Pediatric caudal anesthesia with reference to spread and intracranial effects

Background: Caudal anesthesia is, since 1933, the most common regional anesthetic technique used in children and has hitherto been described as a safe and straightforward method. Recent research has shown that a high-dose (1.5 ml kg-1) caudal block creates an elevated intracranial pressure produced by cerebrospinal fluid (CSF) being shifted cranially by the initial injection of the local anesthetic. The subsequent caudal return of the CSF from the intracranial space to the spinal canal induces a second wave of cranial spread of the local anesthetic. This sequence is named the “CSF rebound mechanism” and explains the biphasic spread of local anesthetic within the spinal canal. Better insight into this spinal spread mechanism has opened new avenues for caudal block research. Aims: First, to provide anatomical data of the caudal-epidural space, to help determine adequate volume dosing (Study 1). Second, to better understand the mechanism behind the biphasic nature of the intraspinal spread, we searched for an anatomical explanation for the initial spread restriction at the thoracolumbar junction (Study 2). Third, to in better detail investigate the effects on cerebral blood flow following a continuous vs. interrupted high-volume caudal injection (Study 3). Lastly, to investigate if a high-volume caudal block produces actually effects on cerebral function, potentially implying transient cerebral ischemia (Study 4). Methods: In Study 1, Magnetic resonance imaging (MRI) was used to assess the volume of the caudal-epidural space in children zero to three years of age judged to have normal spinal anatomy. The volume of caudal, lumbar, and thoracic spinal segments was determined from the sum of the axial MRI slices. In Study 2, the location of the spinal lumbar enlargement, “tumenescence,” of the spinal cord was similarly determined from MRI scans as in Study 1. In Study 3, ultrasound Doppler measurements of middle cerebral artery flow velocities were used as a surrogate to depict changes in cerebral blood flow in relation to continuous or intermittent injection. In Study 4, electroencephalography (EEG) was used to determine potential changes in EEG patterns associated with the injection of a high-volume caudal block, thereby indicating if the caudal block produces a change in cerebral function. Particular emphasis was put on changes in relative delta activity. Results: The median volumes of the epidural space per vertebral segment were; Thoracic: 0.60 ml (95%CI 0.38-0.75); Lumbar: 1.18 ml (95%CI 0.94-1.43) and Caudal: 0.85 ml (95%CI 0.56-1.18), with a linear correlation to both length and weight. The volumes needed to reach L1, Th 10, and Th 6 were 1.3, 1.6, and 1.8 ml kg-1. The lumbar tumenescence was found to be consistently located at Th 11. The cerebral blood flow velocities started to diminish exponentially after injecting approximately 0.5 ml kg-1 of local anesthetics during the continuous injection. Instead, a twopause injection regimen produced a linear and more limited reduction of final blood flow velocities. The EEG showed an immediate increase (within 10 minutes post-injection) in relative delta activity following the caudal injection in most patients. Conclusions: The segmental volumes at different spinal levels are significantly different. The consistent locations of the lower spinal enlargement at Th 11 (together with the effects of the CSF rebound mechanism) may help explain why caudal injections of varying volumes are initially halted at the thoracolumbar junction. Using injection pauses when administering a high-volume caudal block will limit the effect on cerebral blood flow. Furthermore, a high-volume caudal block is associated with a transient change in cerebral function that shows similarities to what is observed during cerebral ischemia. All these findings have clinical implications for the further use of caudal anesthesia in small children