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The epidural space surrounds the spinal cord and the meninges. It extends from the foramen magnum to the sacral hiatus. It is limited posteriorly by the vertebral laminae and the ligamentum flavum, and it communicates with the paravertebral spaces and the perineural space surrounding the spinal nerves. In the dural cuff region near the spinal ganglia, it is intimately associated with the subarachnoid space because of the protrusion of arachnoid granulations, which are easily traversed by local anesthetics. It contains blood vessels and lymphatics and is filled with fat in infants; it is more densely packed and less permeable to local anesthetics in children older than 7 to 8 years. The epidural veins are numerous and plexiform. They are connected with the azygous system and the inferior vena cava, and because they have no valves, an inadvertent intravascular injection results in immediate systemic distribution. In the cadaver, the epidural space is triangular in transverse sections, with the widest part lying posteriorly in the median sagittal plane. Midline approaches are usually considered safer than paramedian approaches.
Under physiologic conditions, the epidural space probably has only a potential volume, and epidural anesthesia should be considered as a dynamic process because any epidural injection produces changes in pressure and a variation in the normal anatomy.[173] The subatmospheric pressure used for detecting epidural penetration results from displacement of the dura mater by the block needle. Because subcutaneous tissues and vertebral ligaments are less densely packed in infants and young children, the LOR felt as the needle penetrates the ligamentum flavum is less significant than in adults and influences the choice of LOR technique. Previous extradural anesthesia may result in proliferation of connective tissue, adhesions between the dura mater and the ligamentum flavum, and granulation and changes in the ligamentum flavum due to inflammation, which may reduce the spread of local anesthetics during further epidural anesthesia.[174]
Epidural anesthesia can be used for operations of the lower extremities and almost any part of the trunk,[72] [97] [98] including the chest,[175] but it is used mostly for low abdominal and retroperitoneal surgery. The selection among caudal, sacral intervertebral, and lumbar routes may be difficult. Most operations on the pelvis and lower limbs in infants and young children are performed under caudal anesthesia, provided that a single-shot technique is adequate. In patients older than 6 years or when a reinjection catheter is mandatory, intervertebral epidural approaches are preferred. Specific contraindications include severe malformations of the vertebral column and the spinal cord such as complete spina bifida or meningocele. Patients with a history of hydrocephalus or severe convulsive disorders and those with vertebral implants, reduced intracranial compliance, or elevated intracranial pressure should not be given epidural anesthesia, although these are not absolute contraindications.[112]
The midline lumbar intervertebral approach is the usual technique, with the patient in the sitting position if awake or, more commonly, the lateral decubitus position with the side to be operated lying lowermost and the hips and knees flexed 90 degrees ( Fig. 45-4 ). The spine is bent to open the interspinous spaces and enlarge the interlaminar space. Because the spinal cord ends at a lower level in children, it is safer to select the L4-5 or the L5-S1 interspace (i.e., Taylor approach). The line joining the two iliac crests crosses the skin projection of the fifth lumbar vertebra that allows easy identification of the fifth and fourth spinous processes by palpation. The site of puncture is at the mid-distance of both spinous processes and on the midline. After an intradermal wheal of local anesthetic in conscious children, the epidural needle is inserted at right angles to the skin until it contacts the interspinous ligament. The introducer needle is then removed, and the syringe used for detecting the epidural space is connected. The epidural needle is then slowly advanced toward the laminae while constant pressure (with fluid) or intermittent pressure (with gas) is exerted on the barrel of the syringe. Increased resistance is detected when the needle touches the ligamentum flavum, followed by a sudden LOR as the tip of the needle
Figure 45-4
Epidural block procedures: sacral intervertebral approach
(1), lumbar approach (i.e., midline route) (2), and thoracic approach (i.e., midline
route) (3).
The next step consists of injecting the local anesthetic slowly through the epidural needle or inserting an epidural catheter for repeat or continuous infusion of local anesthetic. Tunneling of the catheter facilitates its immobilization and decreases the risks of accidental removal and bacterial contamination.[177] As a rule, no more than 2 to 4 cm should be introduced within the epidural space, but occasionally, as for caudal catheters, a longer distance can be inserted to reach upper lumbar of thoracic neuromere when a more cephalad intervertebral approach to the epidural space must be avoided.[178] Some anesthesiologists recommend using electrical stimulation to locate more precisely the final position of the tip of the catheter.[179]
Difficulties may be encountered during the procedure because of an inappropriate insertion route, insufficient flexion of the spine, or vertebral deformities, but these problems are uncommon. Like adults, conscious children may rarely complain of paresthesias, which have long been assumed to be related to potential spinal damage even though the spinal cord has no intrinsic innervation and no nociceptors. These paresthesias result from irritation or trauma to epidural blood vessels.[180]
The recommended volume of anesthetic solution depends on the upper level of analgesia necessary for completion of the surgery. Blocking one neuromere requires approximately 0.1 mL per year of age.[181] In practice, after the administration of 1 mL/kg (up to a maximum of 20 mL), the final height of sensory block ranges from T9 to T6 in more than 80% of cases.
