In the January and March issues of Veterinary Medicine, we discussed several local and regional anesthetic techniques to help in the analgesic management of patients. This article discusses techniques associated with oral surgery—blocking the infraorbital and maxillary nerves in the maxilla and the mental and mandibular (inferior alveolar) nerves in the mandible. Appropriately applied nerve blocks can reduce nociceptive input, thereby facilitating preemptive and multimodal analgesia and potentially reducing a patient's inhalant anesthetic and postoperative analgesic requirements. Look for the final part of this series, which will cover epidural anesthesia and analgesia, later this year.
INDICATIONS AND COMPLICATIONS
The trigeminal nerve and ganglion carry nociceptive input from the head to the brainstem. The trigeminal nerve has three branches: ophthalmic, maxillary, and mandibular. Infraorbital, maxillary, mental, and mandibular (inferior alveolar) nerve blocks provide anesthesia to the upper or lower jaw and are commonly performed in practice for dental extractions. These blocks are also used for any surgical procedure of the upper or lower jaw (Table 1).
These blocks can be difficult to perform in obese and brachycephalic animals because the landmarks are more challenging to palpate. Possible complications of infraorbital, maxillary, mental, and mandibular (inferior alveolar) blocks include trauma to the nerve, resulting in neurapraxia, and inadvertent intravenous or intra-arterial injection of local anesthetic; retrobulbar hemorrhage could occur with the infraorbital block. As for all local and regional anesthesia techniques, aspiration before injection is imperative to avoid intravenous or intra-arterial injection.
Table 1: Indications and Drug Dosages for Infraorbital, Maxillary, Mental, and Mandibular (Inferior Alveolar) Nerve Blocks in Dogs and Cats. Note: In the Dosages column, the drug doses are the total maximum dose, to be divided among all injection sites.
DRUGS AND PREPARATION
The local anesthetics used most commonly in these blocks are 2% lidocaine, 0.5% bupivacaine, or a combination of both (Table 1). Onset of anesthesia with 2% lidocaine occurs in five to 10 minutes and lasts for about one to one-and-a-half hours. The duration of anesthesia can be extended up to two hours by adding epinephrine to the lidocaine (0.1 ml of 1:1,000 [0.1 mg] epinephrine per 20 ml of local anesthetic). Onset of anesthesia with bupivacaine occurs in 20 to 30 minutes and lasts for about four to six hours. The combination of lidocaine and bupivacaine results in a more rapid onset but may reduce the duration of anesthesia. Adding an opioid increases the efficacy and prolongs the duration of analgesia with the blocks.
With these blocks, the needle is usually inserted percutaneously, but the infraorbital and mental blocks may be performed through the oral mucosa, depending on patient anatomy. Sterile preparation of the area is not usually performed, but a sterile needle and syringe must be used.
INFRAORBITAL AND MAXILLARY NERVE BLOCKS
Blocking the infraorbital nerve, the continuation of the maxillary nerve, as it exits the infraorbital foramen (Figure 1, A) anesthetizes the upper lip, nose, roof of the nasal cavity, and skin as far caudal as the infraorbital foramen. The maxillary incisors are inconsistently blocked with this technique, particularly in dogs.1-3
1. The position of the needles to block the maxillary nerve in dogs and cats. Local anesthetic administration at the infraorbital foramen (A) will provide anesthesia rostral to the foramen. Local anesthetic administration where the maxillary nerve courses perpendicular to the palatine bone, between the maxillary foramen and the foramen rotundum (B), will provide anesthesia to the entire upper jaw, including the teeth, on one side.
This block should be performed cautiously in brachycephalic dogs and cats (e.g. Himalayans, Persians) because of the proximity of the orbit to the foramen and the potential for penetrating the globe. Retrobulbar hemorrhage leading to proptosis is a potential complication of this technique.
To desensitize the infraorbital branch of the maxillary nerve at its point of emergence from the infraorbital foramen, insert a 25- to 29-ga needle, either percutaneously or through the buccal mucosa, into the foramen, which is usually found dorsal to the third premolar, and advance it 1 to 2 mm. Elevate the head, aspirate before injection, and apply digital pressure over the foramen as you slowly inject the local anesthetic to facilitate its movement caudally into the foramen, causing more effective nerve blockade. Advancing the needle farther into the foramen is not recommended because this increases the chance of lacerating the nerve.
To anesthetize the entire upper jaw, including all of the teeth, block the maxillary nerve where it courses perpendicular to the palatine bone, between the maxillary foramen and the foramen rotundum (Figure 1, B).2,3 Insert the needle through the skin at a 90-degree angle, in a medial direction, ventral to the border of the zygomatic arch and about 0.5 cm caudal to the lateral canthus, and then advance it toward the pterygopalatine fossa. Frequently, the needle will contact the ramus of the mandible; if you do so, walk it off the ramus cranially. Slowly inject the local anesthetic after test aspiration. This technique is more difficult to perform in cats.