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Spinal anaesthesia
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Spinal anaesthesia
Backflow of cerebrospinal fluid through a 25 gauge spinal needle after puncture of the arachnoid mater during initiation of spinal anaesthesia
MeSHD000775

Spinal anaesthesia (or spinal anesthesia), also called spinal block, subarachnoid block, intradural block and intrathecal block,[1] is a form of neuraxial regional anaesthesia involving the injection of a local anaesthetic with or without an opioid into the subarachnoid space. Usually a single-shot dose is administrered through a fine needle, alternatively continuous spinal anaesthesia through a intrathecal catheter can be performed.[2] It is a safe and effective form of anesthesia usually performed by anesthesiologists and CRNAs that can be used as an alternative to general anesthesia commonly in surgeries involving the lower extremities and surgeries below the umbilicus. The local anesthetic with or without an opioid injected into the cerebrospinal fluid provides locoregional anaesthesia: true anaesthesia, motor, sensory and autonomic (sympathetic) blockade. Administering analgesics (opioid, alpha2-adrenoreceptor agonist) in the cerebrospinal fluid without a local anaesthetic produces locoregional analgesia: markedly reduced pain sensation (incomplete analgesia), some autonomic blockade (parasympathetic plexi), but no sensory or motor block. Locoregional analgesia, due to mainly the absence of motor and sympathetic block may be preferred over locoregional anaesthesia in some postoperative care settings. The tip of the spinal needle has a point or small bevel. Recently, pencil point needles have been made available (Whitacre, Sprotte, Gertie Marx and others).[3]

Indications

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Spinal anaesthesia is a commonly used technique, either on its own or in combination with sedation or general anaesthesia. It is most commonly used for surgeries below the umbilicus, however recently its uses have extended to some surgeries above the umbilicus as well as for postoperative analgesia. Procedures which use spinal anesthesia include:[citation needed]

Spinal anaesthesia is the technique of choice for Caesarean section as it avoids a general anaesthetic and the risk of failed intubation (which is probably a lot lower than the widely quoted 1 in 250 in pregnant women[4]). It also means the mother is conscious and the partner is able to be present at the birth of the child. The post operative analgesia from intrathecal opioids in addition to non-steroidal anti-inflammatory drugs is also good.

Spinal anesthesia may be favored when the surgical site is amenable to spinal blockade for patients with severe respiratory disease such as COPD as it avoids the potential respiratory consequences of intubation and ventilation. It may also be useful in patients where anatomical abnormalities may make tracheal intubation relatively difficult.

In pediatric patients, spinal anesthesia is particularly useful in children with difficult airways and those who are poor candidates for endotracheal anesthesia such as increased respiratory risks or presence of full stomach.[5]

This can also be used to effectively treat and prevent pain following surgery, particularly thoracic, abdominal pelvic, and lower extremity orthopedic procedures.[6]

Contraindications

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Prior to receiving spinal anesthesia, it is important to provide a thorough medical evaluation to ensure there are no absolute contraindications and to minimize risks and complications. Although contraindications are rare, below are some of them:[5][6]

  • Patient refusal
  • Local infection or sepsis at the site of injection
  • Bleeding disorders, thrombocytopaenia, or systemic anticoagulation (secondary to an increased risk of a spinal epidural hematoma)
  • Severe aortic stenosis
  • Increased intracranial pressure
  • Space occupying lesions of the brain
  • Anatomical disorders of the spine such as scoliosis (although where pulmonary function is also impaired, spinal anaesthesia may be favored)[7]
  • Hypovolaemia e.g. following massive haemorrhage, including in obstetric patients
  • Allergy

Relative Contraindication

Risks and complications

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Complications of spinal anesthesia can result from the physiologic effects on the nervous system and can also be related to placement technique. Most of the common side effects are minor and are self-resolving or easily treatable while major complications can result in more serious and permanent neurological damage and rarely death. These symptoms can occur immediately after administration of the anesthetic or be delayed.[8]

Common and minor complications include:[6]

  • Mild hypotension
  • Bradycardia
  • Nausea and vomiting[9]
  • Transient neurological symptoms (lower back pain with pain in the legs) [10]
  • Post-dural-puncture headache or post-spinal headache[5] – Associated with the size and type of spinal needle used. A 2020 meta analysis recommended use of the 26G atraumatic spinal needle to lower the risk of PDPH – specifically, the Braun Atraucan 26G needle.[11]

Serious and permanent complications are rare but are usually related to physiologic effects on the cardiovascular system and neurological system or when the injection has been unintentionally at the wrong site.[6] The following are some major complications:

Technique

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Regardless of the anaesthetic agent (drug) used, the desired effect is to block the transmission of afferent nerve signals from peripheral nociceptors. Sensory signals from the site are blocked, thereby eliminating pain. The degree of neuronal blockade depends on the amount and concentration of local anaesthetic used and the properties of the axon. Thin unmyelinated C-fibres associated with pain are blocked first, while thick, heavily myelinated A-alpha motor neurons are blocked moderately. Heavily myelinated, small preganglionic sympathetic fibers are blocked last. The desired result is total numbness of the area. A pressure sensation is permissible and often occurs due to incomplete blockade of the thicker A-beta mechanoreceptors. This allows surgical procedures to be performed with no painful sensation to the person undergoing the procedure.[citation needed]

Some sedation is sometimes provided to help the patient relax and pass the time during the procedure, but with a successful spinal anaesthetic the surgery can be performed with the patient wide awake.

