Themicturition reflex should result in a few ml left in the bladder. The main exceptions to this are in the case of neurological abnormality where the co-ordination is lost (detrusor-sphincter dyssynergia) or benign prostatic hyperplasia (BPH), the swelling of the prostate gland with age in the male leading to BOO. The urethra passes through the prostate and thus swelling of the prostate can impede flow. Although the detrusor may contract and the sphincter relax, the obstructive resistance increase due to prostate may be too great for adequate emptying of the bladder. BPH has an incidence of approximately 30 % in the age range 50-60.
Although presenting symptoms - frequency of micturition and nocturia (nighttime passing of urine) - are initially only inconveniences - unless the patient is a long distance lorry driver- the long term complications of untreated bladder output obstruction include urinary retention which requires hospitalisation and ureteric reflux leading to hydronephrosis and possibly kidney failure. It is therefore important that obstruction be correctly diagnosed and treated. Potentially an enormous number of surgical procedures is involved. Patients are initially referred because of their symptoms but ultimately these are a notoriously unreliable guide to bladder output obstruction. ( See scatter graphs from SGH flow clinic -appendix 1).
The ideal investigation for BPH is the pressure flow study (Abrams &Griffiths 1979), where simultaneous flow and pressure measurements can be used to accurately diagnose bladder output obstruction. However the pressure flow is invasive, with either a urethral or suprapubic catheter being inserted. The freeflow investigation is therefore the method of choice for most routine patients. It is totally non-invasive. According to Turner Warwick -
"It has to be questionable whether surgeons who do not use uroflowmetry should advise and practise obstruction relieving operations - the clinical situation is almost comparable to treating hypertension without using a sphygmomanometer"
However the interpretation of the flow study is not without its pitfalls. Various artefacts can give rise to an artificially high value for Qmax. Abdominal straining for example or waggling of the stream. Further some effort has to be made to encourage patients to void a large enough volume. Flow curves from small volumes are unreliable as the flow mechanism is never seen at full power.
In clinical practice,uroflowmetry has been found to offer reasonable sensitivity and specificity if carried out correctly.
The definitive way to diagnose bladder outlet obstruction is by apressure flow study.