Bladder outflow
obstruction
This is a urodynamic concept based on the
combination of low flow rates in the presence of high voiding pressures. It can
be diagnosed definitively only by pressure—flow studies. This is because
symptoms are relatively nonspecific and can result from detrusor instability,
neurological dysfunction and weak bladder contraction. Even low measured peak
flow rates (<10—12 ml/second) are not absolutely diagnostic because in
addition to BOO, weak detrusor contractions or low voided volumes (owing to
instability) can be the cause..
Urodynamically proven bladder outlet obstruction may result from:
• BPH
• bladder neck stenosis
• bladder neck hypertrophy
• prostate cancer
• urethral strictures
• functional obstruction due to neuropathic conditions
Primary effects of BOO on the bladder
• Urinary flow rates decrease [for a voided volume >200 ml; a peak flow rate of >15 mI/second is normal (Fig. 66.6), one of 10—15 mI/second equivocal and one <10 mI/second low (Fig. 66.7)
• Voiding pressures increase [pressures >80 cmH2O are high (Fig. 66.8), pressures between 60 and 80 cmH2O are equivocal, pressures <60 cmH2O are normal]
The long-term effects of bladder outflow
obstruction are as follows.
1.
The bladder may decompensate so that detrusor contraction becomes
progressively less efficient and a residual urine develops.
2.
The bladder may become more irritable during filling with a decrease in
functional capacity partly caused by detrusor instability (see Chapter 65)
which may also be caused by neurological dysfunction or ageing, or be
idiopathic.
Aside
from symptoms, the complications of BOO are as follows.
1.
Acute retention of urine is
sometimes the first symptom of BOO. Postponement of micturition is a common
precipitating cause; overindulgence in beer, confinement to bed on account of
intercurrent illness or operation are other causes.
2.
Chronic retention. In patients
in whom the residual volume is >250 ml or so (Fig.
66.9), the tension in the
bladder wall increases owing to the combination of a large volume of residual
urine and increased resting and filling bladder pressures (a condition known as
high-pressure chronic retention. The increased intramural tension results in
functional obstruction of the upper urinary tract with the development of
bilateral hydronephrosis (Figs 66.10 and 66.11). As a result, upper tract
infection and renal impairment may develop. Such men may present with overflow
incontinence, enuresis and renal insufficiency. These symptoms should alert
the doctor to the presence of this condition.
3.
Impaired bladder emptying. If
the bladder decompensates with the development of a large volume of residual
urine, urinary infection and calculi are prone to develop.
4.
Haematuria. This may be a
complication of BPH. Other causes must be excluded by carrying out an IVU,
cystoscopy, urine culture and urine cytological examination.
5. Pain is not a symptom of BOO and its presence should prompt the exclusion of
acute retention, urinary infection, stones, carcinoma of the prostate and
carcinoma in situ of the bladder.