Regional anesthesia of the equine head (Proceedings)
Regional or local anesthesia of the equine head greatly facilitates performing standing procedures that are anticipated to elicit pain in the patient. With effective local anesthesia, less systemic sedatives may be required for standing surgeries (e.g. dental extractions, laceration repairs, incisor avulsion repairs), patients under general anesthesia can be run at a lighter plane of anesthesia, and postoperative pain may be lessened if effective preemptive analgesia is in place. There appears to be a steady increase in the number of procedures being described performed in the standing, sedated horse, that classically were done under general anesthesia. Certain patients are too great a risk for general anesthesia. Additionally, costs are reduced and patient morbidity is reduced in general anesthesia is avoided, so a working knowledge of how to affect regional anesthesia of the equine head to do standing procedures is very useful.
Anatomy and general comments
The major sensory innervation of the head is the trigeminal nerve (CN V) which has three main branches: ophthalmic, maxillary, and mandibular. The maxillary nerve enters the maxillary foramen and continues in the infraorbital canal and beyond as the infraorbital nerve. Alveolar branches are distributed to all upper arcade teeth from within the canal. The mandibular nerve enters the mandibular foramen and becomes the inferior alveolar nerve, supplying branches to the lower arcade teeth and then emerging as the mental nerve at the mental foramen. The supraorbital nerve is the continuation of the frontal nerve as it exits the supraorbital foramen. The frontal nerve is a branch of the ophthalmic portion of CN V. The palpebral nerve is a branch of the facial nerve and provides motor innervation to the muscle of the upper eyelid. The internal auricular and great auricular nerves are branches of the facial and second cervical nerve respectively.All of the following described nerve blocks are more readily and accurately performed by referring to a skull for anatomical relationships before inserting the needle. The horse should be sedated before blocks are performed and additional restraint (e.g. twitch) may be necessary for some horses. Skin preparation is routine. Clipping of the hair is optional but a surgical scrub is recommended for all blocks. It is typical for the horse to jerk its head to varying degrees if a nerve is directly stimulated by needle penetration – the veterinarian should be wary of this occurring to avoid injury to personnel and the risk of shearing off a needle or causing other injuries to the horse. One common reason for not fully appreciating the benefits of doing nerve blocks is not giving the local anesthetic enough time to have its fullest effect. Accuracy of the nerve block (i.e. how far does the local anesthetic have to diffuse to reach the actual nerve) and the size of the nerve are factors that will impact onset of activity. The maxillary, mandibular and infraorbital nerves are relatively large and at least 15-20 minutes should be allowed and is needed for the anesthetic agent to work most effectively. Mepivacaine (Carbocaine) is favored for its quick onset of action, low tissue irritation, and 2-3 hour duration of activity. Complications related to performing the blocks are rare however the possibility of infection, nerve irritation (which could result in self mutilation of the face), and hematoma formation (see maxillary nerve block) exists.