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10. Regional Anesthesia for Infants and Children
Keywords
Local anesthetic pharmacology in childrenPediatric epiduralCaudal blockSacral hiatusDorsal nerve blockRegional anesthesia is an important part of pediatric anesthesia. A comfortable child is less likely to be agitated after surgery, less likely to dislodge dressings and drains, and less likely to be psychologically traumatized by their experience. Continuous regional analgesia is useful where pain is likely to be severe and prolonged and or difficult to assess such as in children with severe cerebral palsy and subsequent communication difficulties. This chapter will concentrate on areas specific to pediatric regional anesthesia, and it is assumed the reader is familiar with the various blocks also used in adults. Rather than repeating their description here, the focus will be on important differences when performing peripheral nerve blocks in children.
10.1 The Pharmacology of Local Anesthetic Agents in Infants and Children
As is the case with many drugs, the pharmacokinetics of local anesthetic agents are different in neonates and young children compared with adults. The three most important differences are reduced protein binding, reduced metabolism and increased volume of distribution.
Plasma concentration of bupivacaine increases towards toxic levels over time in neonates, but plateaus in infants and children older than 6 months. Based on Meunier et al. Pharmacokinetics of bupivacaine after continuous epidural infusion in infants with and without biliary atresia. Anesthesiology 2001;95: 87–95
Keypoint
There is a high risk of local anesthetic toxicity in neonates and infants during infusions because protein binding and metabolism are reduced. Regional infusions should be stopped after 24–36 h in neonates and infants. The dose of a single, bolus injection is similar in all ages.
Other developmental changes affect local anesthetics in children. Nerve fibers at birth are thin, about half the diameter of adult nerves, and they are less myelinated during the first several years. As a result, low concentrations of local anesthetics in children achieve a block of similar duration and intensity to higher concentrations in adults. The volume of local anesthetic needed is however relatively large due to increased tissue blood flow and tissue spread, and more rapid local clearance.
Maximum doses for single injection blocks and infusions of ropivacaine, l-bupivacaine and bupivacaine in neonates and children
Age group | Maximum bolus injection dose (mg/kg) | Maximum infusion dose (mg/kg/h) |
|---|---|---|
Neonate and infant <6 months | 2.5 | 0.2 |
Child | 2.5 | 0.4 |
Keypoint
The recommended maximum single dose of ropivacaine or l-bupivacaine in children is 2.5 mg/kg.
10.2 Additives to Regional Blocks
Additives to regional blocks and infusions prolong the duration of the block, improve analgesia and provide concurrent sedation. In general, they are avoided in neonates and infants less than 3 months of age due to the risk of sedation and apnea.
Adrenaline (epinephrine) is now rarely used because of concerns about spinal cord ischemia, although it mildly prolongs the duration of caudal analgesia.
Clonidine provides postoperative sedation for the difficult toddler group, as well as analgesia. It can be added to a single shot caudal (1–1.5 μg/kg) or to epidural infusions. A simple mix is to add 1 μg/mL of clonidine to the local anesthetic infusion and run it at the usual rate. Clonidine should be used with caution in infants under 12 months as they can become very sedated, but is otherwise free of side effects apart from occasional mild bradycardia. It prolongs and improves the quality of peripheral blocks in adults, but there is not strong evidence of these effects in children. Small studies in children have not always shown a significant benefit of clonidine, and a large review of a regional block database in Philadelphia found benefit only when very dilute local anesthetic solutions had been used.
Opioids may be added to epidural infusions just as with adults. An alternative is to use opioids by the oral or IV routes to supplement the epidural as required. Some centers do not allow nursing staff to titrate epidural infusion doses. By combining epidural and systemic analgesic techniques, the epidural can provide background analgesia, and opioids can be titrated by nursing staff as required. Dexamethasone is not recommended as an additive in children because safety and efficacy have not yet been established. Ketamine prolongs caudal analgesia, but is not used due to possible neurotoxicity.
