Secondary spread of caudal block as assessed by ultrasonography

Redistribution and secondary spread after the initial injection of local anaesthetics (LAs) are important factors that contribute to the final spread of caudal block in children. However, to date, these phenomena have yet not been studied in detail. Thus, the aim of this observational study was to define patterns of secondary spread and redistribution of a caudal block by means of real-time ultrasonography scanning and cutaneous testing.

Ultrasound assessment of LA spread within the caudal-epidural space and epidural pressure was followed during 15 min after initial injection (1.5 ml kg(-1), ropivacaine 0.2%) in 16 infants. At 15 min post-injection, cutaneous testing was also performed to assess the cranial dermatomal level of the block (at end-tidal sevoflurane 2.5%).

The median ultrasound-assessed cranial spread was Th10 and Th8 at 0 and 15 min, respectively, and the sensory level at 15 min was Th4. The caudal injection was initially found to compress the terminal part of the dural sac, later followed by a partial re-expansion as epidural pressure was returning towards pre-injection values. An intrasegmental redistribution from the dorsal to the ventral compartment of the epidural space was also observed.

Two separate patterns of secondary spread of caudal block could be observed, being horizontal intrasegmental redistribution and longitudinal cranial spread. The observed bi-directional movement of cerebrospinal fluid (coined 'the CSF rebound mechanism') does explain a major part of the difference between the initial ultrasound-assessed cranial level and the final level determined by cutaneous testing.