Epidural Anesthesia

Anatomy and Physiology

The Spinal Cord and Epidural Space

The adult spinal cord is approximately 45 cm shorter than the spinal canal. The spinal cord ends at the L1 vertebra in 50% of adults and at the L2 vertebra in about 40%. Although it was previously believed that the newborn's spinal cord extended to the L2 or L3 vertebrae, recent studies have shown that the average neonate's conus medullaris is also at the L2 vertebra. Below this level, the lumbar and sacral nerves converge to form the cauda equina. The spinal cord is suspended in cerebrospinal fluid and surrounded by the arachnoid mater. The arachnoid mater and subarachnoid space extend caudally to S2 in adults, S3 in children, and S4 in newborns. The arachnoid mater is closely approximated to the dura mater, which is attached to the spine by its outer endosteal portion. The arachnoid mater envelopes the brain intracranially and the spinal cord and extends through the foramina intervertebral to the epineural connective tissues of the spinal nerves.[2]

The spinal epidural space extends from the foramen magnum at the base of the skull to the sacral hiatus and contains fatty and connective tissues, blood vessels, and lymphatics. The blood vessels may dilate with pregnancy or ascites, increasing the risk of a traumatic or bloody puncture. The epidural space is further divided into the anterior and posterior epidural spaces. The anterior epidural space contains the ventral spinal nerves, the basivertebral veins, and the internal vertebral venous plexus. In contrast, the posterior epidural space contains the dorsal spinal nerves, the intervertebral veins, and the sinuvertebral nerves.[5] 

The boundaries of the epidural space are formed by the dura mater and arachnoid mater internally, the ligamentum flavum and the vertebral periosteum externally, and the intervertebral foramina laterally.[5] The distance between the skin and the spinal epidural space is variable. Depending on a patient's age and amount of subcutaneous fat, this distance can range from 4 cm in normal BMI adults to >8 cm in obese patients. The dorsal border of the spinal epidural space is the ligamentum flavum. Moving superficially, the remaining layers are the interspinous ligament located between the spinous processes, the supraspinal ligament situated on the surface of the spinous processes, subcutaneous tissue, and the skin.[2]

Relevance to Epidural Anesthesia

The anatomy of the epidural space is of paramount importance to the administration of epidural anesthesia. By targeting specific spinal segments and structures within the epidural space, anesthesiology clinicians can achieve targeted pain relief and minimize the risk of complications. In addition, understanding the anatomical variations and the location of blood vessels, spinal nerves, and other structures within the epidural space is essential for safe and effective epidural catheter placement.[6]

Techniques to Locate the Epidural Space

Various techniques have been developed to locate and access the epidural space safely and accurately. The traditional landmark-based approach relies on identifying anatomical landmarks, including the spinous processes, interspinous spaces, and the midline, to guide epidural needle insertion.[6] In addition, the loss-of-resistance technique is commonly employed to confirm the location of the epidural space, which involves feeling resistance to the gentle injection of air or saline with the epidural needle when not in the epidural space and the sudden loss of resistance when the space is entered.[6]

The caudal epidural block is a specific technique that involves the administration of local anesthetic agents into the caudal epidural space located at the sacral hiatus.[5] This technique is often used in pediatric anesthesia and specific lower extremity and perineal procedures in adults. The most common shape of the sacral hiatus is an inverted U shape, seen in both males and females. The length from the apex of the sacral hiatus to the first sacral spine is slightly longer in females compared to males; however, the sacral cornua's width is similar in both sexes. Understanding these morphometric parameters is crucial for successful epidural anesthesia procedures. Variations in the sacrum can contribute to lower backache and may impact surgical procedures. The sacral canal is formed by the union of the pedicle and lamina of the 5 sacra and contains important structures such as the cauda equina and sacral nerves.[7] Correct needle placement can only be achieved by identifying anatomical landmarks (eg, the sacral cornu, the lateral sacral crests, and the apex of the sacral hiatus). Even with identifying anatomical landmarks, clinicians fail to place a caudal epidural block 30% of the time, secondary to anatomical variations; often, ultrasound is used to guide placement.[8] Therefore, clinicians must have a good understanding of the anatomy of the sacral region to have any chance of successfully administering a caudal epidural block.

