After a fistulotomy I have decreased awareness of bladder filling but can initiate a normal urinary stream through the penis; what is the likely cause and how should I evaluate and manage this altered bladder sensation?

Altered Bladder Sensation After Fistulotomy

Your decreased awareness of bladder filling with preserved ability to initiate normal voiding most likely represents incomplete cauda equina syndrome (CESI) or peripheral nerve injury from the fistulotomy, and you require urgent MRI of the lumbosacral spine and immediate urological evaluation to prevent progression to permanent bladder dysfunction. 1

Critical Red Flags Requiring Immediate Action

Any new bladder dysfunction—including impaired bladder sensation, hesitancy, or altered urethral sensation—even with preserved voluntary control of micturition, represents incomplete cauda equina syndrome (CESI) until proven otherwise and demands emergency MRI. 1

• The British Journal of Neurosurgery explicitly states that "impaired bladder or urethral sensation" with preserved control of micturition defines CESI, which if treated promptly prevents progression to cauda equina syndrome with retention (CESR) where permanent neurological damage occurs 1
• Your ability to initiate a normal stream does NOT exclude serious neurological injury—CESI patients retain voluntary control but have other disturbances like reduced bladder sensation, which precisely matches your presentation 1

Most Likely Diagnosis: Incomplete Cauda Equina Syndrome vs. Peripheral Nerve Injury

Cauda Equina Syndrome (Most Urgent Consideration)

Reduced bladder filling sensation with preserved voiding ability is a defining feature of incomplete cauda equina syndrome (CESI), not a benign symptom. 1

• CESI patients have "objective evidence of CES but retain voluntary control of micturition" with disturbances including "reduced bladder or urethral sensation" 1
• If treated at the CESI stage, patients typically achieve "normal, or socially normal, bladder and bowel control" long-term 1
• If CESI progresses to CESR (retention), many patients require "intermittent self-catheterisation, manual evacuation of faeces" permanently 1

Peripheral Nerve Injury from Fistulotomy

Fistulotomy can directly injure pudendal nerve branches that provide sensory innervation to the bladder neck and urethra, causing altered sensation without affecting detrusor contractility. 1, 2

• The pudendal nerve provides sensory innervation to the perineum, urethra, and contributes to bladder sensation 1
• Peripheral neuropathy affecting bladder function presents with "impaired bladder or urethral sensation" while motor function (ability to void) may remain intact initially 2

Immediate Diagnostic Workup (Within 24 Hours)

Step 1: Emergency MRI Lumbar Spine

Obtain MRI of the lumbosacral spine without contrast as an emergency—do not wait for symptoms to worsen. 1, 3

• MRI cannot diagnose CES (which is clinical) but identifies "significant compression of the cauda equina roots" requiring emergency surgery 1
• The British Journal of Neurosurgery states that "MR imaging is required, as an emergency, in patients who have 'red flag' symptoms and/or signs of CES" 1

Step 2: Assess for Additional Cauda Equina Features

Perform focused examination for bilateral radiculopathy, perineal sensation changes, anal tone, and progressive leg weakness. 1, 3

• Check for bilateral radicular pain or sensory loss in legs 1, 3
• Test perineal sensation systematically—"subtle impairment of perineal sensation is easily missed or misinterpreted" 1
• Perform digital rectal examination for anal tone, though "findings have low interobserver reliability" 1
• Document any leg weakness or gait changes 1, 3

Step 3: Measure Post-Void Residual

Obtain post-void residual (PVR) measurement immediately—either by bladder ultrasound or catheterization within 30 minutes of voiding. 4, 5

• PVR >250-300 mL confirms significant urinary retention and indicates risk for kidney damage 4, 5
• Repeat PVR measurement 2-3 times due to "marked intra-individual variability" to confirm findings 5
• Normal PVR does not exclude neurological injury but helps stratify urgency 4

Step 4: Urological Consultation

Arrange urgent urological consultation for pressure-flow urodynamic studies if MRI is negative for cauda equina compression. 4, 5

• Urodynamics are "mandatory before invasive therapy" when there is "concomitant neurologic disease affecting bladder function" 5
• Pressure-flow studies are "the only method to distinguish detrusor underactivity from bladder outlet obstruction" 5

Management Algorithm Based on Findings

If MRI Shows Cauda Equina Compression

Proceed to emergency neurosurgical decompression within 12 hours if any perineal sensation or anal tone is preserved. 1

• "Recovery of function is more likely if there is some perineal sensation preoperatively" 1
• Surgery at CESI stage prevents progression to CESR with permanent dysfunction 1

If MRI is Negative but PVR >250-300 mL

Initiate intermittent catheterization every 4-6 hours immediately to prevent bladder overdistension and upper tract damage. 4, 5

• Keep individual catheterization volumes <500 mL to "maintain physiologic bladder capacity" 5
• Proceed to urodynamic studies to differentiate obstruction from detrusor underactivity 4, 5

If MRI Negative and PVR Normal

Consider peripheral pudendal nerve injury from fistulotomy and manage conservatively with close monitoring. 1, 2

• Peripheral neuropathy-associated bladder dysfunction may improve spontaneously over weeks to months 2
• "Management of urinary symptoms in patients with bladder dysfunction is usually supportive" 2
• Monitor with repeat PVR measurements every 4-6 weeks 5

Critical Pitfalls to Avoid

Never dismiss altered bladder sensation as "just part of recovery" from pelvic surgery—this is a neurological emergency until proven otherwise. 1, 3

• "Dismissing mild bladder symptoms as 'just part of sciatica' is a critical error, as even subtle changes in bladder function require emergency evaluation" 3
• "White flag features can be a diagnosis made too late"—waiting for urinary retention or incontinence means permanent damage has already occurred 1

Do not start antimuscarinic medications (for presumed overactive bladder) without first measuring PVR—this will worsen retention and can precipitate acute urinary retention. 4

Do not base clinical decisions on a single PVR measurement—always confirm with repeat testing due to substantial variability. 5

Do not assume normal voiding ability excludes serious neurological injury—CESI is defined by preserved voluntary control with other bladder disturbances. 1

Prognosis and Expected Outcomes

If this represents CESI and is treated urgently (within 48 hours), you have excellent chance of complete recovery of bladder function. 1

• Patients treated at CESI stage typically achieve "normal, or socially normal, bladder control" long-term 1
• Delay beyond CESI stage results in "severe impairment of cauda equina function with a paralysed, insensate bladder" requiring permanent catheterization 1

If this represents isolated peripheral nerve injury from fistulotomy, spontaneous recovery over 3-6 months is possible with supportive management. 2