Abstract
Urodynamic investigations provide a valuable tool to aid diagnosis and management of lower urinary tract symptoms by evaluating bladder function during the filling, storage and voiding phases. As well as covering basic urinary tract anatomy and physiology, this article provides a comprehensive overview of urodynamic principles, from a gynaecological perspective. It discusses an overview of the various different tests including cystometry, uroflowmetry and pressure flow studies including understanding abdominal pressure, bladder pressure and detrusor pressure significance. It provides case studies that include useful tips for interpreting urodynamic studies, including two single best answer questions, in order to aid professionals preparing for the MRCOG exam or those participating in urogynaecology clinics.
Introduction
Urodynamic investigations study the function and dysfunction of the lower urinary tract with the aim of reproducing patients’ symptoms to offer a diagnosis and help guide management. This article aims to provide a comprehensive overview of urodynamic studies, particularly within Gynaecology, and is aimed at the general trainee, consultant and especially those revising for their MRCOG examinations.
Understanding the basic principles and applications of urodynamic studies is essential for all professionals involved in general gynaecology or urogynaecology clinics. We will explore the anatomy and physiology of the female urinary system, the different types of urodynamic tests, the indications for these and their clinical relevance.
A clear “urodynamic question” should be asked and answered when performing urodynamic testing. Investigations should be performed by qualified clinicians, who are encouraged to follow the International Continence Society standards for good urodynamic practice. Results not only provide invaluable insight into conditions such as incontinence, overactive bladder and urinary retention, but also help to inform patient management plans in context with the overall clinical picture.
What is urodynamics?
Definition
Urodynamic tests assess the filling, storage and voiding function of the bladder, urethra and urethral sphincters by evaluation of bladder pressure, bladder volume capacity and urine flow rates. The results help to diagnose urinary disorders and optimize the approach for patient treatment.
History
The field of urodynamics has evolved significantly since beginning in the late 1800s, when canine bladder manometry was used to investigate the relationship between bladder pressure and urine flow. General understanding and interest gradually grew over the next 100 years. The large number of people who sadly experienced spinal cord injuries during World War II provided an opportunity to explore atonic and autonomic bladder dysfunction.
The term ‘urodynamics’ was first used in around 1954 to describe the function of the urinary tract. Gradually, the need for an objective, scientific urinary tract assessment tool grew. Bates et al described the bladder as ‘an unreliable witness’ in 1970, referring to the early limited association between patients’ symptoms and their urodynamic diagnosis.
In recent years, two major advances have increased our urodynamic testing proficiency. The first is supplementing basic urodynamic testing with simultaneous radiological imaging in video urodynamics. The second is standard setting for urodynamic procedures and equipment. Worldwide, this is largely done by the International Continence Society (ICS), established in 1971. The ICS standardises terminology and publishes guidelines, including ‘Good Urodynamic Practice’, published in 2016 and available for free from the ICS website. This document describes the technical skills required to carry out high quality, reliable and reproducible urodynamic studies and provides guidance for interpretation of results.
Applications
Urodynamic testing is used in a variety of hospital settings to benefit a diverse patient cohort. Although within the Gynaecology field, and therefore the focus of this article, it is primarily used to investigate female Lower Urinary Tract Symptoms (LUTS), it is widely used in the male or paediatric populations, as well as those with neurological disorders.
LUTS are divided into three groups: storage, voiding and post-micturition symptoms. Symptoms are generally not specific and there is often significant overlap between related medical diagnoses.
Common lower urinary tract symptoms include the following.
Storage symptoms:
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Urgency: Sudden, compelling desire to pass urine which is difficult to defer.
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Nocturia: Complaint that urine is passed during the main sleep period. Each urination must be followed by sleep or the intention to sleep. This should be quantified using a bladder diary.
Voiding symptoms:
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Urinary Incontinence: Involuntary loss of urine experienced during the bladder storage phase.
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Stress urinary incontinence: involuntary loss of urine on effort or physical exertion including sporting activities, sneezing, coughing.
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Urgency urinary incontinence: involuntary loss of urine associated with urgency.
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Mixed urgency incontinence: both stress and urgency urinary incontinence. Can be urge-predominant or stress-predominant.
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Coital urinary incontinence: involuntary urine loss during or after coitus, may occur during penetration or at orgasm.
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Intermittency: Stopping and starting of urine flow on one or more occasions during one voiding episode.
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Straining: Needing to strain to initiate or maintain urine flow.
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Dysuria: pain, burning or other discomfort during voiding. May be intrinsic to urinary tract i.e. bladder/urethra, external or referred from other adjacent structure i.e. lower ureter.
Post-micturition symptoms:
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Feeling of incomplete emptying: self-explanatory term for a feeling experienced after passing urine.
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Post micturition dribble: involuntary loss of urine immediately after finishing passing urine i.e. after rising from the toilet (in females).
A comprehensive clinical history including assessment for common symptoms as listed above, utilization of a validated patient questionnaire for symptom and bother scores and a physical examination including pelvic and focussed neurological examination is vital. Relevant past medical and surgical history and current medications should be noted. A bladder diary should be completed prior to urodynamics, for three full days (consecutively or non-consecutively) and should include timing and volume of voids, fluid intake (volume and type of fluid), incontinence episodes and the degree of incontinence. It may also include symptoms such as urgency or what activity preceded the involuntary urine loss.
This extensive assessment leads to the formation of a “urodynamic question” which is an essential pre-requisite to undertaking testing.
