Can muscle relaxants cause postoperative urinary retention after general anesthesia?

Muscle Relaxants and Postoperative Urinary Retention

Muscle relaxants themselves do not directly cause postoperative urinary retention (POUR), but the anticholinergic agents used to reverse them—specifically glycopyrrolate given with neostigmine—are a significant and modifiable cause of POUR after general anesthesia.

The Mechanism: It's About the Reversal Agent, Not the Relaxant

The issue is not the neuromuscular blocking drugs (rocuronium, cisatracurium, atracurium) used during surgery. Rather, the problem lies with the traditional reversal strategy using neostigmine plus glycopyrrolate 1. Here's why:

• Neostigmine is an acetylcholinesterase inhibitor that reverses neuromuscular blockade by increasing acetylcholine at the neuromuscular junction
• Glycopyrrolate (an anticholinergic) must be co-administered with neostigmine to prevent unwanted muscarinic side effects like bradycardia and bronchospasm
• The problem: Glycopyrrolate decreases bladder detrusor muscle contractility, directly impairing the ability to void postoperatively 2

The Evidence: Striking Differences in POUR Rates

The most recent and highest quality evidence demonstrates dramatic differences based on reversal agent choice:

A 2024 multicenter retrospective cohort study of 157,398 patients found that neostigmine/glycopyrrolate reversal was independently associated with:

• 2.2-fold increased risk of POUR in inpatients (5.0% vs 2.4% with sugammadex)
• 2.6-fold increased risk in outpatients (0.9% vs 0.4%)
• Five-fold higher POUR-related emergency visits within 2 days of discharge 3

A 2024 single-institution study showed even more striking results:

• 0% POUR rate with sugammadex vs 8.4% with neostigmine/glycopyrrolate in minimally invasive hernia repair
• 25-times higher odds of POUR with neostigmine/glycopyrrolate after controlling for age and intravenous fluids 4

A 2022 retrospective review confirmed:

• 4.3-times higher POUR rate with neostigmine/glycopyrrolate compared to sugammadex 5

Clinical Algorithm for Preventing POUR

Step 1: Identify High-Risk Patients

Patients at elevated risk for POUR include 6:

• Age >60 years (25.4% POUR rate vs 10.2% in younger patients)
• Male sex (19.6% vs 9.3% in females)
• Diabetes mellitus (24.0% POUR rate)
• Pre-existing lower urinary tract symptoms (33.3% POUR rate)
• Spinal anesthesia (23.6% vs lower rates with general anesthesia)
• Surgery duration >90 minutes (26.2% POUR rate)
• Perioperative IV fluids >1500 mL (24.1% POUR rate)
• Postoperative opioid use (21.9% POUR rate)

Step 2: Choose Reversal Strategy Based on Risk

For all patients, but especially high-risk patients:

• Use sugammadex for reversal of rocuronium or vecuronium whenever feasible 3, 4, 5, 2
• Sugammadex directly encapsulates steroidal muscle relaxants without requiring anticholinergic co-administration 1
• Dose sugammadex according to depth of blockade (2-8 mg/kg based on train-of-four monitoring) 1

If neostigmine must be used:

• Wait for spontaneous recovery to at least 4 tactile responses to train-of-four stimulation before administering neostigmine 1
• Use standard dose of 0.04 mg/kg (40 mcg/kg) with atropine 0.02 mg/kg 1
• Anticipate 10-20 minutes for complete reversal (TOF ratio ≥0.9) 1
• Recognize that POUR risk will be substantially elevated regardless of technique refinements

Step 3: Postoperative Monitoring

The 2013 ASA Practice Guidelines recommend assessment of urine output and voiding during emergence and recovery 7. However, objective assessment is superior to subjective reporting:

• Monitor for inability to void within 6-8 hours postoperatively 6
• Consider bladder ultrasound for post-void residual measurement in high-risk patients 8
• Watch for suprapubic discomfort and bladder distension 6

Important Caveats and Pitfalls

Common Misconception: "The Muscle Relaxant Caused Retention"

This is incorrect. The neuromuscular blocking drugs (when properly reversed) do not cause POUR. The culprit is the anticholinergic component of traditional reversal 2.

Residual Neuromuscular Blockade

While not directly causing POUR, inadequate reversal of neuromuscular blockade is associated with:

• Higher morbidity and mortality within 24 hours postoperatively 1
• Greater risk of critical respiratory events
• Greater risk of postoperative pneumonia 1

Always ensure complete reversal (TOF ratio ≥0.9) through quantitative monitoring at the adductor pollicis 1.

Special Populations

• Renal failure patients: Use standard sugammadex doses despite renal elimination; clinical efficacy is maintained 1
• Elderly patients: Sugammadex efficacy may be slightly decreased; monitor carefully 1
• Bariatric surgery: One study paradoxically found less neostigmine associated with increased POUR, but this likely reflects confounding by indication 9

The Bottom Line

To minimize POUR after general anesthesia, preferentially use sugammadex rather than neostigmine/glycopyrrolate for reversal of neuromuscular blockade. This single intervention can reduce POUR rates by 50-100% across diverse surgical populations, translating to fewer catheterizations, urinary tract infections, emergency visits, and prolonged hospital stays 3, 4, 5, 2. The muscle relaxants themselves are not the problem—it's how we reverse them that matters for bladder function.