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.

Maxillary nerve block

The purpose of this nerve block is to provide anesthesia of the ipsilateral upper dental arcade (cheek teeth through to the first incisor) and maxillary soft tissues. Analgesia of the paranasal sinuses on the blocked side can also be anticipated. The nerve is blocked as it traverses the pterygopalatine fossa and just before it enters the maxillary foramen, which is located axial (deep) and ventral to the globe. The nerve is large and closely related to accompanying vessels. At least three different approaches to this nerve are possible and personal preference usually dictates which one is used. A 20 or 22 g, 3.5 or 5 inch spinal needle is used for all approaches. Approach 1: The needle is inserted from a dorsal to ventral direction at the palpable inside edge of the external frontal crest (temporal line of frontal bone). The needle is angled axially to "walk" down the vertical face of the frontal and palatine bones keeping in close approximation with their surfaces until the needle is inserted about 3.5 inches (Figure 1). With a 3.5 inch needle it is very unlikely the maxillary nerve will be penetrated from this approach. Approach 2: The needle is inserted just below the zygomatic arch (avoid the transverse facial vessels) and 3 cm caudal to the lateral canthus of the eye (Figure 2A) and directed towards the midpoint of the contralateral facial crest in a slightly rostroventral plane (Figure 2B). If bone is felt after 1-2 inches of insertion the rostral edge of the vertical ramus of the mandible has been contacted and redirection more rostrally is required. If the nerve is not contacted (head jerk reaction) the needle may be passed to reach the palatine bone (about 3-3.5 inches deep in the average sized horse) and then it is withdrawn a few millimeters before injection. Approach 3: A perpendicular path from the lateral aspect of the head can be followed when the needle is inserted just below the temporal process of the zygomatic bone at the level of the caudal third of the eye (Figure 3). The needle is advanced until it contacts the nerve (head jerk reaction) or the palatine bone, and then it is withdrawn a few millimeters before injection. A modification of approach 3 recommends inserting the needle in a slightly ventral direction from the perpendicular plane only to a depth of 45-50 mm (1.75-2 inches). This places the needle tip in the extraperiorbital fat tissue and presumably allows for effective diffusion of local anesthetic to the target nerve with much less risk of puncturing a local vessel or puncturing the maxillary nerve. For the maxillary nerve block 10-20 ml of local anesthetic is slowly infused once the needle is inserted to the right locale. Allow 15-20 minutes for the block to take full effect. Infrequently, a vessel may be punctured (maxillary artery or its branches, and the deep facial vein) with the deep insertion techniques, and this can lead to a noticeable swelling caudal to the dorsal bony orbit, bulging of the globe, and swelling of the lateral aspect of the masseter just below the zygomatic arch. This swelling is typically self-limiting and resolves over 2-3 days.

Figure 1: Approach 1 to the maxillary n.

Inferior Alveolar Nerve Block at the Mandibular Foramen

The purpose of this nerve block is to provide anesthesia of the ipsilateral lower dental arcade (cheek teeth through to the first incisor) and mandibular soft tissues. The mandibular foramen is located on the medial, slightly concave surface of the mandible at the intersection of the line drawn along the buccal occlusal edge of the upper cheek teeth arcade and a perpendicular line passing through the lateral canthus of the eye (Figure 4A). The foramen is situated about 3.5 inches rostral to the back edge of the vertical ramus of the mandible, on the line along the buccal edge of the upper cheek teeth arcade. A 20 gauge 8 inch spinal needle is inserted on the medial side of the most caudal aspect of the horizontal ramus of the mandible and directed slowly, slightly dorsolaterally towards the location of the mandibular foramen (ventral approach, Figure 4A and 4B). The needle should be inserted between the facial neurovascular bundle and parotid salivary duct, and the medial surface of the mandible. The approximate depth of insertion should be premeasured on the outside of the mandible (typically about 5 inches). Bone is occasionally felt and the needle is withdrawn a few millimeters before injection if it is at the correct depth. Twenty millimeters of local anesthetic is deposited and 15-20 minutes is required for effective onset of analgesia. If the needle is not passed in close proximity to the concave medial aspect of the mandible (by angling it slightly dorsolaterally) it is possible for the needle to enter the caudal oral cavity and local anesthetic will flow out of the mouth when it is injected. Alternatively a 3.5 inch 20 or 22 gauge spinal needle may be inserted from the caudal medial aspect of the vertical ramus of the mandible (caudal approach, Figure 4A and 4B) and directed towards the pre-determined location of the foramen.

