Local Anesthetic Nerve Blocks and Oral Analgesia
Judy Rochette Canada
We have come a long way in our treatment of animal pain, especially within the last decade. Research has assisted in extending to veterinary medicine many of the practices and products previously used in the human field. Our clients are familiar with these practices and expect us to be adept in these procedures and to extend them to their pets. A client's willingness to comply with our suggestions for oral care may be determined by their perception of pain control for their pet.
REASONS TO USE LOCAL BLOCKS
Enhanced client compliance is just one reason to perform local anaesthetic nerve blocks. By controlling intraoperative pain, the need for deep anesthesia is eliminated. The risk of vagally mediated reflex bradycardia in removed, as is the attendant hypotension and hypoventilation of higher gas concentrations. This translates to a safer anesthetic experience for the patient.
By preventing the �wind-up� phenomena of pain and inhibiting the release of noxious stimuli the level of discomfort the patient experiences is reduced both during the procedure and after. This translates to a faster and less eventful recovery; a reduced need for extra medication in the immediate post-operative period; and will reduce the amount, type and frequency of post-operative analgesic needed to keep the patient comfortable. Reduced gas consumption and lower need of technical and pharmaceutical involvement in patient care mean monetary savings for the practitioner.
LOCAL ANESTHETIC AGENTS
Local anesthetic agents enter and occupy ion channels in a nerve cell membrane, preventing depolarization. This prevents or retards conduction of pain impulses. Uptake of the local agent is improved with the addition of hyaluronidase, and duration of effect increased with vasoconstrictors. Epinephrine (adrenaline) and L-norepinephrine (levarterenol) are used at 1:50 000 (1 mg/50 ml saline) or 1:200 000 (1 mg/200 ml saline). The lower concentration is sufficient for efficacy, but the higher concentration offsets the instability of epinephrine and improves shelf life. All local anesthetic agents work poorly in an acidic environment such as an infected area.
The most commonly used local anaesthetic agents are mepivicaine, lidocaine and bupivacaine. Mepivacaine, with or without epinephrine, takes effect within minutes, and begins to attenuate after 1.5�2.0 hours. Lidocaine 2% (with epinephrine 1:200 000) has an onset of two to five minutes, and provides sensory blockade for two hours. Bupivacaine, with or without epinephrine, requires approximately five minutes for onset, but provides analgesia for six hours. Mepivacaine is less toxic than lidocaine and bupivacaine provides a greater safety margin, especially in cats. All three agents can be purchased in ampules, which contain 1.8 ml, or in larger, more economical, multi-dose bottles.
The total dose of mepivacaine, lidocaine, or bupivacaine that can be given at one time to a dog or cat is 2 mg/kg. This total dose must be divided over the number of sites that need to be anesthetised. For example, bupivacaine can be purchased in a 0.5% solution and therefore has 5 mg/ml. A 5.0 kg cat could tolerate 10 mg of bupivacaine, or could take a total volume of 2 cc of solution. If four sites need to be desensitized then the maximum volume to use per site is 0.5 ml. In practice, 0.25�0.3 ml of local agent per site is adequate to achieve full desensitisation in cats and small dogs. A 45 kg dog could tolerate 90 mg of mepivacaine, lidocaine or bupivacaine. In a 2% solution of lidocaine, there is 20 mg/ml. The total volume of solution this dog could accept is 4.5 mls. If two sites needed desensitizing, then 2.25 mls could be used per site. Clinically, 0.75�1.0 ml at a site is sufficient to achieve complete analgesia. A one or three ml syringe with a 25-gauge needle is usually adequate for placing the blocks.
The vast majority of sensory input is received via branches of the trigeminal (fifth cranial) nerve. The major sensory branches of concern to oral and dental surgeons are the maxillary and mandibular divisions. The maxillary division leaves the trigeminal ganglion and exits the cranial cavity through the foramen rotundum, courses through the alar canal, and crosses the pterygopalatine fossa to enter the infraorbital canal. Just before entering the caudal limit of the infraorbital canal, the nerve sends off branches that will become the major and minor palatine nerves. These nerves innervate the hard and soft palates, their mucosa, and the nasopharynx. These branches are desensitized with the maxillary nerve block.
