Hello, I am Beth Ann Clayton and I welcome you to the vertebral anatomy presentation. The learning objective for this presentation is to identify the anatomic structures and considerations encountered during the performance of neuraxial anesthesia. I have no financial relationships with any commercial interests related to the content of this activity. I will not discuss off-label use during my presentation. To begin, central neuraxial block involves the administration of local anesthesia onto the spinal cord itself via spinal or adjacent to the spinal cord via the epidural space. Spinals and epidurals share the same anatomy and physiology but are distinct from one another due to their unique anatomic, physiologic, and clinical features. Knowledge of anatomic landmarks and underlying structures of the vertebral column aids in forming a three-dimensional mind's eye picture. This picture coordinates the feel of structures and tissues in the vertebral column against your fingers and the needle and facilitates accurate placement of the medications into the desired neuraxial space. The vertebral column extends from the base of the skull and the frame and magnum to the tip of the coccyx. Its purpose is to protect the spinal cord and support and transmit body weight. It is comprised of 33 vertebrae that are interconnected. There are 7 cervical vertebrae, 12 thoracic, 5 lumbar, 5 fused sacral, and 4 fused coccygeal vertebrae. It's important to understand the vertebral column curves because the curves affect the medication flow distribution in the neuraxial spaces. The cervical and lumbar curves convex anteriorly and the thoracic and sacral curves concave anteriorly. The high points of the vertebral column in the supine position are C5 and L3 and the low points of the vertebral column in the supine position are T5 and S2. When administering a hyperbaric local anesthetic, the local anesthetic will tend to accumulate in the low points and it is harder for it to rise as the curve rises up. The individual vertebral bodies are stacked on top of one another. They are separated by fibrocartilaginous intervertebral discs. This provides support for the cranium and the trunk and it also allows for truncal flexibility. We will now describe the anatomy of the cervical, thoracic, and lumbar vertebrae. The body of these vertebrae lies anteriorly. The vertebral arch is formed by two pedicles which project anteriorly from the body and then two lamina which connect the pedicles. This vertebral arch creates the vertebral foramen which allows the passage and the protection of the spinal cord. The bony projections of the transverse processes and the spinal process allow for attachment of muscles and ligaments. The two transverse processes are created by joining the pedicles and the spinous process is formed by the fusing of the lamina. When viewing the images to the right, you see that the pedicles and the processes of each vertebrae contain a superior and an inferior articular process and also lateral notches. In the vertebrae stack, the notches and the articulating surfaces create a facet joint from the intervertebral foramina. This provides safe passage of the spinal nerves passing from the spinal cord. The articular services of the facet joints are covered by hyaline cartilage which permits gliding motion between the vertebrae. The nerve branches from closely associated spinal nerves innervate these facet joints. If the facet joint is injured, however, the associated spinal nerves may be affected leading to pain along dermatomes or muscle spasm along the associated myotomes. The size and shape of lamina and spinous processes differ among cervical, thoracic, and lumbar vertebrae. In the cervical and thoracic vertebrae, the spinous process is angled acutely caudate, tightly overlapping from one to the next. This allows for added protection to the spinal cord when standing erect. The lumbar vertebrae, however, are the largest of the vertebral bodies and the largest spinous processes. These spinous processes are almost horizontal and they have less overlap between the spinous processes, providing large gaps bridged by the ligaments. The sacrum is wedged between the iliac crests. It is a triangle-shaped structure formed by the fusion of five sacral vertebrae. In most people, the laminar arches of the sacrum are missing, creating the sacral hiatus. The sacral hiatus is formed by failure of the fifth sacral lamina to fuse at midline. The sacral hiatus is the site of caudal anesthesia. It is covered by a thick ligament, making it easily palpated in kids, allowing caudal anesthesia to be straightforward in the pediatric population. The coccyx is four small segments of bone that become fused into two bones between the ages of 25 and 30. The bony prominences seen and felt along the midline of the back are the spinous processes. The first prominent spinous process is cervical vertebrae number seven. C7 is where the cervical and thoracic vertebrae junction. The thoracic vertebral level, T7, occurs at a line across the inferior angle of the scapula. And the thoracic vertebral level, T12, is palpated at the bottom of the rib cage at the 12th rib. The lumbar vertebrae number four can be identified when you feel for the iliac crest and bring your thumbs or finger across to the midline of the back, identifying L4. Ligaments are tough bands of fibrous connective tissue, which connect bones to other bones, providing support and stability while also limiting excessive movement. There are several vertebral ligaments, describing them from the posterior to the anterior. You have the supraspinous ligament, followed by the intraspinous ligament, then the ligamentum flabum, then the posterior longitudinal ligament, and lastly, the anterior longitudinal ligament. You first encounter the supraspinous ligament. It is a strong cord-like ligament, connecting the tips of the spinous processes from C7 to the sacrum. It is wide and thick in the cervical and thoracic vertebrae. It consists of three