The sacral hiatus is the external lower opening of the spinal canal

The sacral nerves provide innervation of the deep and superficial pelvic floor and the viscera of the pelvic cavity

The caudal space is visualised underneath the sacral part of the supraspinous ligament

The caudal (epidural) space is located underneath the sacral part of the supraspinous ligament

The ligament is deep to the iliocostalis and longissimus thoracis muscles

The sacral hiatus and the caudal space can be visualised with ultrasound in the transverse view

The caudal block is performed by injection of local anaesthetic via the lower external opening of the vertebral canal – called the sacral hiatus

Jean Francois Fernel (1497-1558) a French physician, was the first who described the vertebral canal

The left and right supraspinous ligaments extend from the ischial spines to the sacrum and coccyx

The sacral part of the supraspinous ligament is the caudal end of the supraspinous ligament, that connects the spinous processes from the seventh spinous process to the coccyx

The sacral part of the supraspinous ligament covers the sacral hiatus – the caudal opening of the vertebral canal

The caudal block is performed by inserting the needle through the supraspinous ligament into the sacral hiatus

The sacral hiatus is located between the prominent fused articular processes – the cornua – and underneath the sacral part of the supraspinous ligament

The transversalis fascia (TF) covers the deep surface of the transverse abdominis muscle (TAM), the quadratus lumborum muscle QLM) and the psoas major muscle (PMM).

Anteriorly it covers the deep surface of the rectus abdominis muscle; it is covered by the peritoneum

The layers of the abdominal wall from the outside are: external oblique muscle (EOM), internal oblique muscle (IOM) and TAM

The latissimus dorsi muscle (LDM) and the erector spinae muscle (ESM) are superficial to the QL muscle

The patient is placed in the lateral position with the side to be anaesthetized turned upwards

A curved array transducer (6-2 MHz) is placed in the transverse plane at the abdominal flank immediately cranial to the iliac crest