title: The fuctional anatomy of the trigeminal nerve of the horse
Creators: name: Anne Newton, Stacey
Publisher: name: University of Liverpool Repository
Year: 2023

description: It was hypothesised that the aetiopathology of the condition of headshaking involves abnormalities of either the structure, function or behaviour of the trigeminal nerve. Specifically it is believed that the second division of the trigeminal nerve, represented by the caudal nasal nerve (CNN) is involved. In addition it is hypothesised that such cases are equine analogues of the condition of trigeminal neuralgia (TgN) in humans. To answer these hypotheses the following work was undertaken: 1. Studies of a series of clinical cases of headshakers to assess similarities/dissimilaritie swith TgN. 2. Studies of the pharmacokinetics of carbarnazepine (CBZ) in headshakers. 3. Isolation and purification of equine P2 myelin protein and its use in investigating the serum levels of P2 myelin protein antibody in the normal population, cauda equina neuritis (CEN) and headshakers. Serum antibodies to P2 myelin protein have been identified in demyelinating diseases in humans - Guillain-Barré Syndrome (GBS), CEN and headshakers. 4. Studies of the macroscopic and microscopic anatomy of the trigeminal nerve in normal controls and headshakers. 5. Studies of the immunocytochernistry of the trigeminal ganglia, brainstem and root areas of normal controls and headshakers. 6. Magnetic Resonance Imaging (MRI) studies of the equine head. 7. Studies investigating total counts and sizes of nerve fibres of the trigeminal nerve. 8. Equivalent comparative studies of the physiology of the trigeminal nerve. The results of the clinical study support the hypothesis that headshaking is similar to TgN since both syndromes show evidence of seasonality and unilateral presentation of clinical signs, the presence of a trigger zone and a positive response to CBZ therapy in several cases. In headshakers, the trigger zone appears to be in the nasal cavity and the positive response to CNN anaesthesia supports the hypothesis that this branch of the trigeminal nerve is affected. CBZ therapy is effective in some cases of headshaking but pharmacokinetic studies demonstrate that the non-response in some cases is likely due to under dosing and problems of interference with absorption. Isolation of P2 myelin protein was successful but P2 myelin protein antibodies were not identified in CEN and headshaking cases. This suggests that there is no evidence that the aetiopathogenesis of headshaking involves an immune-mediated mechanism elicited by antibodies to P2 myelin protein. No gross or microscopic abnormalities were identified in the headshakers. Some differences were observed between normal controls and headshakers in total nerve counts and the identification of neuropeptides in the trigeminal system. However the significance of these findings is unknown due to the small numbers of cases involved. No vascular compression was identified at the trigeminal root entry zone (REZ) as reported in TgN cases. MRI techniques are of no use in post mortem specimens of the equine head for examination of the neurovascular relationships of the trigeminal nerve. More research on the use of intravascular contrast in post mortem head specimens is required, though in preference, the use of anglography with MRI in the anaesthetised animal, would likely be much more successful. Initial studies of the neurophysiology of the trigeminal nerve have demonstrated that the technique of evoked CNAPs applied to human TgN cases can be applied successfully to the horse. The equine trigeminal nerve has a very fast conduction velocity, faster than that reported in previous studies of other species. This work fails to conclusively support the hypothesis that headshaking is similar to TgN since no pathology or vascular compression at the REZ was identified. However, the clinical presentation and response to CBZ are similar in both syndromes. Further investigation of trigeminal nerve CNAPs in the horse appears to be the most promising method to investigate any alteration in function in headshaker cases as reported in TgN cases.