Indications

The primary indications for epidural anesthesia include obstetrical anesthesia during labor and surgical anesthesia for thoracic, major intra-abdominal, or spine surgeries, provided there is no need for muscle relaxation. Epidural anesthesia may also be used as an adjunct in intraoperative or postoperative pain management. Patients at higher risk for postoperative complications, such as those affected by ischemic heart disease, have demonstrated increased benefits (eg, decreased postoperative pulmonary complications and faster intestinal return to function) with epidural anesthesia in the past.[1][9]

A systematic review with a meta-analysis comparing intravenous and epidural routes of patient-controlled analgesia (PCA) in abdominal surgery demonstrated that epidural PCA was associated with better postoperative pain relief, reduced opioid consumption, and fewer postoperative complications than intravenous PCA.[10] Additionally, it has been shown that combined epidural anesthesia and analgesia reduced postoperative cognitive impairment in patients receiving general anesthesia.[11] Furthermore, investigations have been conducted on the effects of perioperative epidural analgesia on cancer recurrence and survival, suggesting a potential association between epidural analgesia and improved survival outcomes.[12] However, recent studies have shown that the benefits of epidural analgesia during surgery compared to current alternative types of anesthesia (eg, catheter wound infusions and peripheral nerve blocks) are not as great and carry a higher risk of complications.[4] In limited studies, epidural anesthesia demonstrated slightly more effective postsurgical pain control than systemic medications. Therefore, other than for obstetrical indications, epidural anesthesia may be more indicated as an adjuvant therapy; further research is needed before definitive conclusions can be reached.[13]

Contraindications

Epidural anesthesia is considered relatively safe for most patients; however, alternative types of anesthesia may be more appropriate for some individuals. 

Absolute Contraindications

• Patient refusal
• Local infection at the puncture site
• Increased intracranial pressure
• Traumatic spinal cord injury [14]

Relative Contraindications

• Hemodynamic instability
• Obstructive cardiomyopathy
• Uncorrected coagulopathy or therapeutic anticoagulation
• Thrombocytopenia
• Inability to maintain positioning for epidural placement
• Anatomic spinal abnormalities [15][14]

Equipment

Epidural Needles

Multiple types of epidural needles have been designed (eg, Tuohy, Hustead, Crawford, and Weiss). However, the Tuohy epidural needle is most often used. These epidural needles are usually 17 G or 18 G and are 3.5 in long; epidural needles up to 6 in are available and can be used in patients with increased subcutaneous adiposity.[16]

Loss of Resistance Syringe

Loss of resistance syringes are made of glass or plastic and have very low friction between the plunger and the barrel to detect the change in resistance at the epidural space. Syringes may be loaded with air, saline, or both; the injected medium does not affect the success of identifying the epidural space, nor does it change the complication rate.[17][16]

Epidural Catheters

Epidural catheters are used for continuous epidural anesthesia or analgesia. The catheters may be flexible or stiff and may have single or multiple perforations for the perfusion of the analgesic agent. In most commercially available kits, syringe-catheter connections (eg, Luerlock or Luer-slip connectors) are also included.[18][16]

Epidural Procedure Site Preparation

Aseptic technique when placing an epidural catheter and dressing the insertion site is critical for infection prevention. Supplies required include:

• Sterile gloves
• Surgical cap and mask
• Sterile draping
• Individualized skin preparation solution packet (eg, chlorhexidine with or without alcohol or povidone-iodine with or without alcohol)
• Sterile occlusive dressing [19]

Medications

The preference of anesthesiology clinicians will dictate which medications are used in a patient's epidural anesthesia. However, a combination of local anesthetic (eg, bupivacaine, ropivacaine, and an opioid (eg, fentanyl, sufentanil).[14] IV fluids are also typically administered before an epidural is performed to prevent hypotensive episodes. Additionally, IV ephedrine or phenylephrine should be available to treat hypotension during neuraxial anesthesia.[20]

Ultrasound-Guided Epidural Anesthesia

Clinicians may choose to place an epidural catheter with ultrasound guidance. Ultrasound technology allows real-time visualization of the relevant anatomical structure, potentially improving the accuracy and safety of epidural catheter placement. The advantages of using ultrasound guidance for epidural anesthesia include better identification of the epidural space, fewer needle passes and attempts, and a lower incidence of complications such as unintentional dural puncture, infection, and hematoma formation. Recent advancements in ultrasound-guided techniques have shown promise in improving the safety and efficacy of epidural anesthesia. Ultrasound guidance can reduce complications and improve the procedure's success by providing real-time visualization of the relevant anatomical structures. However, further research is needed to establish the superiority of ultrasound-guided epidural anesthesia over traditional landmark-based techniques.[21]

