19.1 Neuraxial Techniques: Spinal, Epidural, and Caudal Anaesthesia

19.1 Neuraxial Techniques: Spinal, Epidural, and Caudal Anaesthesia

Neuraxial anaesthesia—encompassing spinal (subarachnoid), epidural, and caudal blockade—constitutes the cornerstone of regional anaesthetic practice in modern surgical care. Mastery requires a precise understanding of vertebral column anatomy, the biophysics of drug distribution within the cerebrospinal fluid (CSF) and epidural space, the physiological consequences of sympathetic denervation, and the immediate recognition and protocolized management of life-threatening complications.

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1. Neuroanatomy and Mechanisms of Neuraxial Blockade

Anatomical Layers and Drug Diffusion

During needle advancement from skin to the subarachnoid space along the midline, the needle traverses the following structures in sequential order:

1. Skin and subcutaneous adipose tissue
2. Supraspinous ligament (dense fibrous band bridging vertebral spinous tips)
3. Interspinous ligament (thin membranous collagenous bands)
4. Ligamentum flavum ("yellow ligament" rich in elastic fibers, offering distinct tactile resistance before a sudden release)
5. Epidural space (potential space containing internal vertebral venous plexus of Batson, lymphatics, fat, and mixed spinal nerve roots)
6. Dura mater (tough fibroelastic outer meningeal layer)
7. Arachnoid mater (avascular delicate cellular membrane; closely applied to the inner surface of the dura)
8. Subarachnoid space (containing CSF, spinal cord, and trabeculae)

In spinal anaesthesia, local anaesthetic (LA) is deposited directly into the subarachnoid space, mixing with CSF. The drug acts primarily on spinal nerve rootlets (radicles) within the subarachnoid space, which lack a thick protective epineurium and perineurium, rendering them exquisitely sensitive to conduction block. Secondary sites of action include the dorsal root ganglia, the posterior horn superficial laminae (Lissauer's tract and substantia gelatinosa), and the superficial ascending and descending long tracts of the spinal cord.

In epidural anaesthesia, the solution is deposited into the epidural space outside the dural sac. To achieve neural blockade, the local anaesthetic must diffuse across the dural sleeves into the CSF, as well as transit through the intervertebral foramina into the paravertebral space to act directly on mixed spinal nerves and dorsal root ganglia.

Differential Nerve Blockade

Nerve fibers exhibit differential sensitivity to local anaesthetics based on axonal diameter, degree of myelination, and conduction velocity. This gives rise to predictable spatial and functional gradients across spinal dermatomes:

+-----------------------------------------------------------------------------------------------------+
|                                 DIFFERENTIAL NEURAXIAL BLOCKADE                                     |
|                                                                                                     |
|  Cephalad Level   [ Sympathetic Block ]  (B fibers, small unmyelinated C fibers)                    |
|        ^          - 2 to 4 dermatomes higher than sensory block                                     |
|        |          - Results in peripheral venodilation and loss of vasomotor tone                   |
|        |                                                                                            |
|   Middle Level    [ Sensory Block ]      (A-delta & C fibers: cold/pain; A-beta: light touch)       |
|        |          - Temperature (cold) loss occurs 1 to 2 segments higher than pinprick             |
|        |          - Pinprick sensation lost before light touch and proprioception                   |
|        v                                                                                            |
|   Caudad Level    [ Motor Block ]        (A-alpha & A-gamma fibers: heavily myelinated, large)      |
|                   - 2 dermatomes lower than sensory block                                           |
|                   - Preserves deep tendon reflexes at higher segmental levels                       |
+-----------------------------------------------------------------------------------------------------+