10.3 Ultrasound Guidance of Local Anesthetic Blocks
Similar to adult practice, ultrasound facilitates regional anesthesia in children. Ultrasound enables smaller doses of local anesthetic to be more easily, safely and accurately placed in close proximity to nerves, whilst avoiding inadvertent damage to adjacent structures. Ultrasound based techniques are similar to those described in adults with a few exceptions. Smaller probes, depths and needle lengths are often required to optimize the image. Anatomical structures and tissue planes are generally smaller, shallower and better defined. Particular care needs to be taken not to ‘overshoot’ when passing through shallow elastic tissue structures. Maintaining needle visualization throughout a short tissue trajectory can be challenging. Using a sharp hypodermic needle to puncture through the tough elastic skin before inserting a blunt short-bevel needle, helps maintain needle visualization and avoid inadvertent overshoot.
10.4 Complications of Local Anesthetic Blocks
Most pediatric regional blocks are placed after the induction of anesthesia to provide post-operative analgesia. Performing blocks with the child asleep is safe, and may even be safer than with the patient awake. Several large surveys with more than 100,000 patients show complications after blocks in children are uncommon. The most recent prospective survey in 2018 from the Pediatric Regional Anesthesia Network (PRAN), showed complications were uncommon, with a similar, low incidence of complications in peripheral and neuraxial blocks.
10.4.1 Overdose and Systemic Toxicity
These complications are usually due to arithmetic error or accidental use of the wrong strength of solution. It is best to always calculate the maximum allowable amount in milligrams and never draw up more than this, regardless of the volume required. A simple and conservative rule is to limit dosage to 1 mL/kg of ropivacaine 0.2%, l-bupivacaine 0.25% or bupivacaine 0.25%, giving a maximum dose of 2.5 mg/kg (Table 10.1). Overdose with cardiac arrest or convulsion is rare, but infants are more likely to develop these complications. Real-time ultrasound guided blocks reduce the volume required by 30–50% and allow visualization and avoidance of vessels, reducing the risk of intravascular injection. In an audit of over 100,000 pediatric regional blocks by the PRAN group, the rate of severe local anesthetic toxicity was 0.76: 10,000.
Test doses containing adrenaline (epinephrine) have been extensively studied in children, but changes in heart rate, BP and T waves are not sufficiently sensitive or specific, and vary between different volatile agents and propofol. Aspiration tests are also unreliable. It is therefore prudent to give larger volumes of local anesthetic in divided doses, watching for changes in respiration, T wave amplitude, ST segments and heart rate or onset of nodal rhythm.
10.4.2 Neurological Injury
There are few pediatric series of sufficient size to draw definite conclusions, but lasting injury after major plexus and single shot caudal block appears to be extremely rare. Infants less than 4 months of age and pre-teens appear to be most at risk. In a prospective multicenter cohort of more than 100,000 peripheral nerve and neuraxial blocks in children there were no cases of permanent neurological deficit associated with regional anesthesia. The rate of transient neurological deficit was low at 2.4 per 10,000. A UK pediatric epidural audit of 10,000 epidurals reported only one incident with residual effects 12 months after surgery in a 4 month old.
10.4.3 Injury to Visceral Structures
Is a concern during blocks of the anterior abdominal wall. Rectal damage during caudal block has also been reported.
10.4.4 Minor Complications
Pressure areas during continuous blocks in children are not uncommon, usually manifesting as heel redness or rarely skin loss. Urinary retention is common with continuous epidural blockade and warrants catheterization at all ages. Lower limb weakness and delayed ambulation may occur after caudal blockade with high concentrations of local anesthetic, or after Iliohypogastric block with inadvertent spread to the femoral nerve. Block failure is usually due to depositing the local anesthetic too deeply, as most peripheral nerves are quite superficial. Accurate visualization of nerves using ultrasound may reduce the incidence of this to zero.
10.5 Neuraxial Blocks
The anatomical curves of the spine are absent at birth and not fully fixed until puberty, altering the spread of spinal and epidural local anesthetics.
There is less variation in the angulation of the spinous processes in children, allowing easier access to thoracic and lumbo-sacral epidural spaces.
The spinal cord ends at L3 at birth and moves to the adult position around L1 by 12 months of age.
The sacrum is not fully ossified with intervertebral spaces still present, allowing sacral epidural access.
The dural sac ends at S3-4 in the neonate, moving to the adult level around S2 by 12 months of age. This is variable and it occasionally extends to the sacral hiatus in infancy.
The line joining the two superior iliac crests (the intercristal line) is through L5 in children and L5-S1 in neonates.
