Regional anesthesia and pain management are fundamentally important skills to successfully leverage for the benefit of our dentistry and oral surgery patients. Despite these patients being under general anesthesia while undergoing procedures, there are inherent benefits to utilizing techniques that will allow for the reduction of inhalant anesthesia, improve recovery and improve the patient’s comfort at the time of discharge. Anatomy of the face and mouth is complex. There are many locations and combination of locations where local anesthetics can be administered which will result in regional anesthesia.

Regional anesthesia can offer many benefits by reducing the animal’s response to painful stimuli during the procedure as well as provide postoperative analgesia. Primarily speaking, benzodiazapines, phenothiazines and general anesthetics have no primary analgesic activity. These medications alter the state of consciousness and abolish the perception of pain. Peripheral sensitization, or a reduction in the threshold necessary for stimulus transduction occurs due to the effects of tissue injury and inflammation. Inflammatory mediators including prostaglandin E2, bradykinin, neurotrophic factors (NGF) and the activation of mast cells contribute to peripheral sensitization. These inflammatory mediators lower the activation threshold and increase the amount of Na+ flowing across the channel. Once general anesthetic medications are metabolized, the patient is vulnerable to the sensation of pain. In recent years, significant advancements have developed aiding our ability to assess pain in cats. Evaluating facial expression in these patients not only help us identify painful patients prior to treatment but also to assess response to treatment and post-operative efforts for analgesia.

Local anesthetics work by inhibiting transmission through their effects on Na+ channels. By preventing depolarization and propagation of neural signals to the brain, pain can be effectively blocked. While local blocks reduce the amount of perceived pain and amount of required general anesthesia and associated unwanted side effects, the patient’s comfort can be improved. Effective local blocks are not a replacement for safe, effective general anesthesia. The addition of local blocks to the anesthesia and analgesia protocol will provide the benefits of polypharmacy which can be recognized as threefold. These drugs: (1) prevent peripheral and central sensitization, (2) reduce the adverse effects associated with larger doses of medication and (3) provide better postoperative pain management to smooth out the recovery of the patient.

Studies measuring minimum alveolar concentration have shown that administration of local anesthetics reduce the amount of inhalant necessary to keep 50% of patients asleep during a given stimulus. The use of local anesthetics preventing the propagation of nerve impulses may be beneficial on its own but may be further improved through the addition of opioids or alpha-2 agonists administered locally.

When given as a local anesthetic, the lidocaine family of drugs provides a variety of options with different onsets of action and different durations of action. Doses should not exceed 5mg/kg in dogs and 2mg/kg in cats. Lidocaine is commonly used in human regional and local anesthesia because a quick onset and short duration of action is desirable. Compliance with human patients for taking oral medications is quite good and return to function (frequently the workplace) is important. In canine and feline dentistry, bupivacaine is a popular medication used off label because of its longer duration of action. Depending on placement the duration of action may be 6 to 10 hours. Recent study information suggests that duration of action may vary by individual dog, but that bupivacaine may last 1-3 days in some patients. