Regional Anesthesia for the Dentistry and Oral Surgery Patient

For most canine and feline patients, dental cleanings and thorough evaluation of the oral cavity is recommended at least annually.¹ For these patients, general anesthesia is required for an accurate assessment of the health of the oral cavity and for a thorough performance of dental cleaning.^1,2 According to the 2013 American Animal Hospital Association Dental Care Guidelines for Dogs and Cats, general anesthesia with a secured airway is necessary for proper assessment and treatment of canine and feline patients.¹

However, general anesthesia in veterinary patients is not to be taken lightly. Clients have significant concerns and anxiety when thinking about their pets being placed under general anesthesia.

As practitioners, to help reduce the incidence of anesthetic-related complications, we should perform an accurate presurgical/preanesthetic assessment of each patient. A thorough physical examination, baseline screenings, and appropriate diagnostic testing to identify any underlying conditions will help us optimize the condition of the patient before the procedure.

In addition, we should consider adding regional anesthesia to the anesthetic protocol. Regional anesthesia decreases the patient’s dependence on general anesthesia, which benefits both the patient and the practitioner. This article emphasizes the benefits and describes the drugs and techniques involved in proper administration of regional anesthesia.

Assessing Pain

As veterinarians, we do not have the luxury of asking our patients if they are experiencing pain. However, the International Association for the Study of Pain states that the inability to communicate pain does not necessarily negate the possibility that pain is being experienced.³ Dogs and cats with dental disease experience discomfort that the client and general practitioner may not appreciate. According to the 2015 American Animal Hospital Association/American Association of Feline Practitioner Pain Management Guidelines for Dogs and Cats, an animal’s inability to self-report pain and discomfort leaves the assessment and recognition of pain with the veterinary professional.⁴ According to a 2014 survey of veterinary surgeons in the United Kingdom, among the top 3 perceived common causes of chronic pain in their patients was dental disease.⁵ This pain may not be appreciated by the practitioner or client until after the disease has been treated and the patient has resumed its prepain normal behavioral activities.⁴

What Is Pain?

As described by the International Association for the Study of Pain, pain is a sensory or emotional stimulus associated with actual or apparent tissue trauma that is perceived as unpleasant.⁶ The purpose of pain is to elicit a reaction from the body to prevent additional damage to the affected area. Response to a painful stimulus can range from hyperalgesia, an exaggerated response to a stimulus normally perceived as painful,⁷ to allodynia, a painful response to a stimulus that is not normally perceived as painful.⁷ When pain is incurred for an extended duration, central sensitization (windup) can occur. This response occurs when the associated neurons repeatedly fire at a lower than normal threshold⁸ and is commonly seen in patients suffering from chronic oral pain (e.g., severe periodontal disease).⁹

When tissue is injured, the painful response is recognized by the central nervous system through a process called nociception. Recognition and processing of the painful stimulus occur in 4 steps: transduction, transmission, modulation and perception.¹⁰

1. Transduction is the transformation of energy from a painful stimulus into nerve impulses by pain receptors.¹⁰ Transduction can be inhibited by multimodal pain-relieving methods (e.g., use of local anesthetics, nonsteroidal anti-inflammatory drugs, and opioids).¹¹
2. Transmission is the movement of nerve impulses to the spinal cord and then to the brain.¹⁰ This process can be inhibited by use of local anesthetics, opioids, and alpha-2 agonists.
3. Modulation is the transmission of the painful stimulus at the spinal cord to be transmitted to the brain as pain or to inhibit further transmission to the brain.¹⁰
4. Perception is the method by which impulses are recognized as pain.¹⁰ Perception can be inhibited by administering opioids, alpha-2 agonists, anesthetics, and inhalants.¹¹

Ideally, multimodal pain management techniques should be used to try to lessen the amount of general anesthesia that is needed for a given procedure. For dentistry and oral surgery, one of the most effective ways to help block a painful stimulus is to use regional anesthesia.

