How do you interpret a urodynamic study?

Interpreting Urodynamic Studies

Urodynamic studies should be interpreted as an interactive diagnostic assessment of lower urinary tract function, with the primary goal of reproducing the patient's symptoms and determining their cause through specific measurements and observations. 1

Components of Urodynamic Testing

• Post-void residual (PVR): Assesses the possibility of significant bladder and/or outlet dysfunction, particularly important in patients with neurological conditions 1
• Uroflowmetry: Provides information about voiding function; at least two flow rates should be obtained with volumes greater than 150 ml for reliability 2, 3
• Cystometry (CMG): Evaluates bladder filling and storage function, identifying conditions like detrusor overactivity 1, 4
• Pressure-flow studies (PFS): Assesses the relationship between detrusor pressure and urine flow rate during voiding, crucial for diagnosing bladder outlet obstruction 1, 2
• Videourodynamic studies: Combines imaging with pressure measurements for comprehensive assessment 1, 5
• Electromyography (EMG): Evaluates pelvic floor muscle activity, though technically challenging and should be interpreted with caution 2, 5
• Urethral function tests: Includes Valsalva leak point pressure and urethral pressure profile measurements 1, 6

Interpretation Framework

For Bladder Storage Phase

• Evaluate bladder sensation, compliance, and capacity 4, 5
• Identify detrusor overactivity (DO), characterized by involuntary detrusor contractions during filling 2, 7
• Note that absence of DO on a single study does not exclude it as a cause of symptoms 1, 7

For Voiding Phase

• In men, obstruction is likely with the classic "high pressure-low flow" pattern 2, 8
• In women with elevated detrusor voiding pressure and low flow, consider obstruction, especially with new symptoms after surgery 1, 2
• Distinguish detrusor underactivity from obstruction by relating detrusor pressure at maximum flow to the maximum flow rate 2, 7

Clinical Applications

• For stress urinary incontinence: Urethral function tests help assess the severity and mechanism of leakage 6, 5
• For urgency incontinence: Cystometry can identify DO, though its absence doesn't exclude overactive bladder 1, 7
• For neurogenic bladder: Complex CMG and pressure flow studies are essential during initial evaluation and follow-up 1
• For LUTS in men: PFS are recommended before invasive therapy when Qmax is >10 ml/second to confirm obstruction 2, 8
• For mixed incontinence: UDS helps determine the predominant component and guide treatment 1, 6

Critical Interpretation Principles

• Reproduce the patient's symptoms during testing for accurate diagnosis 1
• Interpret findings in the context of global assessment, including examination, diaries, and PVR measurements 1, 7
• Recognize technical limitations and potential artifacts, particularly in EMG testing 2, 4
• Consider multichannel subtracted pressure measurement over single channel cystometrogram to minimize artifacts 2, 8
• Formulate specific urodynamic questions before testing: what information is needed and which technique is most appropriate 1

Important Caveats

• Urodynamic findings alone may not precisely predict treatment outcomes 1, 6
• Consider risks of infection and, in spinal cord injury patients, autonomic dysreflexia 1, 2
• Recognize that each patient is unique, requiring individualized interpretation of results 1, 5
• Be aware that symptoms and physical findings often do not adequately predict the underlying pathophysiology 1