SPECIFICITY OF THE PEDIATRIC PERIOD

Developmental Considerations

Many organs and their functions are not fully developed at birth. During the embryonic period, the spinal cord occupies the entire spinal canal, but from the fetal period onward, the growth of the spinal canal exceeds that of neural structures; the termination of the spinal cord and that of the dural sac project at progressively higher levels (L4 and S3-4, respectively), up to the end of the first year of life (see Chapter 58 and Chapter 60 ). During the second year, it approaches adult levels (L1 and S2, respectively). As a consequence, a lumbar epidural approach above L4 should be avoided in infants because the spinal cord can be damaged by the needle.

As for all bones, ossification of the vertebral column is not achieved at birth. The vertebral arches fuse posteriorly during the first year of life, and their junction with the vertebral body becomes ossified from the third to the sixth year.^[2] The posterior fusion remains incomplete at the level of S5 and often at S4, delimiting a V-shaped space called the sacral hiatus that is covered by the sacrococcygeal membrane, which is the continuation of the ligamentum flavum at this level. This membrane gives easy access to the epidural space in infants and young children (for caudal anesthesia).

The sacrum is made up of distinct vertebral pieces, the fusion and ossification of which is not fully achieved before the 25th year of life. Posterior approach to the epidural space is possible at sacral levels in the same way as at lumbar levels. However, because the sacrum is cartilaginous, it can be easily traversed by sharp needles, with possible subsequent damage to ossification nucleus and pelvic or retroperitoneal organs.

Myelinization begins during the fetal period in cervical neuromeres and extends progressively but is not fully completed until the end of the 12th year of life.^{[3] [4]} This lack of myelin in young patients makes the penetration of local anesthetics easier within the nerve fibers. Diluted solutions of local anesthetics can produce consistent nerve blockade because the nerve fibers are thinner, which reduces the distance between successive nodes of Ranvier.

At birth, the spine has a single regular flexure, and whatever the intervertebral space, any epidural needle is inserted with the same orientation. With the development of the cervical flexure (i.e., head sustained) and then that of the lumbar lordosis (i.e., sitting position), the orientation of the epidural needle must be modified accordingly. Another important feature is the loose attachment of fascias and sheaths to nerve trunks in young patients. This allows extended spread of local anesthetics, resulting in high-quality nerve blockade whatever the technique and occasionally unwanted or undesirable distant nerve blocks. The main pediatric particularities that influence the selection or performance of a regional block are summarized in Table 45-1 .