For the broader O&G Magazine readership, Q&A seeks balanced answers to those curly-yet-common questions in obstetrics and gynaecology.

Urodynamics: who, what, when and how?

It is often said that ‘the bladder is an unreliable witness’, whereby the symptoms reported by the patient may not necessarily correlate to the underlying pathophysiology. Urodynamic studies (UDS) provide a dynamic and interactive assessment of bladder function to aid in delineating and investigating symptomatology. This Q&A article will aim to answer some of the common questions relating to urodynamics.

What are the aims of urodynamics?

As with all investigations, urodynamics should not be used in isolation and must be correlated with the clinical picture. The general aims when performing UDS is to determine the ability of the bladder to store urine, empty effectively and to determine the ability of the urethra to provide continence, including during exertion.

What are urodynamics?

As a tool to investigate lower urinary tract function or dysfunction, UDS is an umbrella term incorporating all invasive and non-invasive tests. This includes uroflowmetry, post void residual, cystometry and urethral pressure profile.

The most commonly used method of urodynamics in Australia is conventional urodynamics where the patient’s bladder is filled artificially in the outpatient clinic setting. Ambulatory urodynamics, where the patient’s bladder is allowed to fill naturally; or videourodynamics, which involves the use of fluoroscopy, will not be covered in this brief Q&A.

How is a UDS performed?

UDS is most simply divided into a filling and a voiding phase. Some tests are also conducted at bladder capacity. There are two catheters usually placed into the patient’s bladder and vagina or rectum. At the beginning of the filling phase the bladder is confirmed empty with a catheter, artificial filling of the bladder commences. This occurs through a bladder catheter.

Pressure measurements are taken through a pressure catheter placed in the bladder (vesical pressure or p_ves) and through the vaginal or rectum (abdominal pressure or p_abd). The true detrusor pressure (p_det) is measured indirectly using the formula: P_det = p_ves – p_abd. This gives the pressure that is actually exerted by the detrusor muscle without the influence of external pressures such as through straining/coughing/sneezing etc.

What can we tell from a UDS?

During the filling phase, as the bladder is slowly filled, there should be constant interaction between the urodynamicist and the patient so that subjective sensations and symptoms are captured and correlated at the time points in which they occur.

The sensations that are recorded include: first sensation of bladder filling, first desire to void and strong desire to void. Other sensations including absent, reduced or increased bladder sensation as well as pain can also be documented. The detrusor function during filling is classified as stable (normal) or unstable (when detrusor overactivity or involuntary contractions are present). Bladder capacity (maximum cystometric capacity) is documented at the end of filling.

Bladder compliance is also calculated during the filling phase. Compliance describes the relationship between the change in detrusor pressure and the change in bladder volume or more simply, how well a bladder is able to adapt to stretch. Low bladder compliance can be seen in female patients with chronic inflammation (painful bladder syndrome, radiation cystitis), outlet obstruction and neurological conditions (cauda equina syndrome, multiple sclerosis and spinal cord injury) and is important as it increases the risk of upper renal tract dysfunction.^{1 2}

Provocative measures are performed during filling and at the end of the filling phase when maximum cystometric capacity is reached. This is when the patient feels that she can no longer delay micturition. Provocative measures such as postural changes and hand washing aim to provoke detrusor overactivity. Coughing aims to provoke the sign of urodynamic stress incontinence when there is an increase in abdominal pressure that occurs without a detrusor contraction.

Urethral function can be assessed during urodynamics using urethral pressure profile measures. It aims to estimate the ability of the urethra and the surrounding supportive tissue to keep the bladder outlet closed. There are multiple measures used to quantify the mechanism along the whole urethra or at fixed points including MUCP or leak point pressures. Generally, low pressures define intrinsic sphincter deficiency which has been associated with poorer outcomes following continence surgery and increased risk of repeat surgery.^{3 4} Poor urethral mobility can be assessed clinically and on USS imaging⁵ and is also associated with poor outcomes at continence surgery.^{3 6}

During the voiding phase, the volume voided, flow rate, voiding time and voiding pressures can be measured. A post void residual (PVR) is measured at the end of voiding either at the start and/or at the end of UDS. PVRs should be measured immediately post micturition and when elevated is consistent with voiding dysfunction. Urodynamics assessment of voiding pressures in those with elevated residual volumes allows further sub-classification into obstructive voiding in those with high detrusor pressures (eg. post continence surgery) or detrusor underactivity/acontractile detrusor in those with low detrusor pressures (eg. neurogenic causes).

In women with prolapse considering surgical intervention, reducing a prolapse may also be able to demonstrate occult stress urinary incontinence. This assists in counselling for potential concurrent continence procedures.

What are some important indications for UDS?

Urodynamics have a pivotal role in facilitating the diagnosis and management of female urinary incontinence. It is recommended in those that have complicated histories, those that have failed to respond to initial treatments and prior to continence surgery, except in those with pure stress urinary incontinence demonstrated on history and examination. In 2012 and 2013,^{7 8} two papers reported no advantage to the routine preoperative utilisation of urodynamics in women with pure, uncomplicated primary stress urinary incontinence with the sign of stress incontinence on examination. These studies excluded patients with concurrent voiding dysfunction, prolapse or an elevated post void residual of > 150mL on ultrasound or catheterisation. The reader should be aware that ‘uncomplicated’ stress incontinence as described by these studies represents only approximately a third of women presenting with stress urinary incontinence⁹ and that the conclusions from these papers lack generalisability to most women considering continence surgery.