Anatomy

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In spinal anesthesia, the needle is placed past the dura mater in subarachnoid space and between lumbar vertebrae. In order to reach this space, the needle must pierce through several layers of tissue and ligaments which include the supraspinous ligament, interspinous ligament, and ligamentum flavum. Because the spinal cord (conus medullaris) is typically at the L1 or L2 level of the spine, the needle should be inserted below this between L3 and L4 space or L4 and L5 space in order to avoid injury to the spinal cord.

Positioning

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Patient positioning is essential to the success of the procedure and can affect how the anesthetic spreads following administration. There are three different positions which are used: sitting, lateral decubitus, and prone. The sitting and lateral decubitus positions are the most common.

Sitting – The patient sits upright at the edge of the exam table with their back facing the provider and their legs hanging off the end of the table and feet resting on a stool. Patients should roll their shoulders and upper back forward.

Lateral decubitus – In this position, the patient lies on their side with their back at the edge of the bed and facing the provider. The patient should curl their shoulder and legs and arch out their lower back.

Prone – The patient is positioned face down and their back facing upwards in a jackknife position.

Limitations

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Spinal anaesthetics are typically limited to procedures involving most structures below the upper abdomen. To administer a spinal anaesthetic to higher levels may affect the ability to breathe by paralysing the intercostal respiratory muscles, or even the diaphragm in extreme cases (called a "high spinal", or a "total spinal", with which consciousness is lost), as well as the body's ability to control the heart rate via the cardiac accelerator fibres. Also, injection of spinal anaesthesia higher than the level of L1 can cause damage to the spinal cord, and is therefore usually not done.

Differences with epidural anaesthesia

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Schematic drawing showing the principles of spinal anaesthesia

Epidural anaesthesia is a technique whereby a local anaesthetic drug is injected through a catheter placed into the epidural space. This technique is similar to spinal anaesthesia as both are neuraxial, and the two techniques may be easily confused with each other. Differences include:

  • A spinal anaesthetic delivers drug to the subarachnoid space and into the cerebrospinal fluid (CSF), allowing it to act on the spinal cord directly. An epidural delivers drugs outside the dura (outside CSF), and has its main effect on nerve roots leaving the dura at the level of the epidural, rather than on the spinal cord itself.
  • A spinal gives profound block of all motor and sensory function below the level of injection, whereas an epidural blocks a 'band' of nerve roots around the site of injection, with normal function above, and close-to-normal function below the levels blocked.
  • The injected dose for an epidural is larger, being about 10–20 mL compared to 1.5–3.5 mL in a spinal.
  • In an epidural, an indwelling catheter may be placed that allows for redosing injections, while a spinal is almost always a one-shot only. Therefore, spinal anaesthesia is more often used for shorter procedures relative to procedures which require epidural anaesthesia.
  • The onset of analgesia is approximately 25–30 minutes in an epidural, while it is approximately 5 minutes in a spinal.
  • An epidural often does not cause as significant a neuromuscular block as a spinal, unless specific local anaesthetics are also used which block motor fibres as readily as sensory nerve fibres.
  • An epidural may be given at a cervical, thoracic, or lumbar site, while a spinal must be injected below L2 to avoid piercing the spinal cord.

Injected substances

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Bupivacaine (Marcaine) is the local anaesthetic most commonly used, although lidocaine (lignocaine), tetracaine, procaine, ropivacaine, levobupivicaine, prilocaine, or cinchocaine may also be used. Commonly opioids are added to improve the block and provide post-operative pain relief, examples include morphine, fentanyl, diamorphine, and buprenorphine. Non-opioids like clonidine or epinephrine may also be added to prolong the duration of analgesia (although Clonidine may cause hypotension). In the United Kingdom, since 2004 the National Institute for Health and Care Excellence recommends that spinal anaesthesia for Caesarean section is supplemented with intrathecal diamorphine and this combination is now the modal form of anaesthesia for this indication in that country. In the United States, morphine is used for cesareans for the same purpose since diamorphine (heroin) is not used in clinical practice in the US.

Baricity refers to the density of a substance compared to the density of human cerebrospinal fluid. Baricity is used in anaesthesia to determine the manner in which a particular drug will spread in the intrathecal space. Usually, the hyperbaric, (for example, hyperbaric bupivacaine) is chosen, as its spread can be effectively and predictably controlled by the Anaesthesiologist, by tilting the patient. Hyperbaric solutions are made more dense by adding glucose to the mixture.

Baricity is one factor that determines the spread of a spinal anaesthetic but the effect of adding a solute to a solvent, i.e. solvation or dissolution, also has an effect on the spread of the spinal anaesthetic. In tetracaine spinal anaesthesia, it was discovered that the rate of onset of analgesia was faster and the maximum level of analgesia was higher with a 10% glucose solution than with a 5% glucose spinal anaesthetic solution. Also, the amount of ephedrine required was less in the patients who received the 5% glucose solution.[12] In another study this time with 0.5% bupivacaine the mean maximum extent of sensory block was significantly higher with 8% glucose (T3.6) than with 0.83% glucose (T7.2) or 0.33% glucose (T9.5). Also the rate of onset of sensory block to T12 was fastest with solutions containing 8% glucose.[13]

History

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Main article: History of neuraxial anesthesia

The first spinal analgesia was administered in 1885 by James Leonard Corning (1855–1923), a neurologist in New York.[14] He was experimenting with cocaine on the spinal nerves of a dog when he accidentally pierced the dura mater.

The first planned spinal anaesthesia for surgery on a human was administered by August Bier (1861–1949) on 16 August 1898, in Kiel, when he injected 3 ml of 0.5% cocaine solution into a 34-year-old labourer.[15] After using it on six patients, he and his assistant each injected cocaine into the other's spine. They recommended it for surgeries of legs, but gave it up due to the toxicity of cocaine.

