Urodynamics
Urodynamics turns vague voiding complaints into measured facts.
The big picture
Urodynamics turns vague voiding complaints into measured facts. Where the history tells you what the patient feels, urodynamics shows you what the bladder and outlet are actually doing during the two phases of bladder life — filling/storage and emptying/voiding. Its purpose is to answer concrete questions: does the bladder store at safe pressures, does it contract to empty, and is the outlet obstructed? Each test maps onto one of those questions.
The framework: match each test to a phase — uroflowmetry and post-void residual (emptying screen), cystometry (storage), pressure-flow studies (emptying, obstruction), EMG (sphincter), and the leak-point pressures.
Mechanism pathway
Tap any step to see why it happens.
How to read a urodynamic trace
The four channels of a urodynamic trace
Everything is recorded against the same time axis. Pdet is not measured directly — it is Pves minus Pabd, which is how a true detrusor contraction is told apart from abdominal straining.
VLPP vs DLPP — side by side
VLPP / ALPP vs DLPP — side by side
Two completely different leak point pressures: one tests the sphincter (a continence question), the other tests storage safety / the kidneys.
| VLPP / ALPP | DLPP | |
|---|---|---|
| Pressure measured | Abdominal / intravesical (the strain) | Detrusor (the bladder's own pressure) |
| What drives the leak | Abdominal straining (Valsalva/cough) | Poor bladder compliance (rising storage pressure) |
| Detrusor contraction? | None (must be absent) | None (must be absent) |
| Structure tested | The outlet / sphincter | The storage system / upper tracts |
| Clinical question | Is the sphincter weak? (continence) | Are the kidneys in danger? (safety) |
| Abnormal value | LOW <60 cm H₂O → intrinsic sphincter deficiency (ISD) | HIGH >40 cm H₂O → upper-tract (renal) risk |
| Typical patient | Woman with stress incontinence | Neurogenic, poorly compliant bladder |
Diagnostic algorithm
Each step answers one question. Tap to expand.
Board traps
Uroflowmetry/flow alone cannot distinguish obstruction from a weak detrusor — that needs a pressure-flow study.
Cystometry measures storage: sensation, capacity, compliance, overactivity.
Poor compliance (steep pressure rise on filling) endangers the upper tracts.
Pressure-flow: high pressure + low flow = obstruction; low pressure + low flow = weak detrusor.
EMG detects detrusor–sphincter dyssynergia.
Pdet = Pves − Pabd — the rectal (abdominal) line is what stops a strain masquerading as a detrusor contraction.
Poor compliance (≈ <12.5 mL/cm H₂O) and sustained storage pressure >40 cm H₂O endanger the upper tracts.
ALPP quantifies stress-incontinence sphincter weakness; DLPP flags dangerous storage pressures.
Videourodynamics (added fluoroscopy) localises obstruction (e.g. primary bladder neck obstruction) and detects reflux.
VLPP / ALPP = abdominal pressure at leakage on Valsalva (no detrusor contraction); low <60 cm H₂O = ISD — a continence/sphincter measure.
Don't confuse them: VLPP tests the sphincter (<60 = ISD); DLPP tests storage safety (>40 = upper-tract risk).
Pressure-flow nomograms (Abrams-Griffiths / Schäfer) grade outlet obstruction in men.