Auricular vagus nerve stimulation halved rebound pain after interscalene block

Design

single-centre, randomised, participant- and assessor-blinded, sham-controlled trial

Population

120 adults having elective arthroscopic shoulder surgery with single-shot interscalene block

Primary outcome

incidence of rebound pain within 24 hours of surgery

Effect

20.0% vs 43.3%; absolute risk reduction 23.3% (95% CI 7.2 to 39.5); RR 0.46 (0.26 to 0.82); NNT 4.3 (2.5 to 13.9)

One hundred and twenty adults having elective arthroscopic shoulder surgery under ultrasound-guided single-shot interscalene block were randomised 1:1, participant- and assessor-blinded, to active transcutaneous auricular vagus nerve stimulation or sham. Stimulation was applied perioperatively and continued on postoperative days 1 and 2. The primary outcome was rebound pain within 24 hours.

Rebound pain occurred in 12 of 60 (20.0 per cent) with active stimulation and 26 of 60 (43.3 per cent) with sham — absolute risk reduction 23.3 per cent (95% CI 7.2 to 39.5), relative risk 0.46 (0.26 to 0.82), number needed to treat 4.3 (2.5 to 13.9), P = .006. Resting pain was lower by 0.50 points (−0.63 to −0.37) and movement-evoked pain by 0.39 (−0.65 to −0.14). Quality of recovery on days 1 and 2 and sleep quality on the night of surgery and the first postoperative night were better. Cumulative sufentanil consumption did not differ significantly (geometric mean ratio 0.67, 0.44 to 1.01, P = .056). No stimulation-related adverse events.

Rebound pain is the unsolved problem of single-shot blocks, and the interventions proposed so far — dexamethasone, catheters, scheduled analgesia — all have costs. A non-drug ear electrode with a number needed to treat of four and no adverse events is worth taking seriously, and the equipment is cheap enough to matter in an Indian day-case unit. The caution is that this is one single-centre trial with a subjective primary outcome and a sham that participants may have been able to distinguish; the pain score differences, at half a point, are not themselves clinically meaningful. It is a strong enough result to pilot and to replicate, not yet to standardise.

• Consider piloting perioperative auricular stimulation in shoulder arthroscopy lists with a high rebound rate
• Measure your own baseline rebound pain incidence first — 43 per cent is the comparator here
• Do not expect opioid sparing; that outcome did not reach significance
• Keep the pre-block counselling and scheduled oral analgesia regardless
• Treat the half-point pain score differences as secondary; the incidence reduction is the finding

Why it matters

Rebound pain is the main thing that erodes the benefit of a single-shot block, and nothing cheap has worked on it before.

Don't overread it

A single-centre sham-controlled trial with a subjective primary outcome — this needs replication before it becomes routine.

The statistics, in plain English

A number needed to treat of 4.3 is large for a perioperative intervention, but its confidence interval runs to 13.9, so the true benefit could be a third of the headline. Rebound pain is patient-reported, so blinding integrity carries the result: if participants could tell active from sham stimulation, the effect would be inflated. The opioid result at P = .056 is not a negative finding, just an underpowered one.

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