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Spinal anaesthesia

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from Grokipedia
Spinal anaesthesia, also known as subarachnoid block or intrathecal anaesthesia, is a regional anaesthetic technique in which a local anaesthetic agent is injected directly into the within the subarachnoid space surrounding the , resulting in temporary loss of sensation and motor function in the lower body. This method provides rapid onset of anaesthesia, typically within 3 to 8 minutes depending on the agent used, and is commonly employed for surgical procedures involving the lower , , , or lower extremities, such as caesarean sections, or surgeries, and urological operations. Unlike , it allows patients to remain conscious while achieving profound analgesia and muscle relaxation below the level of the block, often reducing the need for systemic opioids and facilitating quicker postoperative recovery. The technique was first successfully performed in 1898 by German surgeon , who used as the local anaesthetic, marking a significant advancement in for . Since then, safer synthetic agents like bupivacaine and lidocaine have become standard, with bupivacaine offering a duration of 90 to 150 minutes and lidocaine providing 60 to 90 minutes of effect. The procedure involves positioning the patient in a sitting or lateral decubitus posture, performing aseptic skin preparation, and inserting a fine spinal needle (typically 25- to 27-gauge) through the lumbar interspace—most often L3-L4 or L4-L5—to access the subarachnoid space, confirmed by flow, before injecting the anaesthetic. Baricity of the solution (hypobaric, isobaric, or hyperbaric) influences the spread of anaesthesia, allowing tailoring to the surgical site. Indications for spinal anaesthesia include any surgery below the umbilicus where rapid, reliable blockade is desired, particularly in patients with conditions that contraindicate , such as respiratory compromise. Absolute contraindications encompass patient refusal, at the injection site, severe hypovolaemia, and increased , while relative ones include , spinal deformities, or pre-existing neurological disorders. Advantages over include lower risk of airway complications, better postoperative pain control, and reduced incidence of due to preserved lower limb circulation; however, potential complications such as (from sympathetic blockade), post-dural puncture headache (affecting up to 25% of cases), and rare instances of total spinal anaesthesia or must be managed vigilantly. Overall, when performed by trained anaesthetists, spinal anaesthesia is a , effective option with a high success rate for appropriate procedures.

Introduction

Definition and Overview

Spinal anaesthesia, also known as subarachnoid block or intrathecal anaesthesia, is a regional anaesthesia technique that involves the injection of a local anaesthetic directly into the subarachnoid space surrounding the , thereby blocking the transmission of impulses to produce sensory and motor below the level of injection. This method targets the roots exiting the , providing targeted numbness and paralysis without affecting higher neural functions. Introduced in the late , spinal anaesthesia was first successfully performed by in 1898, marking a significant advancement in surgical by allowing procedures without the risks associated with . It is primarily employed for surgical interventions involving the lower body, including lower abdominal surgeries, pelvic operations, and procedures on the lower limbs, where effective analgesia and muscle relaxation are essential. Key advantages of spinal anaesthesia include its rapid , typically within minutes, which facilitates quick surgical , as well as profound skeletal muscle relaxation that enhances operative conditions. Additionally, it reduces the reliance on , thereby potentially lowering the incidence of associated complications such as issues and postoperative cognitive dysfunction. The local anaesthetic spreads within the to achieve the desired dermatomal level of blockade.

Mechanism of Action

Spinal anesthesia achieves its effects through the direct administration of local anesthetics into the subarachnoid space, where the agent diffuses across the to contact the roots. There, local anesthetics reversibly bind to voltage-gated sodium channels in the axonal membranes, primarily in their open or inactivated states, preventing sodium influx and thereby inhibiting the generation and propagation of action potentials. This blockade temporarily interrupts the conduction of sensory, motor, and autonomic nerve impulses along the spinal nerves, resulting in loss of sensation, muscle relaxation, and sympathetic inhibition below the level of injection. A key feature of spinal anesthesia is the phenomenon of differential blockade, where nerve fibers exhibit varying sensitivities to the local anesthetic based on their , myelination, and function. Smaller-diameter, thinly myelinated fibers, such as Aδ nociceptive fibers responsible for and sensation, are blocked at lower concentrations and more rapidly than larger, heavily myelinated fibers like Aα motor fibers or Aβ fibers mediating touch and . Sympathetic preganglionic B fibers are the most sensitive, often blocked first, leading to and , while unmyelinated C fibers (involved in dull ) show relative resistance. This results in sensory blockade preceding motor blockade in clinical practice. The extent and predictability of the blockade are modulated by the baricity of the solution relative to , which influences its distribution within the subarachnoid space under . Hyperbaric solutions (denser than CSF) tend to settle dependently, producing a more controlled, unidirectional spread when the patient is positioned appropriately (e.g., sitting), ideal for lower abdominal procedures. Isobaric solutions ( similar to CSF) spread more evenly regardless of position, while hypobaric solutions (less ) rise cephalad in the , useful for unilateral blocks in lateral decubitus. Patient positioning during and immediately after injection further affects spread, with factors like lumbar curvature and injection speed contributing to the final dermatomal level. Pharmacokinetically, spinal anesthesia has a rapid onset of 2 to 5 minutes due to the direct proximity of the to nerve roots, with peak effect occurring within 10 to 15 minutes. The duration of blockade typically lasts 1 to 3 hours, varying with the agent's lipid solubility and protein binding, which delay vascular uptake and prolong tissue residency. Systemic absorption is minimal initially but increases as the diffuses into epidural and systemic circulation, with clearance influenced by factors such as dose volume and vasoactive additives that alter local blood flow.

