◊ BACKGROUND:
A stimulation current of no more than 0.5 mA is regarded as safe in avoiding nerve injury and delivering adequate stimulus to provoke a motor response. However, there is no consistent level of stimulating threshold that reliably indicates intraneural placement of the needle. The authors determined the minimally required stimulation threshold to elicit a motor response outside and inside the most superficial part of the brachial plexus during high-resolution, ultrasound-guided, supraclavicular block.
◊ METHODS:
After institutional review board approval, ultrasound-guided, supraclavicular block was performed on 55 patients. Patients with neurologic dysfunction were excluded. Criteria for extraneural and intraneural stimulation were defined and assessed by independent experts. To determine success rate and any residual neurologic deficit, qualitative sensory and motor examinations were performed before and after block placement. At 6 month follow-up, the patients were examined for any neurologic deficit.
◊ RESULTS:
Thirty-nine patients met all set stimulation criteria. Median +/- SD (interquartile range) minimum stimulation threshold outside was 0.60 +/- 0.37 mA (0.40, 1.0) and inside 0.30 +/- 0.19 mA (0.20, 0.40). The difference of 0.30 mA was statistically significant (P < 0.0001). Stimulation currents of 0.2 mA or less were not observed outside the trunk in any patient. Significantly higher thresholds were observed in diabetic patients. Success rate was 100% after 20 min. Thirty-four patients had normal sensory and motor examination at 6 months. Five patients were lost to follow-up.
◊ CONCLUSIONS:
Within the limitations of this study and the use of ultrasound, a stimulation current of 0.2 mA or less is reliable to detect intraneural placement of the needle. Furthermore, stimulation currents of more than 0.2 and no more than 0.5 mA could not rule out intraneural position.
◊ Reviewed by Z. J. Koscielniak-Nielsen, MD, DMSc, FRCA
Associate Professor, Regional Anaesthesia Research, Rigshospital,
Copenhagen, Denmark
There is increasing evidence from ultrasound-guided nerve blocks that local anaesthetics (LA) may be injected intraneurally when muscle twitches are elicited with minimum stimulating currents (MSC) of 0.3 – 0.5 mA and 0.1 ms duration. This even more applies to patients with neuropathies e.g. due to diabetes mellitus or uraemia. Four out of seven diabetic patients in this study had intraneural MSCs of 0.3 – 0.4 mA. On the other hand, case series and newer studies employing imaging techniques suggest that intraneural injections at peripheral nerves are not as dangerous as generally believed. The present investigation follows the already published one on axillary block of the same group [1]. Although only 39 patients were included and five of them were lost to long-term follow-up the results add another brick to our understanding of intraneural injections and MSC amplitudes.
The most relevant findings of this study are that MSC amplitudes of <0.2 mA always indicate intraneural needle position and that none of the 34 patients followed-up for 6 months had neurological sequelae attributable to intraneural injections. However, it is reassuring that the maximum LA volume injected into the brachial plexus trunk was only 5 ml and that low LA and adrenaline concentrations were used: bupivacaine 2.5 mg/ml, lignocaine 10 mg/ml, adrenaline 3 mg/l. Furthermore, patients were fully awake and the injections were slow (0.33 ml/s) and terminated in case of high resistance or paraesthesia.
The discussion of the brachial plexus (BP) anatomy at the supra-clavicular level is very insightful, but the definitions of intraneural injections can be misleading. Anatomy at this level is highly variable and intraneural injections into the plexus are not the same as into a single, terminal nerve. As accurately pointed out by the authors, over 50% of the cross-sectional area of the BP is occupied by a non-neural component. It is a relatively loose connective tissue, containing fat and small blood vessels, and is generally known as epineurium. In the supra-clavicular area it has clear borders and can easily be identified. This is not the case in the infra-clavicular and axillary areas. The authors also postulate that puncturing this epineural layer opens the inner environment of the nerve or trunk. This is not necessarily what others have found. Selander et al. [2] stated that the perineurium which surrounds axon-containing fascicles maintains the inner environment of a nerve. Intraneural LA injections not disrupting the perineurium, especially those not containing high concentrations of LA or adrenaline, may be harmless. However, even high-resolution ultrasound does not always allow differentiation between intra- and extra-fascicular injections. High injection pressures and/or pain on injection may be better indicators of potentially damaging injections [2, 3]. The authors avoided both of them suggesting that all injections were extra-fascicular. Moreover, the needles had atraumatic tips and the blocks were performed by a highly experienced anaesthesiologist. All these precautions have minimized the risks of nerve injury.
In my opinion the variability of MSC amplitudes (0.3 – 1.6 mA) upon a visually confirmed needle-nerve contact as shown in this study confirms that the generally accepted threshold of 0.5 mA should be taken with caution. This variability may be even higher for other nerves and patient populations. High-resolution ultrasound is a valuable help to determine location of a needle tip. However, over 30% of patients in this study were excluded because of inadequate images. Intraneural injections at the supra-clavicular level, even extra-fascicular ones, may have dire consequences for patients. With the current level of evidence, every precaution should be taken to avoid these injections and/or to minimize their consequences: gentle needle handling, atraumatic tips, termination of injection upon elicitation of painful paraesthesia or against high pressure, frequent aspiration, and good verbal contact with the patient. Neither nerve stimulator nor ultrasound is foolproof.
◊ REFERENCES
1. Bigeleisen PE.
Nerve puncture and apparent intraneural injection during ultrasound-guided axillary block does not invariably result in nerve injury. Anesthesiology 2006; 105: 779-83.
2. Selander D, Sjostrand J.
Longitudinal spread of intraneurally injected local anesthetics. An experimental study of the initial neural distribution following intraneural injections. Acta Anaesthesiol Scand 1978; 22: 622-34.
3. Hadzic A et al.
Combination of intraneural injection and high injection pressure leads to fascicular injury and neurologic deficits in dogs. Reg Anesth Pain Med 2004; 29: 417-23.
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