* Introduction:Overactive bladder (OAB) is a symptom syndrome characterized by urgency frequency with or without urgency incontinence. Usually no metabolic or anatomical disorders can be found in patients with OAB, and the condition may have a great impact on quality of life. Antimuscarinics are the first line treatment and yield a success rate of more than 70%. Urothelial dysfunction and abnormalities of sensory receptor expression or transmitter release in the suburothelial nerves might contribute to OAB which is refractory to antimuscarinics. In patients who failed current antimuscarinic treatment, intravesical botulinum toxin A (BoNT-A) injection provides an chance of improvement. Previous studies reported success rates of BoNT-A injection for OAB ranged from 60 to 80%. Intravesical treatment to inhibit abnormal receptor expression or transmitter release in the sensory nerve terminals of the suburothelial space can provide good therapeutic effects in the treatment of OAB. However, patients might develop large postvoid residual (PVR) and subsequent urinary tract infection (UTI) after BoNT-A injections, therefore, this treatment is usually left for patients who are refractory to antimuscarinic therapy. However, how to define antimuscarinic refractory OAB remains controversial. How many different types of antimuscarinics should be prescribed before we call the case failure has not been elucidated. The aim of this study is to evaluate the effect of flexibly adding-on oxybutynin ER (5mg QD) in patients with OAB who were refractory to monotherapy with the first antimuscarinic agent (tolterodine 4mg QD).
* Methods: A total of 200 patients with refractory OAB will be included in this prospective, open label protocol. Inclusion criteria are persistent OAB symptoms (frequency urgency with/without urgency urinary incontinence) after behaviour therapy and an optimized dose of one antimuscarinic agent (Tolterodine 4mg) for at least 3 months. Patients with neurogenic diseases, untreated bladder outlet obstruction, recurrent UTI, large PVR (\>150ml) will be excluded from this study. Patients will be randomized to allocate in the treatment group (receiving tolterodine 4mg QD and oxybutynin 5mg to 15mg QD) or control group (tolterodine 4mg QD) in 1:1 ratio based on the permuted block randomization code. Oxybutynin ER 5mg to 15 mg once daily will be flexibly adding-on from baseline to the third month in the treatment group, depending on patient's effectiveness and tolerability to adverse events. At the baseline, 1 and 3 months after oxybutynin ER adding-on, we will assess the International Prostate Symptom Score (IPSS), Overactive Bladder Symptom Score (OABSS), Patient Perception of Bladder Condition (PPBC), the Urgency Severity Scale (USS) questionnaires, uroflowmetry and PVR. The therapeutic effect will be considered as successful if there was a reduction of PPBC of 2 from baseline and a reduction of USS of 1 from baseline, or the USS value is 0 at 3 months. The adverse events and tolerability of this combined therapy will also be assessed.
* Expected Results: Compared with baseline, total IPSS, IPSS storage subscore, quality of life indexes, OABSS, USS and PPBC will be expected to significantly decrease at 1 and 3 months. PVR might be increased at 3 months after adding-on treatment. The changes of IPSS voiding subscore, peak urinary flow rate and voided volume might be increased or comparable to the control group during the follow-up. We expect at least one-third of patients can have a successful therapeutic effect without significantly increased adverse events. However, the other patients might withdraw from the study due to adverse events such as severe dry mouth, constipation, large PVR, UTI, severe difficult urination or acute urinary retention.

Adverse events should be cautiously monitored during the treatment course.