Figure 2A: Approach 2 to the maxillary n.

Infraorbital Nerve

Blockade of the infraorbital nerve alveolar branches (one or both sides as indicated) allows interdental wiring techniques for correction of maxillary incisor avulsion injuries to be performed in the standing horse. Also restoration procedures and extractions can be performed on incisor or canine teeth, and cheek teeth, if the local anesthetic has reached caudally enough. Rostral maxillary soft tissues can be desensitized by this block, e.g. for laceration repairs. The infraorbital foramen is palpable dorsorostral to the rostral end of the facial crest and in line with the medial canthus of the eye (Figure 5). The caudal lip of the foramen and the exiting nerve can be palpated when firm digital pressure is applied after gently displacing the levator labii superioris muscle dorsally. Using a 22 gauge 1.5 inch needle, 5 mls of local anesthetic can be deposited at the foramen to effectively reduce sensitivity of soft tissues rostral to that point (i.e. upper lip, nares). Recall that alveolar branches to the cheek teeth and canine and incisor teeth depart the nerve before it exits the foramen. Therefore effective anesthesia of these teeth requires the insertion of a needle into the infraorbital canal and depositing 5-10 ml of local anesthetic in the canal to provide analgesia of successive teeth the further caudal the local anesthetic diffuses. A slightly less invasive method is to insert a needle just within the canal and then while applying digital pressure at the foramen to seal the exit, local anesthetic can be slowly injected with the aim of forcing the solution to travel caudally along the canal. Horses are particularly prone to sudden head rearing reaction when performing the infraorbital nerve block because it is very likely that the nerve will be directly stimulated by the needle. Placing a bleb of local anesthetic over the nerve at the level of the foramen and allowing it 10 minutes to take effect may facilitate deeper insertion of a needle into the canal for more caudal anesthesia. Fifteen to 20 minutes should be allowed for this block to take full effect.

Figure 2B: Approach 2 to the maxillary n.

Mental Nerve

Blockade of the mental nerve(s) before it exits from the mental foramen, allows interdental wiring techniques for correction of mandibular incisor avulsion injuries to be performed in the standing horse, and restoration and extraction procedures of incisor or canine teeth. The rostral mandibular soft tissues are also desensitized by this block. The mental foramen is located on the lateral aspect of the mandible about 3-4 cm rostral to the mesial (rostral) surface of the first cheek tooth (#306 or #406) (Figure 5). The foramen is palpated after displacing the tendinous insertion of the depressor labii inferioris muscle ventrally. The mental nerve may be blocked as it exits the foramen and this will provide anesthesia of the lower lip and adjacent soft tissues. A 22 gauge, 1 or 1.5 inch needle, is inserted towards the foramen from a rostral to caudal direction and 3-5 mls of local anesthetic is deposited at the foramen. Threading a needle into the foramen is required to affect anesthesia of the canine or incisor teeth on that side and this can be very difficult as the canal is relatively narrow and access is limited by anatomical curvatures in the mandible. Alternatively, inserting a needle just inside the foramen and using digital pressure to seal the canal exit and force 3-5 mls of local anesthetic caudally will improve the chances of an effective block. Again, horses will react suddenly when performing this block as the nerve is typically directly stimulated. About 10-15 minutes should be allowed for this block to take full effect.

Figure 3: Approach 3 to the maxillary n.

Supraorbital (Frontal) Nerve

The nerve is blocked to reduce sensory innervation of the medial two thirds of the upper eyelid. Approximately 1-2 ml of local anesthetic is deposited subcutaneously over the palpable opening of the supraorbital foramen, located 1 cm or so caudomedial to the dorsal rim of the orbit in the zygomatic process of the frontal bone (Figure 5). This nerve block also affects branches of the palpebral nerve, thus helping to decrease upper eyelid motor activity.

