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Overactive Bladder and Incontinence
Figure 3: Voluntary Closure Mechanism
Bladder Emptying Is Achieved by Active Muscle
Contraction of the Longitudinal Muscle of the Anus and
Levator Plate, Relaxation of Pubococcygeus Muscle and
Rhabdosphincter and Detrusor Contraction
Micturition is not only sphincter relaxation and detrusor contraction: the
funnelling of the bladder neck and proximal urethra is caused by contraction
of the LP and LMA posterior muscles. This active opening out of the urethral
outflow tract vastly reduces resistance to flow, according to the fourth
power law of Hagen-Poiseuille (see Figure 5). By understanding these
important physiological components of micturition we can explain bladder-
emptying problems and masked SUI in patients with connective tissue
defects at the pelvic floor.
Abnormal Bladder Emptying
One important cause is the inability of the downward vector (arrow) (see
Figure 6) to open out the urethral outflow tract, a result of a lax USL.
Therefore, the bladder must expel urine against a urethral resistance
N = stretch receptors at bladder base. The voluntary contraction of the puborectalis muscle
increased by the fourth power of the narrowing. Another cause is kinking
which lies below levator plate (LP) lifts the whole LP muscle and all organs, rectum, vagina
of the urethra by a ballooning cystocele. Furthermore, the funnelling can be
and bladder base, upwards and forwards, causing ‘squeezing’.
hindered by a sling at the proximal urethra or bladder neck or after elevation
Figure 4: Urethral Closure Mechanism of the bladder neck by colposuspension. Cystoceles or loose USLs can
prevent the LP or LMA from opening the bladder neck, because the muscles
need tight ligaments and fascias to work optimally.
The Impact of the Fourth Power Law of Hagen-Poiseuille
on Abnormal Emptying and Continence
Anatomical dissection at mid-urethra
Figure 5 schematically indicates the impact of this law on urinary
Flow ~ radius
4
M. pubococcygeus (Hagen-Poiseuille) continence and evacuation.
15,16
If the striated muscle closure mechanism
can close the urethral tube to half its diameter, say D/2, then the expulsion
Pressure ~ 1/radius
pressure required rises by the fourth power (2x2x2x2) to 2,560cm H
2
O (see
Rhabdosphincter
(Laplace)
Figure 2). If the urethra can be opened out to 2D by the striated muscle
mechanism demonstrated in Figure 5, the expulsion pressure drops by the
Vagina tight
Cross-section at
fourth power (2x2x2x2) to 10cm.
mid-urethra (blue)
Vagina loose Masked Stress Urinary Incontinence
S
Vaginal wall (purple)
Masked SUI can also be explained by muscle action. In prolapsed patients,
the posterior muscles LP and/or LMA cannot contribute optimally to open
The anatomical dissection shows the ventral vaginal wall as a hammock below the
the bladder neck for funnelling. In patients with a loose hammock, a
mid-urethra, where the rhabdosphincter is concentrated. The diagram shows a tight
vagina and a loose vagina with increasing diameter within the rhabdosphincter when continent state between SUI and abnormal bladder emptying can occur.
the vagina is loose.
After repairing the prolapsed cystocele or vaginal vault or prolapsed
Figure 5: Influence of the Law of Hagen-Poiseuille on
uterus, these muscles can assist in funnelling. The SUI patient may need a
Continence and Evacuation
mid-urethral sling, which can be placed at the same time as the prolapse
surgery or in a second session.
Tethered Vagina Syndrome
2560cm 160cm10cm
The vaginal wall below the bladder neck is called the ‘zone of critical
elasticity’. It is stretched by the forward (PCM) and dorsocaudal (LP
and LMA) action of the muscles. If excess scarring in this region is
present after surgery, the vaginal wall is shortened and cannot be
stretched sufficiently. The more powerful backward forces are
tethered to the weaker forward forces. The urethra is forcibly pulled
D/2 D 2D
open on getting out of bed in the morning, when the pelvic floor
contracts to support all of the intra-abdominal organs. The classic
symptom of tethered vagina syndrome is commencement of
uncontrolled urine leakage as soon as the patient’s foot touches the
D represents the urethral tube at rest, D/2 during closure (see Figure 2) and 2D during floor. In these cases, restoration of elasticity at the bladder neck area
micturition (see Figure 6). The bladder pressures displayed to expel urine on each are nominal
figures only, and represent only the changes that follow the change in diameter with active
of the vagina by plastic surgery is required, generally by insertion of a
striated muscle closure or opening (directional arrows).
Martius skin graft.
64 EUROPEAN UROLOGICAL REVIEW
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