Equine laryngeal hemiplegia. Part IV. Muscle pathology.

This study confirmed that neurogenic muscle pathology exists in intrinsic laryngeal muscles supplied by the recurrent laryngeal nerves in horses subclinically and clinically affected with laryngeal hemiplegia. An important additional observation was the occurrence in three out of four laryngeal hemiplegic horses of neurogenic muscle changes in a hindlimb muscle, the extensor digitorum longus, a muscle supplied by another long peripheral nerve. This finding suggests that a polynenropathy exists in laryngeal hemiplegic horses, and supports the classification of this disease as a distal axonopathy. Comparison of the degree of pathology in the intrinsic laryngeal muscles and that of the recurrent laryngeal nerves innervating them, demonstrated a strong correlation between the extent of damage in the distal left recurrent laryngeal nerve and the overall degree of muscle pathology. The muscle damage in clinically affected horses is a reflection of the nerve damage present in the most distal portion of the recurrent laryngeal nerve. The more variable pathological changes found in proximal levels of the left and right recurrent laryngeal nerves probably reflects the ongoing nature of the pathological process affecting nerve fibres. The existence of a subclinically affected group of horses, the earliest involvement of an adductor, the left cricoarytenoideus lateralis muscle, and the presence of changes in the right intrinsic laryngeal muscles all confirmed the findings of previous workers.