layers. The first layer is the superficial layer, which extends over several vertebral spinous processes. The second layer is the middle layer, connecting two to three spinous processes. And the inner layer connects the neighboring spinous processes. The intraspinous ligament attaches in a posterior cranial direction along the spinous processes, fusing posteriorly with the supraspinous ligament and anteriorly with the ligamentum flabum. Absent or poor quality of the intraspinous ligament occurs in the cervical area, and it is also very thin in the lumbar area. The ligamentum flabum is an elastic, flabemous, continuous ligament from the foramen magnum to the sacral hiatus. It connects anterior and inferior aspects of one lamina to the posterior and superior aspect of the lamina below. It is very dense compared to the other ligaments, and it can measure from five to six millimeters thick in the lumbar region. It is responsible for our upright posture, and it is the ligament that we pass through immediately preceding the epidural space. The spinal cord lies within the vertebral canal. If you remember, the vertebral canal is protected by the vertebral arch, which is created by the two pedicles and the two lamina. The spinal cord is covered by three protective membranes, collectively called meninges. The first layer, directly outside of the spinal cord, is the pia mater. The pia mater is then surrounded by the arachnoid mater, and the most outside meninge layer is the dura mater. The meninges are non-nervous support tissue, which provides protective covering for the cord and the nerve roots from the foramen magnum to the base of the cauda equina. The pia mater is the first layer directly outside the spinal cord. It intimately surrounds the spinal cord and terminates as a delicate filament known as the phylum terminale, which secures the cord to the upper coccyx. The pia mater also covers the spinal roots and nerves as far laterally as their intravertebral foramen, and it is a very highly vascular meninge. The arachnoid mater lies directly outside of the pia mater. The arachnoid mater is a thin, avascular, cobweb-like tissue, and it closely adheres to the dura mater. The arachnoid mater continues past the end of the spinal cord and ends at sacral level 2. It also forms the cylindrical-shaped column that contains the cerebral spinal fluid. The dura mater is the outermost, tough membrane, and it extends from the foramen magnum to sacral level 2. It continues laterally along the spinal nerve roots as they leave their intravertebral foramen, and the tissue becomes thin as it leaves the foramen, where it becomes the epineurium and the perineurium of the spinal nerves. The fibers run longitudinally. The dura mater is the tissue that you feel immediately prior to entering the subarachnoid space. The epidural space is the space between the meninges and the vertebral canal. The boundaries of the epidural space are in the superior at the foramen magnum, and caudally at the sacral hiatus or sacrococcygeal ligament. Anterior boundary is the posterior longitudinal ligament, and laterally, boundary is the pedicles, and posterior is the ligamentum flabum and the lamina. The distance from the skin to the lumbar epidural space is typically lies between 2 1⁄2 centimeters to eight centimeters, averaging about five centimeters. The largest midline of the mid-lumbar region is at five to six millimeters deep. It is narrow in the thoracic area at three to five millimeters, and very small in the lower cervical region at 1 1⁄2 to two millimeters in thickness. Located in the epidural space are veins, lymphatics, small arteries, and fat. There may also be a band of connective tissue along the midline of the epidural space. If this band of connective tissue exists, it can prevent free passage of the epidural catheter or produce a repeated one-sided block. The epidural space is slightly negative pressure, and there's more negative pressure in the thoracic area compared to the lumbar area, and there is no pressure difference noted in the sacral region. The subdural space is a potential space between the dura mater and the arachnoid mater. It is actually a pretty difficult location to end up, as the dura and the arachnoid mater are in very close contact. However, if you inject medication into the subdural space, you can end up with a very inconsistent block or a very spotty block. The subarachnoid or interthecal space lies between the arachnoid mater and the pia mater. The subarachnoid space is filled with cerebral spinal fluid, and this is the location we wish to enter our neuraxial medications for spinal anesthesia. Cerebral spinal fluid is clear and colorless. It's produced by the choroid plexus and eliminated by the arachnoid villi. It has a specific gravity average of 1.007 at 37 degrees Celsius. The cerebral spinal fluid is also isotonic, but it does contain glucose and protein as well as electrolytes. If you're trying to determine if the fluid coming out of your needle is cerebral spinal fluid or possibly saline, the cerebral spinal fluid will feel warm since it's at 37 degrees Celsius, and it also will feel sticky because of the glucose. If the liquid that you see is clear and colorless, but cool and slippery, then it probably is not cerebral spinal fluid. The volume of cerebral spinal fluid in an adult is 120 to 150 milliliters, and there is approximately 20 to 35 milliliters found in the spinal subarachnoid space at any given time. A total of 500 milliliters of CSF is produced daily, or approximately 21 milliliters per hour. The cerebral spinal fluid serves as a mechanical buffer to protect the brain and the spinal cord. There are 31 pairs of spinal nerves that arise from the posterior and the anterior nerve roots along the cord. The nerve roots exit from their intervertebral foramen and innervate various structures of the body. Skin area that is innervated by spinal nerve is termed a dermatome. Each spinal nerve contains motor, sensory, and usually autonomic