Regional Anesthesia

For the dentistry and oral surgery patient, regional anesthesia can eliminate transduction and transmission, thereby decreasing pain perception and central sensitization.¹¹ A study conducted in 2013 found that use of regional anesthesia reduced the minimum alveolar concentration of isoflurane needed without causing any adverse effects on the hemodynamic state of the patients.¹² Keeping patients in a lighter plane of anesthesia increases client satisfaction because the patients recover quickly from the effects of anesthesia and are therefore less likely to be discharged with a drug “hangover.” Regional anesthesia helps create a painless transition from general anesthesia to consciousness and continues to work after the procedure to reduce patient discomfort and allow oral pain medication to begin working.

Commonly Used Medications

Lidocaine

A short-acting medication that is commonly used for regional anesthesia is lidocaine. This medication is usually supplied as a 2% solution (20 mg/mL). It has a rapid onset of action (within 5 minutes)¹³ and a relatively short duration of action (60 to 120 minutes).¹⁴ If you want intraoperative pain relief only and would like to have the local anesthetic metabolized by the time the patient is awake, this drug would be the most appropriate choice for regional anesthesia. Cats are significantly more sensitive than dogs to this medication. Reported doses are 6 mg/kg for dogs and 3 mg/kg for cats.¹⁵ However, we prefer to use a maximum dose of 2 mg/kg in the dog and 1 mg/kg in the cat (TABLE 1). Note that the total dosage of this medication is additive. If lidocaine is being used as part of an induction agent or maintenance pain medication (e.g., constant rate infusion), the total dosage must not exceed 5 mg/kg in dogs.16 Our preference is to keep the dosage below 2 mg/kg in both canine and feline patients. A potential side effect of lidocaine administration is central nervous system excitation, which could result in convulsions.¹⁷

Bupivacaine

A longer-acting medication used for regional anesthesia is bupivacaine. This medication comes in different concentrations: 0.25% (2.5 mg/mL), 0.5% (5 mg/mL), and 0.75% (7.5 mg/mL). The onset of effect has been reported to be as short as 8 minutes and as long as 30 minutes, and the effect has been reported to last from 4 to 10 hours, depending on where it is placed. For cats, the toxic dose of this medication is greater than 2 mg/kg total dose.¹⁵ As with lidocaine, cats are also extremely sensitive to bupivacaine. The maximum dosage of this medication should never be exceeded. For dogs and cats, the recommended total dosage of bupivacaine is less than 2 mg/kg (Table 1).¹⁵ Unlike lidocaine, bupivacaine is not used in constant rate infusions because it is highly cardiotoxic. However, in dogs and cats, the total dosage for local blocks is additive. Complications include neurotoxic and cardiotoxic complications (e.g., tremors, seizures, coma, respiratory depression, profound cardiac depression, ventricular fibrillation, and asystole).¹⁸

When a regional anesthetic drug is injected into canine and feline patients, it is imperative that the administrator first aspirate the syringe to avoid inadvertent intravenous injection. We have never experienced any adverse side effects or complications when careful technique is used and recommended dosages are followed.

Addition of an Opioid

An adjunctive medication that can be added to bupivacaine to extend the action of the regional block is buprenorphine. We recommend that buprenorphine be added to the regional block at a dose of 15 mcg per patient. Studies have shown that when bupivacaine was combined with an opioid such as buprenorphine, it increased the duration of action of the local anesthetic agent by threefold.^19,20 In a study evaluating the effect of adding buprenorphine to bupivacaine in an infraorbital nerve block and its effects on the minimum alveolar concentration of isoflurane, the anesthetic requirement for patients that received the combination was less than that for patients that received bupivacaine alone.²¹ The same study also provided support for prolonged regional anesthetic effects.²¹ The authors speculated that the addition of buprenorphine to a regional anesthetic block may extend the duration of the block to 48 to 96 hours after administration instead of 4 to 10 hours without buprenorphine.²¹

Regional Anesthesia Technique

Regional anesthesia is safe for the patient as long as the administrator has a grasp of the anatomy surrounding the region to be injected and uses safe technique. Because regional anesthesia is administered when the patient is under general anesthesia, it can be invasive if careful technique is not used. Another consideration is whether regional anesthesia should be used when biopsy samples are being collected or a resection is being performed in an area that may contain neoplastic cells. Placement of regional anesthesia in an area containing neoplastic cells could push those cells away from the original tumor margins; therefore, caution should be exercised when performing regional blocks in an area that may be neoplastic.

