The two functions of the bladder are to store and void urine. The process of micturition involves neural circuits (afferent and efferent neural pathways and central and peripheral neurotransmitters) in the brain and spinal cord that coordinate the anatomic components of the lower urinary tract. However, the direct connection and contribution of these elements are not completely understood. 

As bladder volume increases, involuntary contractions of the detrusor muscle are often associated with overactive bladder—though such urodynamically demonstrable detrusor overactivity is not a prerequisite for a diagnosis of the condition. The mechanisms underlying these involuntary contractions have not yet been fully elucidated, although increasing attention is being focused on the role of sensory afferent nerves in the process.⁸ 

Overactive bladder has sometimes been referred to as the “hidden condition”¹ because, despite the impact on their lives, patients are often embarrassed by the condition and do not want to discuss it with their physicians.⁵ Many patients also consider overactive bladder to be a normal part of aging and may assume nothing can be done to alleviate their symptoms.⁹ 

To manage overactive bladder effectively, physicians must first encourage patients to speak openly with them. Given the prevalence of this condition, annual physical examinations or medical check-ups may be a good opportunity to discuss urinary problems with patients. A simple question such as “Are you bothered or worried by your urine control?” may be sufficient to initiate discussion. If a patient is concerned about his or her urine control, the physician should ask the patient to fill out a diary detailing the time, estimated volume of urine voided, and experience of each urination (ie, whether or not the urination was associated with urgency). Such a diary can be useful at the diagnosis stage to establish the extent and severity of any urinary problems. 

Achieving a differential diagnosis is a relatively straight-forward process that can be performed in the physician's office using a simple diagnostic algorithm for overactive bladder (Figure 1). For patients presenting with one or more symptoms of this condition (frequency, urgency, urge incontinence, or nocturia), a patient history should be obtained—including information about past genitourinary disorders—to exclude symptoms resulting from comorbid conditions or concomitant medications. 

A physical examination of the abdomen, rectum, pelvis, and genitalia should be performed to exclude obvious pathologies such as inflammation, prolapse, or masses. Urinalysis should be performed to exclude conditions that are sometimes associated with overactive bladder (eg, bacteria in the urine would suggest an infection, excess glucose in the urine could be a result of diabetes, and blood or protein in the urine may indicate kidney disease). In instances where urinalysis results suggest an associated condition, it may be necessary to refer the patient for additional diagnostic testing. For example, if a patient has unexplained hematuria, a urology referral for cytoscopy is recommended. In the absence of infection or other pathologies, including bladder cancers, a diagnosis of overactive bladder can be made. 

Urodynamic assessment can also be used in the diagnosis of overactive bladder, though the value of such assessments for this purpose is controversial. A recent report⁴ suggests that a positive urodynamic assessment for incontinence does not influence treatment success, indicating that these expensive, invasive, and complex tests should be reserved for patients with unusual symptoms or when initial therapy is unsuccessful.⁴ 

Treatment of overactive bladder can include nonpharmacologic interventions, pharmacologic interventions, or a combination of both.¹⁰ Approaching these options in consideration of the whole patient allows physicians to determine the best therapy. 

Nonpharmacologic interventions in the treatment of patients with overactive bladder usually involve “bladder retraining,” which generally consists of patient education, scheduled voiding, and urge-suppression techniques. Educating patients about bladder function is important in detering patients from using harmful coping behaviors, such as restricting fluid intake. Patients should also be alerted that consuming certain products (eg, alcohol, caffeine, spicy foods, and tomato-based products) may exacerbate their condition. In addition, scheduled voiding can help patients correct the habit of frequent urination and increase bladder capacity. Finally, urge suppression techniques such as pelvic floor muscle exercises (Kegel exercises), which involve tightening and relaxing the muscle around the rectum, and biofeedback-assisted training can strengthen and improve voluntary control of muscles. 

