CE Vol. 24, No. 6 June 2002 V 439 Article #1 (1.5 contact hours) Refereed Peer Review Comments? Questions? Email: compendium@medimedia.com Web: VetLearn.com Fax: 800-556-3288 KEY FACTS Due to its long duration of action, bupivacaine is the agent of choice for regional oral analgesia in companion animals. The maximum cumulative dose of bupivacaine is 2 mg/kg. The maxillary nerve block may be preferred over the infraorbital nerve block in providing anesthesia to the maxillary molars. Regional Nerve Blocks for Oral Surgery in Companion Animals Burnt Store Animal Hospital, Punta Gorda, FL Brett Beckman, DVM Northwest Veterinary Dental Services, Ltd., North Vancouver, BC Loïc Legendre, DVM, FAVD, DAVDC, DEVDC ABSTRACT: Regional nerve blocks provide a practical means of peri- and postoperative analgesia as an adjunct to general anesthesia for painful oral procedures in companion animals. Regional nerve blocks are technically simple to master, quick and easy to administer, and have a low incidence of complications. Few materials are needed to perform regional nerve blocks, and many practices may already have the necessary supplies. The mental, inferior alveolar, infraorbital, and maxillary nerves can be blocked alone or in combination to provide the desired analgesia to any location in the oral cavity. In addition to the benefits derived from lower inhalation anesthetic concentrations, postoperative analgesia allows less aggressive systemic pain management during recovery. Sensory fibers that innervate the bone, teeth, and soft tissue of the oral cavity originate from the maxillary and mandibular branches of the trigeminal nerve. 1 Four regional nerve blocks can be easily performed and provide analgesia to the oral cavity. Common procedures in which local pain control would be beneficial include surgical and nonsurgical extractions, bone and softtissue reconstruction following trauma, mass excisions, biopsies of oral tissue, root canal therapy, oronasal fistula repair, cleft palate repair, and periodontal flap surgery. Knowledge of the anatomy of the oral cavity aids the practitioner in effective administration, and careful technique minimizes complications. BENEFITS The ability to control perioperative pain with regional nerve blocks allows reduction in the concentration of inhalant anesthetic, 2 which minimizes complications from hypotension, bradycardia, and hypoventilation. 3 Patients maintained on lower anesthetic concentrations consequently recover more quickly and with fewer complications. Preoperative regional nerve blocks continue to provide analgesia in the postoperative period, thereby increasing patient comfort and decreasing the requirement for systemic pain medication. 4,5
440 Small Animal/Exotics Compendium June 2002 Figure 1 Injection site for a mental nerve block in a dog. MATERIALS The materials needed to perform regional nerve blocks are minimal and inexpensive. A tuberculin syringe; a 25- to 27-gauge, 3 4- to 1-inch needle; and a regional anesthetic agent are all that are needed. Human dentists and some veterinary dentists commonly use stainless steel dental syringes. The plunger has a thumb ring, so aspiration and injection using one hand can be readily accomplished. These syringes use single-dose vials, which are significantly more expensive than multidose vials. AGENT AND DOSE Bupivacaine is the regional anesthetic of choice in veterinary dentistry due to its long duration of action compared with that of other agents. Bupivacaine 0.5% with or without 1:200,000 epinephrine is a common choice. The duration of action varies from 3 to 10 hours. 