Failed Spinal Anaesthesia - Causes and Mechanisms

1. Technical / Needle Placement Failures

a) Pseudo-successful lumbar puncture

• The most common mechanism: CSF appears in the needle hub, giving false confidence of correct placement
• The needle tip may be at the dural margin (subdural or peridural) rather than truly intrathecal
• Injection into the subdural space produces a delayed, patchy, or unexpectedly extensive block; injection into the epidural space produces no spinal block at all

b) Incorrect needle placement

• Needle tip exits the subarachnoid space as the syringe is attached (movement artifact)
• Injection directed through a lateral recess or dural fold rather than the true CSF pool
• Bevel partly outside the dura: part of injectate lost to epidural space

c) Blocked or damaged needle

• Manufacturing defects: burred tip, blocked lumen, or occluded orifice despite CSF aspiration
• A needle with a blocked lumen can still show CSF in the hub but injectate does not enter the CSF

d) Needle too low (sacral interspace)

• Injection at L5-S1 or below places the drug far from the main CSF column
• Caudal anatomy at low levels creates a dural sac that is narrow and drug may pool without spreading

2. Patient Anatomical Factors

• Miller's Anesthesia (10e) notes that "many of these factors are not controllable by the anesthesiologist, leading to significant interpatient variability."

3. Drug (Local Anaesthetic) Factors

a) Inadequate dose

• The single most important modifiable cause
• The 2024 Demilie study confirmed that a bupivacaine dose ≤10 mg was an independent predictor of failure
• Under-dosing is more common in elderly patients, short procedures, or when trying to minimise hypotension

b) Baricity mismatch with patient position

• Baricity = density of local anaesthetic / density of CSF (CSF density = 1.00059 g/mL at 37°C)
• Hyperbaric solutions spread to dependent regions; hypobaric solutions spread to non-dependent regions
• A hyperbaric solution injected with the patient sitting and then left seated produces a saddle block - appropriate for perineal surgery but fails if a higher block (e.g., T4-T6 for caesarean section) is required
• Conversely, a hypobaric solution injected in Trendelenburg may spread too high
• Plain (isobaric) bupivacaine 0.5% may actually be slightly hypobaric at body temperature (37°C), leading to unpredictably high or patchy blocks if posture is not optimised

c) Dilution or loss of injectate

• Leakage at the needle-syringe junction during injection
• Excessive barbotage diluting the drug with CSF
• Loss of drug into a paravertebral vessel (intravascular injection) without realising

d) Drug degradation or contamination

• Old, contaminated, or improperly stored local anaesthetic
• Mixing errors (wrong drug or wrong concentration drawn up)
• Inadvertent dilution with saline when preparing the syringe

4. CSF-Related Factors

• Bloody CSF: A 2024 systematic review identified bloody CSF as an independent predictor of failure; blood may alter local anaesthetic pH, reducing the unionised (active) fraction, or physically dilute the drug
• Increased CSF pressure (obese patients, large intra-abdominal tumours, severe straining): forces drug back toward the needle and limits cephalad spread
• Low CSF volume: conversely causes unexpectedly high spread - but is occasionally associated with patchy block in the presence of spinal adhesions

5. Patient Position During and After Injection

• Failure to adjust patient position correctly after injection to utilise the baricity effect
• Moving the patient too quickly (before drug is fixed to neural tissue) can redistribute a hyperbaric solution away from the target dermatomes
• Prolonged sitting position after hyperbaric injection restricts block to sacral level (saddle block) when a higher level is needed
• Trendelenburg after hypobaric injection can cause an unintended high block

6. Operator and Experience-Related Factors

• Provider inexperience (<2 years) was confirmed as an independent predictor in the 2024 Demilie multicenter study
• Emergency surgery settings are associated with 7x higher failure rates (AOR 7.01 in one study) - likely reflecting time pressure, positioning difficulties, and less optimal drug selection
• Omission of intrathecal adjuvants (fentanyl, morphine, clonidine) increased failure five-fold in the Demilie study - adjuvants enhance block quality and density

7. Local Anaesthetic Resistance (Rare)

• Rare case reports exist of patients with apparent resistance to local anaesthetics
• Postulated mechanism: sodium channel mutations (channelopathies) altering Nav1.x receptor function
• However, as NYSORA emphasises: this has not been demonstrated at the cellular level and no confirmed Nav mutations have been found in otherwise asymptomatic patients
• More likely explanations (inadequate dose, technical failure) should always be excluded first

8. Block Insufficient Height vs. Complete Failure

Management Principles

1. Posture manipulation - for inadequate height with hyperbaric solution: Trendelenburg + flex hips/knees to flatten lumbar lordosis
2. Repeat spinal - cautiously, with awareness that drug already in the CSF means the total dose will be higher; only 2 attempts recommended to avoid neural injury
3. Epidural top-up (if CSE technique used) - preferred rescue option
4. Surgeon infiltration - for limited, superficial supplementation
5. Systemic sedation/analgesia - ketamine or opioids as a bridge
6. Conversion to general anaesthesia - mandatory if above fail, especially for urgency or patient safety

• Miller's Anesthesia 10e, Chapter 41 - Factors affecting block height (Drug factors, baricity, patient factors)
• Fettes PD, Jansson JR, Wildsmith JA. Failed spinal anaesthesia: mechanisms, management, and prevention. Br J Anaesth 2009;102:739-748
• Demilie et al. (2024) - multicenter prospective study, BMC Anaesthesiology
• Girard T, Savoldelli GL. Failed spinal for caesarean. Curr Opin Anaesthesiology 2024;37(3):207-212