The role of quantitative electromyography (EMG) in horses suspected of acute and chronic grass sickness.
Authors: Wijnberg I D, Franssen H, Jansen G H, van den Ingh Th S G A M, van der Harst M R, van der Kolk J H
Journal: Equine veterinary journal
Summary
# Editorial Summary: Quantitative EMG in Equine Grass Sickness Grass sickness has long presented a diagnostic challenge due to its variable clinical presentation and suspected autonomic involvement, yet direct electrophysiological evidence of motor neuron pathology has been limited. Wijnberg and colleagues used quantitative electromyography to characterise lower motor neuron involvement in 14 horses with acute, subacute or chronic grass sickness, comparing their findings against established reference data for healthy horses and those with confirmed lower motor neuron disease. Electrophysiological neuropathy was evident in 12 of the 14 horses, with characteristic changes including prolonged motor unit action potential duration (ranging from 11.0–20.3 msecs depending on muscle site), increased phases and turns, and pathological spontaneous activity in all affected animals; post-mortem examination confirmed autonomic ganglia degeneration alongside minor spinal motor neuron changes in all eight necropsied cases, with neurogenic muscle atrophy evident in four of seven biopsied muscles. For practitioners managing horses with unexplained weight loss, colic, or suspected neurogenic disease, quantitative EMG provides an objective ante-mortem diagnostic tool that strengthens differential diagnosis and may help distinguish grass sickness from other neuropathic conditions, though biopsy and autopsy findings suggest the primary pathology remains autonomic rather than purely somatic motor neuron disease.
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Practical Takeaways
- •Quantitative EMG can provide ante mortem clinical and electrophysiological evidence of neuropathy in horses with suspected EGS, aiding definitive diagnosis before necropsy.
- •The combination of EMG findings (pathological spontaneous activity and characteristic MUP parameters) with clinical signs of weight loss and colic can help differentiate neurogenic disease in equine practice.
- •EGS involves both autonomic ganglia and lower motor neuron degeneration; EMG detects the motor neuron component that was previously not recognized clinically.
Key Findings
- •Quantitative EMG demonstrated neuropathy in 12 of 14 horses (86%) clinically suspected of EGS, with specific MUP duration ranges of 11.0-13.7 msec (subclavian), 14.8-20.3 msec (triceps), and 12.2-17.2 msec (lateral vastus).
- •Pathological spontaneous activity was observed in all 14 horses examined, indicating motor neuron involvement.
- •Post mortem examination of 8 horses confirmed degenerative changes in autonomic ganglia combined with minor spinal lower motor neuron degeneration in all cases.
- •Muscle biopsies revealed neurogenic atrophy consistent with lower motor neuron disease in 4 of 7 horses (57%).