Anatomy and Physiology

Spinal Cord and Meninges

The is a cylindrical structure composed of that extends from the to the region, serving as the primary conduit for neural signals between the brain and the periphery. In adults, it typically terminates at the level of the L1-L2 vertebral interspace, forming the , beyond which the and sacral nerve roots descend as the within the lumbar cistern. This anatomical arrangement is crucial for spinal anaesthesia, as it allows needle insertion below the cord's termination to access neural elements without direct injury. The spinal cord is enveloped by three protective meningeal layers: the outermost dura mater, a tough fibrous membrane; the middle arachnoid mater, a delicate avascular layer; and the innermost pia mater, which adheres closely to the cord's surface. The subarachnoid space, located between the arachnoid and pia mater, contains cerebrospinal fluid (CSF) and is the site of anaesthetic injection during spinal anaesthesia, enabling the agent to surround the cauda equina nerve roots. To minimize the risk of , for spinal anaesthesia is performed at the L3-L4 or L4-L5 intervertebral space, which lies inferior to the cord's typical endpoint. In term infants, the typically terminates at the adult level (L1-L2 interspace), with variations possible. Pathological conditions, such as tethered cord syndrome, may result in an abnormally low termination (below L2-L3), potentially altering procedural safety and requiring imaging for confirmation.

Cerebrospinal Fluid and Spread of Anaesthetic

(CSF) is a clear, colorless, and acellular fluid that occupies the subarachnoid space, providing mechanical cushioning and buoyant support to the while facilitating nutrient transport and waste removal. In adults, the total CSF volume is approximately 150 mL, with roughly 125 mL distributed in the subarachnoid spaces surrounding the and and the remaining 25 mL in the cerebral ventricles. Within the context of , the relevant portion is the lumbosacral CSF volume in the lower subarachnoid space, which typically ranges from 40 to 80 mL and directly impacts the dilution and distribution of the injected anesthetic. The spread of the anesthetic agent through the CSF determines the extent and predictability of the sensory and motor blockade achieved during spinal anesthesia. Key factors influencing this spread include the volume and baricity (density relative to CSF) of the injectate, which affect how the solution mixes or settles within the fluid column; patient posture at the time of injection and immediately after, which governs gravitational distribution; intra-abdominal pressure, which can compress the dural sac and reduce effective CSF volume by displacing fluid cranially; and inherent CSF flow dynamics driven by cardiac pulsations, respiration, and postural changes. These elements interact to enable controlled propagation of the anesthetic along the , primarily via and bulk flow within the narrow subarachnoid space. In , the cephalad (upward) spread of hyperbaric solutions is particularly predictable when the patient is positioned sitting, as the denser injectate pools dependently in the curve before ascending upon positioning, allowing clinicians to titrate doses for targeted levels such as T10 for lower abdominal procedures or T4 for cesarean sections. This predictability stems from the consistent interplay of injectate properties and posture, though interpatient variability in CSF volume can alter outcomes. Unpredictable spread, often due to variations in the above factors, can result in excessive cephalad migration, leading to high spinal blockade where the anesthetic reaches cervical levels, compromising intercostal muscle function and potentially causing , , or . Such complications underscore the importance of precise dosing and monitoring to maintain the desired segmental distribution within the CSF.

Clinical Use

Indications

Spinal anesthesia is primarily indicated for surgical procedures involving the lower , , , and lower extremities, where it provides effective sensory and motor below the level of the umbilicus. Common elective surgeries include cesarean sections, total hip and knee replacements, repairs, lower limb amputations, , and foot and ankle procedures. It is particularly favored in obstetric contexts, such as cesarean deliveries and vaginal births, allowing the patient to remain awake for immediate bonding with the neonate while minimizing fetal exposure to general anesthetics. In orthopedic settings, it supports repairs in elderly patients, reducing the need for general anesthesia in those with comorbidities. The advantages of spinal anesthesia in these indications include rapid onset of dense analgesia, lower intraoperative blood loss, decreased risk of and pulmonary complications compared to general , and faster postoperative recovery with reduced requirements. For instance, in and , it has been associated with a 15% reduction in needs and improved control, facilitating earlier mobilization and bowel function return. It also avoids general risks such as airway complications and aspiration, making it suitable for patients with respiratory issues. Relative indications extend to acute in trauma, such as fractures in geriatric populations, and obstetric emergencies requiring urgent intervention. It is increasingly used in outpatient settings for procedures like due to its cost-effectiveness and rapid recovery profile, enabling same-day discharge. In high-risk cardiac patients undergoing noncardiac surgery, such as those with moderate-to-severe or , neuraxial techniques like spinal anesthesia are recommended to reduce systemic opioid use, lower systemic vascular resistance, and minimize pulmonary and cardiac morbidity, including and . Evidence from a 2022 meta-analysis of randomized controlled trials supports lumbar epidural variants in cases for decreasing major adverse cardiac events, though outcomes may vary by procedure. The 2024 AHA/ACC guidelines endorse these approaches for select high-risk groups to optimize perioperative outcomes.