Anatomy Considerations

Anatomic landmarks for the most commonly used regional anesthesia techniques are the infraorbital foramen, mandibular foramen, angular process, and the facial vascular notch (FIGURE 1). Innervation to the maxilla, and hence the nerve(s) that innervate the dental structures and surrounding soft tissues, is supplied by the infraorbital nerve, which is a branch of the maxillary nerve and its associated branches. Innervation to the mandible is supplied by the inferior alveolar branch of the mandibular nerve.

FIGURE 1. (A) Canine skull showing infraorbital foramen (arrowhead), angular process (star), facial vascular notch (arrow). (B) Feline skull showing infraorbital foramen (arrowhead) and angular process (star). (C) Canine skull showing facial vascular notch (arrow). (D) Feline skull showing facial vascular notch (arrow). (E) Feline mandible, lingual aspect, showing mandibular foramen (arrowhead) and angular process (star). (F) Canine mandible, lingual aspect, showing mandibular foramen (arrowhead) and angular process (star); note that the specimen is missing a mandibular third molar.

Cranial Infraorbital Nerve Block

The cranial infraorbital nerve block inhibits stimulation to the following nerves: infraorbital, incisivomaxillary, rostral superior alveolar dental, external nasal, internal nasal, and superior labial.¹⁴ This block desensitizes the maxillary first, second, and third premolars, canine, and incisor teeth on the same side on which the block is administered. It also desensitizes the associated soft tissues, skin of the muzzle, and the upper lip on the ipsilateral side of block administration.¹⁴

To perform the cranial infraorbital block in the dog, palpate the infraorbital foramen as a depression in the alveolar mucosa apical (dorsal) to the distal root of the maxillary third premolar or the mesial root of the maxillary fourth premolar. In the cat, the infraorbital foramen is located at the mesial aspect of the third premolar. The needle should be inserted just into the canal (FIGURE 2), parallel with the canal or directed slightly ventral to it. If you insert the needle apically (dorsally), it could penetrate the retrobulbar space or the globe of the eye. If you insert the needle too far ventrally, it could contact the floor of the infraorbital canal, preventing insertion deeper into the canal. The chosen drug, after appropriate dose calculation, should then be slowly injected into the canal. The drug should be infused into the canal rather than infiltrated directly into the nerve. To prevent intravascular infusion, after inserting the needle into the canal, rotate the syringe 360 degrees, aspirating at each quarter turn. After injection into the canal, remove the syringe and apply digital pressure to the opening of the infraorbital canal for 1 minute.

FIGURE 2. Location of the infraorbital canal opening and appropriate penetration into the canal for the cranial infraorbital (cranial maxillary) nerve block. (A) Canine skull. (B) Canine cadaver. (C) Feline skull.

Caudal Infraorbital (Maxillary) Nerve Block

The caudal infraorbital nerve block inhibits stimulation to the following nerves: maxillary; infraorbital; caudal, middle, and superior alveolar dental; incisivomaxillary; and rostral superior alveolar dental.¹⁴ This block desensitizes the maxillary 1st and 2nd molars and all premolars, canine, and incisors of the ipsilateral quadrant.¹⁴ Also blocked are the bone and soft tissues of the maxilla on the ipsilateral side of block administration, along with the skin of the nose, cheek, and upper lip on the ipsilateral side.¹⁴

The technique for the caudal infraorbital nerve block is identical to that for the cranial infraorbital nerve block. The needle should be inserted into the canal and directed approximately half the length of the zygomatic arch (FIGURE 3). The direction of the needle should be parallel with the canal. If the needle is inserted apically (dorsally) or ventrally, the problems described above can occur. The same injection procedure described for the cranial infraorbital block should be followed, ending with digital pressure to the rostral opening of the canal for 1 minute.

FIGURE 3. Location of the infraorbital canal and appropriate depth of penetration into the canal for the caudal infraorbital (caudal maxillary) nerve block. (A) Canine skull. (B) Canine cadaver. (C) Feline skull. (D) Feline skull, dorsal view. (E) Feline cadaver.

Other approaches to the caudal infraorbital block include the subzygomatic approach and a technique using the maxillary tuberosity. However, because of variations in skull type and breed, we prefer the approach described above.