Anatomy and physiology of the urinary system
Basic anatomy
The urinary system is a complex network of organs that primarily produce, store and expel urine, in addition to physiological maintenance of homeostasis. The kidneys, ureters, bladder, urethra and urethral sphincters are highly specialized to optimize function.
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Kidneys:
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Two retroperitoneal bean-shaped organs, roughly each the size of a fist.
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Contain approximately 1 million nephrons, which filter the blood to remove waste products, excess water and ultimately, form urine.
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Produce hormones i.e. erythropoietin, calcitriol.
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Ureters:
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Narrow muscular tubes, approximately 25 cm long that are lined with a smooth muscle layer to perform peristalsis.
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Bladder:
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Hollow muscular organ within the pelvic cavity.
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Typical capacity is 300–500 ml, although it can stretch to accommodate a larger volume. Stretch receptors in the walls send signals to the brain that stimulate the urge to urinate.
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The ability to store and expel urine can be assessed i.e. via cystometry.
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Lined with transitional epithelium.
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Detrusor smooth muscle layer contracts to allow bladder emptying. Detrusor functionality and coordination can be assessed.
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Urethra:
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Female urethra is approximately 4 cm long
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Role can be assessed via pressure-flow studies, helping to identify potential obstruction or dysfunction.
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Urethral sphincters:
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Internal urethral sphincter is made of smooth muscle, located where the bladder connects to the urethra. It is involuntary i.e. under autonomic nervous system control, and relaxes during micturition to allow for the flow of urine.
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External urethral sphincter is made of skeletal muscle and surrounds the urethra. It is voluntary, i.e. under conscious control, and allows the individual to hold urine until an appropriate time and place for urination is available.
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Discrepancies in the function of these sphincters can indicate various disorders such as stress urinary incontinence or detrusor overactivity.
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Physiological function
As part of the urinary system, the kidneys play a vital role in maintaining the body’s internal environment via blood filtration, waste excretion, internal regulation and hormonal production. The kidneys filter approximately 180 litres of blood daily, which is critical to maintaining internal chemical balance and preventing accumulation of toxic substances. The kidneys also provide an essential regulatory function via the renin-angiotensin-aldosterone system (RAAS). This adjusts blood pressure by controlling fluid volume and electrolyte concentrations within the blood. The kidneys are also responsible for acid-base balance within the body. Finally, the kidneys also produce hormones i.e. erythropoietin, which stimulates red blood cell production, and calcitriol, the active form of vitamin D, which helps regulate calcium levels.
Types of urodynamic test
Overview
Urodynamic tests vary in complexity and are chosen based upon the clinical picture including: patient symptoms, examination findings, bladder diary results. Patients often present with multiple different lower urinary tract symptoms. Prior to performing any test, the “urodynamic question” should be clearly formed.
The primary types of urodynamic tests include cystometry, uroflowmetry, pressure flow study and postvoid residual measurement .
The pressure flow study is a combination of cystometry and uroflowmetry and is typically seen when observing ‘urodynamics’.
Urodynamics can be:
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Invasive i.e. involving insertion of one or more catheters/transducers into bladder and/or other body cavity.
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Non-invasive i.e. other urodynamics done without catheter insertion, for example, uroflowmetry, post-void residual measurement.
Transducer and catheter systems: there are three types of transducer and catheter systems used in clinical practice currently.
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Fluid-filled catheters and external transducers
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Air-filled catheters and external transducers
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Catheter-tip transducers
Current ICS guidelines recommend fluid filled catheters for cystometry (see Figure 1 ). A fluid-filled catheter is passed into the bladder and attached to an external transducer at the height of the pubic symphysis (upper edge). The exact position of the catheter tip within the bladder is not critical, due to the continuous column of warmed saline along the catheter. The catheter measures the pressure change within the bladder by passing that change in pressure down the column of water to an external pressure sensor. The transducer then converts this pressure sensor value to a real-time pressure trace.
Artefact signals within the trace must be minimized in order to maximize the accuracy of result interpretation. Possible causes of artifact include patient movement or slipped catheter. Patient position change necessitates adjustment of the transducer back to the level of the pubic symphysis.
Detailed descriptions
ICS definitions of terms used in uroflowmetry are shown in Box 1 .
Flow rate is the volume of fluid expelled via the urethra per unit time (ml/s)
Maximum flow rate (Qmax) is the maximum measured flow rate, after correction for artefacts
Voided volume is the total volume emptied
Time to maximum flow is the time taken to reach maximum flow after flow onset, typically <5–10 seconds
Flow time is the time over which measurable flow occurs
Average flow rate is the voided volume divided by flow time
Uroflowmetry (free flow): uroflowmetry assesses the rate and pattern of urine flow i.e. it is a voiding assessment only. The patient voids into a special device called a uroflowmeter, in a private, comfortable environment, which aims to ensure a typical void. The uroflowmeter measures the weight of urine with respect to time, in millilitres per second (ml/s), to obtain the flow rate. This can be represented in a graphical format with time (s) on the x-axis and flow (mL/s) on the y-axis; examples demonstrated in Figures 2–5 . A voided volume of >150 ml ensures curve accuracy and reliability. Normally, a ‘bell-shaped’ curve is seen, with maximum flow rate (Qmax) >15 ml/s and a post-void residual (PVR) <30 ml ( Figure 2 ). PVR is measured either by bladder ultrasound scan or urethral catheterization.