Within the primary care setting, bladder retraining can be labor-intensive and time-consuming for both the patient and the physician, and successful implementation requires considerable motivation and commitment on the part of the patient. Despite these obstacles, bladder retraining should always be considered in the management of overactive bladder, as it is a safe treatment option from which many patients can benefit.¹¹ Moreover, behavioral modifications that occur as a result of bladder retraining may enhance the outcomes obtained with pharmacologic interventions.⁹ 

While a range of drugs have been used in the past for the management of overactive bladder—from smooth muscle relaxants to tricyclic antidepressants—antimuscarinic agents now form the mainstay in the armamentarium used to control the symptoms associated with this condition. Conventional wisdom¹² holds that antimuscarinics act by blocking the muscarinic receptors on the detrusor muscle (which are normally stimulated by acetylcholine released from parasympathetic nerves) thereby diminishing the intensity of involuntary detrusor muscle contractions. Emerging research,¹² however, suggests that anti-muscarinic agents mainly act during the storage phase, decreasing urge and increasing bladder capacity, during which time there is normally no activity in the parasympathetic nerves. Studies^12-14 are now focusing on the ability of antimuscarinics to modulate afferent impulses as a means of effectiveness. 

Antimuscarinic agents differ at the structural and molecular level, resulting in different metabolism, absorption, potency, and selectivity profiles, as follows: 

An understanding of these differences—particularly in how they impact the efficacy and safety of the agents—is important and allows physicians to make informed decisions about the most suitable treatment option for their patients' needs.^15-20 A summary of these features is provided in Figure 2. 

There are currently five antimuscarinic agents available in the United States for the treatment of overactive bladder: oxybutynin, tolterodine tartrate, darifenacin, solifenacin (all tertiary amines), and trospium (a quaternary amine). The tertiary amines are predominantly metabolized by the cytochrome P-450 enzymes in the liver with little active compound being excreted in the urine. Of administered doses, <0.1%, <1%, and <15% of unchanged oxybutynin, tolterodine, and solifenacin, respectively, is excreted in the urine.^22-24 For darfenacin, 3% of the dose excreted in urine is unchanged.²⁵ By contrast, trospium (quaternary amine) is minimally metabolized in the liver, with the majority (60%) of the absorbed compound being excreted in its active form.²⁶ The different pharmacologic profiles of these antimuscarinic agents^22-27 may contribute to the differences in safety and efficacy seen in clinical practice studies (Table).^28-34 

Oxybutynin has been available for the treatment of overactive bladder for more than 30 years and is effective in a range of patients. It is available as immediate- and extended-release oral tablets (oxybutynin chloride) and as a transdermal patch.^30,35 The most commonly reported adverse event for extended-release oral oxybutynin is dry mouth, experienced by up to 68% of patients and causing some patients to discontinue therapy.³⁶ Transdermal oxybutynin results in a far lower incidence of dry mouth (<10%) but is associated with local skin reactions such as pruritis.³⁰ This lower incidence of dry mouth is believed to result from the lack of presystemic metabolism to form an active metabolite with similar properties to the parent compound (N-desethyl-oxybutynin). 

 

Certain attributes of oral oxybutynin, such as its relatively small size, highly lipophilic nature, and its neutral charge, make it more likely to cross the blood-brain barrier than other antimuscarinic agents available for the treatment of overactive bladder. Therefore, central anticholinergic effects, which could result in cognitive impairment, are a concern in patients (particularly elderly patients) taking oxybutynin.³⁷ 

Tolterodine, which is orally administered, has been available in the United States since 1998. Originally launched as an immediate-release formulation, tolterodine is now available as an extended-release formulation. Initiated and maintained at a dose of 4 mg once daily, tolterodine extended-release formulation is efficacious in the treatment of overactive bladder.³⁸ Tolterodine has comparable efficacy to oxybutynin, but with a reduced incidence of adverse events (eg, dry mouth).^37,39,40 Although preliminary reports^41,42 have drawn an association between tolterodine and impaired cognitive functioning, tolterodine is less lipophilic than oxybutynin and is therefore less likely to cross the blood-brain barrier.⁴³ 

