Intraneural injection was previously regarded as a strong predictor of prolonged neurological deficit following peripheral nerve blocks, and consequently, the prevailing teaching for many years was to do all possible to avoid injecting LA inside nerves. However, several reports emerged during the last decade of so-called "intraneurally" injected LA confirmed by US that did not result in subsequent neurological deficit. 1-6 This has been in association with both low 1-3 and high 5-7 local anaesthetic volume. Consequently, it has lead to the suggestion that the injection of small volumes of local anaesthetic beneath the epineurium but outside the perinerium is an acceptably safe practice. 8
The most influential of these reports was a prospective series of axillary nerve blocks, 1 where a traditional (non-US guided) technique was used, but the subsequent LA injection was monitored with US. A high proportion of injections were, in fact, intraneural. If intraneural injection was observed, LA injection was aborted after a few mls had been injected. None of the patients in the study demonstrated persistent neurological symptoms (> several months). Further studies conducted at the popliteal sciatic nerve level have suggested similar findings – even when a large volume of LA has been injected. 7, 9, 10
One needs to view these results cautiously. Peripheral nerves consist of axons surrounded by endoneurium, which are assembled into fascicles. A tough connective tissue layer, the perineurium, envelops the fascicles, which are themselves organised into peripheral nerves – the nerve is surrounded by the epineurium (Fig. 1).
Fig. 1. Microscopic structure of a peripheral nerve: endoneurium/perineurium and epineurium.
Intrafascicular injection is widely accepted as hazardous and strongly associated with prolonged neurological deficit. Current US technology cannot distinguish between intrafascicular and extrafascicular injection, and it is highly likely that previous reports of intraneural injection involved extrafascicular injection. At the axillary level of the brachial plexus 11 and the popliteal level of the sciatic nerve, 12 the nerves have a relatively high proportion of connective tissue, which probably afforded significant protection in the case of intraneural injection - intraneural injection likely remaining extrafascicular. The same findings may not apply to proximal, peripheral nerves such as the interscalene brachial plexus and the subgluteal sciatic nerve, where the proportion of connective tissue within the nerve is much lower. For the axillary brachial plexus study, early follow up was not conducted, therefore, transient neurological symptoms lasting less than a few months were likely missed. Finally, controversy exists regarding the fascial lining of the sciatic nerve at the popliteal level. What may have been believed to be subepineural injection (i.e. intraneural), was almost certainly a subfascial but extraneural injection as the sciatic nerve at this level is surrounded by a complex fascial sheath.
An often overlooked consequence of intentional intraneural injection is the well documented trauma that occurs to a nerve when the epineurium is breached regardless of whether LA is injected. It has been shown that the intentional intraneural placement of very fine (28-32G) electroneurography needles are associated with subsequent electrophysiological conduction disturbances, 13 but symptoms are reported in only about 10% of patients. 14 These symptoms typically persist for 3-7 days but can very occasionally persist for several weeks. 14 Regarding reported data regarding the effects of intraneurally placed (larger calibre) nerve block needles without an associated injection, it has been shown that those needles used for regional anaesthesia produce a proportionately greater degree of axonal injury than microneurography needles when fascicles are accidentally impaled. 15, 16 Long bevelled needles inserted with the bevel transverse to the long axis of the nerve fibres, also cause more fascicular damage than those inserted longitudinally to the fibres. 17 These observations all point to a likely greater disruption of nerve integrity with intraneurally placed larger calibre needles compared with intraneurally placed electroneurography needles. More frequent and more severe symptoms might therefore be expected.
Short bevelled needles are preferred to long bevelled needles – a practice that is often based on animal studies showing that short bevelled needles are less likely to penetrate both the epineurium (intraneural) and perineurium (intrafascicular). 17, 18 However, if a short bevelled needle penetrates a fasciculus, it has been shown that this results in more axonal damage than a long bevel. 16 Regardless of bevel type, when nerve impalement occurs, all needle types have been associated with axononomesis and interruption of the myelin sheath. 19
Our practice is to avoid intraneural injection for the above reasons. We believe the purported advantage of a more rapid block onset does not justify the potential increased risk of prolonged neurological deficit. If rapid onset surgical anesthesia is needed, we either place supplementary blocks, 20 or administer a light GA. The sciatic nerve deserves special comment. For popliteal sciatic block, we have found that to produce a reliable block, the fascia surrounding the nerve needs to be penetrated. Such needle placement has been previously described as "intraneural" or "subepineural", however, it is more correctly a (extraneural) “subfascial” injection.
