Spinal Anaesthesia — Comprehensive Review

1. Anatomy

Relevant Structures (Surface to Subarachnoid Space)

• Skin → subcutaneous fat → supraspinous ligament → interspinous ligament → ligamentum flavum → epidural space → dura mater → subdural space → arachnoid mater → subarachnoid space (CSF)
• The spinal cord typically terminates at L1–L2 in adults (conus medullaris); the dural sac extends to S2. Below L2, the cauda equina floats freely in CSF — this is why lumbar puncture at L3–4 or L4–5 is safe.
• Neonates have a lower cord termination (L3) and relatively larger CSF volume (~10 mL/kg vs 1.5–2 mL/kg in adults), requiring proportionally higher local anesthetic doses.

Cerebrospinal Fluid

• CSF density: 1.00059 g/mL at 37°C
• Lumbosacral CSF pressure: ~15 cm H₂O
• CSF volume accounts for 80% of the variability in peak block height and regression of blockade. Body weight (BMI) inversely correlates with CSF volume (less CSF in obese patients)

2. Mechanism of Action

• Spinal nerve roots (in the subarachnoid space) — most accessible and most easily blocked
• Dorsal root ganglia
• Superficial and deep portions of the spinal cord

Differential Nerve Block (order of susceptibility — most to least):

3. Physiological Effects

Cardiovascular

• Hypotension — the most common cardiovascular complication; results from sympathectomy causing vasodilation and decreased venous return. Sympathetic block extends 2–6 dermatomes above the sensory level.
• In healthy normovolemic patients: systemic vascular resistance decreases ~15–18% even with near-total sympathectomy.
• In elderly patients with cardiac disease: SVR decreases ~25%, cardiac output decreases ~10%.
• Bradycardia — results from:
  • Blockade of T1–T4 cardiac accelerator fibres
  • Reduced venous return → reduced stretch of right atrial chronotropic receptors → increased vagal tone
  • The Bezold–Jarisch reflex (small end-diastolic LV volume triggers mechanoreceptor-mediated bradycardia) — particularly in young, healthy, hypovolaemic patients
• Risk factors for exaggerated bradycardia (40–50 bpm): baseline HR <60, age <37, male sex, β-blockade, prolonged case duration
• Cardiac arrest is rare but more likely in young, healthy, conscious patients with high spinal when T1 is blocked

Coronary Circulation

• High thoracic block (T4) can improve myocardial function in ischaemic patients by reducing LV afterload and myocardial oxygen demand.

Respiratory

• Low/mid spinal blocks: minimal respiratory effect (the diaphragm is innervated by C3–5)
• Very high blocks: progressive impairment; intercostal muscle paralysis leads to reduced tidal volume and limited ability to cough
• Total spinal: apnoea from brainstem involvement

Renal

• Modest, clinically unimportant decrease in renal blood flow
• Urinary retention: S2–S4 block inhibits the detrusor muscle; intrathecal opioids amplify the effect

4. Indications

• Lower extremity surgery (arthroplasty, fracture fixation)
• Perineum and pelvic girdle surgery
• Lower abdominal surgery (inguinal hernia, appendicectomy)
• Urological surgery (TURP, cystoscopy)
• Obstetric surgery (caesarean section — preferred over general anaesthesia due to lower maternal morbidity/mortality, better neonatal outcomes)
• Lumbar spine surgery
• Laparoscopic cholecystectomy (described)
• When the patient wishes to remain conscious, or when comorbidities (severe respiratory disease, difficult airway) make general anaesthesia higher risk
• Ex-premature infants undergoing hernia repair (reduces post-operative apnoea risk)

5. Contraindications

Absolute

• Patient refusal
• Localised infection at the injection site (risk of meningitis/abscess)
• Allergy to planned drugs
• Severe uncorrected hypovolaemia (high risk of cardiovascular collapse)
• Raised intracranial pressure with mass lesion (risk of brainstem herniation)
• Severe coagulopathy / therapeutic anticoagulation (see ASRA guidelines)

Relative

• Uncooperative patient
• Sepsis (bacteraemia — risk of haematogenous seeding)
• Pre-existing neurological disease (medicolegal concerns)
• Severe aortic stenosis / hypertrophic cardiomyopathy (fixed cardiac output cannot compensate for vasodilation)
• Prior spinal surgery / anatomical abnormalities (technical difficulty, unpredictable spread)
• Anticoagulation (timing per ASRA Fourth Edition guidelines)

