2014A07 Describe the local anatomy relevant to the performance of a lumbar epidural block.

Epidural space

·   Potential space

·   Catheter inserted via a needle

·   Local anaesthetic +/- other drugs injected

Boundaries

·   Superior: fusion of dura to skull

·   Inferior: sacrococcygeal membrane

·   Anterior: dura mater for posterior epidural space, posterior longitudinal ligament for anterior epidural space.

·   Posterior: ligamentum flavum, laminae

·   Lateral: pedicles, intervertebral foramina

Contents

Fat:

·   ↑ in obese

·   ↓ in elderly

Loose areolar connective tissue:

·   May cause septation, patchy block

Arteries:

·   Lateral location

·   Supply from spinal branch of lumbar artery

·   Damage -> expanding haematoma

Venous plexus:

·   Anterior > posterior location

·   Drain to: lumbar veins

·   Engorged if uterine contraction

·   Risk of puncture, catheter insertion and local anaesthetic systemic toxicity (LAST)

Lymphatics

Spinal nerve roots

Midline layers

·   Skin -> fat -> supraspinous-ligament -> interspinous ligament -> ligamentum flavum -> epidural space

·   Further: -> dura mater -> subdural space -> arachnoid mater -> subarachnoid space

Midline procedure

·   Sitting and hunched

·   Tuffier’s line = between upper outer iliac crests = L4/5 interspace

·   Needle entry midway between two spinous processes

·   Angle anteriorly (spinous processes direct posterior)

·   Loss of resistance to saline on entry into epidural space

Anatomical complications

·   Subcut placement -> failed block

·   In subdural space -> unreliable, patchy

·   In subarachnoid space -> dense motor block, high block, hypotension, post-dural puncture headache

·   In epidural vein -> LAST

·   In epidural artery -> expanding epidural haematoma

·   In paravertebral space -> unilateral block