Contraindications

Spinal anesthesia carries specific contraindications that must be carefully evaluated to prevent serious complications such as , hemorrhage, or neurological . These are categorized as absolute, where the procedure should not be performed under any circumstances, and relative, where the risks may be weighed against benefits in select cases with multidisciplinary input. Absolute contraindications include patient refusal, as is paramount and the procedure cannot proceed without it. Local at the puncture site, such as or , poses a high risk of introducing pathogens into the , leading to or epidural . Severe , defined as significant volume depletion without correction (e.g., from hemorrhage or ), can exacerbate hemodynamic instability due to sympathectomy-induced . Increased , often from mass lesions or obstructive , is contraindicated because cerebrospinal fluid drainage during puncture may precipitate . Relative contraindications encompass conditions where spinal anesthesia may be avoided or modified, but could be considered if benefits outweigh risks. , for instance, including with platelet counts below 50,000/μL or active anticoagulation, increases the risk of , though thresholds vary by guideline and patient factors. or systemic infection heightens the potential for bacteremia and involvement, though stable patients on antibiotics may be assessed individually. Fixed cardiac output states, such as severe , can lead to profound or ischemia due to reduced preload tolerance. Prior spinal complications, including deformities or instrumentation, may complicate needle placement and increase trauma risk. Pre-procedure assessment protocols, updated in 2025 guidelines, emphasize comprehensive evaluation including tests (e.g., platelet count, INR, aPTT) to identify bleeding risks, alongside neurological examinations to document baseline deficits and rule out progressive disease. These protocols also involve reviewing for infection or and performing a focused back examination for site suitability. When contraindications are present, alternatives such as epidural anesthesia (if relative and feasible) or general anesthesia are recommended to achieve surgical goals safely.

Procedure

Patient Preparation and Positioning

Patient preparation for spinal anesthesia involves several key pre-procedure steps to ensure safety and efficacy. is obtained after thoroughly discussing the procedure, its benefits, risks, and alternatives with the patient. Intravenous (IV) access is established using a wide-bore (16- or 18-gauge) to facilitate fluid and medication administration if needed. Baseline , including , , and , are recorded, alongside a comprehensive history and that assesses for allergies, prior exposures, status, and any spinal abnormalities such as infections or deformities. If the patient exhibits significant anxiety, mild with an agent like may be provided, while ensuring equipment for and circulatory support is immediately available. Continuous monitoring is essential throughout the preparation and procedure to detect and manage hemodynamic changes promptly. Standard monitors include (ECG) for cardiac rhythm, for , non-invasive blood pressure measurement cycled every 1-2 minutes initially, and if supplemental oxygen or ventilation support is anticipated. These devices are applied prior to positioning, with checked and documented immediately after setup and at regular intervals, such as every 5 minutes intraoperatively. Optimal positioning maximizes lumbar interspace exposure while minimizing patient discomfort and procedural risks. The lateral decubitus position is frequently utilized, with the patient lying on their side (typically the left for non-pregnant individuals), knees flexed toward the chest, and the spine curved forward in flexion to widen the L3-L4 or L4-L5 interspaces. The sitting position serves as an alternative, especially for hyperbaric anesthetic solutions or to achieve more controlled spread; here, the patient sits upright at the bed's edge with feet supported on a stool, shoulders slumped forward, chin tucked to the chest, and hands clasped over the dependent knee to promote maximal flexion. Anatomical landmarks, such as the iliac crests marking the L4 spinous process, guide interspace selection in both positions. Special considerations adapt preparation and positioning to individual patient factors. In pregnant patients undergoing procedures like cesarean delivery, the left lateral decubitus position with a 15-degree table tilt is recommended to relieve aortocaval compression from the gravid , enhancing maternal during setup. For obese patients, the sitting position often improves palpation of spinous processes despite excess , potentially requiring additional flexion or supportive pillows for stability. Patients with necessitate preoperative imaging or detailed palpation to account for spinal curvature, with positioning adjusted—such as enhanced lateral flexion—to align asymmetric interspaces effectively.

Injection Technique

The injection technique for spinal anaesthesia involves precise placement of a needle into the subarachnoid space to deliver the anaesthetic agent, typically performed under aseptic conditions following patient preparation and positioning. Essential equipment includes a 25- to 27-gauge spinal needle, such as a Quincke (cutting bevel) or Whitacre (pencil-point) type, along with 3 mL or 5 mL syringes and a local anaesthetic like 1% lidocaine for skin infiltration. in alcohol is used for skin antisepsis, and a sterile drape is applied to maintain the field. The procedure begins with thorough skin antisepsis using chlorhexidine, followed by subcutaneous infiltration with 1% lidocaine to create a wheal at the insertion site. The spinal needle is then advanced using either a midline or paramedian approach, typically at the L3-L4 or L4-L5 interspace to avoid the spinal cord. In the midline approach, the needle is inserted perpendicular to the skin and angled 5 to 15 degrees cephalad, passing through the supraspinous and interspinous ligaments, ligamentum flavum, and dura mater. For the paramedian approach, suitable for patients with calcified ligaments or obesity, the needle enters 1 to 2 cm lateral to the midline spinous process and is directed medially and cephalad, bypassing the interspinous ligaments. In difficult cases, such as severe scoliosis, a modified Taylor approach at the L5-S1 interspace may be employed, inserting the needle paramedian just caudal to the sacral cornua and directing it cephalad toward the midline. Subarachnoid entry is confirmed by the free flow of clear (CSF) from the needle hub, which may require gentle aspiration with a for smaller-gauge needles; the patient is usually positioned sitting or lateral decubitus to facilitate this, as detailed in preparation guidelines. Obsolete methods like the hanging drop test, where saline in the hub is drawn in by negative subarachnoid pressure, are rarely used today. Loss of resistance, primarily associated with epidural techniques, is not standard for confirming subarachnoid placement. Once confirmed, the anaesthetic solution is injected slowly over 10 to 15 seconds to minimize complications like high spinal block, with the needle stabilized to prevent movement. The needle is then withdrawn, and the site is inspected for bleeding.

