Mechanisms of continence and voiding, with international continence society classification of dysfunction.

The two functions of the lower urinary tract are storage of urine within the bladder and timely expulsion of urine from the urethra. Bladder filling occurs with little or no increase in intravesical pressure, despite large increases in urine volume. This process, called accommodation, results from viscoelastic properties of the bladder wall and reflex inhibition of detrusor contractility. Urinary continence is maintained at rest by outflow resistance generated by the urethral sphincteric mechanism. With increases in intra-abdominal pressure, urethral resistance is augmented by passive pressure transmission to the bladder neck and proximal urethra and possibly by reflex muscular activity within the urogenital diaphragm. Normal voiding is a reflex act under voluntary control that involves relaxation of the urethra and sustained contraction of the bladder until emptying is complete. The ICS has classified lower urinary tract dysfunction as disorders of the filling or storage phase and as disorders of the emptying phase. Although urodynamic testing is not always necessary, some evaluation of both bladder and urethral function in each phase is required for accurate diagnosis.