Personnel

• Anesthesiologist or Nurse Anesthetist
• Assisting personnel

Preparation

Before an epidural anesthesia procedure, the patient should be positioned and the area prepared appropriately. Emergency equipment and medications need to be readily available in the room. Furthermore, intravenous access must be placed for fluid and medication administration. Standard American Society of Anesthesiologists (ASA) preanesthesia guidelines should be followed (eg, preoperative testing, hemodynamic monitoring). The patient may be positioned in a sitting or lateral decubitus position; back arching is advised for better exposure to intervertebral interspaces. Aseptic technique and field preparation must be used throughout epidural placement.[22]

With any technique used to place an epidural, the patient should be positioned, prepped, and draped in the standard sterile fashion. The correct intervertebral space is then identified using anatomic landmarks, a loss of resistance needle, and sometimes ultrasound guidance. A systematic review of random controlled trials to evaluate preprocedural lumbar neuraxial ultrasound compared to the traditional landmark-based approach demonstrated that ultrasound guidance reduced the number of needle passes, attempts, and traumatic procedures. Additionally, ultrasound-guided techniques were associated with a shorter procedure time and a higher success rate.[23] A recent literature review on ultrasonography for lumbar neuraxial block, including epidural anesthesia, reported similar findings. Ultrasound-guided techniques were also associated with improved procedural outcomes, such as increased first-attempt successes, fewer needle passes, and a decreased incidence of complications.[21]

Technique or Treatment

The clinician's approach to performing epidural anesthesia depends on which area of the spine the epidural space will be accessed. Due to the difficult angulation of the thoracic spinous processes, epidural anesthesia performed above the T11 vertebra generally uses the paramedian approach. Those performed below the T11 level typically use the midline approach.[24][25]

Medial and Paramedian Approach

In the medial (ie, midline) approach, the needle insertion site is midline between the spaces created by the vertebral spinous processes. Upon locating the desired spot, lidocaine 1% must be injected into the skin and underlying tissues to decrease the discomfort with the advancement of the epidural needle. Once localized anesthesia is achieved, the epidural needle is advanced with a stylet in place and with its bevel point cephalad, ultimately guiding the epidural catheter to the proper location. The epidural needle must be advanced through the skin, subcutaneous tissue, supraspinous, and interspinous ligaments. Once there, the stylet is removed, and the loss of resistance syringe that is filled up with saline, air, or both must be attached to the needle. As the needle advances, the clinician applies pressure to the plunger so that once the ligamentum flavum is pierced, a loss of resistance pressure is noted, confirming entry into the epidural space. On average, the distance from the skin to the ligamentum flavum is approximately 4 cm.[25] The anesthesiology clinician will usually inject 5 to 10 cc of saline to expand the epidural space and decrease the risk of vascular injury.[26]

In the paramedian approach, the needle insertion site is 1 cm lateral to the vertebral interspace. Local anesthetic is injected along the predicted path of the epidural needle as described for the medial approach. The epidural needle is then advanced through the paraspinal tissues. Given this location, the needle will not transverse the supraspinous or interspinous ligaments. The advancement of the needle must stop upon feeling the engagement in the ligamentum flavum. The loss-of-resistance syringe is attached, and the epidural space is located similarly to the median approach.[25]

Once the epidural space is reached by either the midline or paramedian approach, the epidural catheter is advanced through the introducer needle after removing the loss-of-resistance syringe. The epidural catheter should be advanced to the 20-cm mark on the outside of the catheter. The epidural needle is removed after noting the distance from the skin to the epidural space as indicated by the marked epidural needle. The epidural catheter is then partially withdrawn, leaving approximately 5 to 6 cm of the tip in the epidural space. However, no consensus exists on the precise catheter length that should be left within the epidural space.[27] The clinician can calculate the length by adding 5 to 6 cm to the distance previously measured by the epidural needle; the resulting total is the number up to which the epidural catheter should be withdrawn.[28]

Once the catheter is in its final position, a 3-cm syringe is used to see if cerebral spinal fluid can be gently aspirated. Aspiration of cerebral spinal fluid indicates the epidural catheter is within the intrathecal space and should be relocated. A test dose to exclude a misplaced catheter is then performed by injecting 3 mL of 1.5% lidocaine with 5 mcg/mL of epinephrine. An increase in a patient's heart rate of 20 to 30 bpm or 15 to 20 mm Hg in systolic blood pressure may indicate intravascular positioning.[29]