2,3,6,7 The volume administered per site should range from 0.1 to 0.5 ml for dogs and 0.1 to 0.3 ml for cats. 4 An additional 50% of the anesthetic agent can be administered for an infraorbital nerve block. The maximum cumulative dose of bupivacaine is 2 mg/kg. 2,4 This dose is not generally encountered in routine regional infiltration in large patients, but care should be taken in small patients. For example, a 2-kg patient should receive a maximum of 0.8 ml of 0.5% bupivacaine. If 0.25 ml per site is administered for four regional blocks, the maximum dose is exceeded by 0.2 ml (1 mg). A 20-kg dog, however, can receive as much as 40 mg (8 ml). Even using the maximum recommended 0.5-ml dose per site for four sites, the calculated amount is 10 mg, which is well below the maximum dose. REGIONAL NERVE BLOCKS OF THE ORAL CAVITY Mental Nerve Block The mental nerve block affects the bone, teeth, and Figure 2 Injection site for an inferior alveolar nerve block (extraoral technique) in a dog. soft tissue rostral to the second premolar on the infiltrated side (Figure 1). The middle mental foramen is the largest of the three mental foramina; therefore, it is used for the mental nerve block. Its location varies slightly depending on the breed, size, and species of the patient. In dogs, it can be found just caudal to the mandibular labial frenulum in the ventral third of the mandible on the buccal aspect. The injection should be administered in the area just ventral to the rostral (mesial) root of the second premolar. Palpate the middle mental foramen. Insert the needle into the submucosa in a rostral to caudal direction starting just behind the frenulum. Advance the needle to the opening of the foramen. Aspirate and very slowly inject the agent. Hold digital pressure over the injection site for 60 seconds to ensure maximum caudal diffusion of the agent into the mandibular canal. Because the anesthetic is injected into the canal, pain may induce a rise in heart rate unless the agent is injected very slowly. The middle mental foramen may be difficult to palpate in small dogs. Ideally, the foramen is located with the aid of dental radiography. As an alternative, the needle should be placed in the submucosa at the ventral third of the mandible at the level of the diastema between the first and second mandibular premolars. Aspirate and slowly inject the agent. Hold digital pressure over the injection site for 60 seconds. In cats, the middle mental foramen is located at the level of the mandibular labial frenum and cannot be readily palpated. The needle should be placed into the submucosa at the rostral border of the mandibular labial frenulum halfway between the dorsal and ventral borders of the mandible. Advance the needle to the midfrenulum adjacent to the mandible. Aspirate and slowly inject the agent. Hold digital pressure over the injection site for 60 seconds. Inferior Alveolar (Mandibular) Nerve Block The inferior alveolar (mandibular) nerve block affects
Compendium June 2002 Nerve Blocks for Oral Surgery 441 Figure 3 Injection site for an inferior alveolar nerve block (intraoral technique) in a dog. the mandibular bone, teeth, soft tissue, and tongue on the infiltrated side. There are two techniques for administration: extraoral and intraoral. Extraoral Technique In dogs, it is usually possible to palpate the notch of the caudal ventral mandible cranial to the angular process (Figure 2). 4 In cats and some dogs, the notch cannot be palpated. If the notch cannot be palpated, the point on the ventral mandible is located on a vertical plane with the midpoint of the zygomatic arch. This midpoint should be approximately located on a vertical plane with the lateral canthus. Insert the needle at the lingual aspect of the ventral mandible at this point. Advance the needle dorsally along the lingual aspect of the mandible to the midpoint between the ventral and dorsal borders of the mandible. The needle will now be in the vicinity of the mandibular foramen where the inferior alveolar nerve enters the mandibular canal. If desired, a finger can be placed inside the mouth to palpate the needle, ensuring its location at the opening to the foramen. Aspirate and slowly inject the agent. Intraoral Technique Palpate the mandibular foramen intraorally (Figure 3). It resides on the lingual surface of the mandible two thirds of the distance from the last molar to the angular process of the mandible. The angular process can be palpated extraorally at the caudal- and ventral-most projection of the mandible. Once the foramen is palpated, enter the needle intraorally on the lingual surface of the mandible adjacent to the foramen. Aspirate and inject slowly. Infraorbital Nerve Block The area affected by the infraorbital nerve block depends on the degree of caudal diffusion of the anesthetic agent (Figure 4). If done as described here, the Figure 4 Injection site for an infraorbital nerve block in a dog. bone, soft tissue, and dentition rostral to the maxillary first molar will be affected. Knowledge of the anatomic relationship of the infraorbital nerve and its branches is important when performing this nerve block. The infraorbital nerve branches from the maxillary trunk of the trigeminal nerve at the level of the pterygopalatine fossa. 