• Sympathetic Autonomic Blockade: Mediated by small preganglionic myelinated B fibers ($1 - 3\text{ }\mu\text{m}$) and unmyelinated C fibers. Because critical conduction block requires exposure of fewer nodes of Ranvier (or lower threshold concentrations), the sympathetic block extends 2 to 4 dermatomal segments cephalad to the sensory block level.
• Sensory Blockade: Mediated by thinly myelinated $A\delta$ fibers ($2 - 5\text{ }\mu\text{m}$, mediating fast/sharp pain and temperature) and unmyelinated C fibers ($0.4 - 1.2\text{ }\mu\text{m}$, slow/burning pain), followed by myelinated $A\beta$ fibers ($6 - 12\text{ }\mu\text{m}$, touch and pressure). Clinically, loss of cold sensation (ice or alcohol swab) occurs approximately 1 to 2 segments higher than loss of sharp pinprick sensation.
• Motor Blockade: Mediated by large, heavily myelinated $A\alpha$ motor neurons ($12 - 20\text{ }\mu\text{m}$) and $A\gamma$ spindle fibers. Complete conduction failure requires a high concentration of local anaesthetic bathed across $\ge 3$ consecutive nodes of Ranvier. Consequently, motor block terminates approximately 2 dermatomes caudad to the sensory block.

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2. Spinal Anaesthesia: Baricity, Positioning, and Hemodynamics

Baricity and Spread Dynamics

Baricity is the ratio of the density of the injected local anaesthetic solution to the density of the patient's CSF at body temperature ($37^\circ\text{C}$):

$\text{Baricity} = \frac{\rho_{\text{solution}}}{\rho_{\text{CSF}}}$

Normal human CSF has a density of $1.0003 - 1.0008\text{ g/mL}$ at $37^\circ\text{C}$ (mean specific gravity $\approx 1.0063$).

Spinal Curvature and Positioning

In the supine horizontal position, the vertebral column exhibits two distinct anatomical curvatures: the lordotic cervical and lumbar curves, and the kyphotic thoracic and sacrococcygeal curves. The two most dependent (lowest) points in the supine canal are T4 (thoracic kyphosis apex) and S2 (sacral hollow), whereas the highest point is L3 (lumbar lordosis apex). Consequently, hyperbaric solutions injected at L3-L4 gravitate downward into the sacral curve and cephalad over the L3 peak into the thoracic kyphosis, naturally settling at the T4-T5 dermatomal level.

Cardiovascular and Respiratory Sequelae of Neuraxial Blockade

1. Hemodynamic Consequences:
   • Systemic Vasodilation: Sympathetic preganglionic fibers emerge from the spinal cord between T1 and L2. Blockade of these fibers obliterates vascular smooth muscle tone. Arteriolar vasodilation causes a modest decrease in systemic vascular resistance (SVR); however, venodilation produces profound venocapacitance pooling in the splanchnic and lower extremity vascular beds, precipitously reducing venous return ($VR$), stroke volume ($SV$), and cardiac output ($CO$).
   • Profound Bradycardia: When the block ascends to encompass T1 - T4, preganglionic cardioaccelerator sympathetic fibers are blocked. Unopposed vagal parasympathetic innervation produces marked bradycardia. Furthermore, precipitous drops in right atrial filling pressure and central venous pressure ($CVP$) deactivate stretch receptors, triggering paradoxical vagal bradycardia via the Bainbridge reflex and the Bezold-Jarisch reflex (intracardiac mechanoreceptors sensing an underfilled, hypercontractile left ventricle, causing abrupt vagally mediated bradycardia, peripheral vasodilation, and cardiac arrest).
2. Respiratory Consequences:
   • The diaphragm is innervated by the phrenic nerve arising from the cervical plexus (C3, C4, C5; "C3, 4, 5 keeps the diaphragm alive"). High thoracic spinal blockade sparing the cervical plexus preserves resting tidal breathing and diaphragmatic excursion.
   • However, blockade of thoracic spinal nerves ($T1 - T11$) paralyses intercostal muscles, and blockade of $T6 - L1$ paralyses abdominal wall musculature. This severely compromises active expiration, forceful coughing, and clearance of bronchial secretions, causing peak expiratory flow rate ($PEFR$) and forced expiratory volume to drop by $20 - 30\%$.
   • Patients frequently complain of subjective air hunger or dyspnea despite normal $SpO_2$ and arterial blood gas values due to the loss of sensory proprioceptive feedback from the moving thoracic chest wall.

