Motor pathway evaluation by transcranial magnetic stimulation in Swedish horses with acquired equine polyneuropathy

Background: Acquired equine polyneuropathy in Nordic horses (AEP) is the most prevalent equine polyneuropathy in Norway, Sweden, and Finland and is characterised by pelvic limb knuckling due to metatarsophalangeal extension dysfunction. Objectives: To evaluate the function of descending motor pathways in AEP using transcranial magnetic stimulation (TMS). Study Design: An analytical, observational cohort design. Methods: Clinical findings and TMS results of 20 horses from an AEP outbreak in Sweden were evaluated at 5-month intervals. Latency time (LT) in milliseconds (ms) between coil discharge and onset of muscle potential was recorded for thoracic and pelvic limbs. Results: Fourteen affected horses showed knuckling, 2 horses showed lameness, and 6 horses were neurologically sound and showed no clinical signs at the first visit. Thirteen of 14 neurologically affected horses had improved clinically 5 months later, four no longer showed knuckling. Motor neurological dysfunction with increased LT was confirmed by TMS in all 14 affected horses at both visits. Mean difference in LT from normalised reference values (ΔLT) in the pelvic limbs of affected horses was +12.95 ms (+38%) at the first examination (1.9–29.6 ms; SD 1.23; n = 14), and +8.1 ms (+24%) 5 months later (1.0–18.9 ms; SD 1.21; n = 14), cutoff >0.8 ms. Eleven of 14 affected horses also presented delayed TMS responses in the thoracic limbs, with up to 14% ΔLT increase. Neurologically sound, non-lame horses (n = 8) showed mean ΔLT −0.5 ms (−1.8 to 0.2 ms; SD = 0.64) in pelvic, and −0.35 ms (range, −0.7 to 0 ms; SD = 0.26; n = 8) in thoracic limbs, cutoff >0.2 ms. Limitations: Examinations were only repeated once. Conclusion: This study confirms the involvement of motor pathways in AEP and adds to the previously established involvement of sensory nerve fibres. Sensory and motor involvement contributes to the mismatch of ascending and descending nerve signals and to the clinical manifestations. TMS may be useful in evaluating clinical and subclinical cases of AEP.