In a patient with recent anorectal surgery who now has loss of fine rectal sensation and new urinary urgency, would urodynamic testing help rule out causes such as detrusor overactivity, underactivity, outlet obstruction, or early neurogenic bladder?

Urodynamic Testing in Post-Anorectal Surgery Urinary Urgency

Yes, urodynamic testing would be valuable in this clinical scenario to differentiate between detrusor overactivity, detrusor underactivity, bladder outlet obstruction, and early neurogenic bladder, particularly given the concerning combination of lost rectal sensation and new urinary urgency following anorectal surgery. 1

Immediate Priority: Rule Out Cauda Equina Syndrome

Before proceeding with urodynamics, emergency MRI of the lumbosacral spine within 24 hours is mandatory for any new bladder or urethral sensory disturbance to exclude incomplete cauda equina syndrome that may require urgent neurosurgical decompression. 2 This is critical because:

• Reduced bladder sensation should never be dismissed as a benign postoperative finding 2
• Any new sensory change must be treated as potential incomplete cauda equina syndrome until imaging excludes it 2
• Patients with cauda equina syndrome can paradoxically present with detrusor overactivity (32.8% of cases) rather than the expected acontractile bladder, particularly when the highest level of injury is L2 or above, suggesting combined conus medullaris involvement 3

Role of Urodynamics After Imaging

When Urodynamics Are Indicated

Multichannel filling cystometry is appropriate when determining if detrusor overactivity, altered compliance, or other urodynamic abnormalities are present in patients with urgency incontinence, especially when invasive or irreversible treatments are being considered. 1 In your patient, urodynamics serve multiple purposes:

• Detrusor overactivity identification: Multichannel subtracted pressure measurement can detect involuntary detrusor contractions during filling that may explain the new urgency symptoms 1, 4
• Detrusor underactivity assessment: Pressure-flow studies can distinguish between detrusor underactivity and bladder outlet obstruction by relating detrusor pressure at maximum flow to the maximum flow rate 1, 4, 5
• Neurogenic bladder evaluation: Complex cystometry and pressure-flow studies are essential during initial evaluation of patients with relevant neurological conditions 1, 6

Critical Diagnostic Considerations

The absence of detrusor overactivity on a single urodynamic study does not exclude it as a causative agent for symptoms. 1, 4 Therefore:

• Attempt to replicate the patient's exact symptoms during testing for accurate diagnosis 6
• Interpret urodynamic findings in the context of global assessment, including physical examination, voiding diaries, and post-void residual measurements 1, 6
• Consider that symptoms and physical findings often do not adequately predict underlying pathophysiology 6

Specific Testing Components Recommended

1. Post-void residual (PVR) assessment: Essential to evaluate for significant bladder and/or outlet dysfunction, particularly important given the neurological concerns 1, 6

2. Multichannel cystometry: Preferred over single-channel cystometrogram, which is subject to significant abdominal pressure artifacts 1, 4

3. Pressure-flow studies: Can differentiate detrusor underactivity from bladder outlet obstruction, which may be relevant if sphincter dysfunction occurred during surgery 1, 4, 5

4. EMG testing: Should be performed in combination with cystometry in patients with neurologic disease and elevated PVR or urinary symptoms to detect detrusor-external sphincter dyssynergia 1

Differential Diagnosis Framework

The combination of lost rectal sensation and new urinary urgency suggests several possibilities that urodynamics can help clarify:

• Pudendal nerve injury: Must be excluded first, as it produces identical sensory deficits without affecting detrusor contractility; if MRI is negative and voluntary voiding is preserved, this diagnosis is made and managed conservatively 2
• Neurogenic detrusor overactivity: Can occur with conus medullaris or upper cauda equina lesions (L2 or above) 3
• Detrusor underactivity: More common with lower cauda equina lesions (L3 or below), occurring in 91.2% of such cases 3
• Bladder outlet obstruction: May result from sphincter hypertonicity or surgical changes 1

Adjunctive Diagnostic Testing

Anorectal manometry should be performed before biofeedback therapy to document rectal hyposensitivity using balloon-distension protocols that provide objective measurements of first-sensation thresholds. 2 Elevated first-rectal sensory threshold volumes signify sensory impairment that warrants intervention. 2

Important Caveats

• Urodynamic testing carries risks including infection and urethral trauma from catheterization 1
• EMG testing is technically challenging and nonspecific; artifacts are common and require interpretation in context with fluoroscopy, cystometry, and flow rate 1, 4
• Urodynamic findings alone may not precisely predict treatment outcomes 1, 6
• Electrophysiology might be the most useful assessment tool for predicting neurogenic bladder type in patients with suspected cauda equina involvement 3