What are the causes of a unilateral epidural block?

Causes of Unilateral Epidural Block

Unilateral epidural block is most commonly caused by lateral catheter placement in the epidural space, with the catheter tip positioned in the intervertebral foramen on one side of the spine, leading to asymmetric distribution of local anesthetic. 1

Anatomical and Technical Factors

Catheter-Related Factors

• Lateral catheter placement: The most common cause, with catheter tips often found lateral to the dura in the intervertebral foramen 2
• Excessive catheter advancement: Insertion beyond 4-5 cm into the epidural space increases risk of lateral placement 1
• Catheter migration: Post-insertion movement of initially well-positioned catheters

Anatomical Barriers

• Epidural septum/plica mediana dorsalis: Midline connective tissue that may impede lateral spread
• Epidural adhesions: From previous surgery, inflammation, or trauma
• Epidural space abnormalities: Hematoma, abscess, spinal stenosis, or disc herniation 1
• Previous spinal surgery: Can alter epidural anatomy and increase risk 1

Technical Factors

• Operator experience: Less experienced providers have 3.77 times greater odds of complications including unilateral block 1
• Time of day: 6.33 times higher risk of complications during night shifts (19:00-08:00) 1
• Approach technique: Midline approach reduces risk of lateral placement compared to paramedian 1

Patient-Related Factors

Positioning

• Lateral positioning: During or immediately after injection can cause gravity-dependent spread of local anesthetic 1
• Patient movement: Movement after catheter placement may affect distribution

Patient Demographics

• Obstetric patients: More likely to experience problematic unilateral block during labor analgesia 1
• Anatomical variations: Individual differences in epidural space anatomy
• Spinal pathology: Conditions like scoliosis can affect distribution

Pharmacological Factors

Local Anesthetic Properties

• Volume administered: Insufficient volume may not spread adequately
• Concentration gradient: Affects spread pattern in epidural space 1
• Baricity of solution: Affects gravitational spread 1
• Viscosity differences: Can influence distribution patterns 1

Prevention and Management

Prevention Strategies

• Limit catheter advancement to 4-5 cm into epidural space 1
• Use midline approach to reduce lateral placement risk 1
• Administer local anesthetic through the epidural needle before catheter insertion to improve quality and reduce complications 3

Management of Unilateral Block

• Catheter manipulation: Withdraw catheter 1-2 cm if unilateral block is suspected 1
• Patient repositioning: Turn patient to the unblocked side to promote gravity-dependent spread 1
• Additional medication: Administer supplemental local anesthetic with patient positioned appropriately

Complications and Considerations

• Unilateral block may lead to inadequate analgesia on one side
• Hypotension due to sympathetic blockade may be less pronounced with unilateral block 4
• Unintended dural puncture can occur in approximately one-third of cases 1
• Motor block assessment using straight-leg raising test or Bromage scale is recommended 1

Pitfalls and Caveats

• Unilateral block should not be confused with failed epidural block
• Asymmetric distribution is common and can be compatible with uniform anesthesia 2
• Air or fat in the epidural space can interfere with solution spread 2
• Excessive catheter manipulation may increase risk of vascular injury or dural puncture
• Rapid administration of additional local anesthetic may increase risk of high or total spinal block 4

Remember that remarkable interindividual variability in patterns of spread is normal, even in successfully functioning epidural analgesia 2. Proper assessment and management techniques can help convert a unilateral block to bilateral anesthesia in many cases.