Botulinum toxin relieved painful foot dystonia without costing motor function

Design

single-centre, randomised, double-blind, placebo-controlled trial with 12-week open-label extension

Population

33 adults with unilateral Parkinson disease and painful foot dystonia refractory to medication optimisation

Primary outcome

change in King's Parkinson's Disease Pain Scale item 5 at 6 weeks

Effect

mean difference −3.60 at 6 weeks, sustained at 12 (p<0.001); Hedges' g −1.33 (95% CI −2.13 to −0.53); responders 75.0% vs 11.8%

Pain from foot dystonia in Parkinson disease usually arrives in OFF periods and often survives every reasonable adjustment of dopaminergic therapy. This single-centre trial randomised 33 adults with unilateral Parkinson disease and medication-refractory painful foot dystonia to onabotulinumtoxinA 100 units or placebo, injected under guidance into tibialis posterior, extensor hallucis longus and flexor digitorum brevis, with assessment at 6 and 12 weeks and an open-label extension.

The primary outcome, item 5 of the King's Parkinson's Disease Pain Scale, fell by a mean 3.60 points more with toxin than placebo at six weeks and the difference held at 12 weeks (p < 0.001), with a large between-group effect size (Hedges' g −1.33, 95% CI −2.13 to −0.53). Three-quarters of the toxin group reached a 30% pain reduction at six weeks against 11.8% of controls, and 5.6% at 12 weeks. Both the severity and frequency subitems improved. Visual analogue scale pain improved at six weeks but not significantly at 12. Motor scores, quality of life and adverse events did not differ; adverse events were mild.

The finding that matters alongside the pain result is the absence of motor cost. The reason this intervention is often not offered is a fear of weakening a foot in someone whose gait is already fragile, and in this trial the MDS-UPDRS III and IV scores did not move. Thirty-three patients from one centre is small, so the effect size is imprecise — but the direction and the safety signal are both clear enough to make the offer reasonable in a symptom with nothing else behind it.

• Ask specifically about foot pain in OFF periods — patients often report it as cramp rather than pain
• Consider guided botulinum toxin where dopaminergic optimisation has failed
• Reassess at 6 and 12 weeks, using the same pain measure each time
• Set expectations that the analogue-scale benefit faded by 12 weeks even where the dystonia scale did not

Why it matters

The usual reason for not offering this — fear of weakening an already unstable foot — was not borne out.

Don't overread it

A single-centre trial of 33 patients establishes a plausible, sizeable effect, not a precise one.

The statistics, in plain English

Hedges' g of −1.33 is a large standardised effect, but with 16 and 17 patients per arm the interval of −2.13 to −0.53 is wide: the effect is real and its size is not well pinned down. The divergence between the dystonia-specific scale and the visual analogue scale at 12 weeks suggests the benefit is specific to dystonic pain rather than to pain in general — which is consistent with the mechanism, but it also means a patient may not report feeling globally better.

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