We advocate using an 18G Tuohy needle for most adult blocks (continuous and single shot), with the exception of perhaps distal upper limb blocks and the ankle block. Needle calibre should obviously be modified for small infants. Compared to the traditional 22G short bevel needle, a large calibre (e.g. 18G) very blunt (e.g. Tuohy) is preferred primarily because of improved block safety.
A larger calibre blunt needle (e.g. 18G Tuohy cf. 22G short bevel) reduces the potential for significant complications for three main reasons:
1. Reduced risk of intraneural needle placement, and in particular intrafascicular injection – a well recognized predictor of prolonged neurological deficit following peripheral nerve blocks.
2. Reduced risk of inadvertent subarachnoid needle placement (total spinal anesthesia and possibly devastating spinal cord syrinx formation).
3. Reduced risk of inadvertent intravascular injection (systemic LA toxicity – central nervous system and cardiac).
Reduced risk of intraneural needle placement
Intraneural needle placement should be avoided because such placement causes disruption to a nerve’s structural integrity. 1 Intrafascicular (within an individual fascicle) local anesthetic injection is also predictive of long-lasting neurological deficit. 2 Fine bore (e.g. 22G) short bevel needles can certainly be placed within nerve fascicles.
Despite years of research and innovation to detect intraneural needle placement (nerve stimulation, ultrasound, injection pressure monitoring), the reality is that no one technology can reliably detect intraneural needle placement. However, use of an 18G (or larger) very blunt (e.g. Tuohy) needle can prevent intrafascicular (and probably also intraneural) needle placement/injection: the needle’s calibre is such that it is not possible to place within a nerve fascicle. Excluding the very large sciatic nerve, its calibre and tip shape also virtually eliminates the possibility of intraneural placement.
If one accepts that injection pressure monitoring has a role in detecting/preventing intrafascicular injection, a secondary advantage of a large calibre needle is that because a large calibre needle has a lower resistance to injection, subjective assessment of injection pressure will be more readily appreciated. This issue is especially relevant for long needles that have an inherently high injection resistance – subjective assessment of injection pressure is practically impossible with the dreaded 22G 10 cm needle.
Reduced risk of inadvertent subarachnoid/nerve root needle placement
Peripheral nerve block complications related to subarachnoid or nerve root needle placement have been some of the most serious, 3 and include total spinal block, permanent neurological deficit (including hemiplegia secondary to injection related syrinx formation) and death. The spinal cord dura can extend along the nerve roots outside the vertebral column (dural “sleeves” or “cuffs”). Therefore, blocks associated with subarachnoid injection include paravertebral block (including lumbar plexus block) and interscalene block. It has been proposed that an intracordal syrinx may arise secondary to intraneural nerve root injection with subsequent proximal spread of the local anesthetic into the spinal cord. 3
A large calibre very blunt needle carries a lower risk of dural puncture compared to a thinner/sharper needle (e.g. 22G standard short bevel). 4, 5 Furthermore, with the exception of the large diameter lumbar nerve roots, an 18G Tuohy needle cannot penetrate the substance of a nerve root, virtually eliminating the risk of intracordal syrinx formation and the consequent severe neurological deficit.
Reduced risk of inadvertent intravascular injection.
This argument is analogous to the reasoning for the use of a large calibre needle to minimize the risk of intrafascicular injection. A large calibre (preferably short) needle has a lower aspiration resistance than a thinner (or longer) needle. Therefore, negative pressure (generated by the syringe to detect intravascular needle tip placement) is more likely to be transferred to the needle tip. Intravascular needle placement is therefore more likely to be detected (as blood aspirating into the syringe). In fact, intravascular placement of an 18G needle will almost always manifest as blood in the syringe with the slightest withdrawal of the syringe plunger. Again, the dreaded 22G 10 cm needle is associated with significant false negatives for intravascular placement.
Another advantage of a larger calibre needle is that these needles are more easily sonographically visualized than smaller calibre needles. Further, the are not as prone to bending during attempted manipulation within the tissues.
Theoretical disadvantages of the 18G Tuohy needle (compared to the common 22G short bevel) include:
Increased muscle trauma: There is no evidence in the peer-review literature for this to be a clinically relevant problem. We have performed thousands of blocks using the 18G Tuohy (single shot and continuous) without observing block site (needle tract) pain as a significant issue.
Increased blunt nerve trauma: Recent animal studies have demonstrated that repeated needle-nerve contact causes histological nerve changes reflecting nerve trauma. 6 A larger calibre needle would probably cause more nerve trauma (cf. a 22G short bevel) if repeatedly advanced towards the nerve. However, the clinical significance of extraneural nerve trauma and consequent histological change is unclear, but it would probably manifest as only minor transient neurological symptoms e.g. numbness +/- tingling. The only histological change shown to correlate with prolonged deficit is intrafascicular injection and the resulting neuronal death.
Both theoretical problems are of doubtful clinical significance. Therefore, in our opinion, the benefits of using a larger calibre Tuohy needle (cf. a 22G short bevel) for nerve blocks far outweigh the theoretical problems.
A final note about cost: In some markets, cost may be a a prohibitive factor to using a large calibre insulated Tuohy needle. The Contiplex Tuohy catheter kit has a retail price of approx. $US40 compared to approx. $US15 for a standard stimuplex needle. Echogenic needles are even more expensive, while non-insulated non-echogenic epidural Tuohy needles remain inexpensive.
1. 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.
2. Hadzic A, Dilberovic F, Shah S, 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.
3. Benumof JL. Permanent loss of cervical spinal cord function associated with interscalene block performed under general anesthesia. Anesthesiology. 2000;93:1541-4.
4. Cohen JM, Gray AT. Functional deficits after intraneural injection during interscalene block. Reg Anesth Pain Med. 2010;35:397-9.
5. Liu SS, Yadeau JT, Shaw PM, Wilfred S, Shetty T, Gordon M. Incidence of unintentional intraneural injection and postoperative neurological complications with ultrasound-guided interscalene and supraclavicular nerve blocks. Anaesthesia. 2011;66:168-74.
6. Steinfeldt T, Poeschl S, Nimphius W, et al. Forced needle advancement during needle-nerve contact in a porcine model: histological outcome. Anesth Analg. 2011;113:417-20.