Causes of Unilateral Epidural Block
The most common causes of unilateral epidural block are catheter malposition (particularly lateral placement in the epidural space or within the intervertebral foramen) and anatomical barriers that prevent uniform spread of local anesthetic.
Anatomical and Technical Factors
Catheter Positioning
- Lateral catheter placement: The most frequent cause of unilateral block is catheter tip positioned laterally in the epidural space or within an intervertebral foramen 1
- Distance from midline: Greater lateral deviation from midline increases likelihood of unilateral block
- Depth of insertion: Excessive advancement of catheter can lead to unilateral placement
Anatomical Barriers
- Midline septum: Connective tissue bands in the posterior epidural space may impede spread of local anesthetic
- Epidural fat: Uneven distribution of epidural fat can create asymmetric channels for medication flow
- Dural folds: Posterior dural folds can create compartmentalization within the epidural space 1
Technical Factors
- Injection speed: Rapid injection increases likelihood of uniform spread
- Patient positioning: Lateral positioning during or immediately after injection promotes gravity-dependent spread
- Keeping patients in lateral position for 15-20 minutes after injection increases likelihood of unilateral block 2
- Volume of injectate: Smaller volumes (≤5ml) more likely to produce unilateral effects
- Needle type and direction:
- Needle bevel direction affects initial spread of local anesthetic
- Pencil-point needles (Whitacre) vs cutting needles (Quincke) can influence distribution 2
Pathological Causes
Space-Occupying Lesions
- Epidural adhesions: From previous surgery, inflammation, or trauma
- Epidural hematoma: Blood accumulation compressing dural sac unilaterally
- Epidural abscess: Localized infection causing asymmetric compression
Other Pathological Conditions
- Spinal stenosis: Narrowing of spinal canal affecting distribution
- Disc herniation: Protrusion causing asymmetric epidural space
- Previous spinal surgery: Scar tissue altering epidural anatomy
Procedural Complications
Inadvertent Placement
- Subdural injection: Medication spread between dura and arachnoid layers
- Intrathecal catheter migration: Partial entry of catheter through dura
- Unrecognized dural puncture: Approximately one-third of dural punctures go unrecognized 3
Medication-Related Factors
- Concentration gradient: Higher concentration on side of injection
- Baricity of solution: Hyperbaric solutions flow with gravity
- Viscosity differences: Affects spread pattern in epidural space
Prevention and Management
Prevention Strategies
- Midline approach: Reduces risk of lateral placement
- Pre-catheter injection: Administering local anesthetic through the epidural needle before catheter insertion improves block quality and reduces complications 4
- Proper catheter advancement: Limited to 4-5 cm into epidural space
- Test dose: Careful assessment after initial injection
Management Approaches
- Catheter manipulation: Withdraw 1-2 cm if unilateral block suspected
- Patient repositioning: Turn patient to unblocked side to promote gravity-dependent spread
- Supplemental injection: Additional volume may improve spread
- Catheter replacement: Consider if persistent unilateral block affects analgesia quality
Clinical Assessment
- Straight-leg raising test: Recommended as screening tool for motor block 3
- Bromage scale: For formal documentation of motor block if detailed assessment needed 3
- Sensory level testing: Assess bilateral temperature and pinprick sensation
Special Considerations
- Obstetric patients: Unilateral block more problematic during labor analgesia
- Operator experience: Less experienced providers have higher rates of inadvertent dural puncture (3.77 times greater odds) 3
- Time of day: Higher risk of complications during night shifts (19:00-08:00) with 6.33 times higher relative risk of inadvertent dural puncture 3
Recognizing the cause of unilateral epidural block is essential for appropriate management and prevention of inadequate analgesia. Careful technique, proper catheter positioning, and understanding of epidural space anatomy are key to achieving bilateral, effective epidural anesthesia.