Neuraxial Anesthesia

Maged D. Fam

Praveen Dharmapalan Prasanna

Introduction

Neuraxial blockade is an effective method for providing anesthesia and analgesia to a variety of surgical patients. It involves accessing and depositing local anesthetics and other adjuvant drugs around the spinal cord and nerve roots. Depending on the location of placement of drugs, neuraxial blockade could be broadly discussed under subarachnoid (spinal), epidural, or caudal blocks. Although these are anatomically located closely, there are significant differences when it comes to the dosage and spread of local anesthetics. There are also differences in clinical end points such as sensory, motor, and sympathetic blockade. Among these, subarachnoid blockade typically requires the least amount of drugs compared to epidural or caudal techniques, which require a higher dose and volume of local anesthetics.

History

The presence of fluid in the subarachnoid space was known from the times of the Roman empire; however, the technique of doing a dural puncture was first described by the late 19th century. This was shortly followed by Augusta Carl Bier, a German surgeon, injecting cocaine in the subarachnoid space for patients undergoing lower lip surgery.¹ The first described incident of postural puncture headache was also described by Bier after performing a procedure upon himself. From this point onward, neuraxial anesthesia had been evolving constantly for the next hundred years. Spinal anesthesia quickly gained popularity during the dawn of the 20th century and was widely practiced in Europe and the United States by 1940s. The technique of spinal anesthesia was better understood and refined during this period, compared to the epidural blockade that was explored later. This was likely due to lack of deep understanding of the epidural anatomy, unavailability of specialized epidural needles and catheters, and the fact that spinal anesthesia produced reliable dense sensory and motor blockade in an era where muscle relaxants were not yet invented. The increasing popularity of subarachnoid block suffered a setback in the mid 1940s where case reports emerged of patients suffering permanent paraplegia following spinal anesthesia.² Although the real cause of permanent nerve damage in these cases were not fully understood, subsequent studies solidified the safety of spinal anesthesia, leading to its revival in the 1950s.³ The development of the epidural technique somewhat trailed that of subarachnoid spinal block. Though the anatomy of epidural space was well studied and described by the end of the 19th century, the technique of continuous epidural blockade had to wait until the mid-20th century to be developed, by the Cuban anesthesiologist Manual Martinez Carbelo. Carbelo improvised the Tuohy needle placement with a 3.5-French catheter that was inserted into the epidural space.⁴

Advancements in pharmacology resulted in the development of safer local anesthetic agents. Moreover, the invention of less traumatic needles helped reduce common side effects
such as postdural puncture headache (PDPH). Neuraxial anesthesia has matured and refined over the last century and is currently an integral part of perioperative patient care. The use of neuraxial anesthesia has gained increased attention in the last few years, as an effective tool for achieving opioid-sparing analgesia. Many of the Enhanced Recovery After Surgery (ERAS) pathways have integrated epidural analgesia as a key strategy.⁵ A growing body of research has shown reduction in morbidity and mortality associated with the use of neuraxial blockade in patients undergoing various surgical procedures. These include reduction in cardiorespiratory complications, blood loss, and venous thromboembolism.⁶

Patient Selection

As with any other procedure, patient refusal is an absolute contraindication for neuraxial anesthesia. Patients with altered mental status may not be able to stay immobile during the procedure, increasing the risk of inadvertent injury to neural structures. Other absolute contraindications are confirmed severe allergy to local anesthetics or presence of active local infection at the site of injection. Spinal and epidural anesthesia should be avoided in patients with raised intracranial pressure owing to increased risk of brain herniation. One exception to this rule is intracranial hypertension due to pseudotumor cerebri syndrome also known as idiopathic intracranial hypertension.

When it comes to relative contraindications, careful risk/benefit analysis should guide the decision-making in the choice of regional anesthesia. Patients who are coagulopathic may be at increased risk of developing spinal or epidural hematoma following neuraxial blockade. Coagulopathy is considered a relative contraindication depending on the severity and the type of the underlying coagulation abnormality.

For example, thrombocytopenia is often considered a relative contraindication for neuraxial procedures; however, there is no universally accepted platelet count lower limit. Common clinical practice in the obstetric population is to not perform spinal/epidural blockade in patients with PLT count under 70 000. Recent outcome reports suggest that the risk of epidural hematoma substantially increases below the 70 000 threshold.⁷ Note that not only the absolute number of platelets but the functional quality of platelets is also important for coagulation in clinical scenarios such as HELLP syndrome. The timing of these coagulation tests is also important since the levels of various coagulation factors in the cascade could fluctuate significantly within hours.