1 The infraorbital nerve gives off the caudal superior alveolar nerve before entering the infraorbital canal. The caudal superior alveolar nerve then branches to supply the maxillary fourth premolar and molars. Once it enters the infraorbital canal, the infraorbital nerve branches into the middle superior alveolar nerves that innervate the premolars. The rostral superior alveolar nerves branch just before the infraorbital nerve exits the infraorbital canal to innervate the canine and incisor teeth. Unless the agent diffuses beyond the caudal borders of the infraorbital canal, adequate anesthesia cannot be provided to the maxillary fourth premolar and molars. It is therefore recommended that, if anesthesia to the caudal cheek teeth is desired, the maxillary nerve block be used. The maxillary nerve block also provides anesthesia to the hard and soft palate. An additional 50% of the recommended amount of regional anesthetic agent is used for the infraorbital nerve block to allow adequate diffusion to the middle superior alveolar nerves that lie within the infraorbital canal. Palpate the infraorbital foramen in the maxilla dorsal to the caudal (distal) root of the third maxillary premolar. Insert the needle through the buccal mucosa in a caudal direction. Advance the needle to the entrance of the foramen, aspirate, and (while holding direct digital pressure behind the needle tip and over the foramen) slowly inject the agent. Withdraw the needle and continue to hold digital pressure at the infraorbital canal for 60 seconds. Maxillary Nerve Block The maxillary nerve block affects the maxillary bone,
442 Small Animal/Exotics Compendium June 2002 Figure 5 Injection site for a maxillary nerve block in a dog. teeth, soft tissue, and palatal tissue on the infiltrated side (Figure 5). With the patient s mouth open, palpate the notch at the rostral ventral aspect of the zygomatic arch as it meets the bone surrounding the last maxillary molar. Insert the needle directly adjacent to the bone at this level so that the needle walks along the caudal aspect of the notch. The needle should be directly perpendicular to the horizontal line of the palate. Advance the needle dorsally to a level just beyond that of the root tips of the last molar. Aspirate and slowly inject the agent. COMPLICATIONS Complications resulting from oral nerve blocks have been described in human dentistry and oral surgery literature. Although extremely rare, permanent damage to the inferior alveolar nerve, lingual nerve, or both has occurred as a result of inferior alveolar nerve blocks. The incidence has been reported to be between 1:26,762 and 1:160,571. 8 Toxic doses of bupivacaine have been reported to cause cardiovascular toxicity and death, although this is extremely rare. 9 Local anesthesia is not used in humans with malignant hyperthermia because of the potential for complications. 10 Neurologic complications include facial nerve palsy, transient amaurosis, Horner s syndrome, transient nerve paralysis, and unilateral deafness. 11 Although the incidence of systemic toxicity and local complications with bupivacaine is low in humans, care should be taken to ensure a safe infiltration. Gentle advancement of the needle will minimize damage to soft tissue. Drawing back on the syringe before injection will ensure that the needle has not been inserted into blood vessels, thereby avoiding direct intravenous injection. With frequent use, these blocks can become a valuable part of the anesthetic protocol, providing added safety and patient comfort. REFERENCES 1. Evans HE, Christensen GC: Miller s Anatomy of the Dog. Philadelphia, WB Saunders Co, 1979, pp 903 974. 