Motor pathway evaluation by transcranial magnetic stimulation in Swedish horses with acquired equine polyneuropathy.
Authors: May, Hanche-Olsen, Goehring, Matiasek, Jäderlund, Zablotski, Gröndahl
Journal: Equine veterinary journal
Summary
# Motor Pathway Dysfunction in Acquired Equine Polyneuropathy: TMS Evidence from a Swedish Outbreak Acquired equine polyneuropathy (AEP) has emerged as the leading polyneuropathy across Nordic countries, presenting clinically as pelvic limb knuckling attributable to metatarsophalangeal extension failure; however, the underlying neurological mechanisms remained incompletely characterised. Using transcranial magnetic stimulation (TMS) to measure motor pathway conduction latency, researchers evaluated 20 horses from a Swedish AEP outbreak at baseline and 5-month intervals, demonstrating that affected animals exhibited markedly prolonged latency times in pelvic limbs (+12.95 ms or 38% above reference values initially, improving to +8.1 ms or 24% by follow-up), with 11 of 14 clinically affected horses also showing delayed responses in thoracic limbs (up to 14% increase). Notably, whilst 13 of the 14 neurologically affected horses improved clinically over five months and four resolved their knuckling, TMS abnormalities persisted in all affected animals at both timepoints, suggesting subclinical motor pathway involvement continues beyond visible recovery. The findings indicate AEP involves both sensory and motor fibre dysfunction, creating a mismatch between ascending and descending neural signals that explains the characteristic clinical presentation. For practitioners, TMS offers potential diagnostic value in identifying both clinical and subclinical cases where motor pathway involvement may not yet manifest as overt lameness or knuckling, potentially enabling earlier intervention and more accurate prognostication during outbreak investigations.
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Practical Takeaways
- •TMS may help identify subclinical motor pathway involvement in AEP before clinical signs appear, potentially enabling earlier intervention in affected herds
- •Persistent motor neurological dysfunction detected by TMS can occur even when horses show clinical improvement in lameness or knuckling, suggesting ongoing nerve pathway impairment
- •AEP involves both sensory and motor nerve fibre dysfunction; horses with isolated clinical signs may have bilateral motor pathway involvement detectable only by TMS
Key Findings
- •All 14 neurologically affected horses showed increased motor latency time (LT) in pelvic limbs at +12.95 ms (+38% above normal) at first examination, improving to +8.1 ms (+24%) at 5-month follow-up
- •Eleven of 14 affected horses also presented delayed TMS responses in thoracic limbs with up to 14% LT increase, indicating bilateral motor pathway involvement
- •Clinical improvement occurred in 13 of 14 affected horses over 5 months, with four horses no longer showing knuckling despite persistent motor dysfunction on TMS
- •Neurologically sound horses (n=8) showed normal TMS latency values (mean ΔLT -0.5 ms pelvic, -0.35 ms thoracic), confirming TMS can differentiate subclinical from clinical cases