6. Factors Affecting Block Height

Drug Factors (most controllable)

Baricity

• Baricity = density of LA solution / density of CSF
• Hyperbaric (density > 1.00059 g/mL): spread to dependent regions; more predictable; shorter duration than isobaric; dextrose added
• Hypobaric (density < 1.00059 g/mL): spread to non-dependent regions; sterile water added
• Isobaric: not influenced by gravity
• Clinical application: administer hyperbaric solution in lateral decubitus → preferential block of dependent (operative) side

Lumbar Lordosis

• In the supine position, L3–L4 is the most caudal point; T5–T6 the lowest thoracic point
• Hyperbaric solution injected at L3–4 will pool in the lumbar lordosis or flow cephalad to T5–T6, depending on positioning

Patient Factors (less controllable)

• CSF volume (accounts for 80% of variability)
• BMI (high BMI → less CSF volume → higher block)
• Age (elderly → less CSF)
• Pregnancy (engorged epidural veins compress subarachnoid space → higher spread)
• Height (less important than CSF volume)

7. Pharmacology — Local Anaesthetics

8. Additives (Adjuvants)

Opioids

Vasoconstrictors

• Epinephrine (100–200 mcg) and phenylephrine prolong block by causing local vasoconstriction, reducing LA vascular uptake; also have direct spinal cord α₂ effects
• Epinephrine extends bupivacaine and tetracaine block by 30–50%

α₂-Agonists

• Clonidine (15–75 mcg intrathecally): prolongs sensory > motor block; mechanism involves K⁺-channel opening and membrane hyperpolarisation; side effects: hypotension, bradycardia, sedation
• Dexmedetomidine: increasing evidence for prolonging block

9. Technique

Patient Positioning

• Sitting (flexed): best CSF identification, clearest landmarks; CSF drops readily into needle; preferred for obese patients and saddle blocks
• Lateral decubitus: patient more comfortable; allows selective block with hyperbaric/hypobaric solutions

Approach

1. Identify L3–4 or L4–5 interspace (Tuffier's line = iliac crest = L4 vertebral body)
2. Raise skin wheal with local anaesthetic
3. Insert introducer, then spinal needle with bevel parallel to dural fibres (reduces PDPH)
4. Advance through supraspinous → interspinous ligament → ligamentum flavum (resistance) → dura (pop)
5. Confirm free CSF flow; inject slowly, aspirating to confirm position before and after

• Entry point: 1 cm lateral to superior edge of inferior spinous process
• Direction: 10–15° off the sagittal plane, cephalomedially
• Does not traverse supraspinous/interspinous ligaments

Needle Types

• Cutting (Quincke): sharp bevel; easier to insert; higher PDPH rate; bevel should face cephalad (parallel to longitudinal dural fibres)
• Pencil-point (Whitacre, Sprotte, Gertie Marx): blunt tip, lateral opening; lower PDPH rate — preferred; slightly more resistance to insertion
• Gauge: 25–27G for ambulatory (lowest PDPH); 22G for children and difficult cases

Use of Ultrasound

Dermatomal Requirements for Common Surgeries

10. Complications

Hypotension

• Most common complication; incidence up to 33% in non-obstetric and >80% in obstetric cases without prophylaxis
• Mechanism: sympathectomy → vasodilation → reduced venous return → reduced CO
• Management: IV fluid preloading or co-loading, vasopressors (phenylephrine first-line in obstetrics to maintain uteroplacental flow; ephedrine for bradycardia-associated hypotension), left uterine displacement in pregnancy

Post-Dural Puncture Headache (PDPH)

• Caused by CSF leak through dural hole → reduced CSF pressure → traction on pain-sensitive structures
• Character: postural — worse upright, relieved supine; often with neck stiffness, photophobia, diplopia
• Risk factors (Box 41.2 from Miller's):
  • Younger age, female sex, pregnancy
  • Larger needle gauge, cutting needle design
  • Multiple punctures
• Treatment:
  • Conservative: supine position, hydration, caffeine, analgesics, sumatriptan
  • Definitive: epidural blood patch (15–20 mL autologous blood at or below the culprit level) — ~90% initial improvement, 61–75% persistent resolution; best performed ≥24 h after dural puncture

Transient Neurological Symptoms (TNS)