Hypotension, even with extensive block, is uncommon under 8 years of age unless hypovolemia is present. (Related to reduced resting sympathetic tone).
10.5.1 Caudal Epidural Blockade
Caudal blocks are best used as a single shot block for procedures below the umbilicus in infants and small children. The inferior termination of the epidural space can be approached via the sacral hiatus which is covered by skin and the sacrococcygeal membrane (the continuation of the ligamentum flavum). There is usually a clear loss of resistance or ‘pop’ as the membrane is penetrated. The relationship between volume of solution injected and extent of the block are reasonably predictable since only cephalad spread is possible. Recent studies show the anatomical spread of local anesthetic seen on ultrasound is less than the clinical block obtained, perhaps suggesting the mechanism of epidural blockade is still not fully understood.
10.5.1.1 Technique
Anatomy of the caudal block. Injection is through the sacral hiatus, located in the lower sacrum between the two sacral cornua. (a) The sacral hiatus is at the apex of an equilateral triangle formed by it and the two posterior superior iliac spines. (b) The caudal canal is largest cephalad rather than caudally, and the needle is best inserted towards the top part of the triangular sacral hiatus
Tip
If you are having trouble finding the sacral hiatus, you are probably too low on the back.
The sacrum is a flat structure in infants and children and the technique of needle puncture, flattening and advancing is inappropriate and may cause bloody tap or dural puncture. The needle should be advanced at an angle of 45–60° to the skin at the apex of the hiatus and not advanced once the sacrococcygeal membrane is penetrated, as the dural sac may be very close, particularly in babies. Extreme care is needed to prevent needle dislodgement during aspiration and slow injection. This is best done by stabilizing the needle with a hand resting along the child’s back and either making a window between your thumb and index finger or an underhand technique to allow for early visualization of subcutaneous sacral swelling during injection, a warning of incorrect needle placement. Importantly, there should be minimal resistance to injection. If injection is difficult it is invariably an indication of incorrect placement. Many would advocate always using a similar sized syringe to get a consistent feel for injection.
10.5.1.2 Anatomical Difficulties
The anatomy of the sacral hiatus and caudal canal is highly variable. Sacral variations such as absent cornua, a bony septum or a presacral fat pad can make locating the hiatus technically challenging. Difficulty in locating the sacral hiatus has been reported in at least 11% of children under 7 years of age. Occasionally the sacral hiatus extends one or two segments more cephalad than usual, making dural puncture more likely if needle placement is in the most cephalad point of the long, slit-like hiatus. The correct site for needle placement can be judged using the above two methods. Some advocate ultrasound guidance to aid placement, particularly if there are concerns regarding the anatomy, although routine use of this practice is not widespread.
Cutaneous anomalies including sacral angioma, hairtuft, nevus or dimples near the puncture site may indicate abnormalities of the underlying spine. Midline sacral dimples are found in 2–4% of children and are usually of no significance. Rarely, they are associated with an occult spina bifida. Warning signs of an underlying abnormality include multiple dimples, or high on the back (should be below caudal insertion site, less than 2.5 cm above the anal verge), or more than 5 mm diameter, or associated with an underlying lump or a deviated or double gluteal cleft. Ultrasound can be used to clarify the underlying anatomy, or the block can be abandoned and a pediatric opinion obtained after surgery. Finally, if the sacral hiatus doesn’t feel normal, it might be safer to use different analgesia rather than persisting and causing problems.
10.5.1.3 Needles for Caudal Epidural Blocks
The short-bevel styletted regional or spinal needles have the lowest risk of actual and theoretical complications. They give an obvious sensation or ‘pop’ passing through the membrane, and venous and dural puncture is less likely than with standard needles. The 22G needle is suitable for all ages. Fine gauge needles introduce the risk of unrecognized intra-osseous injection in neonates and infants. Intravenous needles (bloody tap rate 10%) and cannulae (require advancement into epidural space and may kink) still remain popular. Needles without stylettes introduce a small theoretical risk of implantation dermoid which most practitioners do not regard as significant.
10.5.1.4 Local Anesthetic Agents and Doses for Caudal Block
Relationship between volume and block height for caudals in children, based on the classical paper: Armitage EN, Anaesthesia 1979;34: 396
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