Caudal Anesthesia

Caudal anesthesia is a variation of epidural anesthesia widely used in children for procedures performed on areas under the umbilicus (eg, circumcision, herniotomy, or orchiopexy). Typically, patients are placed in a lateral decubitus position, and after the site is sterilely prepped, the sacral hiatus is identified. The sacrococcygeal membrane is pierced with a 22 to 25 G venous catheter at a 45-degree angle relative to the body's longitudinal axis, and the epidural catheter advanced until a loss of resistance is felt. No additional advance must be attempted since the distance between the sacrococcygeal membrane and the caudal end of the dural sac may be less than 10 mm. If CSF or blood is noted, the catheter is repositioned. Once properly located, a common test dose with epinephrine 0.5 mcg/kg is given to exclude a malpositioned catheter.[22][30][27]

Complications

Spinal cord injuries (SCI) resulting from anesthesia procedures are rare but significant concerns for surgical patients. These injuries can have devastating effects on the quality of life and may even lead to mortality. Complications associated with anesthesia-induced SCI include temporary or permanent neurological symptoms, epidural hematoma or abscess, direct traumatic spinal injury, and adhesive arachnoiditis. These complications can manifest as motor deficits, sensory loss, pain, paraesthesia, and even permanent paralysis or anesthesia.[14]

High-risk patients for anesthesia-induced SCI include those with spinal canal malformations, extremes of age, obesity, diabetes, immunocompromised or critically ill individuals, and those with previous neurological diseases. It is essential to carefully evaluate these patients preoperatively to identify any clinical conditions that may increase the risk of complications associated with neuraxial techniques. However, managing traumatic SCI patients in emergency rooms can be challenging due to existing tissue damage.[14] Other complications associated with epidural anesthesia include:

• Hypotension
• Nausea and vomiting
• Bronchoconstriction
• Postpuncture headache after dural perforation [31]
• Transient neurological syndrome 
• Nerve injury with possible neuropathy; paralysis is rare
• Epidural hematoma
• Epidural abscess
• Meningitis
• Accidental intrathecal injection with total spinal anesthesia
• Osteomyelitis [22]

Clinical Significance

The epidural technique is one of the earliest performed in the field of anesthesia. When correctly performed, epidural anesthesia is a safe technique that provides multiple benefits, including decreasing the need for general anesthesia, exposure to volatile anesthetics, and the associated risks. In addition, this technique can reduce the opioid requirement during and after a procedure, lowering the incidence of side effects associated with these medications. This is particularly relevant in pediatric anesthesia, where the potential negative impact of certain anesthetic drugs on neurodevelopment is debated. The epidural technique is also valuable for postoperative pain management as part of a multimodal approach.[2] A recent benefit of epidural anesthesia was providing an alternative to aerosol-generating general anesthesia, which was restricted during the COVID-19 pandemic.

Studies have critically examined the balance between the benefits and risks associated with epidural analgesia for postoperative pain management, concluding that while epidural analgesia offers superior pain relief and improved postoperative outcomes, it also carries inherent risks. Therefore, careful patient selection and close monitoring are essential to minimize complications.[4] Despite potential complications, neuraxial techniques, including epidural anesthesia, remain among the best opioid-sparing pain prevention and management options. These techniques can reduce patients' morbidity, improve outcomes, shorten hospital stays, and decrease the risk of chronic pain, ultimately providing economic benefits. Consequently, epidural anesthesia is a valuable technique for pain management, but it is not without risks. Careful patient selection, close monitoring, and timely interventions are essential to minimize the risk of anesthesia-induced SCI and associated complications. Further research and improvement of anesthesia protocols are necessary to enhance patient safety in neuraxial procedures.[15]

Enhancing Healthcare Team Outcomes

A growing body of evidence demonstrates that using epidural anesthesia has positive effects like faster return in bowel function, decreased hormonal stress response to pain, decreased postoperative pulmonary complications, shortening in the length of in-hospital stay, and patient satisfaction. However, the safe and effective use of epidural anesthesia requires an interprofessional team approach, including physicians, specialists, specialty-trained nurses, and pharmacists collaborating across disciplines to monitor and manage patients during the perioperative period.

Collaboration between interprofessional and multidisciplinary teams is essential in the choice of the locoregional anesthesia technique as the chosen technique must take into account various factors, in particular, the opinion of the patient, the surgeon, and the preferences of the anesthesiology clinician who must be able to perform the nerve block without difficulty for patient safety it is advisable to follow specific guidelines and protocols to conduct adequate regional anesthesia procedures. Furthermore, maintaining continuous closed-loop communication between all members of the perioperative care team regarding the need, technique, and potential management issues associated with the regional anesthetic utilized is essential to optimizing patient outcomes. In addition to obtaining thorough informed consent from either the patient or their authorized legal guardian before the placement of any regional anesthesia, all health team members are responsible for voicing their concerns as needed throughout the perioperative period based on their professional discretion.