What is the differential diagnosis (Ddx) for a patient unable to pass urine after a fall?

Differential Diagnosis: Post-Fall Urinary Retention

In a patient unable to pass urine after a fall, the most critical immediate concern is urethral injury, particularly if there is blood at the meatus, which mandates retrograde urethrography before any catheterization attempt. 1

Immediate Assessment Priorities

Rule Out Urethral Trauma First

• Examine for blood at the urethral meatus - this finding is present in 37-93% of urethral injuries and is the single most important clinical sign 1
• If blood is present at the meatus, perform retrograde urethrography immediately before attempting catheterization to avoid converting a partial injury into complete disruption 2, 1
• Pelvic fractures combined with blood at meatus carry a 50% incidence of genitourinary injury 1
• Never perform blind catheter passage when urethral injury is suspected 2, 1

Establish Bladder Drainage Urgently

• The immediate goal is securing catheter drainage regardless of injury type 1
• If urethral catheterization fails or is contraindicated, place a suprapubic tube to establish prompt urinary drainage 2, 3
• Avoid repeated catheterization attempts as this increases injury extent and delays definitive drainage 2

Differential Diagnosis by Mechanism

Traumatic/Obstructive Causes (Most Likely Post-Fall)

Urethral injury - highest priority diagnosis

• Pelvic fracture-associated urethral injury is the classic post-trauma presentation 2, 1
• Straddle injuries from falls carry high risk for delayed stricture formation 1
• Posterior urethral injuries should not undergo immediate sutured repair due to unacceptably high rates of erectile dysfunction and incontinence 1

Bladder injury

• Consider CT cystography if concomitant bladder injury suspected, particularly with pelvic fractures 1
• Bladder rupture can present with inability to void 2

Pelvic hematoma causing external compression

• Large pelvic hematomas from fractures can compress the bladder neck or urethra 2

Spinal cord injury from fall

• Neurogenic bladder from spinal trauma is a critical diagnosis requiring urgent neurologic assessment 4, 5
• Assess for saddle anesthesia, lower extremity weakness, and anal sphincter tone 4

Non-Traumatic Causes (Consider if Mechanism Unclear)

Benign prostatic hyperplasia (BPH) - accounts for 53% of acute urinary retention cases overall

• Most common cause in elderly men, though typically not directly fall-related 4, 5
• Digital rectal examination reveals prostatic enlargement 6

Medications precipitating retention

• Anticholinergics and alpha-adrenergic agonists are common culprits 4, 7
• Review all prescription medications, over-the-counter drugs, and herbal supplements 5

Infection/inflammation

• Prostatitis, cystitis, or urethritis can cause acute retention 4, 7
• Urinalysis and culture should be obtained after catheterization 6

Neurologic causes unrelated to trauma

• Cortical, spinal, or peripheral nerve lesions 4
• Requires monitoring with neurology and urology subspecialists 5

Diagnostic Algorithm

1. Inspect urethral meatus for blood - if present, proceed to step 2; if absent, proceed to step 4 2, 1

2. Perform retrograde urethrography using 12Fr Foley catheter with 20 mL undiluted water-soluble contrast 1

3. Based on urethrography findings:

   • Partial disruption: single attempt at well-lubricated catheter by experienced provider 2, 1
   • Complete disruption: suprapubic tube placement 2, 3

4. If no blood at meatus, assess for:

   • Pelvic fracture on imaging (high suspicion for occult urethral injury) 2, 1
   • Neurologic deficits suggesting spinal cord injury 4, 5
   • Palpable distended bladder with dull percussion note 6

5. Catheterize and measure post-void residual (PVR):

   • No consensus PVR definition for acute retention exists 5
   • Chronic retention defined as PVR >300 mL on two occasions over 6 months 5

6. Post-catheterization workup:

   • Urinalysis and culture 6
   • Renal function assessment for upper tract damage 6
   • Digital rectal examination for prostatic assessment 6
   • Avoid PSA testing immediately as it will be falsely elevated 6

Critical Management Pitfalls

• Never attempt blind catheterization with blood at meatus - this can convert partial to complete urethral disruption 2, 1
• Do not delay bladder drainage - prolonged catheterization >3 days is associated with higher comorbidity and adverse events 8
• Avoid immediate surgical repair of posterior urethral injuries - associated with devastating complications 1
• Do not dismiss the fall mechanism - even minor trauma can precipitate retention in patients with underlying BPH 8, 6

Disposition Considerations

Admit if:

• Urosepsis or systemic illness 6
• Abnormal renal function requiring monitoring 6
• Acute neurological problems 6
• Unable to manage catheter at home 6

Trial without catheter (TWOC):

• Plan within 2-3 days of initial catheterization 8, 6
• Alpha-blocker initiation at time of catheter insertion increases success rates 8, 5
• 23-40% of patients void successfully after TWOC 8