Agents and Dosages

Spinal anesthesia primarily employs local anesthetics administered intrathecally to block nerve transmission in the spinal cord. The most commonly used agent is hyperbaric bupivacaine at a concentration of 0.5%, with typical dosages ranging from 10 to 15 mg for surgical procedures, providing a sensory block duration of 2 to 4 hours. This formulation allows for predictable cephalad spread influenced by baricity within the cerebrospinal fluid. Ropivacaine, an amide local anesthetic with similar potency to bupivacaine, is administered at comparable doses (e.g., 10-15 mg of 0.5% solution) but offers advantages such as reduced cardiotoxicity and less pronounced hypotension due to its differential blockade of sensory versus motor fibers. Adjunct medications are often combined with local anesthetics to enhance analgesia or extend block duration without increasing the primary agent dose. Intrathecal opioids, such as fentanyl at 10-25 mcg, prolong postoperative analgesia by synergizing with the local anesthetic, typically extending sensory block by 1-2 hours while minimizing systemic opioid requirements. Clonidine, an alpha-2 adrenergic agonist, is used as an adjunct at doses of 15-75 mcg to potentiate the sensory block and improve hemodynamic stability, though its primary benefit lies in dose-dependent prolongation of anesthesia. Dosages are tailored to the procedure, patient factors like and weight, and desired block level to optimize efficacy and safety. For cesarean sections, hyperbaric bupivacaine doses of 7.5-12.5 mg are standard, often adjusted by (e.g., an additional 0.15 mg per cm above 150 cm) to achieve a T4 sensory level while reducing maternal risk. In lower extremity surgeries, doses may increase to 12-15 mg for bupivacaine to ensure adequate coverage up to T10. Low-dose (e.g., 7.5 mg of 0.5% hyperbaric solution) is used in settings for procedures like inguinal herniorrhaphy, offering effective short-duration blocks (1-2 hours) with faster recovery and minimized compared to higher doses of bupivacaine or . As of 2025, short-acting agents like mepivacaine (typically 40-80 mg of 1-2% isobaric or hyperbaric solution, duration 1-2 hours) and 2-chloroprocaine (30-50 mg of 1% solution, duration 45-90 minutes) have become preferred options for procedures such as total hip/knee arthroplasty and non-arthroplasty surgeries, enabling faster motor recovery and same-day discharge.
AgentCommon ConcentrationTypical DosagePrimary Use CaseDuration (hours)
Bupivacaine (hyperbaric)0.5%10-15 mg, cesarean2-4
0.5%10-15 mgLower extremity procedures1.5-3
(low-dose)0.5%7.5 mg surgery1-2
Mepivacaine1-2%40-80 mg TJA1-2
2-Chloroprocaine1%30-50 mgShort procedures0.75-1.5
(adjunct)N/A10-25 mcgProlonged analgesia+1-2
(adjunct)N/A15-75 mcgBlock potentiation+0.5-1.5

Immediate Post-Procedure Management

Following the intrathecal injection of the anesthetic agent, immediate post-procedure management focuses on assessing the onset and adequacy of the sensory and motor blockade to ensure readiness for surgery. The level of sensory block is evaluated using pinprick or cold sensation tests along dermatomal landmarks, such as T10 at the umbilicus for lower abdominal procedures or T4 at the nipple line for upper abdominal ones, typically every 2 to 5 minutes until the block stabilizes. Motor blockade is assessed concurrently using the modified Bromage scale, which grades lower limb function from 0 (full flexion) to 3 (complete paralysis), confirming adequate relaxation for the intended procedure. These assessments are performed continuously in the immediate post-injection period to detect any uneven spread or failure of the block. Hemodynamic monitoring is essential due to the sympathectomy induced by spinal anesthesia, which commonly causes from and reduced venous return. Continuous monitoring of , electrocardiogram, and is standard, with proactive interventions to maintain stability. Intravenous fluid co-loading or boluses (e.g., 500-1000 mL crystalloid) are administered to support preload, while vasopressors such as (bolus 50-100 mcg or infusion 25-50 mcg/min) are used as first-line treatment for , particularly in non-obstetric settings, to counteract alpha-adrenergic blockade without excessive . Guidelines recommend prophylactic infusion initiated immediately post-injection to prevent maternal or patient , titrated to maintain systolic near baseline. To optimize block duration and patient comfort during the onset phase, patients are positioned with slight head elevation (10-15 degrees) to facilitate even cephalad spread of the hyperbaric solution, and supplemental oxygen via is provided if saturation falls below 94%. is typically delayed 5 to 10 minutes post-injection to allow full block establishment, varying by agent—such as shorter with lidocaine (60-90 minutes total duration) or longer with bupivacaine (90-180 minutes). Leg elevation may be used briefly if persists despite fluids and vasopressors. Discharge from the immediate recovery area requires confirmation of resolving blockade and hemodynamic stability to ensure safe ambulation. Criteria include full sensory recovery to the S2 dermatome (perianal sensation), motor strength returning to Bromage score 0, stable vital signs (e.g., systolic blood pressure within 20% of baseline, heart rate 60-100 bpm), and the ability to stand unassisted without orthostasis. Patients are monitored until these milestones are met, typically 1-2 hours post-injection for short-acting agents, before transfer to a less intensive setting.

