Ultrasound-Guided Nerve Blocks: A Step-by-Step Guide
The aim of a nerve block is to place local anaesthetic around the target nerve. What ultrasound adds is that the whole of this becomes visible - the nerve can be seen, the needle can be seen, and the spread of the injectate can be seen.
This article follows the procedure in order: recognising the nerve, choosing the probe, planning the needle path, what to watch as the needle advances, and what to look for while injecting.
1. First, recognise what a nerve looks like
To find a nerve you first have to know how it appears on ultrasound, and be able to tell it apart from the tendon and the vessel next to it.
| Structure | Transverse view | Longitudinal view | How to tell it apart |
|---|---|---|---|
| Nerve | Round or oval, with a "honeycomb" interior (hypoechoic fascicles separated by hyperechoic interfaces) | Cord-like, with parallel linear hyperechoic lines inside | Does not move with muscle contraction |
| Tendon | Densely dotted hyperechoic | Densely packed parallel strongly echogenic fibres | Moves with joint movement |
| Vessel | Anechoic (dark) | Tubular and anechoic | Compressible, pulsatile; colour Doppler shows flow |
The two most practical checks:
- Press. Press lightly with the probe - what collapses is a vein, not a nerve.
- Move. Ask the patient to move the relevant joint - the tendon moves with it, the nerve does not.
A suggestion for getting started: begin with nerves that are superficial and in a constant position, so the visual picture of "what a nerve looks like" is established first, then move on to deeper sites where variation is greater.
2. Choosing the probe
Nerve blocks are mostly superficial, so a high-frequency linear probe is the usual choice; deeper nerves need penetration as well as resolution.
| Model | Configuration | Frequency | Detection depth | Where it suits |
|---|---|---|---|---|
| 5P | Linear | 10MHz-H14MHz | 20-30-40-55mm | Superficial nerves where resolution matters |
| 6L | Linear | 7.5MHz-H10MHz | 20-40-60-100mm | Covers both shallow and deeper sites |
| 5MN | Microconvex + linear | Microconvex 5-H7.0MHz / linear 10MHz-H12MHz | Microconvex 60-90-120-160mm / linear 20-40-60-80mm | When both superficial and deep sites have to be covered |
| 5ML | Microconvex + linear | Microconvex 5-H7.0MHz / linear 7.5MHz-H10MHz | Microconvex 60-90-120-160mm / linear 20-40-60-80mm | As above |
The principle:
The more superficial the nerve, the higher the frequency you can use, and the more detail you see; the deeper it is, the more frequency has to be traded for penetration.
3. Plan the needle path (do not rush in once the nerve is found)
Once the nerve is recognised, look around it before deciding where to enter:
- What is around the nerve - vessels, pleura, organs, bone; confirm each one
- Which direction gives the shortest, lowest-risk path
- Which tissue layers the needle will pass through
- Whether the patient's position allows that path
Nerve blocks are usually done in-plane (long axis) - because the distance between needle tip and nerve has to be watched continuously, and in-plane keeps the whole needle on screen.
4. Advancing the needle and locating the tip
- In-plane insertion: keep the needle aligned with the long axis of the probe so the whole length is visible on screen
- Advance in steps: confirm the tip position after each short advance rather than pushing all the way in one movement
- Slow down near the nerve: once the tip is close, reduce the speed and avoid advancing into the nerve itself
- If the needle cannot be seen: first check whether the probe plane is aligned with the needle path - do not keep advancing blindly
5. Injecting and watching the spread (the step most often skipped)
As the local anaesthetic goes in, an anechoic or hypoechoic area should be seen spreading around the nerve - this is the most valuable moment of the whole "visualisation" idea, and the direct evidence of whether the position is right.
Watch four things:
- Whether the injectate spreads around the nerve - the distribution you want
- Whether it stays confined within muscle - a sign the tip may be in the wrong place
- Whether it collects as a round pool - a warning that the position is not right
- How much resistance there is - abnormal resistance means stop and reassess
Practical points:
- Aspirate before injecting, to confirm the tip is not inside a vessel
- Inject a small test volume first, and continue only once the spread is going the right way
- If the spread is poor, adjust the tip position rather than pushing a larger volume
6. Five common problems
| Problem | What to do |
|---|---|
| Nerve cannot be found | Start from anatomical landmarks (vessels, bone, muscle planes) and follow them step by step |
| Image too dark or too bright | Set gain and depth first, then start looking for structures |
| Needle poorly seen | Check that the probe plane is aligned with the needle path; too steep an angle makes the tip harder to see |
| Injectate spreading poorly | Do not increase the volume - adjust the tip position first |
| Patient build makes imaging difficult | Consider lowering the frequency for penetration, or switching to a microconvex configuration |
7. Probe selection reference
For superficial nerve blocks, a high-frequency linear probe is the first choice; where a deeper site also has to be reached (some lumbosacral plexus approaches, for example), a microconvex plus linear dual-head configuration is worth considering.
The exact model is decided together with the depth of the block site, the patient's build and the operator's technique.
The above is the general version. The Probe Configuration Guide for Nerve Blocks and the full Ultrasound Probe Selection Table set out probe and parameter comparisons for different block sites; ask our product team for a copy.
Summary
- Tell nerve from tendon and vessel with two checks: press, and move
- The more superficial the nerve, the higher the frequency; the deeper it is, the more penetration matters
- Once the nerve is found, plan the path first: see what surrounds it, take the shortest and safest direction
- Nerve blocks are usually done in-plane, so the tip and the nerve stay in view throughout
- While injecting, always watch the spread - it is the most direct evidence that the position is right
- If the spread is poor, adjust the tip rather than increasing the volume
