Spinal anaesthesia

What it is

Spinal (subarachnoid or intrathecal) anaesthesia is the injection of a small volume of local anaesthetic, often with an opioid, into the cerebrospinal fluid of the lumbar subarachnoid space. The drug bathes the nerve roots directly, so a dense sensory and motor block starts within minutes from a single injection, lasting according to the agent chosen. The needle goes in below the end of the spinal cord, which lies around L1–L2 in adults and lower in infants, usually at the L3–L4 or L4–L5 interspace; the line joining the top of the iliac crests crosses roughly the L4 spinous process. A fine pencil-point needle passes skin, supraspinous and interspinous ligaments, ligamentum flavum, epidural space, dura and arachnoid; free flow of clear fluid confirms the position. How high the block spreads depends mainly on the baricity of the solution relative to cerebrospinal fluid and on the patient's position. Sympathetic fibres are blocked a few segments higher than sensory fibres, and sensory higher than motor, which explains the hypotension and bradycardia that are the commonest problems. It suits surgery below the umbilicus: caesarean section, lower limb, hip, perineal and urological procedures.

Indications

• Caesarean section and other obstetric surgery, where it avoids airway risk and lets the mother stay awake.
• Hip and knee replacement, hip fracture and other lower limb surgery.
• Transurethral, perineal, anorectal and inguinal procedures.
• Patients with significant lung disease or a difficult airway, when the surgery is below the umbilicus.
• Intrathecal opioid added for prolonged postoperative analgesia after major surgery.

Contraindications and precautions

• Absolute: patient refusal, infection at the puncture site, raised intracranial pressure, and severe uncorrected hypovolaemia.
• Uncorrected coagulopathy, low platelets or anticoagulants given too recently: follow the timing rules for each drug.
• Fixed cardiac output (severe aortic or mitral stenosis): sudden sympathetic block can drop pressure dangerously.
• Systemic sepsis, pre-existing neurological disease, spinal deformity or previous spine surgery need case-by-case judgement.
• Surgery longer than the block will last, or a patient who cannot lie still, may need another plan.

Equipment and preparation

• Consent, intravenous access and standard monitoring (ECG, blood pressure, pulse oximetry) before the block.
• Sterile pack: antiseptic, drape, gown, gloves, mask and hat; local anaesthetic for the skin.
• Fine pencil-point spinal needle (for example 25- or 27-gauge) with an introducer; cutting needles raise headache risk.
• Spinal local anaesthetic of chosen baricity, drawn up and labelled; intrathecal opioid if planned.
• Vasopressors drawn up, airway equipment and resuscitation drugs at hand for high block or collapse.

Step by step (and why)

1. Confirm consent, coagulation status and the WHO sign-in — a wrong side or an anticoagulated patient is a preventable disaster.
2. Place a working intravenous line and start monitoring — hypotension can come within minutes of the injection.
3. Sit the patient or lay them on the side, back curved forward — flexion opens the gaps between the spinous processes.
4. Find the interspace below L2 using the iliac crest line — staying below the spinal cord avoids cord injury.
5. Prepare the skin and drape with full asepsis — meningitis and abscess are rare but devastating.
6. Insert the needle in the midline, aiming slightly cephalad, feeling the dural pop — the needle follows the gap between processes.
7. Remove the stylet and wait for free flow of clear fluid — this confirms the tip is in the subarachnoid space.
8. Inject slowly, aspirate fluid again at the end, and remove the needle — confirms the dose went where intended.
9. Position the patient for the spread wanted and check the level with cold and pinprick — the block must cover the incision.
10. Treat a fall in blood pressure early with vasopressor and fluid — prevents nausea, collapse and, in pregnancy, fetal distress.

Dermatome landmarks for checking the block

Complications

• Hypotension and bradycardia from sympathetic block; the most common problem, worst with hypovolaemia and in pregnancy.
• High or total spinal block: breathlessness, arm weakness, then apnoea and loss of consciousness.
• Post-dural puncture headache, less common with fine pencil-point needles.
• Nausea (often from hypotension), shivering, urinary retention and itching with intrathecal opioid.
• Transient neurological symptoms (buttock and leg pain after lidocaine); rarely nerve injury, haematoma or infection.

Next step

• Record the level reached, the needle, the number of attempts and any paraesthesia.
• Monitor blood pressure frequently in the first minutes and until the block regresses.
• Call a senior at once for rising block with arm weakness or breathlessness, or for a failed block before incision.
• Before discharge from recovery, confirm regression of motor and sensory block and that the patient has passed urine or has a plan.
• Ask about headache, back pain, new weakness or numbness on the follow-up visit; new deficits need urgent review.

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Elsewhere in DurgiPedia

• Lumbar puncture
• Clean and sterile technique: medical and surgical asepsis
• The nerve action potential
• The autonomic nervous system: organization

Connections

← All of anaesthesia

Educational content; it does not replace supervised training, your institution's protocols or the anaesthetist's or intensivist's decision. It contains no doses and no ventilator or infusion settings.

Sources: Spinal Anesthesia (StatPearls (NCBI Bookshelf), 2022).

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Representative model, for educational purposes only.