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URODYNAMICS

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. 

SAMPLE URODYNAMIC RECORDINGS
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.