Just before the maxillary division enters the infraorbital canal, it gives off the caudal maxillary alveolar nerve that supplies the maxillary molars and the buccal gingiva and mucosa, and is blocked with the caudal infraorbital block. After giving off the caudal maxillary alveolar nerve, the maxillary nerve enters the infraorbital canal, where it is called the infraorbital nerve. While the infraorbital nerve is traversing the infraorbital canal, it gives off two more branches that exit ventrally from the canal. The middle maxillary alveolar nerve innervates the premolars and associated buccal gingiva. The rostral maxillary alveolar nerve supplies the canine, incisors, and associated buccal gingiva. The remaining fibers of the infraorbital nerve then exit the cranial extent of the infraorbital canal to innervate the lateral and dorsal cutaneous structures of the rostral maxilla and upper lip. These middle maxillary alveolar, rostral maxillary alveolar, and the infraorbital nerve are blocked by the cranial infraorbital nerve block.
The mandibular division of the trigeminal nerve arises from the trigeminal ganglion, exits the cranium via the foramen ovale, and divides into multiple branches. The divisions include the sensory buccal nerves, the lingual nerve and the mandibular, or inferior alveolar nerve. The buccal nerves receive stimuli from the facial musculature, skin, mucosa of the cheek and buccal gingiva along the posterior aspect of the mandible. The buccal gingiva can be desensitized by the buccal block. The lingual nerve supplies the tongue, the floor of the mouth, the lingual gingiva, and the submandibular salivary gland. The mandibular nerve enters the mandible on the lingual side, via the mandibular foramen. The nerve then courses rostrally within the bone to innervate the mandibular teeth to the midline. This nerve can be blocked with the mandibular nerve block. At the level of the second premolar (dogs) or rostral to the third premolar (cats), the mandibular nerve gives off mental nerve branches. These branches exit through the mental foramina, and serve the cutaneous areas of the chin, lip, and the rostral buccal gingiva and mucosa. These nerves are blocked with the mental nerve block.
SITES TO DELIVER BRANCH NERVE BLOCKS
The mandibular, middle mental, and infraorbital foramina are of most interest in small animal oral procedures. The infraorbital foramen can be divided into cranial and caudal blocks.
Cranial infraorbital. This site is apical to the distal root of the third premolar just ventrorostral to the zygomatic arch where a depression can be felt and the nerve �twanged.� The block will anaesthetise the ipsilateral premolar, canine and incisor teeth and associated soft tissues.
Caudal infraorbital. If firm digital pressure is placed over the cranial end of the infraorbital canal after injection, or the needle is advanced deep into the foramen, then the nerve will be anaesthetised at the orbital end of the canal and before the caudal maxillary alveolar nerve branches off to the molar dentition. This block will anaesthetise all ipsilateral dentition and soft tissues including the molars.
Middle mental. In the canine, this site is palpated ventral to the mesial root of the second premolar. In the feline, it is located under the lip frenulum about equidistant between the third premolar and the canine. If the needle enters the foramen, the block will anaesthetise the ipsilateral soft tissues, canine and incisor teeth. If the anaesthetic is deposited outside of the foramen then only the buccal soft tissues from the canine forward to the midline will receive analgesia. In human anatomy, there is significant innervation of the complete incisor area originating from both the left and right sides of the mandible. Whether this also occurs in veterinary medicine is unclear. If desensitization appears to be incomplete when working in this area, it may be necessary to block both the right and left mental nerves.
Mandibular. This block can be done intraorally or extraorally. The foramen is a depression located on the medial side of the ramus of the mandible. It is approximately equidistant between the rostral and caudal borders of the ramus and at a height between a line level with the crest of the alveolar bone and the mid height of the body of the mandible. The nerve is anaesthetised before it enters the mandible and will block all soft tissues and dentition on that side of the mouth. The intraoral approach involves directing the syringe across the tongue from the opposite side of the mouth and placing the anaesthetic agent in proximity to the foramen. If this approach is awkward then the extraoral approach may be easier. In this approach, the needle will be inserted at right angles to the ventral border of the mandible 0.5�1.0 cm cranial to the rostral edge of the angular process. A line drawn from the lateral canthus of the eye to this entry point should fall across the midpoint of the zygomatic arch. With a finger inserted into the mouth and palpating the foramen the needle should be walked off the medial edge of the mandible and advanced dorsally until it can be felt to be in proximity to the foramen. This block will provide analgesia to the whole hemimandible.
