Retrospective case-control study based on magnetic resonance imaging reveals asymmetry of the trigeminal nerve in horses with headshaking

Introduction: Idiopathic trigeminal-mediated headshaking (ITMHS) in horses may be associated with trigeminal nerve (TN) neuropathy. In human medicine, TN asymmetry was revealed by magnetic resonance imaging (MRI) correlating with nerve atrophy.

Methods: MRI examinations of 20 adult horses diagnosed with ITMHS were retrospectively compared to six horses that underwent MRI of the head for other reasons. All horses presented to the equine hospital of the University of Veterinary Medicine Hannover, Foundation in 2021 and 2022. MRI studies were obtained with a 3 Tesla MRI scanner (Philips Achieva 3T dStream). Standardized cross-sectional area measurements of branches of the TN were performed bilaterally at 4 defined locations (1: rostral to the trigeminal ganglion, 2: close to the pituitary gland, 3: close to the optic chiasm, 4: rostral to the foramen rotundum). Bilateral cross-sectional transverse area differences (TN asymmetry) were calculated and compared between the two groups using a generalized additive model of the Tweedie family.

Results: Horses with ITMHS had 2 to 8.7 times higher TN asymmetry than controls (F[1] = 22.093, P < .001). The difference in TN asymmetry was most pronounced rostral to the trigeminal ganglion—area 1 (back-transformed estimated marginal mean ratio and SE: 8.66 ± 3.975, t[212] = 4.700, P < .0001). There was no effect of age of the horse or duration of ITMHS on the TN asymmetry.

Discussion and Clinical Relevance: The asymmetry of the TN in horses with ITMHS indicates a unilateral enhanced neuropathy similar to findings in humans. Few previous studies provided evidence of atrophy or dysfunction of the TN in horses diagnosed with ITMHS, moreover, this MRI study revealed morphological changes of the TN in horse and provides directions for further studies and diagnostic approaches.