Ultrasound-guided sciatic nerve block using two-point injections may prolong analgesia versus single-point injection

When performing lower limb surgeries, sciatic nerve blocks at the popliteal level are often used, either alone or in combination with femoral nerve blocks, to provide surgical anaesthetic and postoperative pain management. Real-time drug distribution imaging is made possible by ultrasound guidance, which also lessens issues with intravascular and intraneural local anaesthetic (LA) delivery. On the other hand, the success rate of sciatic nerve block guided by ultrasonography is not consistent. By administering a double-point injection at 2 and 6 cm above the sciatic nerve division and comparing it to a single-point injection at 2 cm above the division using current perception threshold (CPT) to evaluate the completeness of sensory block, the study aimed to ascertain the effectiveness of extending the length of the nerve exposed to local anaesthesia.

Sixty patients scheduled for foot surgeries under USG-guided sciatic nerve block were randomly assigned to two groups: Group Double Point received two injections of 10 mL of 1.5% lignocaine with adrenaline, one at the point similar to the first group and the other 6 cm above the first point. Group Single Point received a single injection of 20 mL of 1.5% lignocaine with adrenaline just proximal to the sciatic nerve bifurcation. Evaluations were conducted on the duration of analgesia, the length of the exposed nerve, the time it took to complete motor blockage, the time it took to complete sensory blockade, and more. The statistical software used for the study was version 20 of the Statistical Package for the Social Sciences (SPSS).

In comparison to single-point injection technique, which took 20.89 (12.62) min, 18.78 (5.95) cm, and 344.28 (125.97) min, respectively, double-point injection technique demonstrated a significantly faster time to complete motor blockade [14.46 (9.93) min], increased length of nerve exposed to local anaesthetic [23.23 (7.209) cm], and extended duration of analgesia [420.40 (99.34) min]. There was no significant difference in the duration of sensory blockage or its onset between the two groups.

The key outcomes of the beginning of sensory blockade and the time taken to achieve sensory blockade are not significantly different when LA is injected at two locations as opposed to a single one, according to the study's authors. The time required to complete the sensory block is not appreciably reduced by the double-point injection approach used in conjunction with ultrasound guidance for the popliteal sciatic nerve block. However, there was a notable prolongation of the duration of analgesia and time to complete motor nerve block, which may be advantageous clinically for postoperative analgesia. This may be explained by the sciatic nerve's wide diameter, which would make it take longer for LA to reach the core fibres and lengthen the period until both sensory and motor blockage begin. When LA is injected, it reaches the core fibres quicker, resulting in a faster onset time, since the nerve width decreases as it bifurcates into the tibial and common peroneal components.

Reference –

Nag, Kusha; Ravishankar, M1; Parthasarathy, S1; Thomas, Tina M.1. Quantitative assessment of ultrasound-guided sciatic nerve block – A comparison of a single-point versus two-point injection technique: A randomised controlled, double-blinded trial. Indian Journal of Anaesthesia 67(9):p 802-808, September 2023. | DOI: 10.4103/ija.ija_140_23.