URODYNAMICS
Urodynamic testing is an important part of the evaluation of voiding dysfunction. Urodynamic studies (UDS) use pressure-flow studies and electromyography to evaluate bladder, urethral, and pelvic floor muscle function while simultaneously integrating real-time, subjective sensations experienced by the patient.
There are
numerous indications for UDS, which include:
1. Patients with recurrent
incontinence, in whom surgery is planned
2. Patients with a mix of
stress and urge incontinence
3. Patients with neurologic
disorders and voiding dysfunction
4. Patients with lower
urinary tract symptoms suggestive of bladder outlet obstruction
EQUIPMENT AND SET-UP FOR URODYNAMIC STUDIES
SET-UP
The patient
is seated. For the first part of the examination, known as uroflowmetry, the
patient freely and spontaneously voids into a uroflowmeter. For the remaining
components of the examination, one or more catheters are placed in the bladder
to measure intravesical pressure and infuse contrast, while another catheter is
placed in the rectum or vagina to measure intraabdominal pressure. An image
intensifier is positioned over the patient’s pelvis to obtain fluoroscopic views
of the bladder. Finally, a patch or needle electrode may be placed near the
external urethral sphincter or external anal sphincter to measure activation
potentials. Although both locations generally give similar recordings, the
former is considered more accurate.
COMPONENTS
OF THE EXAMINATION
Uroflowmetry. Uroflowmetry provides a
graphic analysis of urine flow rate over time. For the reading to be valid, a
minimum of 150 mL of urine should be voided. The normal flow shape is a
bell-shaped curve, in which the rate rapidly rises, plateaus, and then
declines. Several values can be calculated from the curve, including total
voided volume, total void time, maximum flow rate, and average flow rate (total
voided volume/total void time). The normal average flow rate from a full bladder
is about 20 to 25 mL/sec in men and 25 to 30 mL/sec in women, although these
values can vary depending on the volume voided and patient age.
Because
urinary flow is the result of detrusor pressure against outlet resistance,
abnormalities may reflect dysfunction in either of these units. Findings
suggestive of obstruction include a low average or maximum urine flow rate (less
than 10 mL/sec), prolonged void time, or a syncopated pattern of flow
(indicating the subject needs to restore adequate intraabdominal pressure to
sustain flow).
Normal flow
patterns may occur even in the presence of voiding abnormalities if
compensatory mechanisms have developed. For example, a low detrusor pressure may be associated with
normal flow rates if there is compensatory low outlet resistance or high
intraabdominal pressure. Likewise, high detrusor pressures may overcome an
outlet obstruction. The use of pressure-flow studies, described later, can
unmask such abnormalities.
Cystometry. In cystometry, fluid is
infused into the bladder while intravesical and intraabdominal pressure is
documented using urethral and vaginal or rectal catheters. Detrusor pressure is
calculated by subtracting intraabdominal pressure from intravesical pressure.
A
single-channel cystometrogram documents intra-vesical pressure as a function of
the volume of fluid infused. Four phases are seen. The first three phases represent bladder
filling. The first phase contains a sharp initial rise in pressure as fluid is
first infused. The second phase, known as the tonus limb, features a smaller
rise in pressure as additional fluid is infused, and it reflects accommodation of
the elastic bladder wall. The third phase contains a more dramatic rise in
pressure that occurs as the bladder wall becomes maximally distended. The
fourth phase is the voiding phase, which occurs when the bladder has reached
its maximum capacity. Throughout this process, patients are asked to comment on
the sensation at first filling and when they experience both their first desire to
void and a strong desire to void. The volumes at hich patients experience these
sensations are noted.
Several metrics can be determined based on a single-channel cystometrogram. Bladder compliance, for example, can be determined by
noting the intravesical pressure and volume at the start of the study and at
the end of the filling phase, then dividing the volume change by the pressure
change. A normal bladder compliance is less than 12.5 mL/cm H2O.
Involuntary and sudden increases in pressure during the filling phase suggest an
overactive detrusor. Bladder capacity, normally 300 to 500 mL, may be
determined by measuring the volume at which the patient has a strong desire to
void and cannot comfortably tolerate further infusion of contrast.
A
multichannel cystometrogram documents intravesical pressure, intraabdominal
pressure, detrusor pressure, urine flow, infused volume, and EMG potential as a
function of time. By examining all of these variables simultaneously,
additional metrics can be determined, such as the Valsalva leak point pressure
(VLPP), which is used to assess for SUI. The VLPP is equal to the abdominal
pressure sustained during a Val-salva maneuver that causes urine to leak around
the urethral catheter in the absence of detrusor contraction or a cystocele. A
VLPP less than 60 cm H2O suggests SUI because of intrinsic sphincter
dysfunction, whereas VLPP more than 90 cm H2O suggests SUI because
of urethral hypermobility. Values between 60 and 90 cm H2O suggest
mixed causes. In patients without stress urinary incontinence, there is no VLPP
at physiologic filling.
The detrusor leak point pressure (DLPP) is the pressure at which urine leaks around the catheter independent of detrusor contraction or the Valsalva maneuver. DLPP is clinically significant because very high pressure (i.e., in excess of 40 cm H2O) suggests pressure is being transmitted to the upper urinary tract, increasing the risk of hydronephrosis and eventual renal atrophy.
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| SAMPLE URODYNAMIC RECORDINGS |
Pressure Flow
Studies. Pressure flow studies use a combination of the above modalities
to examine the relationship between urine flow and detrusor pressure during
emptying. For example, in patients with low urinary flow rates, high detrusor
pressure suggests an outlet obstruction, whereas low detrusor pressure suggests
detrusor hypocontractility.
Electromyography
(EMG).
EMG of the external urethral sphincter can help determine if there is
coordination or discoordination with the detrusor muscle. At the beginning of
cystometry, before bladder filling begins, the patient is asked to demonstrate
volitional control of the sphincter by actively contracting and relaxing this
muscle. The ability to do so indicates intact pyramidal tracts. The
bulbocavernosus reflex may also be tested by squeezing the glans penis or
clitoris, or by pulling on the bladder catheter. A burst of EMG activity,
signifying a positive result, implies an intact sacral arc.
During
micturition, the sphincter should relax. If it does not, and a neurologic
lesion is likely to be present, this abnormality is termed detrusor–external
sphincter dyssynergia (DESD). It
typically occurs because of lesions between the pons and sacral spinal cord,
with interruption of the fibers that normally coordinate the detrusor and the
urethral sphincter. If patients do not have evidence of a neurologic lesion,
the term pelvic floor hyperactivity or dysfunctional voiding is used instead of
DESD.
Cystogram. A cystogram, obtained
using real-time fluoroscopic imaging, may be performed during urodynamics to
provide real-time anatomic correlates to filling or voiding. This imaging modality provides a visual correlate to
the volume instilled, captures the appearance of the bladder and bladder neck
during filling and voiding, and demonstrates whether vesicoureteral reflux (VUR)
if present.
Residual
Urine.
The postvoid residual (PVR) is determined by catheterization, ultrasound, or
cystogram at the end of a voiding event. A high PVR (150 cc) is suggestive of
bladder outlet obstruction or poor detrusor contractility.
