The incidence of spinal anaesthetic failure, in the absence of technical failure or clinician dose/judgement errors, is in the region of 2.7% – 3.9%. Although clusters of cases suggest defects with the local anaesthetic (LA) solution this is not supported by evidence. Suggestions of individual resistance to local anaesthetics also have limited scientific support.
The CSF concentration of LA in patients with successful spinal anaesthesia is highly variable and there is no correlation between CSF concentration and block height. CSF analysis in failed spinals has shown LA concentrations are often adequate and maldistribution of LA is thought to be the principal cause of failure. Dispersion of LA within CSF is influenced by many factors including drug physical characteristics, spinal canal anatomy and gravity. Hidden anatomical restrictions to CSF flow include cysts, restrictive ligaments, adhesions within the spinal canal and extremes of lumbar CSF volume. Lumbar CSF volume is inversely proportional to peak sensory block height and is highly variable between individuals. as it is independent of age, height, and BMI it is difficult to predict preoperatively.
Management of a failed spinal should be pragmatic– if no block has evolved within 15 minutes either repeat the spinal or offer an alternative anaesthetic e.g GA. Assess sensory block level carefully including sacral dermatomes. Optimise patient positioning and If a repeated injection is performed utilise a different, more cephalad puncture level. Anticipate consequences of excessive cephalad spread and limit combined LA dosage from both injections to avoid potential neurotoxic levels LA concentrations.