Urodynamics (UDS) measures the pressure–volume and pressure–flow behavior of the bladder and outlet to reproduce a patient's symptoms and identify dangerous storage. It should be performed only when it will change management; failure to record an abnormality does not exclude it.
Terminology
- Cystometrography (CMG): the bladder pressure/volume relationship during filling. Pdet = Pves − Pabd (Pves in the bladder, Pabd in the rectum/vagina — the bladder is a DIVA: Det = Ves − Abd), distinguishing a true detrusor rise from raised abdominal pressure.
- Physiologic filling rate is below the predicted maximum (body weight in kg ÷ 4).
Role and Dangerous Findings
Potentially dangerous UDS findings requiring intervention: impaired compliance; detrusor–external sphincter dyssynergia (DESD); detrusor–internal sphincter dyssynergia; detrusor leak point pressure >40 cm H₂O; high-pressure detrusor overactivity throughout filling; and poor emptying with high storage pressures. Decide in advance whether to test on or off medication (on, to assess therapeutic effect; off, to uncover a cause).
The 9 Cs of pressure-flow UDS — filling/storage: Contractions (involuntary detrusor), Compliance, Coarse sensation, Continence, Cystometric capacity; emptying: Contractility, Complete emptying, Coordination, Clinical obstruction.
Filling / Storage Phase
- Detrusor overactivity (DO): involuntary contractions during filling. Characterized as neurogenic (SCI, MS) vs idiopathic; spontaneous vs provoked; phasic (rising amplitude with volume — any phasic contraction is DO regardless of amplitude) vs terminal (a single contraction at capacity causing incontinence). Non-phasic pressure changes reflect compliance, not DO. Always interpret against symptoms; failure to demonstrate DO doesn't exclude it.
- Compliance (ΔV/ΔP, mL/cm H₂O): normal 46–124. Impaired by neurologic conditions (SCI, spina bifida — usually via ↑ outlet resistance), long-term BOO, radiation cystitis, and TB. False-positive low compliance from rapid filling; false-negative (seems better than it is) from "pop-off" mechanisms (VUR, diverticula). Absolute pressure matters most — storage pressure >40 cm H₂O harms the upper tract.
- Leak point pressures:
- Abdominal LPP (ALPP) — the Pves at which leakage occurs from raised abdominal pressure without a detrusor contraction; a measure of sphincter strength applicable only to SUI. <60 cm H₂O suggests ISD; 60–90 equivocal; >90 little/no ISD. Current technology cannot distinguish ISD from hypermobility in women (if there is no hypermobility, SUI must be ISD regardless of ALPP).
- Detrusor LPP (DLPP) — the lowest Pdet at which leakage occurs without raised abdominal pressure or a detrusor contraction; higher urethral resistance → higher DLPP → higher upper-tract risk. Most useful with upper-motor-neuron lesions (DO + DESD), decentralization, or low compliance. When treating impaired compliance, aim for pressure well <40 cm H₂O.
Voiding / Emptying Phase
A minimum voided volume of 150 mL is needed to assess uroflow. Pressure-flow studies identify three patterns: normal (low/normal Pdet + high/normal flow); obstruction (high Pdet + low/normal flow); and detrusor underactivity (low Pdet + low flow).
- In men: Bladder Outlet Obstruction Index (BOOI) = Pdet@Qmax − 2(Qmax) — ≥40 obstructed, 20–40 equivocal, ≤20 unobstructed. Bladder Contractility Index (BCI) = Pdet@Qmax + 5(Qmax) — >150 strong, 100–150 normal, <100 weak. ~2/3 of men with symptomatic BOO have DO, resolving in 50–67% after treating obstruction.
- In women: obstruction when Qmax ≤12 mL/s and Pdet@Qmax ≥25 cm H₂O; BOOI underestimates female obstruction (women void at lower pressures).
- Sphincter coordination: EMG activity normally decreases before a voluntary contraction. DESD (involuntary sphincter activity with the detrusor contraction) requires a neurologic lesion between the pons and sacral cord and is a risk factor for upper-tract deterioration; without a lesion it is dysfunctional voiding. Pseudodyssynergia is an EMG flare from attempted voluntary inhibition of an involuntary contraction (not true DSD).
Other Studies
- Urethral pressure profilometry — maximum urethral closure pressure (MUCP) is the maximum urethral-minus-intravesical pressure; in continent women, functional length ~3 cm and MUCP 40–60 cm H₂O.
- Video-urodynamics (VUDS) — the procedure of choice for bladder-neck dysfunction; primary bladder neck obstruction can only be diagnosed on VUDS. Useful for localizing obstruction, detecting occult incontinence, evaluating VUR, and in neuropathic dysfunction. Consider for high-risk patients (neurogenic disease, unexplained female retention, prior radical pelvic surgery/diversion/transplant, prior radiation).
- Clinical use: UDS is not very useful in pure SUI with normal emptying and demonstrable leak; it is most useful with significant urgency/UUI, emptying problems, prior SUI surgery, uncertain diagnosis, prior radiation, neurologic disease, or very severe symptoms. Significantly impaired compliance is the only absolute urodynamic indication to treat BOO. In conditions with potentially harmful storage (SCI, myelomeningocele), UDS is essential before treatment and during follow-up.
Self-Test
1. Define ALPP vs DLPP. ALPP — the bladder pressure at which leakage occurs from raised abdominal pressure without a detrusor contraction (sphincter strength, SUI). DLPP — the lowest detrusor pressure at which leakage occurs without raised abdominal pressure or a contraction (upper-tract risk in low-compliance bladders).
2. List causes of decreased bladder compliance. Neurologic conditions (SCI, spina bifida, usually via raised outlet resistance), long-term BOO, radiation cystitis, and tuberculosis.
3. What can make compliance seem better than it actually is? "Pop-off" mechanisms — vesicoureteral reflux and bladder diverticula.