What is the role of oxybutynin (antimuscarinic) in treating overactive bladder?

From the Guidelines

Oxybutynin is a recommended treatment option for overactive bladder, as it has been shown to be effective in reducing symptoms and improving quality of life, according to the most recent guidelines from the American Urological Association and the Society of Urodynamics, Female Pelvic Medicine and Urogenital Reconstruction (AUA/SUFU) 1. The medication works by blocking acetylcholine receptors in the bladder muscle, reducing bladder contractions, increasing bladder capacity, and decreasing urinary urgency, frequency, and incontinence episodes.

• The typical starting dose is 5 mg taken orally two to three times daily, with extended-release formulations available at 5-30 mg once daily.
• Side effects commonly include dry mouth, constipation, blurred vision, and drowsiness due to the medication's anticholinergic properties.
• Oxybutynin works by specifically targeting M3 muscarinic receptors in the detrusor muscle, preventing involuntary contractions that cause overactive bladder symptoms.
• The medication begins working within 30-60 minutes of administration, with maximum effects seen after several weeks of consistent use.
• Patients should take oxybutynin as prescribed, stay hydrated while avoiding excessive fluid intake, and implement bladder training techniques alongside medication for optimal results.
• Oxybutynin should be used cautiously in elderly patients and those with glaucoma, gastrointestinal disorders, or cognitive impairment due to potential anticholinergic side effects, as highlighted in the AUA/SUFU guideline 1. The AUA/SUFU guideline emphasizes the importance of shared decision-making in selecting the best therapy for patients with overactive bladder, taking into account individual needs, desires, and side effect tolerance 1.
• The guideline also recommends a stepwise approach to treatment, starting with lifestyle modifications and behavioral therapies, followed by pharmacologic treatments, and finally, more invasive procedures if necessary.
• Oxybutynin is one of the pharmacologic treatment options available, and its use should be based on a thorough evaluation of the patient's symptoms, medical history, and potential risks and benefits, as recommended by the American College of Physicians 1.

From the FDA Drug Label

Oxybutynin chloride is indicated for the relief of symptoms of bladder instability associated with voiding in patients with uninhibited neurogenic or reflex neurogenic bladder (i.e., urgency, frequency, urinary leakage, urge incontinence, dysuria). Oxybutynin chloride exerts a direct antispasmodic effect on smooth muscle and inhibits the muscarinic action of acetylcholine on smooth muscle. Oxybutynin chloride relaxes bladder smooth muscle In patients with conditions characterized by involuntary bladder contractions, cystometric studies have demonstrated that oxybutynin chloride increases bladder (vesical) capacity, diminishes the frequency of uninhibited contractions of the detrusor muscle, and delays the initial desire to void

The role of oxybutynin (antimuscarinic) in treating overactive bladder is to:

• Relax bladder smooth muscle
• Increase bladder capacity
• Diminish the frequency of uninhibited contractions of the detrusor muscle
• Delay the initial desire to void 2 2

From the Research

Oxybutynin Mechanism and Effects

• Oxybutynin is a tertiary amine that undergoes extensive first-pass metabolism, with its active metabolite, N-desethyl oxybutynin, playing a significant role in its effects 3.
• It has antimuscarinic, direct muscle relaxant, and local anesthetic actions, with its antimuscarinic action being the primary mechanism for controlling overactive bladder (OAB) symptoms 3.
• Oxybutynin has been shown to increase bladder capacity and produce clinical improvement in patients with hypereflexia and other types of OAB, with few side effects when administered intravesically 3.

Efficacy and Safety of Oxybutynin

• Oxybutynin is effective in controlling OAB symptoms, with studies demonstrating its efficacy in reducing micturitions per 24 hours, incontinence episodes, and improving quality of life 3, 4.
• However, oxybutynin's antimuscarinic activity can lead to adverse effects, such as dry mouth, constipation, and somnolence, which can necessitate treatment discontinuation in up to 25% of patients 3, 5.
• Extended-release formulations of oxybutynin have been developed to minimize fluctuations in plasma concentrations and reduce the risk of adverse effects 5.

Comparison with Other Antimuscarinic Agents

• Oxybutynin has been compared to other antimuscarinic agents, such as tolterodine, solifenacin, and fesoterodine, in terms of efficacy and safety 4, 6.
• Tolterodine has been shown to have equivalent efficacy to oxybutynin, but with less severe adverse effects, particularly dry mouth 4, 6.
• Solifenacin and fesoterodine have also been shown to have superior efficacy to oxybutynin, but with a higher risk of adverse effects, such as dry mouth and withdrawal due to adverse events 6.

Transdermal Oxybutynin

• Transdermal oxybutynin has been developed as an alternative delivery system, which has been shown to be effective and well-tolerated in patients with OAB and urge or mixed urinary incontinence 7.
• Transdermal oxybutynin has been shown to reduce incontinence episodes, urinary frequency, and improve quality of life, with a lower incidence of dry mouth compared to oral oxybutynin 7.