Urodynamics

Diagnostic study of the functioning of the lower urinary tract, assessing objectively both storage and voiding functions

The goal of urodynamics should be, as best able, to reproduce the patient’s symptoms during testing.

 

Indications for urodynamics:

• Neurogenic bladder
• Incontinence or LUTS not responding to conservative/medical treatments
  • “will affect the decision to pursue or avoid an invasive, irreversible or potentially morbid treatment”
• Diagnostic dilemma
  • e., where the findings will help influence management

 

Preparation:

• Thorough history and examination, and formulation of question urodynamics is intending to answer
• Consented patient with explanation of procedure, preferably with time to process information
• Bladder diary is useful
• Pre-procedural urine culture, with consideration of prophylactic antibiotics if high risk (high PVR, long term catheter, previous UTIs)
• Private, well equipped room
• Consider BP monitoring if at risk of autonomic dysreflexia
• Prolapse should be reduced in women

 

Components of urodynamics:

1. History and examination, and bladder diary
2. Pre-testing uroflowmetry and post void residual
3. Filling study (cystometrogram)
4. Voiding study (pressure flow study)
5. Adjuncts – EMG, imaging, UPP

 

 

Uroflowmetry

Can be performed in private to best try to replicate normal voiding behaviour.

Post void residual measurement should proceed after.

Should be noted whether this felt like a ‘normal void’.

Data points gained include:

• Voided volume
  • Should be > 150 mL to give accurate measurement of flow
• Flow rate – mL/sec
• Maximum flow rate – Qmax
• Average flow rate
• Total voiding time
• Time to maximum flow
• Post void residual
• % of volume voided
• Shape of the curve

 

Normal curve – bell shaped.

Fixed obstruction (i.e., stricture) – flattened.

Straining – straining saw-tooth pattern.

Underactive bladder – prolonged slow flow which may peak midway or later

 

Normal Qmax for men is > 20 mL/sec (> 15 mL/sec for older men) and > 25 mL/sec for women.

Flow rate may be reduced by obstruction or reduced contractility, or by abnormal volumes.

 

 

Filling study (cystometrogram)

Vesical pressure measured with bladder catheter

Abdominal pressure measured with rectal catheter (occasionally vaginal or stomal)

Detrusor pressure calculated by subtraction of abdominal pressure from vesical pressure.

 

Typically, water filled catheters are used and lines should be free of air bubbles to avoid artefact, with the transducer at the level of the patient’s bladder.

Some catheters may have ‘catheter tip transducers’ – but readings may vary depending on where in the rectum or bladder the catheter tip is placed – reusable and sterilised.

Single use air-charged catheters are now available – less artefact from movement, no need for flushing or to be set at level of the bladder – but may not give same readings as fluid filled catheters which were used to develop common nomograms etc.

Transducers must be calibrated regularly.

“Zero” the liquid filling lines to atmosphere/air before placing in the patient.

 

Technique for filling study:

• Patient in standing position is preferable, or sitting upright
• Maximal filling rate physiologically is weight/4 mL/min – suggestion of ICS is mL/min of 10 % of largest voided volume on bladder diary
  • In practice – ?50 mL / min
• 3 x sensations recorded – first sensation of filling, first desire to void, strong desire to void
• Provocation/stress tests
  • Usually series of coughs to elicit SUI
  • Can use Valsalva manoeuvres with syringe

 

Normal filling study:

• Begins when filling commences and ideally finishes at cystometric capacity where “permission to void” is given.
• Detrusor pressure should remain near zero throughout the filling phase, without any involuntary contractions or rises in pressure.

 

Artefacts:

• Movement or knocking the lines, patient laughing etc
• Unequal pressure transmission – check with cough test – if not equal, check lines for air bubbles etc
• Rectal contractions – increase in abdominal pressure only, subsequent fall in subtracted detrusor pressure
• Expelled catheters

 

Diagnoses during filling phase

Detrusor overactivity – spontaneous detrusor contraction which can be provoked or unprovoked, with or without sensation of urgency and urge incontinence

• Classified as neurogenic detrusor overactivity in neuropaths
• May be phasic or terminal
• Cough-associated DO may be seen

 

Compliance – relationship between change in bladder volume and change in detrusor pressure