Risks and Complications

Common Adverse Effects

One of the most frequent adverse effects of spinal anaesthesia is , resulting from sympathetic blockade that leads to and reduced venous return. This occurs in approximately 16-33% of cases, with higher rates in obstetric patients due to aortocaval compression. Management typically involves intravenous fluid administration and vasopressors such as or to restore hemodynamic stability. Nausea and vomiting are also common, affecting up to 40% of patients for and 22% for in the postoperative period, often triggered by , administration, or vagal stimulation. These symptoms are usually transient and can be effectively treated with antiemetics like , which blocks serotonin receptors to prevent emetic reflexes. Post-dural puncture headache (PDPH) arises from leakage through the dural puncture site, causing intracranial and typically manifesting as a positional within 48 hours. The incidence is about 1-3% with the use of fine-gauge, pencil-point needles, though it can be higher with larger needles or multiple attempts. Conservative management includes bed rest, hydration, and to promote and reduce symptoms, with most cases resolving within a week. Shivering is another prevalent side effect, occurring in 30-50% of patients, attributed to impaired thermoregulation from the anaesthesia-induced blockade of thermosensitive pathways in the spinal cord. It often begins intraoperatively and can increase oxygen consumption and discomfort. Treatment options include active warming with forced-air devices to normalize body temperature or low-dose meperidine, which acts centrally to suppress shivering without significant respiratory depression.

Rare and Serious Complications

While total spinal anesthesia is a rare complication resulting from excessive cephalad spread of the local anesthetic into the cervical spinal segments, leading to apnea, hypotension, and potential respiratory arrest, its incidence is estimated at less than 0.1% in general spinal procedures. This condition typically arises from high anesthetic volumes, improper patient positioning, or anatomical variations, and immediate treatment involves endotracheal intubation, mechanical ventilation, and hemodynamic support to maintain oxygenation and circulation until the block resolves. Infections such as or represent infrequent but severe risks following spinal anesthesia, with an overall incidence of CNS infections of approximately 1 in 100,000 procedures (as of 2025), primarily due to bacterial contamination during needle insertion. , often caused by like , presents with fever, headache, and nuchal rigidity within hours to days post-procedure, while may develop more insidiously with and neurological deficits. Prevention relies on meticulous sterile technique, including skin preparation with , single-use equipment, and avoiding procedures in bacteremic patients; treatment entails prompt intravenous antibiotics, for culture, and surgical drainage for if neurological compromise occurs. Neurological injuries, including and , are exceedingly uncommon after spinal anesthesia, with recent 2025 data indicating an incidence below 0.01% for permanent deficits, often linked to direct needle trauma, neurotoxic local anesthetics like high-dose lidocaine, or prolonged exposure to agents. , occurring at about 1-2 per 100,000 procedures, particularly in patients with , can cause leading to if not urgently decompressed. manifests as bowel/bladder dysfunction, , and lower limb weakness due to lumbosacral compression or ischemia, requiring urgent MRI imaging and potential neurosurgical decompression. Risk mitigation involves using guidance for needle placement and limiting hyperbaric solutions to avoid maldistribution. Cardiovascular collapse, encompassing severe or , occurs rarely in susceptible patients during spinal , with an incidence of around 2.73 per 10,000 cases, exacerbated by vagal reflexes from sympathetic blockade or . High-risk groups include the elderly, those with cardiac comorbidities, or obstetric patients under emotional stress; manifestations include profound and loss of consciousness. Management includes immediate administration of atropine for , or for vasoplegia, and if arrest ensues, alongside fluid resuscitation and Trendelenburg positioning. Extremely rare delayed complications of spinal anesthesia include iatrogenic epidermoid tumors, which are benign cysts formed from skin cells implanted into the subarachnoid space during the procedure. These slow-growing tumors have been reported in case studies to present up to seven years later with symptoms such as back pain or neurological deficits. Chronic hematomas or fibrosis are also very uncommon after isolated spinal anesthesia injections, being more frequently associated with spinal surgery involving greater tissue trauma; they typically do not present as a palpable back lump and instead may cause internal compression leading to pain or deficits if they occur.

Comparisons with Other Methods

Differences from Epidural Anaesthesia

Spinal anesthesia involves the injection of local anesthetic directly into the subarachnoid space, where it mixes rapidly with (CSF), resulting in a administration that provides profound and reliable blockade. In contrast, epidural anesthesia targets the surrounding the dural sac, allowing for the placement of a to enable intermittent boluses or continuous , which facilitates and prolongation of . This fundamental difference in injection sites—intrathecal for spinal versus peridural for epidural—leads to distinct pharmacokinetic profiles, with spinal anesthesia achieving more uniform distribution due to CSF diffusion. The onset of anesthesia is notably faster with spinal administration, typically occurring within 2 to 5 minutes, owing to the direct access to nerve roots via the CSF, which produces a dense sensory and motor block that is more profound and predictable than in epidural techniques. Epidural anesthesia, however, has a slower onset of 10 to 20 minutes, as the anesthetic must spread through fatty tissues and bathe the nerve roots externally, resulting in a less intense initial block that can be adjusted but may require higher doses for comparable density. This rapid and deeper blockade in spinal anesthesia minimizes the risk of incomplete analgesia during short procedures but increases the likelihood of hemodynamic instability, such as hypotension. Duration of effect is shorter in spinal anesthesia, generally lasting 1 to 3 hours depending on the agent used (e.g., bupivacaine provides 90 to 150 minutes), making it unsuitable for extended analgesia without additional interventions. Epidural anesthesia offers greater flexibility, with effects extendable beyond several hours through catheter-based boluses or infusions, allowing for maintenance of anesthesia or transition to postoperative pain control. Consequently, spinal anesthesia is preferred for brief surgical interventions, such as lower abdominal or lower extremity procedures, while epidural is favored for prolonged operations or scenarios requiring ongoing analgesia, like labor or postoperative management.