1. Bigeleisen PE. Nerve puncture and apparent intraneural injection during ultrasound-guided axillary block does not invariably result in neurologic injury. Anesthesiology 2006;105:779-83.
2. Brull R, Chan VW, McCartney CJ, et al. Ultrasound detects intraneural injection. Anesthesiology 2007;106:1244; author reply 1247.
3. Rodriguez J, Taboada M, Blanco M, et al. Intraneural catheterization of the sciatic nerve in humans: a pilot study. Reg Anesth Pain Med 2008;33:285-90.
4. Russon K, Blanco R. Accidental intraneural injection into the musculocutaneous nerve visualized with ultrasound. Anesth Analg 2007;105:1504-5, table of contents.
5. Sala-Blanch X, Pomes J, Matute P, et al. Intraneural injection during anterior approach for sciatic nerve block. Anesthesiology 2004;101:1027-30.
6. Schafhalter-Zoppoth I, Zeitz ID, Gray AT. Inadvertent femoral nerve impalement and intraneural injection visualized by ultrasound. Anesth Analg 2004;99:627-8.
7. Sala Blanch X, Lopez AM, Carazo J, et al. Intraneural injection during nerve stimulator-guided sciatic nerve block at the popliteal fossa. Br J Anaesth 2009;102:855-61.
8. Bigeleisen PER, M.A. Ultrasound-Guided Intraneural Injection for Regional Anesthesia: Pro. ASRA News (www.asra.com/Newsletters/Feb_09) 2009;February:6.
9. Robards C, Hadzic A, Somasundaram L, et al. Intraneural injection with low-current stimulation during popliteal sciatic nerve block. Anesth Analg 2009;109:673-7.
10. Sala Blanch X, Domingo Triado V, Bargallo X, et al. [Intraneural sciatic nerve puncture in the popliteal fossa demonstrated by ultrasound]. Rev Esp Anestesiol Reanim 2006;53:269-70.
11. Moayeri N, Bigeleisen PE, Groen GJ. Quantitative architecture of the brachial plexus and surrounding compartments, and their possible significance for plexus blocks. Anesthesiology 2008;108:299-304.
12. Moayeri N, Groen GJ. Differences in quantitative architecture of sciatic nerve may explain differences in potential vulnerability to nerve injury, onset time, and minimum effective anesthetic volume. Anesthesiology 2009;111:1128-34.
13. Inglis JT, Leeper JB, Wilson LR, et al. The development of conduction block in single human axons following a focal nerve injury. J Physiol 1998;513 ( Pt 1):127-33.
14. Eckberg DL, Wallin BG, Fagius J, et al. Prospective study of symptoms after human microneurography. Acta Physiol Scand 1989;137:567-9.
15. Rice AS, Andreev NY, McMahon SB. The consequences of microneurography electrode-induced injury of peripheral nerves observed in the rat and man. Pain 1994;59:385-93.
16. Rice AS, McMahon SB. Peripheral nerve injury caused by injection needles used in regional anaesthesia: influence of bevel configuration, studied in a rat model. Br J Anaesth 1992;69:433-8.
17. Selander D, Dhuner KG, Lundborg G. Peripheral nerve injury due to injection needles used for regional anesthesia. An experimental study of the acute effects of needle point trauma. Acta Anaesthesiol Scand 1977;21:182-8.
18. Sala-Blanch XR, T, Rivas, Eva MD; Carrera, Ana M; Gaspa, A; Reina, MHadzic, A. Structural Injury to the Human Sciatic Nerve After Intraneural Needle Insertion. Regional Anesthesia and Pain Medicine 2009;34:201-205.
19. Maruyama M. Long-tapered double needle used to reduce needle stick nerve injury. Reg Anesth 1997;22:157-60.
20. Fredrickson MJ, Ting FS, Chinchanwala S, et al. Concomitant infraclavicular plus distal median, radial, and ulnar nerve blockade accelerates upper extremity anaesthesia and improves block consistency compared with infraclavicular block alone. Br J Anaesth 2011;107:236-42.