Atrophic myopathy secondary to onion bulb-forming lesions in a young horse: histopathological and immunohistochemical findings.

A sixth-month-old male Trotter presented a neurologic disorder from birth. Progressive tetraparesis of the hind limbs and severe bilateral muscle wasting were the main complaints. Prolonged recumbency complicated by abrupt onset of pneumonia required the euthanasia. The most significant findings revealed severe streptococcal necrotizing pneumonia, severe atrophy of selected muscular groups (adducent, semimembranous, semintendinous and gastrocnemius), and moderate thickening of sacral dorsal and ventral spinal cord rootlets. H&E and special stains (Masson’s trichromic, Periodic Acid Schiff, Gomori silver stain, Acid Phosphotungstic Mallory Haematoxylin, Van Gieson, Alizarin S, Luxol Fast Blue) were performed. Glial Fibrillary Acidic Protein (GFAP) and S-100 protein antibodies for immunohistochemistry were applied. Ultrastructural examination was also performed on sciatic nerve and skeletal muscles. In multiple spinal ganglia and spinal roots and peripheral nerves, focal cellular proliferations composed of fusiform cells forming compact onion bulbs were the main findings; some of them were mineralised and some contained collagen. These were concentric structures formed by Schwann or perineurial cells which contained a central axon, that replaced the nervous tissue and resulted in denervation atrophy of skeletal muscles. In the sciatic nerve the onion bulbs stained GFAP and S-100 protein. This case shows similarities with a case report of focal trigeminal hypertrophic neuropathy in a horse and with a group of diseases known as hypertrophic motor and sensory neuropathy in human beings.