How to manage urinary incontinence caused by muscle relaxants, such as cyclobenzaprine (Flexeril)?

Managing Urinary Incontinence Caused by Cyclobenzaprine

Hold cyclobenzaprine on the day of operation and consider discontinuing it entirely if urinary incontinence develops, as its anticholinergic effects directly cause urinary retention and worsen incontinence symptoms. 1

Understanding the Mechanism of Cyclobenzaprine-Induced Incontinence

Cyclobenzaprine is structurally related to amitriptyline and acts as a centrally acting 5-HT2 receptor antagonist with significant peripheral and central anticholinergic activity. 1 The anticholinergic effects include:

• Urinary retention - a direct adverse effect that can lead to overflow incontinence 1
• Dry mouth, constipation, drowsiness, confusion, and hallucinations - all anticholinergic side effects that compound the urinary issues 1

The anticholinergic burden from cyclobenzaprine can paradoxically worsen both stress and urge incontinence by disrupting normal bladder function. 2

Immediate Management Strategy

Discontinue cyclobenzaprine if urinary incontinence develops or worsens during treatment. 1 If the patient has been on long-term therapy:

• Taper over 2-3 weeks to prevent withdrawal symptoms (malaise, nausea, headache lasting 2-4 days) 1
• These withdrawal symptoms are uncomfortable but not life-threatening 1
• Abrupt discontinuation should be avoided in chronic users 1

Treatment of the Underlying Incontinence

Once cyclobenzaprine is discontinued, address the incontinence based on type:

For Urgency/Urge Incontinence:

• Start with bladder training as first-line therapy (strong recommendation, moderate-quality evidence) 3, 4
• If bladder training fails, initiate tolterodine or darifenacin as first-line pharmacologic options due to their placebo-level discontinuation rates and superior tolerability 3, 4
• Avoid oxybutynin - it has the highest discontinuation rate due to adverse effects (NNTH 14-16) and causes significant cognitive impairment in elderly patients 3, 4
• Consider mirabegron (beta-3 agonist) as an alternative with lower anticholinergic side effects and reduced cognitive risk 3, 5

For Stress Incontinence:

• Do NOT use systemic pharmacologic therapy (strong recommendation) 3, 4
• Initiate pelvic floor muscle training (PFMT) as first-line treatment (strong recommendation, high-quality evidence) 3
• Alpha-adrenergic agonists (phenylpropanolamine) may provide modest benefit but have limited evidence 6, 7, 8

For Mixed Incontinence:

• Combine bladder training with PFMT 3, 5
• If pharmacotherapy needed, use tolterodine or darifenacin for the urgency component 3, 5
• Weight loss and exercise if obese (strong recommendation, moderate-quality evidence) 3, 4

Critical Pitfalls to Avoid

Do not substitute cyclobenzaprine with another anticholinergic muscle relaxant (like carisoprodol or other agents with anticholinergic properties), as this will perpetuate the urinary retention problem. 1, 2

Do not use antimuscarinic bladder medications while the patient is still on cyclobenzaprine, as this compounds anticholinergic burden and worsens urinary retention. 1, 2

Screen for polypharmacy - patients taking ≥7 medications have increased risk of adverse effects from any incontinence medication. 3 In these cases, prefer tolterodine, darifenacin, or mirabegron over other options. 3

Alternative Muscle Relaxant Options

If muscle relaxation is still needed after discontinuing cyclobenzaprine:

• Methocarbamol may be considered but has cardiovascular effects (bradycardia, hypotension) and should be avoided in myasthenia gravis 1
• Tizanidine can be used but requires slow tapering if discontinued due to withdrawal risk 1
• Avoid carisoprodol - it has substantial abuse potential, causes sedation and urinary issues, and requires controlled tapering over 4-9 days 1