Cardiac structural and valvular conditions such as severe aortic stenosis and idiopathic hypertrophic cardiomyopathy were considered absolute contraindications to spinal anesthesia in the past. However, this is not the case anymore as current evidence indicates that neuraxial blockade can be safely performed in these patients with the use of appropriate monitoring and adequate resuscitation.⁸ On a similar note, hypovolemia secondary to systemic sepsis is also a relative contraindication. Spinal or epidural anesthesia can be safely performed in these patients as long as the patient is hemodynamically stable. Many clinicians would avoid placing epidural catheters in the presence of systemic infection. When it comes to demyelinating diseases such as multiple sclerosis, the current evidence is inconclusive. Though in vitro studies
have demonstrated worsening of the demyelinating process following exposure to LA, human studies are inconclusive. Multiple sclerosis remains as a relative contraindication among most practitioners when it comes to spinal anesthesia.⁹ Nevertheless, epidural anesthesia with lower concentration of LA is often preferred in patients with multiple sclerosis, especially in the obstetric population.

Neuroanatomy

It is important to note that the anatomical structures are highly variable depending on the age and physical characteristics of the patient. A three-dimensional understanding of the anatomy of the central nervous system is essential for successful and safe placement of spinal, epidural, and caudal anesthetic. The spinal cord is a caudal extension of the medulla oblongata and ends at L1/L2 vertebral level in most adults, and around L3 in children. The spinal cord is covered by three concentric layers of meninges. The innermost layer, pia mater, is extremely thin and closely adherent to the surface of the spinal cord. The second layer, the arachnoid membrane surrounds the pia mater to constitute the subarachnoid space in between the two membranes. The outermost protective layer, the dura matter has the highest thickness and tensile strength due to collagen fibers and is closely adherent to the arachnoid membrane. The subarachnoid space contains cerebrospinal fluid (CSF) and blood vessels that supply the spinal cord and spinal nerve roots. Arachnoid membrane and dura mater together act as a barrier to the spread of local anesthetics from epidural space into the subarachnoid space. The potential space between these two membranes is known as the subdural space and contains loose areolar tissue. Subdural space could potentially be entered during the placement of an epidural or a subarachnoid block, and this could result in the so-called subdural block, which can present as a high spinal or with a patchy sensory/motor block.¹⁰ The primary site of action for subarachnoid block is the spinal cord itself. However, epidural local anesthetic drugs act primarily on the nerve roots. This structural difference is fundamental to the technique and the dosing of the local anesthetic used for each different type of neuro-axial blockade.

The spinal cord continues below L1 and L2 vertebrae after its termination, as a strand of fibrous tissue known as the Filum Terminale. The collection of spinal nerve roots that emerge from the caudal end of spinal cord, known as Conus Medullaris, is referred to as Cauda Equina. The thecal sac that contains the CSF and nerve roots ends at the level of S2 in most adults and S3 in children. The epidural space is the potential space located immediately outside dura mater. This is where the tip of the epidural needle is placed for deposition of LA. It contains loose areolar tissue, adipose tissue, a rich plexus of epidural veins and, most importantly, spinal nerve roots. The epidural space extends from the foramen magnum
to the sacral hiatus and is bounded anteriorly by the posterior longitudinal ligaments, laterally by the pedicles and intervertebral foramen, and posteriorly by the ligamentum flavum. Ligamentum flavum forms an important landmark for the placement of epidural blockade. It is a dense fibroelastic connective tissue that extends caudocranially, being thicker in the low thoracic and lumbar region, and gets thinner cranially. Ligamentum flavum covers the dorsal aspect of the epidural space circumferentially, and it gains attachment to the laminae and extends from one interlaminar space to the other. Posteriorly, ligamentum flavum is formed of two folds, which fuse in the midline and fuse with the interspinous ligament. This fusion of ligamentum flavum becomes less apparent as it extends cranially especially in the upper thoracic and cervical regions. This has clinical implications while placing high thoracic or cervical epidurals where ligamentum flavum may not be appreciable in midline. The thickness of the ligamentum flavum provides tactile feedback during the placement of neuraxial blockade. Note that the thickness of ligamentum flavum is highly variable among individuals. Caudal epidural space is essentially continuation of the lumbar epidural space into the sacral hiatus. This space can be accessed through the sacrococcygeal membrane, which is continuation of the ligamentum flavum caudally.

