Residency · Residency · Urology
Urodynamics: Principles and Interpretation
Introduction
Urodynamic studies (UDS) encompass a series of diagnostic tests designed to evaluate the function of the lower urinary tract during both the filling and voiding phases. These studies provide objective, physiological data that help characterize lower urinary tract dysfunction, especially when the clinical presentation is unclear or when empirical treatments have failed. To ensure consistency and clinical relevance, the International Continence Society (ICS) has established standardized terminology and methodologies for performing and interpreting urodynamic tests.
Indications for Urodynamic Testing
Urodynamic testing is indicated in several clinical scenarios. It is particularly useful in cases of refractory overactive bladder that do not respond to behavioral or pharmacologic therapies. Patients with neurogenic bladder conditions, such as those resulting from spinal cord injury, myelomeningocele, or multiple sclerosis, benefit from baseline evaluation and ongoing surveillance. When mixed urinary incontinence is present and the dominant component is unclear, UDS can help clarify the underlying mechanisms. It is also valuable after failed anti-incontinence surgery to reassess the cause of leakage. In cases of voiding dysfunction where bladder outlet obstruction (BOO) versus impaired detrusor contractility is suspected, urodynamics aids in differentiation. Preoperative evaluation before surgical interventions for stress urinary incontinence or pelvic organ prolapse often includes UDS. Lastly, pediatric patients with lower urinary tract dysfunction, especially those with vesicoureteral reflux or upper urinary tract changes, are appropriate candidates for urodynamic assessment.
Components of Urodynamic Studies
Uroflowmetry
Uroflowmetry is a non-invasive test that measures the volume of urine voided over time. Key parameters recorded include the maximum flow rate (Qmax), average flow rate (Qavg), total voided volume, and the flow pattern. Normal maximum flow rates are generally greater than 15 mL/s in men and greater than 20 mL/s in women, provided the voided volume exceeds 150 mL to ensure reliability. The flow pattern can provide diagnostic clues: a bell-shaped curve is typical of normal voiding, a plateau pattern suggests obstruction, and an intermittent pattern may indicate abdominal straining or detrusor-sphincter dyssynergia. Post-void residual (PVR) urine volume is measured immediately after voiding, either by catheterization or ultrasound, to assess incomplete bladder emptying.
Cystometry (Filling Phase)
Cystometry during the filling phase is typically performed using dual-channel subtracted cystometry. Intravesical pressure (Pves) is measured via a bladder catheter, while abdominal pressure (Pabd) is recorded using a rectal balloon catheter. The detrusor pressure (Pdet) is calculated by subtracting Pabd from Pves, which helps eliminate artifacts caused by coughing, straining, or patient movement. The bladder is filled with sterile saline at room or body temperature at a medium rate of 25-50 mL/min for standard studies. During filling, several key observations are made: the first sensation of filling usually occurs at 100-200 mL, the first desire to void at 200-350 mL, and the strong desire to void at 350-500 mL. The maximum cystometric capacity is the volume at which the patient can no longer delay voiding. Bladder compliance, defined as the change in volume divided by the change in detrusor pressure, is normally greater than 40 mL/cmH2O; values below 20 mL/cmH2O indicate low compliance, which risks upper urinary tract damage. Detrusor overactivity (DO) is identified by involuntary detrusor contractions during filling, which may be phasic or terminal in nature.
Leak Point Pressure
Leak point pressure measurements help assess urethral sphincter function. Abdominal leak point pressure (ALPP) is the lowest increase in abdominal pressure that causes urine leakage without a detrusor contraction. An ALPP below 60 cmH2O suggests intrinsic sphincter deficiency (ISD), whereas an ALPP above 90 cmH2O indicates urethral hypermobility as the primary mechanism of leakage. Detrusor leak point pressure (DLPP) is the detrusor pressure at which leakage occurs and is especially critical in neurogenic bladder patients. A DLPP greater than 40 cmH2O is associated with upper tract deterioration, as established by McGuire’s landmark threshold.
<image>Annotated diagram of a urodynamics setup showing dual-lumen bladder catheter and rectal catheter placement, transducer connections, EMG patch electrodes on the perineum, and the uroflowmeter, with labeled channels for Pves, Pabd, Pdet, flow rate, and EMG on a multichannel trace</image>
Pressure-Flow Study (Voiding Phase)
During the voiding phase, simultaneous measurement of detrusor pressure and urine flow rate is performed. Bladder outlet obstruction is characterized by a high detrusor pressure combined with a low maximum flow rate. In contrast, impaired detrusor contractility presents as low detrusor pressure with a low flow rate. Differentiation between these two conditions is aided by tools such as the Schafer nomogram or the Abrams-Griffiths (AG) number. The AG number is calculated as the detrusor pressure at Qmax minus twice the Qmax value. An AG number greater than 40 indicates obstruction, values between 20 and 40 are equivocal, and values below 20 suggest no obstruction. The bladder contractility index (BCI), calculated as detrusor pressure at Qmax plus five times Qmax, helps assess detrusor strength: a BCI above 150 indicates strong contractility, 100-150 is normal, and below 100 signifies weak contractility or detrusor underactivity.