Caudal Inferior Alveolar Nerve Block

The caudal inferior alveolar nerve block inhibits innervation to the inferior alveolar branch of the mandibular nerve before it dives into the mandible; to the caudal, middle, and rostral mental nerves; and to the incisive nerve.¹⁴ Anesthesia to this region desensitizes all teeth (incisors, canine, premolars, molars), associated labial tissues, the rostral lower lip, and the rostral intermandibular tissues on the side in which it is placed.¹⁴ There are 2 approaches to the caudal inferior nerve block: intraoral and extraoral.

Intraoral Approach: To perform this block, palpate the angular process of the mandible on the external surface of the patient. This is a palpable prominence located at the caudal-most aspect of the mandibular body (FIGURE 1, A, B, E, F). Insert the needle intraorally through the gingiva at the location of the distal aspect of the mandibular third molar in the dog or the mandibular first molar in the cat (FIGURE 4). Then insert the needle on the lingual aspect of the mandible, as opposed to the buccal surface, directed toward the angular process, attempting to palpate and deposit the block at the opening of the mandibular foramen (FIGURE 5). The opening of the mandibular foramen is located half the distance between the alveolar crest distal to the last molar and the angular process of the mandible. After the needle is inserted into the region of the mandibular foramen, the syringe should be rotated 360 degrees, aspirating at each quarter turn. Because this foramen and nerve may be difficult to palpate, you can place the local anesthetic along periosteum of the body of the mandible in the location of the mandibular foramen, which should cause the local anesthetic to spread over a large surface area. After withdrawing the syringe, apply digital pressure to the area of the foramen for 1 minute to allow the block to diffuse within the tissues.

FIGURE 4. Appropriate placement of the needle for the intraoral approach to the caudal inferior alveolar (caudal mandibular) nerve block. (A, B) Canine cadaver. Note that the gloved hand is on the external surface of the patient and is palpating the angular process. (C) Feline cadaver.

FIGURE 5. Intended location of the caudal inferior alveolar (caudal mandibular) nerve block. The block should be administered at the opening of the mandibular foramen. This structure is located half the distance between the location of the alveolar crest distal to the last molar and the angular process. (A) Canine mandible, lingual aspect. (B) Feline mandible.

Extraoral Approach: To perform this block, palpate the facial vascular notch (FIGURE 1, A, C, D). This structure is located on the ventral aspect of the caudal mandible. The needle should be inserted directly through the skin in the middle of this structure (FIGURE 6), directed parallel with the lingual aspect of the mandible and continued dorsally to half the width of the mandible (FIGURE 7). This is the location of the mandibular foramen, similar to the intraoral approach. As described for the intraoral approach because this foramen and nerve may be difficult to palpate, you can place the local anesthetic alongside periosteum of the body of the mandible in the region of the mandibular foramen, which should cause the local anesthetic to spread over a large surface area. Injection technique is the same as above.

FIGURE 6. Location of the facial vascular notch on an anesthetized dog. For the caudal mandibular nerve block, the needle is inserted through the skin and directed on the lingual aspect of the mandible for half the width of the mandible in the location of the mandibular foramen.

FIGURE 7. (A, B) Canine skull showing the location of the mandibular foramen identified by using the facial vascular notch.

Discussion

Multimodal pain-relieving efforts should be pursued for canine and feline patients. Just because our patients cannot communicate their pain to us does not negate the fact that they are experiencing pain. Regional anesthesia, especially when combined with an opioid, can provide a more comfortable procedure for our patients and a more satisfying experience for our clients. When safe technique is used, regional anesthesia not only helps the dentistry and oral surgery patient but also increases client satisfaction. Regional anesthesia reduces the general anesthetic requirement, provides intraoperative and postoperative pain relief, and contributes to a smoother postoperative recovery. These benefits increase client satisfaction by making the patient’s postoperative recovery more comfortable and allowing the patient to be discharged with less drug hangover. Clients expect the same services for their pets as they do for themselves. We can reduce their concerns about use of general anesthesia for dental cleaning patients if at the time of discharge, our patients are awake, pain-free, and able to ambulate as well as when they came in that morning.