Spinal Anaesthesia

Spinal (subarachnoid) anaesthesia is a central neuraxial block produced by injecting local anaesthetic into the cerebrospinal fluid in the subarachnoid space. It is a single-shot, rapid-onset, dense block that is among the most heavily examined regional techniques in NEET PG — favourite stems include the dermatomal level required for a given surgery, the mechanism and treatment of post-dural puncture headache (PDPH), baricity, and absolute contraindications.

Definition & basic concept

In spinal anaesthesia, a small dose of local anaesthetic (LA) is deposited directly into the CSF within the subarachnoid space, producing a block by acting on the nerve roots and dorsal root ganglia bathed in CSF. Because the drug is in direct contact with neural tissue, the dose is small (typically 1.5–3 mL), onset is fast (≈5 min), and the block is dense (excellent motor + sensory + sympathetic blockade).

Contrast this with epidural anaesthesia, where a larger volume is placed in the potential (extradural) space and acts mainly on nerve roots traversing the dural cuffs — slower onset, segmental, larger dose, catheter-based.

High-yield: The order in which fibres are blocked is autonomic (sympathetic) → temperature → pain → touch → motor → proprioception. Sympathetic block is therefore highest and recovers last; the sympathetic level is usually 2 segments above the sensory level, and motor block is 2 segments below the sensory level.

Relevant anatomy & landmarks

The needle traverses (midline approach): skin → subcutaneous fat → supraspinous ligament → interspinous ligament → ligamentum flavum → epidural space → dura → arachnoid → subarachnoid (CSF).

Landmark	Vertebral level	Use
Tuffier's (intercristal) line — between iliac crests	L4 or L3–L4 interspace	Standard line for lumbar puncture/spinal
Termination of spinal cord (conus medullaris) — adult	L1 (L1–L2)	Why we puncture below L2 to avoid cord injury
Termination of spinal cord — neonate/infant	L3	Puncture lower (L4–L5/L5–S1) in children
Dural sac termination	S2	Lower limit of subarachnoid space
Vertebra prominens	C7	Surface counting landmark
Root of spine of scapula	T3	Counting landmark
Inferior angle of scapula	T7	Counting landmark

High-yield: Spinal is performed at L3–L4 or L4–L5 (i.e., below L2) because the cord ends at L1 in adults; puncture above this risks cord damage. Tuffier's line crosses L4 / L4–L5 interspace.

Dermatomal levels needed for surgery

This is the single most repeated MCQ. Memorise the sensory level required:

Surgery	Required sensory level	Memory cue
Upper abdominal surgery	T4 (nipple)	Highest commonly used
Caesarean section	T4–T6	Aim T4 for comfort with traction
Lower abdominal / appendicectomy / hernia	T6–T8	Xiphisternum = T6
TURP, vaginal delivery, hip surgery	T10 (umbilicus)	Bladder distension felt to T10
Lower limb surgery	T12 / L1	—
Perineal / haemorrhoid (saddle block)	S2–S5	Sit upright after hyperbaric drug

High-yield surface markers: Nipple = T4, Xiphisternum = T6, Umbilicus = T10, Inguinal ligament = L1, Perineum = S2–S4. For Caesarean section the target is T4 (a classic answer).

Drugs used & baricity

Baricity = density of the LA solution relative to CSF (CSF specific gravity ≈ 1.003–1.008 at 37 °C).

• Hyperbaric (denser than CSF): made by adding 8% dextrose/glucose (e.g., heavy bupivacaine 0.5%). Sinks with gravity → spreads to the dependent part of the patient.
• Isobaric: stays where injected; level less affected by posture.
• Hypobaric (lighter than CSF): made with sterile water; rises against gravity — used for non-dependent positions (e.g., hip surgery in lateral position).

Drug	Concentration	Duration (plain)	Notes
Bupivacaine (heavy)	0.5% with 8% dextrose	90–120 min	Most commonly used worldwide
Lignocaine 5% (heavy)	5% with 7.5% dextrose	45–90 min	Associated with transient neurological symptoms (TNS) and cauda equina syndrome — largely abandoned
Tetracaine	0.5%	90–120 min	—
Ropivacaine	0.5%	Intermediate	Less cardiotoxic
Procaine	5%	Short	Historical

Adjuvants prolong/intensify block: opioids (fentanyl, morphine — morphine gives long analgesia but risk of delayed respiratory depression up to 24 h), clonidine, dexmedetomidine, and adrenaline (vasoconstriction → prolongs duration).

High-yield: A hyperbaric solution moves to the dependent region. So after a hyperbaric drug, head-down (Trendelenburg) raises the block level; sitting upright concentrates it caudally → ideal for a saddle block. Intrathecal morphine can cause delayed respiratory depression because it is hydrophilic and migrates rostrally in CSF.