Newer oral antimuscarinic agents darifenacin (7.5 mg and 15 mg) and solifenacin (5 mg and 10 mg) were approved for the treatment of overactive bladder and urge incontinence by the FDA in 2003 and 2004, respectively. Both agents are well-tolerated and substantially reduce urge incontinence episodes, frequency, and urgency, and increase the volume of urine voided.^44-47 The most commonly reported adverse events for both darifenacin and solifenacin are dry mouth and constipation. Darifenacin has little affinity for the M₁ muscarinic subtype receptor, a feature that is proposed to minimize cognitive effects that might arise if the compound were to cross the blood-brain barrier. Accordingly, adverse cognitive effects have not been reported for darifenacin.⁴⁸ Solifenacin is a nonselective muscarinic antagonist that binds to the M₁, M₂, and M₃ muscarinic receptors—though with slightly greater affinity for M₁ and M₃ receptors than M₂ receptors.⁴⁹ Animal studies^27,49 suggest that solifenacin has the highest degree of bladder selectivity compared with oxybutynin, tolterodine, and darifenacin. In a recent head-to-head trial,⁵⁰ a flexible dosing regimen with solifenacin was found to be superior to extended-release tolterodine for the majority of efficacy variables investigated. However, because this trial⁵⁰ was designed to show non-inferiority, these results should be viewed with caution. 

Trospium, an antimuscarinic agent approved by the FDA in 2004 for the treatment of overactive bladder, has been available in Europe for more than 20 years. The safety and efficacy of trospium have been demonstrated in clinical trials involving over 3000 patients in the United States and Europe and have been further demonstrated with postmarketing studies involving over 10,000 patients.¹³ Trospium is unique among the new antimuscarinic agents for several reasons: it has the least selectivity and the highest overall affinity for the muscarinic receptor subtypes; it is a positively charged quaternary amine, which prevents transfer across the blood-brain barrier; and it is minimally metabolized by cytochrome P-450 enzymes, with approximately 60% of the absorbed dose being excreted in the urine in active form. In addition, this agent demonstrates a rapid onset of effect, reducing frequency and urge incontinence within the first few days of treatment.^15,34 

Trospium also has comparable efficacy to oral oxybutynin and tolterodine, but with fewer adverse events.⁵¹ The lack of cognitive adverse effects may result from the inability of trospium to transfer across the blood-brain barrier. The lack of adverse drug-drug interactions is a result of the lack of metabolism by cytochrome P-450 enzymes. Evidence supporting the possible additional benefits of a compound that is active in the urine was provided by a recent study¹⁴ in which urine from individuals who had ingested a variety of antimuscarinic agents was injected into the bladders of rats. The results of the study¹⁴ showed that, in accordance with its activity in urine, trospium has a local inhibitory effect on detrusor overactivity. 

Overactive bladder is a widespread disorder that is distressing to patients and that is underdiagnosed and undertreated by physicians. The detrimental impact of this condition on patients' QOL makes it of paramount importance that physicians recognize and treat the symptoms associated with this troublesome condition. By recognizing the symptoms of overactive bladder, it is possible for physicians to diagnose and treat the condition within the primary care setting and without the need for complex urodynamic assessments. 

Nonpharmacologic treatment options, alone or in combination with drug therapy, are effective in treating patients with overactive bladder. The development of new antimuscarinic agents and new formulations of more traditional agents widens the options available for the treatment of this disorder. The differences in efficacy and tolerability profiles among available agents provide the opportunity for physicians to choose the form of therapy that best suits the needs of the individual patient. When treating elderly patients, for example, consideration should be given to the use of agents with low potential to cause cognitive adverse events, and low potential for interaction with concomitant medications.