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3. Epidural and Caudal Anaesthesia

Identification of the Epidural Space

The epidural space is identified using specialized blunted needles (most commonly the Tuohy needle, featuring a curved 15-30° Huber tip designed to push the dura away rather than puncture it, and guide catheter advancement):

• Loss of Resistance (LOR) Technique: The needle is advanced through subcutaneous tissue and engaged into the interspinous ligament and ligamentum flavum, where extreme resistance to injection is felt on the syringe plunger. Upon breaching the inner border of the ligamentum flavum into the epidural space, an unmistakable, sudden "loss of resistance" occurs.
  • Saline vs. Air: LOR with saline is internationally preferred over air. LOR with air introduces risks of venous air embolism (if epidural veins are cannulated), patchy/unilateral analgesia (air bubbles insulate spinal nerve roots), pneumocephalus with catastrophic immediate headache if accidental dural puncture occurs, and spinal cord compression in infants.
• Hanging Drop Technique (Gutierrez's Sign): Relies on the subatmospheric (negative) pressure present within the epidural space (typically $-1\text{ to }-7\text{ cmH}_2\text{O}$), generated by transmission of negative intrapleural pressure through intervertebral foramina, especially in sitting thoracic epidural placements. A drop of saline placed on the hub of the needle is sucked inward upon entering the space.

The Epidural Test Dose

Accidental intravascular injection (into the engorged epidural venous plexus) or accidental intrathecal injection (subarachnoid migration) of full epidural doses can lead to catastrophic local anaesthetic systemic toxicity (LAST) or total spinal arrest. A dedicated test dose is mandatory:

• Standard Formulation: $3\text{ mL}$ of lidocaine $1.5\%$ with epinephrine $1:200{,}000$ ($15\text{ }\mu\text{g}$ of epinephrine and $45\text{ mg}$ of lidocaine).
• Intravascular Detection Criteria: Injection of the $15\text{ }\mu\text{g}$ epinephrine bolus into a blood vessel produces an acute, transient increase in heart rate $\ge 10\text{ bpm}$ (classically $\ge 20\text{ bpm}$) or an increase in systolic blood pressure $\ge 15\text{ mmHg}$ within about $20 - 40\text{ seconds}$, accompanied by systemic symptoms such as palpitations, dizziness, or metallic taste, and ECG changes (T-wave amplitude decrease $\ge 25\%$). In patients on beta-blockers, elderly individuals, or labouring parturients during active contractions, heart rate changes may be blunted; a rise in systolic blood pressure of $\ge 15\text{ mmHg}$ or stroke volume monitoring must be prioritized.
• Intrathecal Detection Criteria: The $45\text{ mg}$ of lidocaine is sufficient to produce rapid, profound sensory numbness and motor paralysis of the lower extremities within $3\text{ minutes}$, confirming subarachnoid placement.

Caudal Anaesthesia in Paediatric and Adult Practice

Caudal anaesthesia involves accessing the sacral epidural space through the sacral hiatus, which is formed by the failure of the fifth (and occasionally fourth) sacral vertebral laminae to fuse in the midline, bounded laterally by the sacral cornua and roofed by the sacrococcygeal ligament.

                               [ SACRAL POSTERIOR ANATOMY ]
                                             |
                             [ Posterior Superior Iliac Spines ]
                                      (PSIS - S2 level)
                                             |
                                             v
                                   [ Sacral Canal / Dura ]
                         (Dural sac terminates at S2 adults, S3 infants!)
                                             |
                                             v
                                 [ Sacral Cornua (Bilateral) ]
                                             |
                                  [ Sacrococcygeal Ligament ]
                                             |
                                             v
                                      [ Sacral Hiatus ]

