Urodynamics and Diagnostic Testing
Components
Urodynamics includes uroflowmetry, cystometry, pressure-flow study, and electromyography. Key measures: detrusor pressure (Pdet = Pves − Pabd), compliance, capacity, detrusor overactivity, and Valsalva leak point pressure (VLPP). A low VLPP (<60 cm H2O) suggests intrinsic sphincter deficiency. Urethral pressure profilometry is less commonly used.
Interpretation and pitfalls
Artifact from abdominal straining can mimic detrusor overactivity; subtract abdominal pressure. Poor compliance (high Pdet at low volume) indicates risk of upper tract injury, especially in neurogenic bladder. Detrusor sphincter dyssynergia (DSD) in neurogenic patients shows involuntary sphincter contraction during detrusor contraction. Postvoid residual and flow rates help distinguish retention from obstruction.
Traps
- Confusing detrusor overactivity (involuntary contraction during filling) with stress leakage.
- Failing to interpret VLPP; low VLPP suggests sphincter deficiency and may influence sling choice.
- Ignoring artifacts from rectal catheter placement or poor abdominal subtraction.
- Assuming urodynamics predicts surgical outcomes perfectly; it informs but does not guarantee results.
Worked example
A patient with neurogenic bladder and high storage pressures at low volume has poor compliance; manage with antimuscarinics, clean intermittent catheterization, and monitoring for upper tract changes, not just symptom control.