Urodynamics

Characteristics

Standard urodynamics (SU)

Ambulatory urodynamic monitoring (AUM)

Environment

Lab with limited privacy

Outside of a lab with less privacy issues

Test length

30–90 min

3–24 h

Patient position

Variable but static. Position, depends on urodynamic equipment and if fluoroscopy is used

Patient moves normally throughout the testing period

Catheters

Catheters attach directly to urodynamic hardware

Catheters have micro-tip transducers that are either attached to, or communicate to a remote device that is attached to the patient

Fill rate

Artificial

Physiologic

Feedback

Patient ideally must be able to verbally describe symptoms

Patient must be capable of indicating symptomatic events

Indications

Urodynamic evaluation of a symptomatic patient should be individualized with the goal of investigating a specific urinary problem. Urodynamic testing should be performed to confirm a clinical diagnosis, to potentially alter the management of a specific problem or to prevent injury to the upper urinary tracts. Urodynamics is also used to investigate outcomes of therapeutic interventions [2].

AUM has a minimal role in the initial urodynamic evaluation of most patients. They are advantageous when the findings during SU are inconclusive [3]. The ICS recognizes four indications to perform AUM (Table 2) [1].

Table 2

ICS indications for ambulatory urodynamics

Failure to reproduce or explain patient’s urinary lower urinary symptoms (i.e. patient describes urgency or stress incontinence that is not demonstrated on SU)

Unable to perform SU

Neurogenic bladder

Evaluate outcomes of therapeutic interventions

Ambulatory Urodynamic Monitoring

Historically AUM was performed with standard catheters with intravesical and intra-abdominal pressures measured using fluid filled lines . These lines limited patient mobility. Catheter mounted micro-tip transducers can be used which allow for better patient mobility during AUM. These are secured in the bladder and rectum or vagina with adhesive tape, suture fixation or purpose designed silicone-fixation devices. There are different companies that manufacture portable microprocessors that receive data from the micro-tip transducers. In the last decade wireless Bluetooth® technology has been adopted by some of these companies to avoid the number of wires and simplify the procedure.

Remote devices also have the capacity to allow event marking and recording of urinary symptoms which can be used to perform a digital voiding diary. Leakage events can also be recorded by electronic pads. Data points can be made available via the internet by many of the companies that manufacture these devices. The majority of results can be read or printed from a personal computer making the process very easy for the patient and the physician. The major limitations of AUM are the amount of data produced and the lack of standardization of interpretation and analysis of the data that is conducted by specific software.

Methodology

A detailed explanation is provided to the patient before the test. Instructions to record all urinary symptoms and events, as well as to identify hardware misplacement or malfunction are essential. When using a micro-tip transducer all transducers must be “zeroed” at atmospheric pressure before the catheters are inserted. Unlike a water filled pressure catheter which has a fixed reference point relative to the symphysis a micro-tip transducer does not have a fixed point (See chapter “The Cystometrogram”). Micro-tip transducers will record an erroneous change in pressures when in direct contact with the organ wall or when in contact with other solid material such as stool. Furthermore, the pressures recorded are affected by the vertical height between the vesical and abdominal transducers which can result in negative detrusor values when the patient is in a supine position [3].

As with SU, testing calibration is performed before starting data recording. Each pressure channel must be tested and the patient is asked to cough and to perform a Valsalva maneuver in different positions (supine, sitting and standing) to verify signal quality. Precise positioning and secure fixation are essential to maintain signal quality. The orientation of the transducer should be documented. Transducers should be readjusted if necessary and periodic quality checks should be performed to assess the signal and the patient data input of symptoms and events. It is recommended to have the patient cough or do a Valsalva at regular intervals during the monitoring. This allows for monitoring of signal quality throughout the test which is helpful in the interpretation of the data during the final analysis.

Prior to starting an AUM study a urinary tract infection is ruled out. The catheters are inserted into an empty bladder and ideally an empty rectum. The patient is then asked to get dressed, ambulate, eat, drink and go to the toilet normally at his/her own convenience. When shorter duration studies, 3–4 h, are used some investigators have used protocols where patients are instructed to consume large volume of fluid intake. Patients are instructed to record all coughs or Valsalva maneuvers, any symptoms of urgency and any leakage event in a voiding diary or digitalized into a recording device by pressing a button.

The ICS recommends that an AUM be continued until at least two voiding occasions have been recorded. Additional urodynamics parameters include recording of initiation of voluntary voids, cessation of voluntary voids, episodes of urgency, episodes of discomfort or pain, provocative maneuvers, time and volume of fluid intake, time and volume of urinary leakage and time of pad change [1]. These data points allow for identification of filling versus voiding phase abnormalities.

There are controversies regarding the timing of analyzing the data which is processed through the specific software. Simultaneous recording of urinary symptoms is essential to interpret the results. There can be error and misinterpretation due to the different changes in pressure including pressure changes in the bladder that are volume related. It has been previously described that a second additional transducer can reduce this artifact; however a second transducer can also introduce more confounders simply by adding more data into the analysis and it increases catheter stiffness which may cause alteration of urethral anatomy [4].

Overactive Bladder Syndrome

Ambulatory urodynamics has the advantage of being more sensitive in diagnosing detrusor overactivity (DO) than standard urodynamics (SU) therefore possibly affording an advantage in this patient population. Reports on natural filling cystometry to identify detrusor overactivity (DO), then termed detrusor instability (DI), date back to the late 1970s [5]. In 1980, Thuroff et al. compared AUM to SU and reported a higher sensitivity (60 %) with AUM in comparison to SD (20 %) when studying 10 symptomatic males [6]. Further retrospective studies in the 1990s confirmed the very high sensitivity of AUM to diagnose DI [1, 7]. During this time period the presence of objective DI was a relative contraindication to perform stress incontinence surgery in women with symptoms of stress incontinence and for this reason, urodynamics had a crucial role in the evaluation of urinary incontinence. In 1983 James published on the importance and significance of AUM to reproduce patient urinary symptoms outside the urodynamic suite. Urinary frequency and urgency could be better captured in a less artificial environment with the aim of looking for DI [8].

A prospective study by Webb et al. found a DI rate of 60 % during AUM in 52 women who despite having urinary urgency did not have DI on two prior SU studies [9]. Another study by Vereecken diagnosed DI in 53 % of 100 symptomatic patients with prior negative SU studies [10].

A major criticism of AU is that it may be too sensitive at detecting DO in patients in whom it may not be clinically significant therefore leading to overdiagnosis. There is no question that AU is highly sensitive to diagnose DO in symptomatic patients. However a prospective study by Salvatore et al. on 26 asymptomatic women (mean age 32 years) showed an abnormally high finding of DO in 17 patients (65 %) which was reduced to 3 patients (11.5 %) when two urethral transducers and a voiding diary were used. In this study AUM testing was carried out for up to 4 h [11]. It is unclear how the investigators analyzed all increases in detrusor amplitude associated with symptoms or leakage using two transducers. There was also a lack of blinding which may have led to investigator bias. Although the authors recommended a number of strategies to help decrease the potential for over diagnosis, these findings have never been reproduced in a larger trial.

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