Equine laryngeal hemiplegia. Part III. A teased fibre study of peripheral nerves.

Individual nerve fibres were isolated from the recurrent laryngeal and some distal hindlimb nerves, in an investigation of equine laryngeal hemiplegia. One hundred teased fibres were obtained from each of three sampling sites on both left and right recurrent laryngeal nerves, from 15 Thoroughbred horses. These fibres were graded descriptively and internode lengths measured. A distal distribution of pathology was demonstrated in all groups studied, but was most severe in the clinical group of horses. The predominant change was one of short thinly myelinated internodes interspersed amongst normally myelinated internodes, indicating remyelination of previously demyelinated areas of nerve fibre. Such pathological change was also reflected by the decreased mean internode length, and its increased variability associated with disease. However, it was determined statistically that these abnormal internodes were grouped along particular nerve fibres, rather than being randomly distributed between all nerve fibres. This is thought to indicate myelin sheath changes secondary to underlying axonal pathology. Thus it was concluded that the primary pathology was likely to be axonal in nature, while the high incidence of demyelination changes was a reflection of the chronic nature of the disease process. Thus, the distal distribution of pathology, the primary axonal involvement, the presence of changes in left and right recurrent laryngeal and distal limb nerves, all support the classification of equine laryngeal hemiplegia as a distal axonopathy.