Patients will be recruited from the outpatient clinic of the study hospital. We will hold information campaigns to encourage referrals once the clinical trial procedure is set. Patients will be assessed for eligibility based on the criteria detailed below. Written informed consent will be obtained from eligible subjects or the subjects' parents if they are younger than the age of 18 years. Baseline clinical and neuropsychological assessments will be done at first. Patients will receive a single dose of memantine 5 mg to test if any allergic reactions to those who have never used it before. And then they will be randomized into 3 groups: the first group receives a target dose of memantine 10 mg/day, the second group receives a target dose of memantine 20 mg/day, and the third is a placebo control group. Both the participants and the clinicians are blinded to the agents and dosage they are taking. The dose titrating schedule for medication groups will be 5 mg/day for the first week with an increment of 5 mg per week to reach their designated targeted dose. So the10 mg add-on group will reach their target dose by the beginning of the second week and the 20 mg/day add-on group will reach their target dose by the beginning of the fourth week. Participants will be scheduled to return visit on week 1, 2, 4, 8, and 12 for dispense of medication and clinical assessments. By the end of the 12-week trial, they will receive all clinical and neuropsychological assessments again.