complete neuropsychological examination;
3. Patients with mental illness;
4. Patients with a history of alcoholism and drug addiction, or traumatic brain injury, epilepsy, encephalitis, normal-pressure hydrocephalus and other neurological disorders causing cognitive impairment;
5. Patients with systemic diseases causing dementia (e.g. liver and kidney insufficiency, endocrine disorders, vitamin deficiency);
6. Patients with a history of ischemic or hemorrhagic cerebrovascular diseases, and vascular diseases (stroke foci) as confirmed by imaging examination;
7. Subjects who are unwilling or unable to abide by the study requirement.