Single-shot epidural anesthesia is appropriate for many pediatric cases, especially when adjuvants such as clonidine (1 to 2 µg/kg), preservative-free ketamine (0.25 to 0.5 mg/kg), and with appropriate indications, morphine (30 µg/kg) are added to the local anesthetic. Placement of an epidural catheter permits intraoperative reinjections (i.e., same volume but one half of the initial concentration of bupivacaine) and then continuous infusion of local anesthetic (i.e., bupivacaine or ropivacaine), plain or with the addition of opioids. Infants younger than 12 months are more prone to accumulation with prolonged continuous infusions, and doses exceeding 0.375 mg/kg/hour for the first 24 hours should be avoided; the infusion rate should be one half of this dose for the following days. Commonly used doses of local anesthetics and opioids for single-shot epidural anesthesia and continuous epidural infusions are displayed in Table 45-9 .
Because of incomplete ossification of the sacrum, there are true posterior sacral interspaces suitable for epidural approach at virtually any sacral level. The easiest approach is through the S2-3 interspace with the patient
| Single-shot injections | Lidocaine or mepivacaine: 5 to 8 mg/kg |
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Bupivacaine: |
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>1 yr old: up to 3 mg/kg of a 0.25 (1.2 mL/kg) to 0.5% (0.6 mL/kg) solution |
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<1 yr old: do not administer more than 2 mg/kg of a 0.25% (0.75 mL/kg) solution or less concentrated solution |
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Ropivacaine: |
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Infants: up to 3 mg/kg of a 0.2% solution (1.5 mL/kg) |
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Children: up to 4 mg/mL of a 0.2% to 0.5% solution (avoid volumes exceeding 20 mL and concentrated solutions when motor blockade is unnecessary) |
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Levobupivacaine: few data available; up to 3 mg/kg in infants and 3.5 to 4 mg/kg in children and adolescents |
| Repeat injection (intraoperatively) | Same volume of injection but in a 50% dilution of the first given local anesthetic |
| Continuous infusions of plain local anesthetics | Avoid using lidocaine and mepivacaine |
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Bupivacaine: |
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>1 yr old: 0.3 to 0.5 mg/kg of a 0.125% or 0.1% solution |
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<1 yr old: no more than 0.375 mg/kg for the first 24 hr, then half these doses |
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Ropivacaine: 0.3 to 0.5 mg/kg 0.2% ropivacaine |
| Continuous infusion of opioids (usually in association with local anesthetics) | Morphine: 4–5 µg/kg/hr (5–6 mg of morphine in 200 mL of solution given at an infusion rate of 0.15 mL/kg/hr) |
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Fentanyl: 1.5 µg/kg/hr (200 µg of fentanyl in 200 mL of solution given at an infusion rate of 0.15 mL/kg/hr) |
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Sufentanil: 0.375 µg/kg/hr (50 µg of sufentanil in 200 mL of solution given at an infusion rate of 0.15 mL/kg/hr) |
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Do not administer remifentanil within the epidural or subarachnoid space. |
Thoracic epidurals are rarely performed in pediatric patients; they accounted for 6% of all epidurals in the ADARPEF prospective study that included 15,013 central blocks.[1] The main indications are for operations on the chest and the upper part of the abdomen. The techniques are similar to those used in adults. For the midline approach, the needle is inserted at the midpoint between adjacent spinous processes and, because of their steep angulation, at a 45- to 60-degree angle to the skin, pointing rostrally but in a strict median plane (see Fig. 45-4 ). Alternatively, the paramedian route can be used. Whatever route is used, thoracic approaches to the epidural space are more hazardous than lumbar approaches because there is a danger of direct needle trauma to the spinal cord. These approaches should be performed only by experienced pediatric anesthesiologists on anesthetized children. A catheter can be safely maintained in place for postoperative pain relief.[183]
Cervical epidural anesthesia has no place in pediatric anesthesia.
In addition to the general overview of complications previously described, high lumbar and all thoracic epidural approaches may result in direct spinal trauma or, less infrequently, in vascular lesions producing compressive spinal hematomas. In clinical practice, both complications (especially spinal hematomas) are rare in children. The most commonly encountered complications are related to catheter displacement and administration of opioids with insufficient postoperative monitoring of patients.
Postdural puncture headache (PDPH) requires special consideration. For decades, this complication was considered to be virtually nonexistent, at least in infants and young children. Leakage of CSF was usually considered the key factor responsible for the complication, even though many uncertainties persisted. Review of the pathophysiology of lumbar puncture headache indicates the main cause may be an abnormal distribution of craniospinal elasticity.[184] Increased compliance at lumbar level causes the hydrostatic indifferent point to move
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