Familiarity with the structure and anatomy of vertebrae is helpful while placing neuraxial blockade since it is essentially a blind procedure primarily guided by tactile feedback. The vertebral column extends from the occiput to the coccyx. The cervical, thoracic, lumbar, and sacral vertebrae form the protective casing for spinal cord. A typical lumbar vertebra has a vertebral body and an arch formed by the laminae. The body of the vertebra and the laminae are connected by two pedicles. The resultant central canal forms the protective case through which the spinal cord and meninges run caudally. The superior and inferior articular processes form part of the intervertebral facet joint, which is an important target for injection for patients with chronic back pain.

The midline or interspinous approach is the most common technique for neuraxial placement. The interlaminar approach, also known as the paramedian approach, is a useful technique, since the interlaminar space provides a wider area to access epidural and subarachnoid space. The paramedian approach has the advantage of not requiring the patient to hyper flex the spine. This is useful especially in patients with limited mobility of the spine. It is an approach that is also useful in patients with scoliosis in which there is a rotational and angulation deformity of the spine. Thoracic vertebrae tend to have steep and narrow spinous processes, with reduced interlaminar distance making both midline and paramedian approaches challenging. Care must be taken in the placement of thoracic epidural blockade since the spinal cord could be damaged inadvertently in case of excessive advancement of the needle.

Physiological Effects of Neuraxial Anesthesia

Cardiovascular

Perhaps the most profound and immediate effect following neuraxial blockade is on the cardiovascular system. Both spinal and epidural blocks affect the cardiovascular system in a dose-dependent manner. These changes are primarily mediated via sympathectomy and affect different components of the cardiovascular physiology such as systemic vascular resistance, venous tone, heart rate, and myocardial contractility. The most immediate response following spinal and epidural anesthesia is reduction in venous tone since most of the blood is stored in venous capacitance vessels. The reduction of venous tone results in a decrease in venous return and reduction in preload. This is considered to be the mechanism of the immediate reduction in systemic blood pressure following spinal anesthetic. The dilatory effect on arteriolar tone usually follows and is due to the sympatholytic effect. This effect is somewhat gradual and less pronounced compared to the effect on the venous capacitance vessels. The
effect on heart rate is highly variable depending on the height of the blockade. Reflex tachycardia may be more pronounced in hypotensive and hypovolemic patients. Spinal anesthesia is often associated with bradycardia immediately following the placement, and this is largely due to Bezold Jarisch reflex, which results from a sudden decrease in preload from venodilation. Myocardial depression could occur with high spinal and epidural by directly affecting cardiac accelerator fibers at T1-T4 level.¹¹ Hypotension following epidural anesthetic is often more gradual compared to a spinal anesthetic. This could be related to the timing of incremental dosing of an epidural as opposed to a single injection of a predetermined amount of local anesthetic into the intrathecal space, in the case of spinal anesthetic. Hypotension, if goes unmanaged, will result in reduction in perfusion to all vital organs including coronary artery blood flow. Hence, it needs to be managed aggressively with fluids and vasopressors. Fluid preloading with crystalloids is no longer recommended, and co-loading is now considered the standard of care.¹²,¹³ The choice of an intravenous vasopressor is dictated by the clinical context, with ephedrine and phenylephrine being the most commonly used first-line agents to treat hypotension secondary to neuraxial blockade. Maneuvers to enhance venous return such as placing the patient in Trendelenburg position is beneficial but need to be mindful about cranial spread of hyperbaric intrathecal local anesthetic drugs. A better approach is to flex the bed or the stretcher at the hip so that the patient sits up with the feet at the level of the heart, so as to enhance venous return without risking the cranial spread of intrathecal drugs. The commonly held belief that spinal anesthetic drops the blood pressure more profoundly compared to epidural has not been validated in clinical studies. However, clinical experience indicates that the blood pressure changes could be more gradual in case of epidural anesthesia, and this could be primarily due to the slow incremental dosing of the epidural anesthetic.