What can’t we tell from UDS?

Whilst UDS are useful to delineate symptoms and inform clinicians, interpretation of a urodynamic trace should always be done in the context of clinical symptoms.

A UDS does not take into account the ‘level of bother’, and while it is a dynamic test, it is not able to reproduce all symptoms eg. nocturia, significant exertion such as running, steps or heavy lifting or coital incontinence.

When performing a UDS, it is important to rule out concurrent conditions like urinary tract infections (UTI) or artefacts that could lead to misinterpretation of a UDS. A UTI can increase positive findings and diagnoses that may not be usually present such as increased bladder pain and sensation, detrusor overactivity and poor bladder compliance. In this instance, a UDS should be performed again as required.

Final thoughts

Urodynamic studies remain an invaluable tool in the evaluation of women with lower urinary tract dysfunction. It also presents a unique opportunity to allow clinicians to show patients in real time the pathophysiology behind their symptoms, allowing them to understand and engage with management plans. However, care must be taken when ordering and interpreting each study to ensure that unnecessary testing is minimised and accurate diagnoses are made to inform appropriate treatment.

Further reading

• Abrams P, Cardozo L, Fall M, et al; Standardisation Sub-Committee of the International Continence Society. The standardisation of terminology in lower urinary tract function: report from the standardisation sub-committee of the International Continence Society. Urology. 2003 Jan;61(1):37-49. doi: 10.1016/s0090-4295(02)02243-4. PMID: 12559262.
• Rosier PFWM, Schaefer W, Lose G, et al. International Continence Society Good Urodynamic Practices and Terms 2016: Urodynamics, uroflowmetry, cystometry, and pressure-flow study. Neurourol Urodyn. 2017;36(5):1243-1260. doi: 10.1002/nau.23124. Epub 2016 Dec 5. PMID: 27917521.
• Working Group of the United Kingdom Continence Society; Abrams P, Eustice S, Gammie A, et al. United Kingdom Continence Society: Minimum standards for urodynamic studies, 2018. Neurourol Urodyn. 2019;38(2):838-856. doi: 10.1002/nau.23909. Epub 2019 Jan 16. PMID: 30648750.
• Lose G, Griffiths D, Hosker G, et al; Standardization Sub-Committee, International Continence Society. Standardisation of urethral pressure measurement: report from the Standardisation Sub-Committee of the International Continence Society. Neurourol Urodyn. 2002;21(3):258-60. doi: 10.1002/nau.10051. PMID: 11948719.
• Haylen BT, de Ridder D, Freeman RM, et al. An International Urogynecological Association (IUGA)/International Continence Society (ICS) joint report on the terminology for female pelvic floor dysfunction. Int Urogynecol J. 2010;21(1):5-26. doi: 10.1007/s00192-009-0976-9. Epub 2009 Nov 25. PMID: 19937315.

1. Abrams,P, Cardozo, L, Wagg, A, Wein, A. (Eds) Incontinence 6th Edition (2017). ICI-ICS. International Continence Society, Bristol UK, ISBN: 978-0956960733
2. Arunachalam D, Heit M. Low Bladder Compliance in Women: A Clinical Overview. Female Pelvic Med Reconstr Surg. 2020;26(4):263-269. doi: 10.1097/SPV.0000000000000666. PMID: 30520742.
3. Haliloglu B, Karateke A, Coksuer H, et al. The role of urethral hypermobility and intrinsic sphincteric deficiency on the outcome of transobturator tape procedure: a prospective study with 2-year follow-up. Int Urogynecol J. 2010;21(2):173-8. doi: 10.1007/s00192-009-1010-y. Epub 2009 Oct 3. PMID: 19802505.
4. Ford AA, Ogah JA. Retropubic or transobturator mid-urethral slings for intrinsic sphincter deficiency-related stress urinary incontinence in women: a systematic review and meta-analysis. Int Urogynecol J. 2016;27(1):19-28. doi: 10.1007/s00192-015-2797-3. Epub 2015 Jul 29. PMID: 26220506.
5. Pirpiris A, Shek KL, Dietz HP. Urethral mobility and urinary incontinence. Ultrasound Obstet Gynecol. 2010;36(4):507-11. doi: 10.1002/uog.7658. PMID: 20503229.
6. Fritel X, Zabak K, Pigne A, et al. Predictive value of urethral mobility before suburethral tape procedure for urinary stress incontinence in women. J Urol. 2002;168(6):2472-5. doi: 10.1097/01.ju.0000036492.11901.0d. PMID: 12441943.
7. Nager CW, Brubaker L, Litman HJ, et al; Urinary Incontinence Treatment Network. A randomized trial of urodynamic testing before stress-incontinence surgery. N Engl J Med. 2012;366(21):1987-97. doi: 10.1056/NEJMoa1113595. Epub 2012 May 2. PMID: 22551104; PMCID: PMC3386296.
8. van Leijsen SAL, Kluivers KB, Mol BWJ, et al; Dutch Urogynecology Consortium*. Value of urodynamics before stress urinary incontinence surgery: a randomized controlled trial. Obstet Gynecol. 2013;121(5):999-1008. doi: 10.1097/AOG.0b013e31828c68e3. PMID: 23635736.
9. Serati M, Topazio L, Bogani G, et al. Urodynamics useless before surgery for female stress urinary incontinence: Are you sure? Results from a multicenter single nation database. Neurourol Urodyn. 2016;35(7):809-12. doi: 10.1002/nau.22804. Epub 2015 Jun 9. PMID: 26061435