• Traditionally associated with lidocaine (incidence 10–40%), especially in lithotomy position
• Bilateral buttock/thigh pain without neurological deficit, appearing 12–24 h post-block, resolving within 1 week
• Mechanism unclear; may involve direct neurotoxicity
• Modern chloroprocaine preparations have similar TNS risk to lidocaine; bupivacaine TNS risk is low

Total Spinal / High Spinal

• Excessive rostral spread of intrathecal LA (or epidural drug entering subarachnoid space)
• Features: profound hypotension, bradycardia → possible cardiac arrest; progressive respiratory paralysis → apnoea; loss of consciousness; dilated pupils
• Management: immediate airway management (intubation), CPR if needed, vasopressors (epinephrine for cardiovascular collapse), left uterine displacement in pregnant patients

Urinary Retention

• S2–S4 block inhibits detrusor contractility; opioid additives worsen this
• Resolves spontaneously when block regresses below S2–3
• Intrathecal morphine strongly associated; routine catheterisation not recommended for hip/knee arthroplasty

Pruritus

• Most common opioid side effect: 30–100% incidence
• Mechanism: central μ-receptor activation, not histamine release
• Treatment: naloxone, nalbuphine, ondansetron, propofol; reducing sufentanil dose to 1.5 mcg reduces pruritus without compromising analgesia

Shivering

• Incidence up to 55%; less severe than with epidural (profound motor block limits shivering)
• Prevention: pre-warm with forced-air warmer ≥15 min; avoid cold IV/epidural fluids; intrathecal fentanyl or meperidine

Neuraxial Haematoma

• Rare but catastrophic; risk increased with anticoagulation
• ASRA Fourth Edition guidelines define mandatory intervals between anticoagulant doses and neuraxial procedures
• Presents as new or progressive neurological deficit, severe back pain
• Requires urgent MRI and surgical decompression within 8 h of symptom onset for best outcome

Meningitis / Epidural Abscess

• Bacterial (usually Streptococcus from provider's oropharynx — strict aseptic technique required) or aseptic
• Requires fever, headache, meningismus, culture/CSF analysis, antibiotics

Cauda Equina Syndrome

• Rare; associated with concentrated LA (5% lidocaine), small-bore catheter for continuous spinal, maldistribution of drug
• Permanent sacral nerve deficit: bladder/bowel dysfunction, perineal anaesthesia, lower limb weakness

11. Special Situations

Obstetrics (Caesarean Section)

• Spinal anaesthesia is the technique of choice in most centres for elective CS
• Standard: hyperbaric bupivacaine 0.5% (10–12.5 mg) + intrathecal fentanyl (15–25 mcg) + morphine (100 mcg)
• Target level: T4 bilaterally (loss of sensation to cold at the xiphisternum)
• Hypotension prevention: phenylephrine infusion + IV co-loading; left uterine displacement

Paediatric Spinal

• Main indication: inguinal hernia repair in ex-premature infants (<60 weeks post-conceptual age) to avoid post-operative apnoea from general anaesthesia
• Failure rate up to 20% — general anaesthesia must be available as backup
• Higher LA doses per kg required (larger CSF volume relative to body weight)
• Haemodynamic instability unusual in children <5 years (immature sympathetic system)
• Drug of choice: hyperbaric tetracaine or bupivacaine

Ambulatory Surgery

• Prefer short-acting agents: chloroprocaine (30–60 mg) — fastest discharge readiness (76 min faster than bupivacaine in one study)
• Avoid lidocaine (TNS risk), avoid bupivacaine (prolonged motor block)
• Low-dose fentanyl (10–25 mcg) added to reduce LA dose while minimising delay to discharge

Continuous Spinal Anaesthesia

• Catheter placed in subarachnoid space (usually via 28G microcatheter after intentional dural puncture with Tuohy needle)
• Indications: uncertain surgical duration, severe cardiac disease (incremental dosing → haemodynamic stability), failed single-shot
• Risk: cauda equina syndrome with maldistribution of concentrated LA

12. Combined Spinal–Epidural (CSE)

• Needle-through-needle technique: spinal injection followed by epidural catheter placement at the same interspace
• Advantages: rapid dense block from spinal component; indefinite duration from epidural catheter; lower total drug dose
• Disadvantages: cannot confirm epidural catheter function before the spinal wears off; epidural saline injection raises intrathecal pressure, increasing block height unpredictably
• Indications: labour analgesia (especially walking epidural), prolonged surgery, cases where duration is uncertain

13. Anticoagulation and Neuraxial Techniques (ASRA 2018 Guidelines — Key Points)

Summary