Figure 4A: Determining location of mandibular foramen and outlining approaches to nerve

Palpebral Nerve

The nerve is blocked to minimize upper eyelid movement. 2 ml of local anesthetic is injected subcutaneously over the usually palpable palpebral nerve as it passes over the dorsal aspect of the zygomatic arch.

Figure 4B: Medial aspect of mandible showing foramen and needle approaches.

Retrobulbar Nerve Block

This block causes anesthesia of the eye (optic, trigeminal and oculomotor nerves), so ocular sensation, blink reflex and vision are all diminished for the duration of the block. It is a useful block for enucleation as a complement to general anesthesia, and also for standing surgical procedures. After the procedure, if the eye was not removed, stall rest and ocular lubricants are recommended for 2-4 hours, while eye sensation and vision return to normal. The orbital fossa caudal to the dorsal orbital rim is clipped and prepped aseptically. A 22 gauge 3.5 inch needle is inserted perpendicular to the skull through the skin just caudal to the bony dorsal orbital rim and advanced slowly. As the needle passes through the dorsal retrobulbar cone fascia the eye will have a slight dorsal movement. The needle is now in the retrobulbar space and 10-12 mls of local anesthetic is gently injected. The globe will be noted to protrude rostrally (slight exophthalmos) indicating an accurate deposition of local anesthetic. Five to ten minutes should be allowed for the block to take full effect.

Figure 5. Location of foramina for nerve blocks.

Ear Densitization

Standing procedures of the ear (e.g. freezing tumors, laser surgery, and laceration repairs) can be performed after instillation of local anesthetic (approximately 20 mls) in a ring block around the base of the external ear using a 22 gauge needle. More precise blocking of the ear can be achieved by infusing 2-3 mls of local anesthetic over the internal auricular nerve and the great auricular nerve, using 22-25 g, 1 inch needles. The internal auricular nerve is blocked at a palpable notch on the lateral aspect of the base of the auricular cartilage. The great auricular nerve is palpable running in a rostrocaudal direction on the caudal aspect of the base of the pinna and is blocked subcutaneously at this site. Allow 5 minutes for the blocks to take effect.

Local Infiltration Techniques

Temporary incisors and premolars, and the first premolar (wolf tooth) can be partially desensitized by injecting 1 ml of local anesthetic submucosally adjacent to the tooth using a 25 gauge 5/8 inch or 22 gauge, 1 inch needle. For temporary incisors a bleb on the labial surface just beyond the mucogingival junction is often appropriate. The palatal or lingual side of the tooth can also be injected submucosally for additional analgesia if needed. Additional rostral anesthesia for the maxillary incisors may be obtained by infusing local anesthetic into the incisive canal found at the junction of the lip and gingival mucosa under the upper lip on midline. Wolf teeth are similarly desensitized by a 1 ml bleb in the buccal and palatal submucosa adjacent the tooth. The temporary premolars are more difficult to access and require local infiltration on both the buccal and palatal or lingual sides. Attaching a short IV extension line to the end of the needle allows more practical injection of local anesthetic in caudal intraoral locations. At least 5 minutes should be allowed for the anesthetic to take full effect. Infusion of local anesthetic subcutaneously is also performed for procedures like sinus trephination or sinusotomy procedures, laceration repairs, mass removals or biopsies. Manipulations within the paranasal sinuses may be better tolerated after instilling 50-60 ml of local anesthetic into the sinus space.

References

Fletcher BW. How to perform effective equine dental nerve blocks. AAEP Proceedings 2004;50:233-236.

Staszyk C, Bienert A, Bäumer W, Feige K, Gasse H. Simulation of local anaesthetic nerve block of the infraorbital nerve within the pterygopalatine fossa: anatomical landmarks defined by computed tomography. Res Vet Sci, 2008;85:399-406.

Gilger BC, Davidson MG. How to prepare for ocular surgery in the standing horse. AAEP Proceedings 2002;48:266-271.

Klugh DO. Infiltration anesthesia in equine dentistry. Compend Cont Educ Pract Vet 2004;26:625-628.

McCoy AM, Schaefer EC, Malone E. How to perform effective blocks of the equine ear. AAEP Proceedings 2007;53:397-398.