Causes of Overactive Bladder (OAB)
Overactive bladder is caused by multiple mechanisms including detrusor muscle instability with spontaneous contractions (myogenic), neurological dysfunction affecting bladder control (neurogenic), bladder outlet obstruction, and idiopathic causes where no clear etiology is identified. 1
Primary Pathophysiologic Mechanisms
Myogenic Causes
- Detrusor muscle instability leading to spontaneous, involuntary contractions is a fundamental myogenic cause of OAB. 1 This represents intrinsic bladder muscle dysfunction where the detrusor contracts inappropriately during the storage phase. 2
Neurogenic Causes
- Neurological diseases or injuries affecting central or peripheral nervous system control of the bladder contribute to OAB. 1 This includes conditions like multiple sclerosis, spinal cord injury, Parkinson's disease, and stroke. 3, 4
- The emergence of new voiding reflexes following neurological injury can trigger overactive bladder symptoms. 2
- Neurogenic detrusor overactivity (NDO) is frequently observed in patients with MS (affecting approximately 50% of patients) and most SCI patients develop some bladder dysfunction. 3
Obstructive Causes
- Bladder outlet obstruction, particularly in men with prostatic enlargement, can contribute to OAB symptoms. 1 The bladder muscle hypertrophies and becomes unstable in response to chronic obstruction. 2
Mixed Pathology in Elderly Patients
- Detrusor hyperactivity combined with impaired contractility commonly occurs in elderly patients, representing a complex mixed etiology. 2
Urethral Dysfunction
- Urethral weakness can contribute to the symptom complex of OAB. 2
Idiopathic OAB
- When no identifiable neurological, obstructive, or other pathological cause is found after appropriate evaluation, the diagnosis is idiopathic OAB. 5, 2 This represents the majority of OAB cases in clinical practice and is a diagnosis of exclusion. 5, 6
Essential Conditions to Rule Out
Before diagnosing OAB, you must exclude:
- Urinary tract infection (UTI) through urinalysis 1, 7
- Hematuria not associated with infection 1
- Nocturnal polyuria (characterized by normal or large volume nocturnal voids, unlike the small volume voids in OAB) 1
- Medication side effects that may affect bladder function 1, 8
- Neurological disorders through targeted history and examination 1
Common Clinical Pitfall
A critical error is misdiagnosing overflow incontinence as OAB, which leads to inappropriate antimuscarinic treatment that worsens urinary retention. 7 Always measure post-void residual (PVR) in patients with obstructive symptoms, history of incontinence or prostatic surgery, and neurologic diagnoses. 1 Antimuscarinics should be avoided or used with extreme caution when PVR exceeds 250-300 mL. 7, 8
Multifactorial Nature
- The causes of OAB are multifactorial and not completely understood, with neurogenic and myogenic bladder dysfunction representing the primary mechanisms leading to urgency, frequency, and urgency urinary incontinence. 9
- The ubiquity of lower urinary tract symptoms across various bladder disorders suggests common underlying mechanisms, though different causes may produce similar symptoms through distinct pathways. 2