• ICS recommends looking at pDet at commencement of filling, and end fill pressure (before any detrusor contraction resulting in leakage)
• End fill pressure is probably a better objective measure cf. specific compliance values.
• Falsely reassuring compliance may be seen with “pop-off” mechanisms like VUR.
• Poor compliance may be artificial if filled too fast – can stop filling for two minutes and see if pressure comes back to baseline.
• Impaired compliance with prolonged elevated storage/filling pressures (classically > 40 cm H2O) is a risk factor for upper tract deterioration

 

Urodynamic stress incontinence (genuine stress incontinence) – involuntary loss of urine with increasing abdominal pressure, without detrusor contraction

• Classically assess with series of stronger coughs, can also be done dynamically with syringe.
• Typically done first at 150 mL filled volume.
• ALPP/VLPP is abdominal pressure at which SUI occurs. The lower the ALPP, the weaker the sphincter.
• ALPP < 60 cm H2O signifies ISD, and > 100 typically more related to hypermobility.

 

Detrusor leak point pressure (DLPP) – is the lowest detrusor pressure at which leakage occurs in the absence of increased abdominal pressure or detrusor contraction.

• Usually measured in neuropaths, and important in those with DO and DSD. The lower the leak point pressure, the safer the bladder, as there will be less time with high storage pressures.
• 40 cm H2O is traditional cut off where above has been shown to be ‘unsafe’ – really should be aiming for as low as achievable.

 

 

5 Cs of filling phase:

• Capacity
• Compliance (look at starting and end fill pressures)
• Contractions (detrusor overactivity – phasic/terminal/with leak)
  • “Detrusor contraction at x mL to a pressure of x cm H20, with(out) associated urgency and leak”
  • “consistent with detrusor overactivity”
• Continence (urodynamic stress incontinence/ALPP) or DLPP
• Conscious sensation

 

Pressure flow studies (voiding study)

Should be done in usual position (seated or standing), with privacy if able and under clear instruction.

Problems identified in the voiding phase are the inability to generate enough pressure (underactive bladder / impaired contractility) or too much pressure/obstruction at the outlet (causing obstruction).

Normal voiding:

• Begins soon after permission to void
• Relaxation of the striated sphincter
• Opening of the smooth sphincter / bladder neck
• Detrusor contraction

There may be “situational inability to void”.

Slow stream may be caused by BOO (high pressure low flow) or detrusor underactivity (low pressure low flow). Ensure the software has taken the appropriate Qmax for nomogram assessment.

Calculate the post void by looking at volume infused and volume voided.

Look for straining patterns and the flow curve.

 

Bladder outlet obstruction index is validated for males only:

• pDet at Qmax – 2 x Qmax
• Over 40 is considered obstructed

 

Bladder contractility index:

• pDet at Qmax + 5 x Qmax
• Under 100 in considered underactive

 

 

Interpreting the voiding phase:

• Total volume voided (out of how much filled)
• Shape of the curve
• pDet at Qmax
• BOOI and nomogram – consistent / not consistent with obstruction
• BCI – consistent with underactivity

 

 

Fluoroscopy and imaging

Filling and voiding fluoro

Information gleaned:

• Determine level of obstruction – bladder neck, external sphincter, urethral stricture
• Examine for reflux or DSD in neuropaths
• Contour of the bladder – diverticula, capacity etc
• Urethral anatomy – diverticula, hypermobility

 

EMG

Aims to measure electrical activity associated with the external sphincter (sphincter muscles, pelvic floor muscle and external anal sphincter all recorded).

Most commonly patch electrodes on perineum.

Needle electrodes more invasive but potentially more accurate.

 

Normal – sphincter activity should be continuous during filling and slightly rising over filling (guarding reflex) or during involuntary contractions, but then should be absent during voiding.

DSD – increase in sphincteric activity during voiding, in neuropaths

Dysfunctional voiding – as above but in non-neuropathic patients

Fowler’s – “complex repetitive discharges”

 

Urethral profile pressures (UPP, profilometry)

Fluid pressure needed to just open the urethra.

Withdrawal of a pressure sensing catheter along the length of the urethra whilst simultaneously measuring bladder pressure.

Urethral closing pressure profile – urethral pressure minus intravesical pressure

Maximum urethral pressure (MUP) – highest pressure measured along urethral closing pressure

Maximum urethral closure pressure (MUCP) – maximum difference between urethral pressure and intravesical pressure

Functional profile length – length of urethra along which the urethral pressure exceeds intravesical pressure in women

 

Most women – functional urethral length 3 cm and MUCP 40 – 60 cm H2O.

MUCP < 20 cm H2O suggestive of ISD.

 

ICS – clinical utility of urethral pressure measurements unclear.