Trigeminal Nerve Root Demyelination Not Seen in Six Horses Diagnosed with Trigeminal-Mediated Headshaking

Abstract

Trigeminal-mediated headshaking is an idiopathic neuropathic facial pain syndrome in horses. There are clinical similarities to trigeminal neuralgia, a neuropathic facial pain syndrome in man, which is usually caused by demyelination of trigeminal sensory fibers within either the nerve root or, less commonly, the brainstem. Our hypothesis was that the neuropathological substrate of headshaking in horses is similar to that of trigeminal neuralgia in man. Trigeminal nerves, nerve roots, ganglia, infraorbital, and caudal nasal nerves from horse abattoir specimens and from horses euthanized due to trigeminal-mediated headshaking were removed, fixed, and processed for histological assessment by a veterinary pathologist and a neuropathologist with particular experience of trigeminal neuralgia histology. No histological differences were detected between samples from horses with headshaking and those from normal horses. These results suggest that trigeminal-mediated headshaking may have a different pathological substrate from trigeminal neuralgia in man.

Keywords: demyelination; headshaking; horse; idiopathic; nerve root; trigeminal-mediated.

Figure 1

Reconstructed computed tomography image to illustrate cuts made in specimen dissection.

Figure 2

Microscopic features of the trigeminal nerve roots and caudal nasal nerves, including portions of the pterygopalatine ganglion, of control and affected horses, showing normal axons and myelination within the nerves and normal ganglion cells and architecture within the ganglia. (A) Photomicrograph control horse normal trigeminal nerve root H&E stain. (B) Affected horse normal trigeminal nerve root H&E stain. (C) Photomicrograph control horse including a portion of normal caudal nasal nerve and pterygopalatine ganglion H&E stained. (D) Affected horse including a portion of normal caudal nasal nerve and pterygopalatine ganglion H&E stain.

Figure 3

Photomicrograph of semi-thin sections through a region of trigeminal nerve root compression in a human patient reveals a zone of demyelination within the proximal, CNS part of the nerve root, close to the junction with the PNS. Several thinly myelinated fibers are present within the zone of demyelination. Toluidine blue. Reproduced with permission from Ref. (14).