• Dural Sac Termination Anatomy:
  • In adults, the dural sac and subarachnoid space terminate at the lower border of S2.
  • In neonates and infants, the dural sac terminates lower, at S3. Because the distance between the sacrococcygeal ligament and the dural cuff in neonates can be as short as $5 - 10\text{ mm}$, the risk of unintentional dural puncture and total spinal is markedly elevated if the needle is advanced steeply into the canal. The needle should be inserted at $45^\circ$ until a "pop" through the ligament is felt, then flattened almost parallel to the skin and advanced no more than $1 - 3\text{ mm}$ into the sacral canal.
• Armitage Dosing Formula (Paediatric Caudal Analgesia): Dosing of bupivacaine $0.125 - 0.2\%$ (with or without epinephrine $1:200{,}000$ or clonidine $1\text{ }\mu\text{g/kg}$):
  • Lumbosacral block (circumcision, hypospadias, inguinal hernia): $0.5\text{ mL/kg}$
  • Thoracolumbar block (up to T10, umbilical hernia, orchiopexy): $1.0\text{ mL/kg}$
  • Mid-thoracic block (up to T6): $1.25\text{ mL/kg}$ (maximum recommended volume: $20\text{ mL}$).

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4. Complications of Neuraxial Anaesthesia

Post-Dural Puncture Headache (PDPH)

• Pathophysiology: Loss of dural integrity results in continuous trans-dural leakage of CSF into the epidural space at a rate that exceeds the compensatory biosynthetic rate of the choroid plexus (normal CSF production is approximately $0.35\text{ mL/min}$ or $\sim 500\text{ mL/day}$). The resulting intracranial hypotension depletes the hydrostatic fluid cushion supporting the brain. When the patient assumes an erect posture, the cerebral hemispheres and brainstem sag caudally. This exerts downward mechanical traction on anchoring, pain-sensitive meninges, tentorium, bridging dural veins, and cranial nerves:
  • CN VI (Abducens nerve): Possesses the longest intracranial intradural course, running across the sharp petrous apex of the temporal bone into Dorello's canal. Downward traction causes mechanical stretch, producing unilateral or bilateral lateral rectus palsy, manifested clinically as diplopia.
  • CN VIII (Vestibulocochlear nerve): Traction or endolymphatic pressure alterations produce low-frequency hearing loss, subjective tinnitus, dizziness, and nausea.
• Clinical Presentation: A severe, fronto-occipital, throbbing headache that is strictly postural: rapidly exacerbated within 15 to 30 seconds of sitting or standing upright, and completely or substantially relieved when assuming the recumbent supine horizontal position. Associated with photophobia, nausea, vomiting, and neck stiffness.
• Needle Risk Factors:
  • Gauge: Larger bore needles yield higher leak rates (16-18G Tuohy carries a $50 - 70\%$ PDPH incidence upon accidental dural puncture).
  • Needle Tip Geometry: Cutting needles (Quincke) slice cleanly through dural and arachnoid fibers, creating non-sealing gaping defects with high PDPH rates. Pencil-point needles (Whitacre, Sprotte) separate and stretch elastocollagenous fibers without cutting, triggering local edema that rapidly seals the defect; 25G or 27G pencil-point needles yield PDPH incidences $< 1\%$.
  • Bevel Orientation: If using a cutting needle, aligning the bevel parallel to the longitudinal axis of the spine separates rather than severs longitudinally oriented dural fibers, halving the incidence of PDPH.
• Management:
  • Conservative: Horizontal bed rest, vigorous hydration ($>3\text{ L/day}$), regular analgesics (paracetamol, NSAIDs), oral caffeine ($300\text{ mg}$ q6h) or IV caffeine sodium benzoate ($500\text{ mg}$ in $1000\text{ mL}$ saline, causing cerebral vasoconstriction).
  • Definitive Treatment: Autologous Epidural Blood Patch (EBP):
    • Technique: Performed under strict aseptic conditions. An operator secures epidural access at the same interspace or one interspace below the primary dural puncture site, while an assistant draws $15 - 20\text{ mL}$ of sterile autologous venous blood from the patient's antecubital fossa. The blood is injected slowly into the epidural space until the patient reports distinct fullness, pressure, or dull ache in the back or buttocks.
    • Mechanism: Dual effect: (1) immediate transmission of hydrostatic volume pressure into the thecal sac, elevating intracranial pressure and halting brain sag; (2) sustained formation of a dense gelatinous fibrin clot over the dural defect, providing permanent mechanical seal while tissue repair occurs.
    • Efficacy: Most patients improve after the first patch, but reported rates of complete and lasting relief vary widely between studies, and some patients need a second patch.

