Introduction
Central neuraxial blocks (CNBs) collectively encompass the subarachnoid/spinal, epidural, caudal epidural, and combined spinal-epidural anesthesia. Before the advent of anesthesia, crude methods have been described in the literature to perform various surgical procedures. After that, with the introduction and discovery of anesthetics into clinical practice, the alleviation of pain and good working conditions by adequate muscle relaxation became widespread. However, the concerns of prolonged recovery after general anesthesia (GA) and certain other complications like postoperative nausea and vomiting (PONV) prompted the need for an anesthetic technique that provided the components of anesthesia while retaining the consciousness. This led to the introduction of CNBs, the first being spinal anesthesia by German surgeon Sir August Bier on August 16, 1898. After that, the use of CNBs expanded and laid the foundation of modern-day regional anesthetic practices.
Historical Perspective
Historical milestones in the field of CNBs are as follows:
200 AD: Neuraxial fluid presence demonstrated by Galen.
1891: Dural puncture described by Essex Wynter.
August 16, 1898: First case of spinal anesthesia administered by German surgeon Sir August Bier using local anesthetic cocaine.
1901: First use of intrathecal morphine by Racoviceanu-Pitesti.
1901: The first description of caudal anesthesia was by Cathleen.
1905: Braun used procaine in spinal anesthesia.
1921: Fiedel Pages described the first lumbar epidural anesthesia in humans.
1930s: Dogliotti described the loss of resistance method for identification of epidural space (also known as Dogliotti’s principle).
1935: Tetracaine used by Sise for spinal anesthesia.
1941: Robert Andrew Hingson (1913–1996), Waldo B. Edwards and James L. Southworth developed technique of continuous caudal anesthesia.
1947: First lumbar epidural catheterization for surgery was done by Manuel Martinez Curbelo.
1949: Lidocaine was used for spinal anesthesia by Gordh.
1952: Chloroprocaine was used in spinal anesthesia by Foldes and McNall.
1961: Mepivacaine was used in spinal anesthesia by Dhuner and Sternberg.
1966: Bupivacaine was used in spinal anesthesia by Emblem.
1979: First use of epidural morphine for analgesia by Behar.
The CNBs are the most commonly employed regional anesthetic or analgesic techniques for a plethora of surgical procedures. Conventionally, CNBs have been practiced, utilizing the anatomical landmark technique, tactile perception of the structures, and visible markers (free flow of cerebrospinal fluid [CSF] in spinal, loss of resistance in epidural). In the absence of any abnormal anatomy, these blocks can be performed with reliable accuracy and provide excellent sensory and motor blockade that wears off, depending upon the concentration and volume of the local anesthesia (LA) used.
Applied Anatomy and Physiology (AS5.1, AS5.2)
The recapitulation of the anatomy of the spinal cord and the surrounding structures is imperative for an insight into the physiological considerations of CNBs. The axis of the human body is formed by the vertebral column that extends from the base of the skull up to the pelvis and is integral for providing support to various organs and structures of the body, aid locomotion, and protect the spinal cord that is encased within the bony structures. The vertebral column has 33 vertebrae (cervical 7, thoracic 12, lumbar 5, sacral 5, and coccygeal 4). It has two primary curves (thoracic and lumbar) and two secondary curves (cervical and sacral) that aid in maintaining the posture and flexibility of the entire body.
A vertebra comprises an anterior body and a posterior vertebral arch (Fig. 12.1). A pedicle forms the vertebral arch on each side that arises from the posterolateral side to fuse with the lamina, in order to enclose the vertebral foramen.
The spinal cord, along with the structures and spinal nerves, runs in the vertebral canal that exits through the lateral space between the pedicles. The transverse process arises from the lamina that fuses in the midline, and a spinous process projects to form an important landmark for the CNBs, with intervertebral spaces that can be palpated through the entire length of the vertebral column. The line joining the highest point of the iliac bones (Tuffier’s line) usually corresponds to L4–L5 intervertebral space.
Proximally, the spinal cord is continuous with the brainstem and terminates distally in the conus medullaris as the filum terminale (fibrous extension) and cauda equina (neural extension). The distal termination of the spinal cord varies from L3 in infants to the lower border of L1 in adults because of differential growth rates between the bony vertebral canal and the central nervous system (CNS) during the growth.
The spinal cord is surrounded by three membranes centrifugally: The pia mater, the arachnoid mater, and the dura mater. The pia mater is a highly vascular membrane that encloses the spinal cord closely. The space between pia mater and arachnoid mater is called the subarachnoid or intrathecal space that is filled with CSF, which is produced by the choroid plexuses of the cerebral ventricles and spinal nerves traversing this space. Approximately 500 mL of CSF is produced daily, out of which 30 to 80 mL occupies the intrathecal space from T11–T12 downward. The arachnoid mater is a nonvascular membrane that acts as the principal barrier to the migration of drugs to CSF. Exterior to the arachnoid mater, between the arachnoid mater and outermost layer dura mater, lies the subdural space, which is a nonuniform space.
The epidural space surrounds the dura mater and extends from foramen magnum to the sacral hiatus. It is bound by posterior longitudinal ligament anteriorly, ligamentum flavum (yellow ligament) posteriorly, and pedicles and intervertebral foramina laterally. The contents of the epidural space include nerve roots with dural extensions, areolar tissue, fat, lymphatics, and blood vessels, including the Batson venous plexus.
Ligamentum flavum, also known as yellow ligament, forms the posterior boundary of the epidural space and extends from foramen magnum to sacral hiatus. It comprises a pair of ligaments (left and right) that fuse in the middle. The thickness of ligamentum flavum varies from 3 to 5 mm being thickest in the lumbar region, followed by thoracic and thinnest in the cervical region. Immediately posterior to ligamentum flavum lies the lamina, spinous process, and interspinous ligament. Further posterior to these structures lies the supraspinous ligament that joins the spinous processes together and runs from external occipital protuberance to the coccyx, which is covered by the skin and subcutaneous tissue (Fig. 12.2).
The dural sac terminates in the sacral canal at S2 level (S3 in children). The volume of the caudal canal in adults, excluding the foramina and the dural sac, is about 10 to 27 mL. This wide variability of volume accounts for the variation in the height of the block in caudal anesthesia.
While spinal/subarachnoid block is performed in the lumbar region, epidural blocks can be performed in the epidural space, extending through the entire length of the vertebral canal, although lumbar and thoracic epidurals are the most frequently performed procedures.
Indications of CNBs (AS5.1)
CNBs are indicated in a variety of surgical procedures in obstetrics, acute, and postoperative pain management (Box 12.1). However, there are certain contradictions to the administration of CNBs as listed in Box 12.2.
A single shot spinal or epidural anesthesia is mainly employed for the lower abdominal or lower limb surgeries. Catheter-based techniques wherein a catheter is placed in the subarachnoid or epidural space are usually employed for perioperative (preoperative, intraoperative, postoperative period) or postoperative analgesia in thoracic, abdominal or lower limb surgeries and for labor analgesia.
The choice and type of CNBs are also dependent on nature and duration of surgery, associated comorbidities, ease of performance of block (patient position or underlying pathology), and the risk-benefit ratio tailored for every patient.
Physiological Effects of CNBs
The physiological effects of CNBs are due to the blockade of the sympathetic and somatic nervous system (including sensory and motor neurons), compensatory reflex mechanism, and unrestricted parasympathetic activity. The physiological effects of epidural anesthesia are similar to those of spinal anesthesia; however, the difference in the characteristics of the CNBs is listed in Table 12.1.
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