Another block that is used occasionally for major procedures requiring analgesia of the complete hemimaxilla, including soft tissues, dentition, and palate, is the maxillary nerve block.
Maxillary. The intraoral approach requires a human aspiration syringe with a 27 or 30-gauge needle and can only be achieved in a larger dog due to the limited size of the major palatine foramen. The foramen is located at the intersection of two lines, the first of which is drawn perpendicular to the palatal midline at a mesiodistal midpoint through the maxillary fourth premolars. The second line is drawn parallel to the midline of the palate equidistant between the dentition and the palatal midline. The needle must be inserted pointing caudolaterally, at a maximum
Buccal. The needle should be placed in the submucosa of the tissue and local anesthetic injected at the level of the second and third mandibular molar in the dog, and the third and fourth premolar in the cat. There is concern that penetration of a foramen with a needle may cause trauma to the nerve. This is especially possible when attempting to block the mental nerve in a small animal. For this reason, utilising the alveolar nerve block may provide better desensitisation of the mandible while avoiding iatrogenic trauma. The same could be said for the intraoral approach to the maxillary nerve.
For post-operative pain control, the analgesic should vary depending on the anticipated level of discomfort. Any patient who receives more than a simple extraction should be considered to have moderate discomfort and a case of stomatitis, multiple extraction, or hard or soft tissue manipulation should be assumed to have a high level of pain.
The opioid family is excellent for controlling significant pain. Use of an opioid in premedication, or intra-operatively, will provide a smoother anaesthetic while waiting for a local block to take affect. It will also work synergistically with a local block where oral manipulation provides stimulation not controlled by the block. Butorphanol has a duration of 1�2 hours, oxymorphone 1.5�3 hours, and morphine 2�4 hours. Buprenorphine may provide 8�10 hours of postoperative analgesia. Oxymorphone can also be sent home for intranasal application at 0.05�0.1 mg/kg q4-6h.
An alternative product to control significant pain is a fentanyl patch. This product may only be available through a human hospital pharmacy. A cat or small dog needs a 25 microgram/hr patch, a mid-sized dog 50 the microgram/hr patch, a large dog the 75 microgram/hr patch, and a giant breed, the 100 microgram/hr patch. Time from application of a patch to onset of analgesia is approximately 6�8 hours, with best effect in approximately 12 hours. The duration of analgesia will last for 4�5 days after which time the patch should be removed. Do not use butorphanol while waiting for onset of analgesia from the patch, as it is a competitive antagonist and will make the patch ineffective. The patches are an excellent analgesic to send home with the patient. Place the patch is in a location where the patient or small children cannot access it, and cover it with a bandage. As this is a human product, its use is �off label.� Overdosing may result in pupillary dilation, tachycardia, and/or dysphoria. In such a case remove the patch and the patient will return to normal within hours. If a patch has been ingested encourage vomiting and use an antagonist, e.g., naloxone.
Medications for moderate discomfort include the non-steroidal anti-inflammatories (NSAIAs). An acetominophen/codeine product can be used in dogs at a dose of 10 mg/kg for the acetominophen and one mg/kg for the codeine. For cats, codeine syrup can be given at 0.5�1 mg/kg q8h. An initial dose should be given in hospital to ensure excitement does not occur. Codeine has an unpleasant taste so formulate the codeine with flavoured syrup. Consider adding extra fiber to the diet of an animal receiving codeine to prevent constipation. A transdermal codeine paste is appropriate for small patients and can be placed on the inside of the ear pinna.
Most clients have concerns that oral procedures will inflict pain on their pets. Our clients' perception of us as competent and caring professionals is often determined by what they see at home after a procedure. Judicious use of analgesics will help your patient, help your client, and help yourself.
WSAVA Contact Information