Factors affecting level of block

The block height is the examiner's trap. Key determinants:

1. Baricity of the solution (most important controllable factor)
2. Patient position during and just after injection
3. Dose / total mass of drug (more important than volume or concentration)
4. Site of injection (higher interspace → higher block)
5. Patient factors: pregnancy (engorged epidural veins reduce CSF volume → higher block, lower dose needed), obesity, raised intra-abdominal pressure, height (short patients → higher spread), increased CSF density.

Factors with little effect: barbotage, speed of injection, needle bevel direction (minor), coughing, added vasoconstrictor (affects duration, not height much).

Stepwise approach to performing a spinal: Consent & monitors → IV access + preload/co-load fluids → position (sitting or lateral) → identify L3–L4 (Tuffier's line) → asepsis & local infiltration → insert spinal needle (bevel parallel to dural fibres) → confirm free flow of clear CSF → inject LA slowly → position patient for desired level → monitor BP/HR/level.

Needles

• Pencil-point (non-cutting) needles — Whitacre and Sprotte — spread dural fibres rather than cutting them → markedly lower incidence of PDPH. Preferred.
• Cutting (bevelled) needles — Quincke — higher PDPH; if used, orient bevel parallel to the longitudinal dural fibres.
• Smaller gauge (25–27G) = lower PDPH but slower CSF flow and more failures.

High-yield: To minimise PDPH use a small-gauge pencil-point (Whitacre/Sprotte) needle. Larger-bore cutting (Quincke) needles in young women give the highest PDPH risk.

Physiological effects

• Cardiovascular: Sympathetic blockade → vasodilatation → hypotension; block above T4 abolishes cardiac sympathetic (cardioaccelerator) fibres (T1–T4) → bradycardia. The Bezold–Jarisch reflex (empty ventricle + reduced venous return) can cause profound bradycardia/asystole.
• Respiratory: Tidal volume preserved; high blocks reduce expiratory reserve (abdominal/intercostal paralysis). True respiratory arrest is usually due to brainstem hypoperfusion ("total spinal"), not phrenic paralysis directly.
• GI: Unopposed vagal tone → contracted gut, nausea (often the first sign of hypotension).
• Urinary: Bladder atony → urinary retention.

Complications

Complication	Mechanism / feature	Management
Hypotension (commonest)	Sympathetic block, venodilatation	IV fluids, vasopressors: phenylephrine or ephedrine (ephedrine preferred in obstetrics for uterine perfusion), leg elevation
Bradycardia	Block of T1–T4 cardioaccelerators; Bezold–Jarisch	Atropine, ephedrine, adrenaline if severe
PDPH	CSF leak through dural puncture → intracranial hypotension, traction on meninges	See below
Total/high spinal	Excess cephalad spread → apnoea, hypotension, unconsciousness	Airway/ventilation, fluids, vasopressors, intubation
Urinary retention	Sacral autonomic block	Catheterise
Transient neurological symptoms (TNS)	Back/buttock/leg pain after recovery; esp. lignocaine, lithotomy position	NSAIDs, reassurance, self-limiting
Cauda equina syndrome	Neurotoxicity (high-dose/5% lignocaine, microcatheters)	Permanent deficit — prevention key
Epidural/spinal haematoma	Bleeding, esp. anticoagulated patients	Emergency MRI + decompressive laminectomy < 8 h
Meningitis / arachnoiditis / abscess	Infection	Antibiotics, drainage
Failed/patchy block	Technical	Repeat or convert to GA

Post-dural puncture headache (PDPH) — a guaranteed question

• Mechanism: Persistent CSF leak through the dural rent → fall in CSF pressure → downward traction on pain-sensitive intracranial structures and compensatory cerebral vasodilatation.
• Classic features: Postural (positional) headache — frontal/occipital, worse on sitting/standing, relieved on lying down. Onset usually 24–48 h post-procedure. May have neck stiffness, photophobia, tinnitus, diplopia (VI nerve palsy from traction).
• Risk factors: Young, female, pregnancy, large-bore cutting (Quincke) needle, multiple attempts, low BMI.
• Treatment ladder: Conservative — bed rest, hydration, oral analgesics (paracetamol/NSAIDs), caffeine. Definitive for severe/persistent PDPH = epidural blood patch (autologous blood injected into epidural space — gold standard, >90% effective).

High-yield: PDPH = postural headache, relieved by lying flat, commonest in young pregnant women with cutting needles. Definitive treatment = epidural blood patch. Caffeine works via cerebral vasoconstriction.

Mnemonic for PDPH risk — "Pregnant Young Females Bleed Quincke": Pregnancy, Young age, Female, low BMI, Quincke (cutting) needle, multiple attempts.