2. Holmstrom SE, Frost P, Eisner ER: Veterinary Dental Techniques for the Small Animal Practitioner, ed 2. Philadelphia, WB Saunders Co, 1998, pp 492 493. 3. Rochette J: Local anesthetic nerve blocks and oral analgesia. Proc 26 th World Congr WSAVA, 2001. 4. Haws IJ: Local dental anesthesia. Proc 13 th Annu Vet Dent Forum, 1999. 5. Kaurich MJ, Otomo-Corgel J, Nagy FJ: Comparison of postoperative bupivacaine with lidocaine on pain and analgesic use following periodontal surgery [periodontal abstract]. J West Soc Periodont 45(1):5 8, 1997. 6. Powell SL, Robertson L, Doty BJ: Dental nerve blocks. Postgrad Med 107(1):233, 2000. 7. Paddleford RR: Manual of Small Animal Anesthesia. New York, Churchill Livingstone, 1988. 8. Pogrel MA, Thamby S: Permanent nerve involvement resulting from inferior alveolar nerve blocks. J Am Dent Assoc 131(7):901 907, 2000. 9. Younessi OJ, Punnia-Moorth A: Cardiovascular effects of bupivacaine and the role of this agent in preemptive dental analgesia. Anesth Prog 46(2):56 62, 1999. 10. Malmahmed SF: Handbook of Local Anesthesia. St. Louis, Mosby, 1990. 11. Crean SJ, Powis A: Neurological complications of local anesthetics in dentistry. Dent Update 26(8):344 349, 1999. ARTICLE #1 CE TEST The article you have read qualifies for 1.5 contact hours of Continuing Education Credit from the Auburn University College of Veterinary Medicine. Choose the best answer to each of the following questions; then mark your answers on the postage-paid envelope inserted in Compendium. 1. Why is bupivacaine the agent of choice for regional anesthesia in companion animals? a. safety b. rapid onset of effect c. ph compatibility with tissue d. duration of action e. comfort during injection 2. What two nerve blocks have been used to provide anesthesia to the caudal superior alveolar nerve? a. maxillary and infraorbital b. maxillary and inferior alveolar c. infraorbital and inferior alveolar d. inferior alveolar and mental e. mental and infraorbital 3. What is the maximum cumulative dose of bupivacaine in dogs and cats? a. 1 µg/kg d. 2 mg/kg b. 2 µg/kg e. 4 mg/kg c. 1 mg/kg
444 Small Animal/Exotics Compendium June 2002 4. Which of the following conditions is not a complication of regional nerve blocks? a. cardiovascular toxicity d. unilateral deafness b. permanent nerve damage e. ataxia c. Horner s syndrome 5. The maxillary molars are innervated by the alveolar nerve. a. middle inferior b. caudal superior c. caudal inferior d. middle superior e. rostral superior 6. Which of the following is not a direct or indirect perioperative benefit of regional anesthesia? a. reduction of inhalant anesthetic concentration b. analgesia c. decreased incidence of hypotension d. decreased incidence of bradycardia e. none of the above 7. What is the recommended dose per site for bupivacaine in feline patients? a. 0.1 to 0.3 ml b. 0.1 to 0.5 ml c. 0.2 to 0.4 ml d. 0.2 to 0.7 ml e. 0.3 to 0.6 ml 8. Which of the following statements about the mental nerve block is incorrect? a. In dogs, the mental foramen is located in the ventral third of the mandible. b. The affected areas include bone, teeth, and soft tissue rostral to the second premolar. c. The needle should be advanced to the opening of the mental foramen. d. In dogs, the mental foramen is located just rostral to the mandibular labial frenulum. e. In small dogs and cats, radiography is the most accurate means of locating the middle mental foramen. 9. Which of the following statements concerning the inferior alveolar nerve block is correct? a. The notch in the caudal ventral mandible is easily palpated in cats. b. It is important to enter the mandibular foramen with the needle to ensure adequate infiltration of the agent. c. The notch in the caudal dorsal mandible is easily palpated in most large dogs. d. The mandibular foramen is located two thirds of the distance from the last molar to the angular process. e. The mandibular foramen can easily be palpated by gently sliding a finger on the buccal surface of the mandible halfway between the last molar and the angular process. 10. What is the maximum volume of 0.5% bupivacaine that can be used for regional nerve blocks in a 3.5-kg patient? a. 1.0 ml d. 1.8 ml b. 1.4 ml e. 2.0 ml c. 1.6 ml