Patients will be closely contacted and monitored by the research assistant by telephone. They will be followed up at out-patient clinic at 1 week, 2 weeks and 4 weeks after BoNT-A injections. After the first month after initial BoNT-A injection, patients will be followed up monthly until the therapeutic effect is gone. Videourodynamic study and UPP study will be performed at 4 weeks after BoNT-A injections. The subjective improvement of voiding condition will be assessed by the obstructive symptom scores (including hesitancy, intermittency, dysuria, small caliber of urine) and quality of life score which are adopted from the International Prostate Symptom Score (IPSS) system.