Electromyography (EMG)
Electromyography involves placing surface or needle electrodes at the external urethral sphincter or peri-anal region to monitor muscle activity. Normally, EMG activity increases during bladder filling as part of the guarding reflex and silences during voiding due to reciprocal relaxation. Detrusor-sphincter dyssynergia (DSD) is characterized by a paradoxical increase in EMG activity during detrusor contraction and is pathognomonic for a suprasacral spinal cord lesion. Non-relaxing urethral sphincter obstruction, which may be behavioral or learned, is identified by failure of EMG silencing during voiding without true neurologic DSD.
Videourodynamics (VUDS)
Videourodynamics is considered the gold standard for evaluating complex cases. It combines multichannel urodynamic testing with simultaneous fluoroscopy. Filling the bladder with dilute contrast allows visualization of bladder morphology, vesicoureteral reflux, bladder neck opening, and urethral anatomy during both filling and voiding phases. Primary indications for VUDS include neurogenic bladder evaluation, complex voiding dysfunction, pediatric cases, and patients with failed prior treatments.
<image>Sample multichannel urodynamic tracing showing five channels (Pves, Pabd, Pdet, flow rate, EMG) during a complete fill-void cycle, with annotated markers for first sensation, detrusor overactivity events, permission to void, and peak flow with corresponding detrusor pressure at Qmax</image>
Artifacts and Quality Control
Several artifacts can affect the accuracy of urodynamic measurements. Rectal contractions cause spikes in abdominal pressure without corresponding changes in intravesical pressure, resulting in negative detrusor pressure artifacts. Catheter displacement manifests as a sudden drop in intravesical pressure to zero and can be recognized by the loss of respiratory variation. At the start of the study, resting pressures should show equal intravesical and abdominal pressures, resulting in a detrusor pressure close to zero; this is verified using a cough test. The cough test is periodically performed to confirm proper pressure transmission, with equal spikes in Pves and Pabd and no change in Pdet. The rate of bladder filling also influences results; rapid filling may provoke detrusor overactivity or falsely reduce bladder compliance.
Special Populations
Neurogenic Bladder
In patients with neurogenic bladder, videourodynamics is preferred to assess bladder compliance, detrusor overactivity, detrusor-sphincter dyssynergia, detrusor leak point pressure, and vesicoureteral reflux. Safe bladder parameters include a DLPP below 40 cmH2O, compliance greater than 40 mL/cmH2O, absence of reflux, and low post-void residual volumes.
Female Stress Urinary Incontinence
Urodynamic studies are recommended in women when the diagnosis is uncertain, when mixed symptoms are present, or after failed prior surgery. Standing cystometry may be necessary to provoke stress leakage. It is important to assess for concomitant detrusor overactivity, which occurs in up to 30% of women with stress urinary incontinence.
Pediatric UDS
Pediatric urodynamic studies use age-appropriate bladder capacity, calculated as (age in years + 2) multiplied by 30 mL. Sedation is generally avoided because it alters detrusor function. Natural fill urodynamics may be more physiologic in young children.
<image>Videourodynamic fluoroscopic image during voiding phase showing an open bladder neck with contrast in the urethra, labeled detrusor-sphincter dyssynergia with a narrowed external sphincter zone, and bilateral vesicoureteral reflux graded on the international system</image>
Key Clinical Pearls
It is essential always to subtract abdominal pressure from intravesical pressure because analyzing intravesical pressure alone can lead to misinterpretation of abdominal straining as detrusor contraction. In neurogenic bladder patients, a detrusor leak point pressure greater than 40 cmH2O is a critical threshold associated with upper tract deterioration and necessitates intervention. A maximum flow rate below 10 mL/s on free uroflowmetry with a voided volume greater than 150 mL warrants a pressure-flow study to differentiate obstruction from detrusor underactivity. Invasive urodynamic studies should not be performed when a clear clinical diagnosis exists and management will not change based on the results. Urodynamic findings must always be correlated with the patient's symptoms and clinical context, as urodynamics serves to confirm or refute a clinical hypothesis rather than provide a standalone diagnosis. In men with lower urinary tract symptoms, the Abrams-Griffiths number remains the most validated tool for diagnosing bladder outlet obstruction.
References
- Rosier PFWM, Schaefer W, Lose G, et al. International Continence Society good urodynamic practices and terms 2016. Neurourol Urodyn. 2017;36(5):1243-1260.
- Nitti VW. Pressure flow urodynamic studies: the gold standard for diagnosing bladder outlet obstruction. Rev Urol. 2005;7(Suppl 6):S14-S21.
- Winters JC, Dmochowski RR, Goldman HB, et al. Urodynamic studies in adults: AUA/SUFU guideline. J Urol. 2012;188(6 Suppl):2464-2472.
- McGuire EJ, Woodside JR, Borden TA, Weiss RM. Prognostic value of urodynamic testing in myelodysplastic patients. J Urol. 1981;126(2):205-209.
