Urodynamics: Indications for the Procedure and Systematic Interpretation
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Urinary incontinence is a common concern for women. Initial evaluation should include a detailed history, physical examination, demonstration of stress incontinence, assessment of urethral mobility, determination of symptom severity, and identification of treatment goals. Patients who screen positive for incontinence should have concurrent urinary tract infections ruled out with a urinalysis (+/- culture). Patients should have a postvoid residual to exclude overflow or retention before proceeding with treatment.
Urodynamic testing is reserved for more complex conditions. Systematic reviews show that patients with uncomplicated stress urinary incontinence (defined as postvoid residual <150 mL, negative urinalysis, and a positive stress test) do not need to undergo urodynamic testing before surgery. Indications for testing include unclear diagnosis after basic evaluation, failure to improve after initial treatment, prior pelvic floor surgery, or symptoms that do not correlate with physical findings.
Urodynamic testing evaluates lower urinary tract function by measuring urinary storage and voiding functionality. Testing includes uroflowmetry, cystometrography, and urethral pressure profilometry. The first step is to perform uroflowmetry. The patient is asked to empty their bladder into a commode connected to a flowmeter. The average woman can empty a full bladder in 15 to 20 seconds with an average flow rate of 20 mL/sec. Voiding dysfunction is diagnosed if the flow rate is less than 15 mL/sec with urine volume retention greater than 200 mL.
Cystometrography is then used to diagnose urinary stress incontinence or overactive bladder. During this procedure, 2 catheters are used: one is placed into the bladder to measure bladder pressure (Pves) and the other is placed in the vagina or rectum to measure abdominal pressure (Pabd). Detrusor pressure (Pdet) is calculated by subtracting abdominal pressure from vesicular pressure (Pdet = Pves – Pabd). Five distinct measurements are obtained from cystometrography: intraabdominal pressure, vesicular pressure, detrusor pressure, bladder volume, and saline infusion flow rate. The bladder is incrementally filled with warm saline, and the patient is asked to cough and/or perform a Valsalva maneuver to assess for stress urinary incontinence. Volumes are recorded when the patient feels the urge to void and at maximal void capacity.
The instantaneous bladder pressure is recorded when a patient leaks urine with cough (cough leak point pressure) or Valsalva maneuver (Valsalva leak point pressure). Cough leak point pressure or Valsalva leak point pressure less than 60 cm H20 is indicative of intrinsic sphincter deficiency. Pressures higher than these values are an indication that the urethral sphincter is functioning normally and that urethral hypermobility is the cause of stress urinary incontinence.
The diagnosis of overactive bladder can be made with or without leakage and is evident by an acute increase in detrusor pressure during the filling phase of cystometrography. There is no set minimum increase to detect a detrusor contraction, but it must be a distinct rise in detrusor pressure. Gradual, sustained rises in detrusor pressure, in comparison, are indicative of noncompliant bladders due to etiologies such as radiation, advanced interstitial cystitis, or surgery. This rise occurs because the bladder cannot appropriately expand as it is being filled. Patients with poorly compliant bladders that reach detrusor pressures above 40 cm H20 (outside of coughing, Valsalva maneuver, or distinct detrusor contraction) are at risk of upper urinary tract disease because such high resting bladder pressures can lead to vesicourethral reflux, hydronephrosis, and eventual kidney damage.
Urethral pressure profilometry is done at maximum bladder capacity following cystometrography by slowly pulling the pressure catheter along the length of the urethra to record the maximum urethral closing pressure. Maximum urethral closing pressure less than 20 cm H20 is indicative of intrinsic sphincter deficiency.
Committee Opinion No. 603: Evaluation of uncomplicated stress urinary incontinence in women before surgical treatment. Obstet Gynecol. 2014 Jun;123(6):1403-7. doi: 10.1097/01.AOG.0000450759.34453.31. PMID: 24848922.
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 May 24;366(21):1987-97. doi: 10.1056/NEJMoa1113595. Epub 2012 May 2. PMID: 22551104; PMCID: PMC3386296.
Initial Publication: November 2020; Revised May 2022
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