Flow Rate and Post-void Residual

212

Practical Urology: EssEntial PrinciPlEs and PracticE

high prevalence of benign prostate enlargement

2. An alternative nomogram has been proposed

in older men, and the availability of transure-

by a different group.39

thral resection surgery to relieve obstruction.

3. The BOOI is applicable in men with benign

This provided suitable circumstances to derive

prostate enlargement and no relevant neuro-

urodynamic nomograms28; the nomogram of

logical problem. Nonetheless, the parameters

Abrams and Griffiths was adopted and adapted

of the BOOI have been evaluated in women,

by the International Continence Society as a

such that the BOOI evaluated in conjunction

standardized criterion for diagnosing male

BOO. Thus a plot of Qmax

against simultaneous

with Qmax could be a usable urodynamic

detrusor pressure (P

Q

) allows derivation of

parameter of female BOO.40 A separate study

det

max

the bladder outlet obstruction index (BOOI),

came to similar conclusions, but cut-off val-

from which a definite diagnosis of BOO, equivo-

ues were different.41

cal BOO or unobstructed can be made, as illus-

4. An alternative compared women with ana-

trated in Fig. 16.1. The concept superseded the

tomical BOO against women with stress uri-

earlier concept of a urethral resistance factor,

nary incontinence,42 with revisions using a

which originated from rigid tube hydrodyn-

amics. Since the urethra is an irregular and

larger study group in 2000,43 and a control

distensible conduit, whose walls and surround-

group without stress incontinence in 2004.44

ings have active and passive elements which

5. A panel of criteria derived during videouro-

influence the flow, a resistance factor does not

dynamic evaluation has been proposed based

reliably provide a valid

comparison

between

on study of a group of women clinically felt

patients.29

to have BOO.45

While measurements to derive a BOOI are

6. Area under the curve of detrusor pressure

generally made by invasive urodynamic testing,

during voiding, adjusted for voided volume, is

noninvasive techniques are now emerging30,31

which may be able to derive an estimate of the

a proposed parameter for diagnosing female

detrusor pressure generating maximum flow

BOO, though more work is needed.46

(i.e., with no catheter within the bladder lumen

7. Abnormalities of free flow pattern are seen in

to measure pressure). Penile cuff intermittent

a greater proportion of women subsequently

urethral compression32 is one approach and a

concluded to be obstructed on urodynamic

nomogram has been developed for the tech-

evaluation.47

nique.33 Similar noninvasive pressure measure-

ment can be derived from a modified condom

collecting device.34 Detrusor wall thickness is

Causes of Bladder Outlet

another parameter which may also indirectly

signify the presence of BOO.35,36

Obstruction

Diagnosis of BOO in women has not been

standardized and remains an area of ongoing

discussion.37 Proposals for urodynamic diagno-

Male

sis of BOO have been made as follows;

The additional anatomical structure of the male

1. Female BOO nomogram38; derived a nomo-

bladder outlet is the prostate gland, which

gram based on study of several hundred

encompasses the urethra between the genital

women defined as obstructed on clinical

and urethral sphincters. With aging, the gland

grounds, with quite a heterogeneous group of

undergoes benign prostate enlargement (BPE),

causative pathologies. The study estimated

which can typically be shown to be a conse-

the prevalence of female BOO at 6.5%. The

quence of benign prostate hyperplasia (BPH);

BPH can only be diagnosed where pathological

nomogram

employs

somewhat

different

specimens by biopsy or resection are available.48

parameters from the male BOOI, using the

The prevalence of BPH determined from autopsy

maximum

detrusor

pressure ascertained

studies shows that it does not occur below the

during invasive urodynamics and the maxi-

age of 30 years and that almost 90% of men have

mum flow rate on free flow testing.

developed BPH by their ninth decade.49 Likewise,

there is a steady increase in both LUTS50 and

placebo, the alpha-blocker terazosin, the 5-alpha

BPE,51 and all three processes are clearly inter-

reductase inhibitor finasteride and combination

related with ageing in men.52

therapy, suggested that factors other than BOO

Implicitly, the intrusion of the prostate into

alone almost certainly contribute to LUTS in

the urethral lumen in BPE (Fig. 16.3) can be

BPH. Recent studies using botulinum injections

expected to provide a degree of anatomical

within the prostate suggest that afferent nerve

impedance to urine flow during voiding by

activity may also be an element of LUTS in

direct flow impedance, and by distortion of the

BPE.58 Furthermore, the persistence of LUTS

bladder outlet. Thus, flow rate does decline with

after effective surgical management of BPE21

ageing in a population of men with BPE.25 The

strongly suggests that age-related changes in

role of anatomical impedance to flow is sup-

bladder function contribute to male LUTS as a

ported clinically by the response to treatment

codependent variable alongside BPE.59 The

aimed at reducing male sex hormones, which

inherent difficulties of controlling for the range

reduces the trophic factors giving rise to BPH

of contributory variables means that the

and thereby can partly reverse the enlargement.

pathophysiology of male LUTS will remain a

Within the prostate, the conversion of andro-

source of considerable scientific debate for the

gens to dihydrotestosterone by 5-alpha reductase

foreseeable future.

is a means by which this can be achieved.53 A

Urethral strictures are another mechanism

second contribution to BOO in BPE is the active

giving rise to BOO in men. They comprise an

contraction of smooth muscle in the prostate

inelastic circumferential fibrous constriction

stroma. The smooth muscle of the prostate

(Fig. 16.3), and thereby differ from BPE in that

expresses alpha-adrenergic receptors, falling

there is not contribution of active muscle con-

into at least two subtypes in the human pros-

traction to the BOO. They can be present at any

tate.54 Clinically, alpha-adrenergic antagonists

point along the urethra; postsurgical bladder

relax prostate smooth muscle, indicating a con-

neck stenosis is a related situation. Strictures

tribution of functional prostate muscle activity

have comparatively little effect on lower urinary

in BOO.55 On this basis, alpha-adrenergic antag-

tract function until they cause significant con-

onists and hormonal therapy may respectively

striction of the lumen. Characteristically, the pat-

decrease BOO by relaxing or shrinking the pros-

tern of urine flow differs from that seen in BPE.

tate. These clinical observations support the

Strictures usually arise as a consequence of ure-

hypothesis that there are both dynamic and

thral trauma,which can be iatrogenic.Congenital

static components of BOO associated with BPE.

male urethral strictures have been described.

The Olmstead County Study of Urinary

Cobb’s collar is a congenital narrowing in the

Symptoms and Health Status56 studied 2,115

male urethral bulb of uncertain etiology.60

men between 40 and 79 years of age and showed

BPE and urethra stricture are by far the most

that the severity of LUTS, peak flow rate, and

common causes of BOO in men. Rare causes of

extent of prostate enlargement were all age-

obstruction in men include congenital urethral

dependent and interrelated. However, the

abnormalities,61,62 foreign bodies,63 uretero-

Veterans Affairs Cooperative Study57 comparing

coele,64 adult onset voiding dysfunction.65