nerve fibers. The spinal cord tapers to the conus medullaris, and the nerve pathways continue on a collection of rootlets called the cauda equina, or the horse's tail. It extends from the L1 to S5. The cauda equina nerve fibers can be very sensitive, and therefore, highly concentrated local anesthetic is not recommended in the subarachnoid space out of concern to causing damage to the cauda equina. Originating from the vertebral artery, the single anterior spinal artery provides the majority of the blood flow to the anterior 2 3rds of the cord. There are also two posterior spinal arteries that supply blood to the posterior 1 3rd of the cord, and it originates from the posterior inferior cerebellar arteries, or the vertebral arteries as well. Additional blood supply comes to the spinal cord via intercostal and lumbar arteries as well. Dermatones are areas of skin on your body that rely on specific nerve connections on your spine. In this way, the dermatones are much like a map. The nature of that connection means that the dermatones can help an anesthesia provider determine the level of neuraxial anesthetic, or a potential neuraxial injury. At a T10 level, you are located at the umbilicus, and the T4 level is at the nipple line. Because of the cauda equina, the direct relationship between the spinal level and the dermatone level is distorted in the lower spinal segments. Skin and muscles of the abdominal wall are supplied by T5 to L2, and the peritoneum is supplied by S4, or close to that. On this diagram, you can see the various dermatone levels that can be assessed from innervation of the spinal nerves at a particular intervertebral layer. Nerve fibers are separated into three groups. There are the A fibers, consisting of A alpha, A beta, A gamma, and A delta, the B fibers, and the C fibers. The fibers are differentiated in their diameter and myelination. The larger the fiber, the faster the speed of impulse and conduction. A fibers are large and myelinated. A alpha fibers innervate motor and proprioception. A beta fibers innervate touch, pressure, and small motor movement. A gamma innervates muscle tone, and A delta receives input for sharp pain, heat, cold, and touch. B fibers are also myelinated. They are autonomic fibers from the preganglionic sympathetic nervous system, and they are small in size and easiest to block. C fibers are unmyelinated, and they are postganglionic sympathetic fibers, and they are small and slow conduction rate. They innervate dull pain, temperature, and touch. With central neuraxial anesthesia and analgesia, sympathetic nerve conduction, especially vasomotor, is blocked in the thoracic and lumbar areas. However, the parasympathetic pathways are spared because they lay outside the spinal canal and are not exposed to local anesthetic in the cranial areas and sacral areas. Stimulation of the sympathetic nervous system includes increased myocardial automaticity, conductivity, and contractility. Also constriction of the coronary and skeletal blood vessels, and it is the principal modulator of vascular tone. The sympathetic nervous system is also influenced mostly by the posterior lateral hypothalamus. Therefore, if neuraxial central blockade is placed, this innervation to the sympathetic nervous system is blocked. In the sympathetic nervous system, the preganglionic fibers may pass through the paravertebral ganglia and synapse with the postganglionic fibers that innervate the heart, stomach, and intestine. A sympathetic block is usually two dermatones above the sensory level, and the motor block is usually two dermatones below the sensory level. Therefore, if you have a T10 sensory level, your sympathetic block will occur at T8, and your motor block occurs at T12. The thoracic vertebrae from T1 to T4 innervates the cardiac accelerator fibers, and if this is blocked, you can see significant bradycardia. A visceral sympathetic nerves leave the spinal cord between T5 and L1. A sympathetic blockade may cause hypotension, bradycardia, and decreased contractility. The decreased venous return due to the increase in volume of a capacitance vessels can be attenuated by preloading the patient with 500 to 1,000 mLs of crystalloid. You also, due to this hypotension, may need to treat with vasopressors. Higher levels of local anesthetic blockade can decrease the ability to cough effectively, and may be a concern in patients with lung disease when accessory abdominal muscles and intercostals are blocked. The phrenic nerve innervates the diaphragm at the C3 through C5 level. Generally, tidal volume, minute ventilation, respiratory rate, and arterial blood gases are unchanged at mid-thoracic levels. The gastrointestinal effects of neuroblockade can occur as well. The gastrointestinal tract is innervated by the parasympathetic nervous system and the sympathetic nervous system. The sympathetic nervous system blockade results in unopposed parasympathetic nervous system activity. There can be constriction of the bowel and normal to increased peristalsis, and there can be an increase in GI blood flow. You may have nausea and vomiting from the increased peristalsis, but many other things can contribute to the nausea and vomiting, including your hypotension, opioids, and antibiotic administration. With advancing age, a diminishing thickness of the intervertebral disc results in decreased height of the vertebral column. Thickened ligaments and osteophytes also contribute to difficulty accessing the subarachnoid and epidural spaces. Adult scoliosis is frequently encountered in older adults. Research has demonstrated that scoliosis is present in approximately 70% of the population age greater than 60 years. In the scoliotic spine, the vertebral bodies are often rotated toward the convexity of the curve, and the spinous processes face into the concavity of the curve. Therefore, a paramedium approach from the convex side may be more successful.

vertebral anatomy

neuraxial anesthesia

spinal cord

vertebrae

ligaments

anesthetic placement