Electrically elicited blink reflex in horses with trigeminal and facial nerve blocks

To reassess reference values for the components of the electrically induced blink reflex, document reference values for facial motor nerve conduction velocity, and demonstrate usefulness of the blink reflex as a diagnostic tool in peripheral facial and trigeminal nerve dysfunction in horses.

10 healthy adult horses (8 males, 2 females) without neurologic abnormalities.

Blink reflex tests were performed by electrical stimulation of the supraorbital nerve and facial (auriculopalpebral) nerve. Reflex and direct muscle-evoked potentials of the orbicularis oculi muscles were recorded from concentric needle electrodes inserted bilaterally in these muscles. Supraorbital and auriculopalpebral nerve blocks were performed by lidocaine hydrochloride injections.

Supraorbital nerve stimulation elicited 2 or 3 ipsilateral and 1 contralateral reflex muscle potential in the orbicularis oculi muscles. Auriculopalpebral nerve stimulation elicited a direct and a reflex potential in the ipsilateral orbicularis oculi muscle. After left supraorbital nerve block, no responses could be elicited ipsilaterally or contralaterally upon stimulation of the blocked nerve, but bilateral responses were obtained upon stimulation of the right supraorbital nerve. After right auriculopalpebral nerve block, no responses were recorded from the right orbicularis oculi muscle upon stimulation of left or right supraorbital nerves.

Reference values for the components of the blink reflex and facial motor nerve conduction velocity will allow application of these tests to assist in the diagnosis of equine neurologic disorders involving the trigeminal and facial nerves, the brainstem, and the cranial end of the cervical segment of the spinal cord. This study reveals the usefulness of the blink reflex test in the diagnosis of peripheral trigeminal and facial nerve dysfunction in horses.