Continuous urethral pressure measurement in women with unstable detrusor

To study the role of the urethra in bladder instability, we performed continuous urethrocystometry in 16 patients with verified unstable detrusor and found two qualitatively different pressure oscillations in the urethra: (1) continuous rhythmic pressure waves, characterized by frequency and pressure amplitude and (2) sudden and consistent urethral pressure drops of 35 cm H₂O occurring on average at mean time intervals of 3.5 seconds prior to the onset of the detrusor contraction. Simultaneous pelvic floor electromyographic (EMG) registration showed that the urethral pressure drop occurred either during decreased, unchanged, or increased pelvic floor activity. Restoration of the urethral pressure to its previous level was also unrelated to the pelvic floor EMG activity. Increased EMG activity at the onset of urethral pressure drops was not sufficient to suppress the urethral pressure drop, but could abolish the detrusor contraction. These findings suggest that the unstable detrusor is of a complex pathophysiological origin and involves both storage and micturition as well as detrusor and urethral function.