Urethral Instability

EDITORIAL COMMENT: In an intact lower urinary tract continence is maintained by a positive pressure gradient between urethra and bladder. Resting pressure is greatest in the midurethra where the striated muscle fibres of the external urethral sphincter are most prevalent. Mean maximum urethral closure pressure declines with age from 75 cm H 2 O (21–30 yrs) to 35 cm H 2 O (> 70 yrs) and reflects the loss of urethral striated muscle and vascularity which occurs with ageing (A). In healthy continent females midurethral pressure is constant at rest and with bladder filling, and any rise in abdominal pressure is transmitted equally to both urethra and bladder. Vascular pulsations synchronous with the heart beat are present in younger women (up to 25 cm H 2 0) but gradually diminish in size until the postmenopausal period when they seldom exceed 5 cm H 2 O. During normal micturition there is a fall in urethral pressure with urethral opening followed by a rise in detrusor pressure. This pattern also occurs in many women with detrusor instability (figure 1A). Less commonly urethral relaxation with total loss of closure pressure results in urinary incontinence in the absence of any detrusor activity. Urethral pressure variations as described in this paper are well known to urodynamic investigators who measure urethral pressure with profilometry and during bladder filling. Care needs to be taken to remove any artifact as a cause of these fluctuations such as movement of the pressure sensor within the urethra or levator ani contraction. Nevertheless these fluctuations do occur in a number of women investigated for urinary symptoms as well as in asymptomatic females. This finding in healthy asymptomatic females does not preclude a clinical significance, as detrusor instability has been reported in up to 10% of asymptomatic females (B). In the past the bladder has been the main focus for overactivity of the lower urinary tract (detrusor instability) but it should be remembered that the bladder and urethra act as afunctional unit and need equal attention. It seems prudent at the present time that stress incontinence surgery be performed only in the presence of significant urethral pressure fluctuations when stress incontinence (leakage synchronous with cough in the absence of detrusor activity) is clearly demonstrable and conservative treatment has failed. A. Dwyer PL, Glenning PP. Anatomy and neurology of the lower urinary tract, Curr Opin Obstet Gynecol 1990; 2: 573–579. B. Turner‐Warwick R. Communication to the Eight International Continence Society Meeting, Manchester, 1978. Peter Dwyer, Mercy Hospital for Women and Royal Women's Hospital, Melbourne Summary: Urethral instability is still evolving as a clinical entity. Using pressure variation of 15 cm water or more at the point of maximum urethral pressure (MUP), urethral pressure profilometry on patients referred for urodynamic assessment for lower urinary tract symptoms revealed urethral instability in 6.4% of 608 patients. The close association between urethral and detrusor instability was noted. Urethral instability appears to be a cause of frequency and urgency of micturition, and its presence increases the risk of urinary incontinence.