Contraindications

Absolute:

1. Patient refusal
2. Coagulopathy / therapeutic anticoagulation (risk of spinal haematoma)
3. Raised intracranial pressure (risk of coning/herniation)
4. Infection at the puncture site (local sepsis)
5. Severe hypovolaemia / shock (sympathetic block worsens hypotension)
6. Severe fixed-output cardiac lesions — severe aortic stenosis, severe mitral stenosis, HOCM (cannot tolerate the drop in preload/afterload)

Relative: sepsis/bacteraemia, pre-existing neurological disease (e.g., MS), severe spinal deformity, uncooperative patient, certain valvular lesions.

High-yield: Raised ICP and severe aortic stenosis are classic absolute contraindications to spinal anaesthesia. Coagulopathy / anticoagulants → risk of spinal/epidural haematoma, a surgical emergency.

Anticoagulant timing (frequently tested): withhold LMWH (prophylactic) for 12 h, therapeutic LMWH for 24 h, before neuraxial block; resume after a safe interval. Remove epidural catheters at troughs of anticoagulation.

Spinal vs Epidural vs Combined

Feature	Spinal (subarachnoid)	Epidural
Space	Subarachnoid (CSF)	Epidural (potential) space
Dose / volume	Small (1.5–3 mL)	Large (10–20 mL)
Onset	Fast (~5 min)	Slow (~15–20 min)
Block density	Dense	Less dense, segmental
Catheter	Usually single-shot	Catheter → top-ups, infusion
PDPH risk	Yes (dural puncture)	Only if accidental dural tap
Level control	Less controllable	Titratable, segmental
Hypotension	More abrupt	More gradual

Key differentials / "spot the diagnosis"

• Postural headache after spinal → PDPH (not migraine, not meningitis — fever and non-postural pattern point elsewhere).
• Headache + fever + neck stiffness + non-postural → meningitis, not PDPH.
• Progressive back pain + leg weakness + sphincter loss after spinal in an anticoagulated patient → spinal haematoma → urgent MRI + decompression.
• Bradycardia + hypotension after high block → high sympathetic block / Bezold–Jarisch → atropine + ephedrine.
• Back/leg pain after lignocaine spinal, normal neuro exam → TNS (benign); with deficits → cauda equina (serious).

Recently asked / exam angle

• Sensory level for Caesarean section = T4 (recurrent single-best-answer).
• Cord ends at L1 in adults, L3 in neonates; spinal done below L2 (L3–L4).
• Tuffier's line = L4 / L4–L5 interspace.
• Order of nerve fibre block and that sympathetic block is 2 segments higher, motor 2 lower than sensory.
• PDPH: postural headache, pencil-point needle reduces it, epidural blood patch is definitive; caffeine as adjunct.
• Hyperbaric solution = LA + dextrose; moves to dependent area — manipulate level by posture.
• Absolute contraindications: raised ICP, coagulopathy, severe aortic stenosis, hypovolaemia, local infection, refusal.
• First sign of hypotension under spinal is often nausea.
• Ephedrine preferred over phenylephrine in obstetric hypotension (maintains uterine blood flow) — though phenylephrine is now widely used too; know ephedrine as the classic answer for fetal wellbeing.
• 5% lignocaine → TNS and cauda equina syndrome — why it is avoided intrathecally.

Rapid revision

1. Spinal needle goes below L2 (adult cord ends at L1; neonate L3); standard site L3–L4.
2. Tuffier's line ≈ L4 vertebra / L4–L5 interspace.
3. Layers: skin → SC fat → supraspinous → interspinous → ligamentum flavum → epidural → dura → arachnoid → CSF.
4. Fibre block order: sympathetic → temperature → pain → touch → motor → proprioception; sympathetic 2 levels higher, motor 2 lower than sensory.
5. Surface levels: T4 nipple, T6 xiphisternum, T10 umbilicus, L1 inguinal ligament.
6. Caesarean section → T4; TURP/hip/labour → T10; saddle block → S2–S5.
7. Hyperbaric = LA + 8% dextrose, sinks to dependent part; hypobaric rises.
8. Hypotension is the commonest complication → fluids + ephedrine/phenylephrine; bradycardia from T1–T4 block → atropine.
9. PDPH = postural headache, worse sitting, relieved lying flat, 24–48 h, young pregnant women, cutting needles → epidural blood patch is definitive.
10. Pencil-point (Whitacre/Sprotte) needles ↓ PDPH vs cutting Quincke.
11. Absolute contraindications: refusal, coagulopathy/anticoagulation, raised ICP, local sepsis, severe hypovolaemia, severe aortic stenosis.
12. 5% lignocaine intrathecally → TNS / cauda equina; intrathecal morphine → delayed respiratory depression.