Intervention: Mirabegron
Trial phase: Phase 3
ATC code: G04BD12
SMILES: Nc1nc(CC(=O)Nc2ccc(CCNC[C@H](O)c3ccccc3)cc2)cs1
Sex: All
Age groups: Child
Healthy volunteers: No
Participants at risk: 35
Group description: Experimental: Add-on Mirabegron Patients without symptom improvement or with partial response under intensive behavioural protocol and medical therapy (at least 2 different antimuscarinic agents) will be recruited. They will keep the antimuscarinic and Mirabegron will be added (dual therapy).

Mirabegron: Patients will keep the antimuscarinic providing some efficacy and that is tolerated and Mirabegron will be added (dual therapy).
Eligibility criteria:
Inclusion Criteria:

Male or female ≥ 5 years old and ≤17 years old
OAB diagnostic according to the International Children Continence Society (ICCS) and less than 65% of the expected mean bladder capacity for age is confirmed (30 + (age in years x 30) mL) on a 3-day voiding diary.
Weight and height are within the normal percentile (3rd to 97th percentile) and weight is ≥ 20 kg (3rd percentile of a 8 y.o. child, boy or girl), according to the CDC growth chart
Ability to swallow pills
Subjects/parents (vs. legal guardian) agree to participate to the following study and sign the informed consent
Subjects/parents (vs. legal guardian) are able to comply with the study requirements and with the medication restrictions.
Female subjects of childbearing potential must have a negative serum or urine pregnancy test at enrollment and must agree to maintain highly effective birth control during the study. Sexually active male subjects agree to use a barrier method of birth control with female partner for the duration of the study and at least one month after ending study treatment. Sexually active male subjects agree to use a condom for the duration of the study and for at least one month after ending study treatment and the female partner to use a reliable form of birth control for the duration of the study and for at least one month after ending study treatment.
Patients without symptom improvement or with partial response under medical therapy (at least 2 different antimuscarinic agents).

Exclusion Criteria:

Subject has a diagnostic of dysfunctional voiding
Post-voiding residue > 20 cc
Polyuria (> 75 ml/kg/b.w./24 hours)
Nephrogenic of central diabetes insipidus
Constipation at screening (if the patient is treated and the treatment is successful, the patient will be eligible to the study)
Urinary tract infection at visit 2-3-4. If UTI is present at the screening visit, the UTI must be treated and the success of the treatment must be documented with a negative urinalysis at visit 2.
QTc interval greater than 460 ms, or any increase of 30 ms on follow-up EKG (mean of 6 separate EKG-3 from visit week-2 and 3 from visit week 0). If a patient meets those criteria in the first month (initial dose), he will be excluded from the study. If the QTc change is noted after the up-titration, the dose will be decreased and EKG will be repeated within 1 week to ensure normalization of QTc.
Clinically significant unstable medical condition or disorder
Subject is pregnant or intends to become pregnant
Serum creatinin more than or equal to 2 times the upper limit of normal
Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) more than or equal to 2 times ULN, or bilirubin more than or equal to 1.5 times ULN.
Known hypersensitivity to mirabegron or any contraindication to the use of the molecule, in accordance to the product monography (to the exception of pediatric age).
Subject is taking medication that interact with mirabegron and this medication can't be discontinued (see appendix 1 of excluded drugs)
Known urological pathology other than OAB that could explain urinary symptoms (as bladder stone…)
Non-treated or non-controlled arterial hypertension