During the years 1971-1979, 127 horses with left laryngeal hemiplegia were studied. The physical characteristics and clinical signs observed in this study were recorded in Part I and Part II of ths series of papers. Of these 127 horses, 81 were treated by the laryngoplasty procedure. Complications of surgery are described and the effects of the operation on respiratory noise and performance are evaluated. In 54.8% of horses the chronic respiratory noise observed during exercise was apparently diminished or eliminated post-operatively. The performance of 44% of horses was apparently improved after surgery. Post-operative racing success occurred in 38% of horses treated. Satisfactory arytenoid adduction as assessed endoscopically within 9 days of surgery was achieved in 77% of cases. Surgical failure appeared to be related to cutting of the laryngeal cartilages by the prosthesis and techniques to minimise this are discussed.

This light microscopic investigation of 15 Thoroughbred horses provided substantial evidence for the classification of equine laryngeal hemiplegia as a distal axonopathy. Morphologic and morphometric examinations were performed on resin embedded recurrent laryngeal nerves from control, subclinical and clinical laryngeal hemiplegic animals. In the latter group of animals some distal hindlimb nerves were also examined. A distally graded loss of myelinated fibres selectively affecting those of large diameter was demonstrated in both left and right recurrent laryngeal nerves. Morphologic evidence of similar pathological changes in long hindlimb nerves was also present. An explanation for the early involvement of the cricoarytenoideus lateralis muscle in the course of laryngeal hemiplegia, was offered by the demonstration of more large diameter fibres in the branch of the recurrent laryngeal nerve innervating it, thus making it more susceptible to the disease process.

A review of the literature on stringhalt in horses is presented, including the aetiology, clinical signs, pathology and treatment of this disease. Three New Zealand outbreaks of stringhalt are reported and discussed. The occurrence of these (and most previously reported outbreaks) in the late summer and autumn, and the association of the disease with various weeds, indicates a likely toxic aetiology.

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.

An endoscopic survey of young race horses was performed to examine the prevalence and character of laryngeal movements during quiet respiration. The main aim was to determine whether those arytenoid movements which could possibly reflect the efficiency of left dorsal cricoarytenoid muscle function changed over a period of time. Of the 452 horses examined, 439 were Thoroughbreds and 23 were Standardbreds, 250 were less than 2 years of age (6-21 months), and 202 were 2 years old. One hundred and nine of these horses were examined again 16 months later. Arytenoid movements were given one of four grades. Grades 1 and 2 were considered normal and unlikely to be the result of abnormal left dorsal cricoarytenoid muscle function, whilst grades 3 and 4 were considered likely, or almost certainly, the result of abnormal left dorsal cricoarytenoid muscle function. The percutaneous prominence of the muscular process of left and right arytenoid cartilages, endoscopic arytenoid movement on left and right sides, age, sex and breed was recorded. Chi squared analysis was used to determine the association between age, breed, sex and the other recorded variables, and the presence or absence of abnormal laryngeal movements. At the first examination, 48% of the horses had grade 1, 37% grade 2, 15% grade 3 and 0.2% grade 4 left laryngeal movements. Of the horses examined I6 months later, 52% had grade 1, 33% grade 2, 14% grade 3 and 1% grade 4 left laryngeal movements. Fifteen percent of horses with grade 1 and 9% with grade 2 initially were found to be grade 3 at the subsequent examination. Conversely, 53% of horses with grade 3 initially were found to be grade 1 and 21% grade 2 at the subsequent examination. One horse that was grade 3 at the initial examination was grade 4 at the subsequent examination. Overall, 43% of horses were graded the same, 29% were given a better grade and 28% were given a worse grade. Age and sex were not associated with abnormal left laryngeal movements. The presence of abnormal arytenoid movements was significantly less in Standardbreds, but significantly higher in those horses that had a more prominent muscular process of the left arytenoid cartilage. The number of grade 2 and 3 laryngeal movements recorded on the left side was significantly higher than the right. It was concluded that asymmetrical laryngeal movements are common in young race horses; at this age laryngeal movements may interchange between what is considered normal and abnormal; the proportion of young horses with normal or minor variations in their left arytenoid movements that develop more obvious degrees of asynchrony is low (12%); and the proportion of horses considered to have endoscopic evidence of deficient left abductor muscle function that eventually develop laryngeal hemiplegia is also low (5%).