Equine laryngeal hemiplegia. Part II. An electron microscopic study of peripheral nerves.

The research focused on the microscopic study of peripheral nerves in horses afflicted with laryngeal hemiplegia, a condition affecting horses’ voicebox. The study finds that this disease leads to a significant loss of myelinated fibres in the peripheral nerves, among other damaging effects, and suggests it can be classified as a distal axonopathy.

Methodology and Findings

• The research involved the examination of the recurrent laryngeal nerves in various horses, including four with subclinical and four with clinical laryngeal hemiplegia, and also in five control horses.
• The analysis was conducted using electron microscopy, a powerful tool for viewing extremely small structures like tissues and cells. This was complemented with light microscopy, which provided general imaging of the nerves.
• The myelinated fibres in the left recurrent laryngeal nerve in horses with laryngeal hemiplegia were found to have distally distributed loss. This was aligned with previous observations from light microscopy.
• Furthermore, the study discovered the presence of active axonal pathology, a nerve damage condition, which was more prevalent than initial observations from light microscopic studies suggested.
• The research also noted onion bulb formations, indicating repetitive damage to nerve fibres, a characteristic of certain pathological conditions.

Further Observations and Conclusions

• Pathological changes were not only localized. They were evident within the distal left recurrent laryngeal nerve, in other parts of the left and right nerves, and also in the distal hindlimb nerves.
• The study confirmed the presence of fibres with unusually thick myelin sheaths compared to their axonal size. This confirmation came from statistical comparison of the axis cylinder perimeter against the number of myelin lamellae.
• The peripheral nerve pathology in equine laryngeal hemiplegia appears to be characterized by the distal loss of myelinated fibres, significant active axonal damage, and axonal atrophy.
• Given these findings, the research concludes that equine laryngeal hemiplegia could potentially be classified as a ‘distal axonopathy’, a type of disease that primarily affects the peripheral regions of axons, the long threadlike part of a nerve cell.