Determination of the early age of onset of equine recurrent laryngeal neuropathy. 2. Nerve pathology.

The left and right recurrent laryngeal nerves and peroneal nerves from two groups of foals, one less than 1 month of age and the other 6 months of age, were examined by light and electron microscopy. While there was no evidence of fiber loss on light microscopy, occasional onion bulbs, regenerating clusters and swollen axons were seen in the recurrent laryngeal nerves. To quantitate these changes, total counts of the main pathological structures were performed using the electron microscope. In all the foals the most common abnormality seen in the laryngeal nerves were Bungner bands, which were most numerous in the left recurrent laryngeal nerve of the older group of foals. Swollen axons which contained accumulations of organelles were seen in both age groups but were more common in the older foals. Likewise, onion bulbs were more frequent in the older animals but were also seen as well-formed structures in a foal less than 2 weeks of age. These findings confirm the previous report of neurogenic changes in the laryngeal muscles of foals and suggest that recurrent laryngeal neuropathy has an early and possible prenatal onset in the horse.