Non-invasive assessment of urethral resistance

Urine, produced in the kidneys, is temporarily stored in the urinary bladder. Under appropriate circumstances, the bladder muscle (detrusor) contracts and expels the urine through the bladder neck and urethra. The course of voiding mainly depends on the contractility of the detrusor and the urethral resistance. Upon stimulation initiated by the central nerve system, the detrusor shortens which increases the pressure in the bladder. At the same time, the muscle fibres in the bladder neck and in the urethral wall relax which reduces the urethral resistance. When the pressure in the bladder is higher than the pressure in the urethra, urine pushes the urethra open and voiding starts. When the bladder is empty, the detrusor contraction stops, the urethra closes and the kidneys refill the bladder. In men, the prostate enlarges with increasing age, mostly as a result of benign prostatic hyperplasia (BPH). As the prostate surrounds the urethra, and may thus obstruct it, a poor urinary flow (or flow rate) and frequent voiding are typical symptoms relating to BPH. In general, these symptoms are called Lower Urinary Tract Symptoms (LUTS). At present, 25% of the Dutch men aged 40 and above are known to have these symptoms. In men older than 65 years, this percentage is increased to 33%. In the Netherlands only, the population of interest approximates 1 million. This number will increase as the population is ageing. In some cases, a poor flow rate is caused by a weakly contracting bladder and not by an enlarged prostate. In these patients, the efficacy of a surgical procedure aimed at the relief of an obstruction is doubtful. To test if a low flow rate results from an increased urethral resistance, the pressure in the bladder during voiding needs to be measured. A high bladder pressure combined with a low flow rate indicates that a patient has an obstructed urethra. A low bladder pressure combined with a low flow rate indicates that a patient has a weakly contracting detrusor.

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