Spinal Epidural Hematoma and Abscess

• Spinal Epidural Hematoma (SEH): A catastrophic neurosurgical emergency caused by rupture of the internal vertebral venous plexus into the non-distensible spinal canal. The expanding hematoma compresses the spinal cord or cauda equina, leading to ischemic necrosis.
  • Clinical Triad: Severe, localized back pain with radicular radiation, rapidly progressive motor weakness (flaccid paraparesis), and sphincter dysfunction (urinary retention or fecal incontinence).
  • Diagnostic & Therapeutic Algorithm: Immediate emergent urgent MRI of the whole spine. If hematoma is confirmed, decompressive surgical laminectomy must be performed within 8 to 12 hours of onset; decompression beyond 12 hours rarely reverses permanent paraplegia.
• Spinal Epidural Abscess (SEA): Triad of severe back pain, fever/leukocytosis, and progressive neurological deficits. Common pathogen is Staphylococcus aureus ($>60\%$). Requires immediate blood cultures, MRI, high-dose IV antibiotic therapy, and urgent surgical evacuation/drainage.

ASRA Coagulation Guidelines for Neuraxial Blocks

To mitigate the risk of spinal epidural hematoma, the American Society of Regional Anesthesia and Pain Medicine (ASRA, 4th edition, Horlocker et al. 2018) recommends timing intervals between anticoagulant administration, neuraxial puncture, and catheter removal. The 2022 ESAIC/ESRA guideline uses similar principles with some drug-specific differences (for example, shorter intervals for low-dose DOAC prophylaxis), so follow the local protocol:

Total Spinal Anaesthesia

• Mechanism: Accidental injection of a massive epidural dose into the subarachnoid or subdural space, or uncontrolled massive cephalad migration of subarachnoid local anaesthetic to the brainstem and cisterna magna.
• Clinical Manifestations: Rapid ascending paralysis within minutes: numbness in upper limbs and fingers ($C6 - C8$), profound arterial hypotension and unrecordable blood pressure, severe bradycardia or asystole ($T1 - T4$ sympathetic denervation), respiratory arrest/apnea (phrenic nerve $C3 - C5$ block and medullary hypoperfusion), loss of consciousness, and fixed, widely dilated pupils (blockade of parasympathetic ciliary ganglion fibers).
• Immediate Resuscitation Protocol:
  1. Airway & Breathing: Call for help immediately; secure airway via rapid sequence endotracheal intubation; deliver $100\%\text{ O}_2$ with positive pressure ventilation.
  2. Circulation: Rapid IV fluid bolus of balanced crystalloid ($1000 - 2000\text{ mL}$); tilt table left/elevate legs; administer immediate IV vasopressors: ephedrine $10 - 20\text{ mg}$, phenylephrine $100 - 200\text{ }\mu\text{g}$, and without hesitation IV epinephrine ($10 - 100\text{ }\mu\text{g}$ boluses) if profound bradycardia and collapse ensue; administer atropine $0.5 - 1.0\text{ mg}$ for vagal bradycardia.
  3. Supportive Care: Mechanical ventilation and vasopressor support must be maintained until the local anaesthetic redistributes and metabolizes (typically $1 - 3\text{ hours}$); patients recover completely without neurological deficit if cerebral hypoxemia and ischemia were prevented.

Transient Neurological Symptoms (TNS) vs. Cauda Equina Syndrome (CES)