Spinal Anaesthesia

Core Principles

• Definition: A form of neuraxial regional anaesthesia involving the injection of local anaesthetic (with or without opioids) into the subarachnoid space.
• Anatomical Landmark: The needle must be inserted below L1/L2 (typically L3–L4 or L4–L5) to avoid injury to the conus medullaris.
• Mechanism: Provides motor, sensory, and autonomic (sympathetic) blockade.
• Blockade order: Thin unmyelinated C-fibres (pain) are blocked first; thick myelinated A-alpha motor neurons are blocked moderately; small preganglionic sympathetic fibers are blocked last.

Clinical Indications

• Primary Use: Surgeries below the umbilicus, including orthopaedic (pelvis, hip, knee), vascular (legs), urology (TURP), and gynaecology (hysterectomy, C-section).
• Caesarean Section: Preferred technique as it avoids the risks of general anaesthesia and failed intubation, and allows the mother to be conscious.
• Respiratory Compromise: Favoured in patients with severe COPD to avoid intubation/ventilation consequences.
• Paediatrics: Useful in children with difficult airways or increased respiratory risks.

Contraindications

• Absolute: Patient refusal, local infection at the injection site, bleeding disorders/systemic anticoagulation (risk of spinal epidural hematoma), severe aortic stenosis, increased intracranial pressure, space-occupying brain lesions, and allergy.
• Relative: Ehlers–Danlos syndrome or other conditions causing resistance to local anaesthetics.

Risks and Complications

• Common/Minor: Mild hypotension, bradycardia, nausea/vomiting, and transient neurological symptoms.
• Post-Dural-Puncture Headache (PDPH): Strongly associated with needle size/type. A 26-gauge atraumatic needle (e.g., Braun Atraucan) is recommended to lower the risk.
• Serious/Rare: Cauda equina syndrome, nerve injuries, cardiac arrest, severe hypotension, spinal epidural hematoma, and epidural abscess/meningitis.

Key Technical Concepts

• Baricity: The density of the anaesthetic solution relative to CSF.
• Hyperbaric (made denser with glucose) is most commonly chosen because its spread is predictably controlled by tilting the patient.
• Positioning: Sitting, lateral decubitus, and prone (jackknife).

Spinal vs. Epidural Anaesthesia

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