Myrbetriq (Mirabegron) is NOT Indicated for Urinary Retention
Myrbetriq (mirabegron) should not be used for chronic non-neurogenic urinary retention in an elderly female patient, as it is indicated exclusively for overactive bladder (OAB) with urgency symptoms, not for retention. In fact, mirabegron relaxes the detrusor muscle during bladder filling, which could theoretically worsen urinary retention by further impairing bladder emptying 1.
Critical Distinction: Retention vs. Overactive Bladder
Urinary retention involves incomplete bladder emptying with elevated post-void residual volumes, requiring interventions that promote bladder contraction and outlet relaxation 2
Overactive bladder (OAB) involves urgency, frequency, and urgency incontinence due to detrusor overactivity during filling—the exact opposite pathophysiology 1
Mirabegron is a β3-adrenoreceptor agonist that increases bladder capacity by promoting detrusor relaxation during the storage phase, making it effective for OAB but potentially harmful in retention 3, 4
When Mirabegron IS Appropriate (Not Your Patient)
If your patient actually has overactive bladder rather than retention, the treatment algorithm would be:
First-Line: Behavioral Interventions
- Bladder training for urgency symptoms (strong recommendation, moderate-quality evidence) 1
- Pelvic floor muscle training combined with bladder training for mixed incontinence 1
Second-Line: Pharmacologic Management
Mirabegron 25 mg daily is particularly appropriate for elderly patients (≥65 years) with multiple comorbidities, producing significant improvements in voiding symptoms with an acceptable adverse event rate of 24.62% 5, 6, 7
The 25 mg dose is specifically recommended by the American Urological Association for older adults, with option to increase to 50 mg after 4-8 weeks if needed and tolerated 5, 6, 7
Mirabegron offers advantages over antimuscarinics in elderly patients by avoiding anticholinergic effects such as dry mouth, constipation, and cognitive impairment 5
Combination Therapy (If Monotherapy Fails)
- Adding solifenacin 5 mg to mirabegron 50 mg provides superior efficacy compared to either drug alone, with effect sizes (0.65-0.95) exceeding monotherapy (0.36-0.56) 6, 8
Critical Safety Monitoring for Elderly Patients
Monitor blood pressure periodically, especially during initial treatment, as mirabegron causes dose-dependent blood pressure increases 5, 6
Mirabegron is contraindicated in severe uncontrolled hypertension 5
Use extreme caution in frail elderly patients with mobility deficits, weight loss, weakness, or cognitive deficits, as the therapeutic index is lower 1
Most common adverse events include hypertension, urinary tract infections, headache, and nasopharyngitis 5
Management of Actual Urinary Retention
For chronic non-neurogenic urinary retention in an elderly female:
Identify and discontinue medications that impair bladder emptying (anticholinergics, opioids, alpha-agonists, calcium channel blockers) 2
Consider intermittent catheterization if post-void residual is significantly elevated 2
Refer to urology for evaluation of anatomic obstruction or neurogenic causes 2
Avoid all medications that relax the detrusor muscle, including mirabegron and antimuscarinics 2
Common Pitfall to Avoid
The most critical error would be prescribing mirabegron for urinary retention based on confusion between retention and OAB. These are opposite conditions requiring opposite therapeutic approaches. Always measure post-void residual volume to distinguish between incomplete emptying (retention) and overactive detrusor (OAB) before initiating any bladder medication 1.