Respiratory System

Spinal or epidural anesthesia is often preferred in patients with respiratory compromise precluding the provision of safe general anesthesia. However, it needs to be noted that the spinal and epidural could result in the motor blockade of some of the accessory respiratory muscles such as anterior abdominal wall and intercostal muscles. This could be more profound in the case of a high spinal.¹² Many patients would complain of dyspnea following neuraxial placement, from the lack of sensory feedback from the chest wall muscles, and this is usually not a result of diaphragmatic dysfunction. The mechanism of apnea in a total spinal is in fact primarily due to the circulatory collapse resulting in severe hypoperfusion of the respiratory centers in the brain stem. These patients often quickly recover with aggressive resuscitation with vasopressors, inotropes, and fluids, and often resume spontaneous ventilation as blood pressure normalizes. Neuraxial blockade usually preserves the tidal volume and respiratory rate but could reduce the peak expiratory flow rate, indicating its effects on accessory respiratory muscles such as abdominal musculature.

Gastrointestinal System

The profound sympathetic blockade often results in a parasympathetic hyperactivity. This could result in increased activity of gastrointestinal smooth muscles, resulting in hyper peristalsis, nausea, and vomiting. Nausea and vomiting could be the result of hypoperfusion to gastrointestinal mucosa and is often relieved by timely use of vasopressors and fluids. Antimuscarinic drugs such as atropine and glycopyrrolate have also been successfully used to treat nausea following spinal anesthesia. Postoperative ileus is a common side effect after abdominal surgery and account for a lot of associated morbidity. It has been shown that the duration of postoperative ileus can be shortened by neuraxial anesthesia because of the blockage of nociceptive afferent nerves and thoracolumbar sympathetic efferent fibers with functional maintenance of craniosacral parasympathetic efferent fibers.¹²

Renal and Genitourinary System

Spinal and epidural blockade are often associated with an increased risk urinary retention causing a delay in discharge of surgical patients from the Post Anesthesia Care Unit (PACU). It also increases the likelihood of prolonged bladder catheterization postoperatively.¹⁴ Micturition is a complex neuromuscular process, and the precise mechanisms of urinary retention following neuraxial blockade is not fully understood. It is believed that reducing the concentration of local anesthetics could help reduce the risk of urinary retention; however, this has not been proven in clinical studies.¹⁵

Spinal Anesthesia

Technique

A good understanding of the three-dimensional anatomy of the vertebrae and surrounding structures is useful when it comes to the placement of spinal anesthesia. Neuraxial anesthesia can be challenging depending on the patient’s physical characteristics, patient cooperation, and structural anatomical limitations of the spine itself. Patient education and setting up expectations is essential and helpful, and is often overlooked.

Positioning

Perhaps the most important aspect of the technique of spinal anesthesia is patient positioning, and it is often the most underestimated. Administration of the spinal anesthetic requires at least two personnel, one operator and an assistant. Spinal anesthesia is commonly placed in a sitting, lateral, or prone position. Sitting up position has the advantage of opening interspinous spaces and is also convenient in patients who do not require heavy sedation. Sitting position also has the advantage of easier identification of the bony landmarks in obese patients and those with structural abnormalities of spine. Patient’s shoulders are depressed with flexion of the cervical and lumbar spine toward the operator. Clear instruction improves patient cooperation in maintaining the position during the procedure. Care must be taken to avoid malpositioning such as excessively leaning forward or with rotation of the spine.

Lateral decubitus position is useful in patients who are unable to maintain a sitting up position and in patients who require moderate amounts of sedation. The patient is positioned to the left or right lateral decubitus with the back of the patient brought closer to the edge of the bed. The assistant would maintain the position of the neck flexion with the flexion at the hip joints. The main advantage of lateral decubitus positioning is patient comfort and the ease of positioning the patient in the operating table after the placement of the neuraxial blockade.

Needle Selection

Spinal needles could be broadly classified into cutting and pencil point based on the bevel design. Examples for pencil-point needles are Whitacre and Sprotte. The risk of PDPH needs to be weighed when choosing the type of spinal needle. Pencil-point or conical shape needles are associated with less incidence compared to cutting needles. The other factors that increase the incidence to PDPH is the size of the needle. Larger gauge needles are associated with higher incidence and more symptomatic PDPHs.¹⁶ It is worth noting, however, that even while using a smaller size needle, the number of dural punctures is also directly proportional to incidence of headache.

Only gold members can continue reading. Log In or Register to continue