Where should the tip of the needle be located in ultrasound-guided peripheral nerve blocks?

Data now exist describing the appropriate positioning of the needle tip and pattern of local anaesthetic spread after injection. The recent literature has been analysed in search of studies on the optimal procedure for common approaches centred on block efficacy, performance time, and safety.

Large peripheral nerves are surrounded by a gliding layer, the adventitia or paraneurium. Ultrasonically, a circumneural spread corresponds to adventitial extraneural injection. Nerve expansion with fascicular separation matches intraneural injection. Deliberate intraneural injection remains controversial, and is not advisable at the present time. For popliteal sciatic nerve blocks, positioning the needle in the common nerve sheath between the tibial and peroneal components and obtaining a circumneural spread surrounding both divisions predict rapid surgical anaesthesia. Using axillary and infraclavicular approaches, ultrasound-guided perivascular injection aiming at circumferential spread around the artery appears a valuable alternative to individual targeted nerve injections. For single injection interscalene block, an injection into the fascial sheath but far from the plexus proved to be as effective as an injection adjacent to the nerve structures. Fascial plane approaches are appealing alternatives for thin nerves that run between muscles and cannot be regularly visualized with the current resolution of ultrasound systems.

The ultrasound appearance of nerves and target injections are better understood. The specific distributions of local anaesthetic spread that predict success are significantly different from one anatomical site to another. It seems advisable to avoid intraneural injection.