Differences from General Anaesthesia

Spinal anesthesia provides regional blockade primarily affecting sensory, motor, and autonomic functions below the T10 dermatome level, targeting procedures in the lower abdomen, perineum, and lower extremities, whereas general anesthesia induces systemic unconsciousness, amnesia, analgesia, and complete skeletal muscle relaxation throughout the body. Key advantages of spinal anesthesia over general anesthesia include avoidance of airway manipulation, which reduces risks associated with intubation and ventilation, and a lower incidence of postoperative nausea and vomiting (PONV), with meta-analyses showing a relative risk reduction of approximately 0.29 within 24 hours post-procedure. Additionally, spinal anesthesia carries a lower risk of respiratory depression compared to general anesthesia, as it spares supraspinal respiratory centers and avoids the ventilatory depressant effects of systemic agents. Despite these benefits, spinal anesthesia is limited to surgeries below the waist and has a of 1-5%, potentially necessitating conversion to general , while general anesthesia offers greater reliability for extensive or upper-body procedures but introduces higher systemic risks such as hemodynamic instability. Spinal anesthesia is often preferred for patients with or (COPD), as it minimizes airway complications and pulmonary risks compared to general , whereas general anesthesia is typically selected for upper abdominal or thoracic surgeries and in emergencies requiring rapid onset and full-body control.

Historical Development

Early Discoveries

In 1885, American surgeon pioneered the use of for peripheral blocks by directly infiltrating the drug around nerves, a technique that demonstrated reliable local anaesthesia and inspired later explorations into spinal applications. This work built on the discovery of cocaine's anaesthetic properties and marked a shift toward targeted inactivation for surgical procedures. German surgeon August Karl Gustav Bier advanced these concepts through systematic experiments, culminating in the first intentional spinal anaesthesia in humans on August 16, 1898, at the of . Prior to human trials, conducted preliminary experiments on animals and self-administration with his assistant to test intrathecal injection, confirming its potential to produce anaesthesia of the lower body. 's procedure involved injecting 15 mg of into the subarachnoid space, enabling successful orthopaedic surgeries such as ankle resections without . The technique quickly spread internationally, with the first spinal anesthesia in the United States performed in 1899 by surgeons Dudley Tait and Guido Caglieri in . Early implementations faced significant hurdles due to cocaine's inherent , which frequently resulted in systemic complications like seizures, cardiovascular collapse, and patient deaths, often from overdose or unintended spread. These risks stemmed from cocaine's narrow therapeutic window and its rapid absorption into the bloodstream, leading to excitation. Consequently, adoption remained confined largely to and parts of in the late 19th and early 20th centuries, where cautious surgeons reported successful outcomes in limb and lower-body operations through detailed case series. Bier's publications, including accounts of five successful limb procedures, encouraged limited clinical use despite the dangers, highlighting the technique's promise for avoiding general anaesthesia's broader risks.

Modern Advancements

In the , the development of amide-based local anesthetics marked a significant pharmacological advancement in spinal , with lidocaine synthesized in 1943 and introduced clinically as a 2% hyperbaric preparation specifically for intrathecal use by the late . Unlike earlier ester-type agents such as , which were prone to allergic reactions due to their into para-aminobenzoic acid, amides like lidocaine demonstrated markedly reduced allergenicity, enhancing and broadening applicability in regional . This shift to amides improved tolerability while maintaining effective blockade duration, paving the way for more reliable spinal techniques. During the 1980s and 1990s, refinements in drug formulation and procedural integration further optimized spinal anesthesia outcomes. Hyperbaric bupivacaine, introduced following the agent's synthesis in 1963, saw standardization through extensive studies on baricity and dosing, with key research in the early 1980s establishing predictable spread and hemodynamic control for lower extremity and abdominal procedures. Concurrently, the combined spinal-epidural (CSE) technique gained prominence in the late 20th century. Building on earlier concepts such as Soresi's 1937 description, the modern approach was first documented in 1979 by Curelaru using two separate interspaces but refined with parallel needle systems in the early 1980s and widely adopted for labor analgesia by the early 1990s, allowing rapid onset from the spinal component alongside prolonged epidural supplementation. These innovations reduced procedural variability and extended the method's utility in complex cases. From the 2000s onward, technological and dosing strategies enhanced precision and suitability for outpatient settings. guidance for emerged as a standard aid, with meta-analyses showing it increases first-pass success rates by approximately 20-30% in challenging patients, such as the obese or elderly, thereby lowering failure and complication rates compared to landmark-based approaches. Complementing this, low-dose regimens—typically 4-5 mg of hyperbaric bupivacaine with adjuvants like —became prevalent for ambulatory surgery, minimizing motor blockade and to facilitate same-day discharge while preserving sensory coverage for procedures like knee arthroscopy. By 2025, spinal anesthesia has integrated into enhanced recovery after surgery (ERAS) protocols, particularly for lumbar spinal fusion, where guidelines emphasize multimodal analgesia incorporating intrathecal s to reduce opioid consumption, shorten hospital stays by 1-2 days, and lower complication rates by up to 39%. Concurrent research addresses potential from local anesthetics like bupivacaine, with studies demonstrating that s, such as NRF2 activators, mitigate and in neural tissues, offering neuroprotective strategies through enhanced expression. These developments underscore an evolving, evidence-based approach prioritizing safety and efficiency.

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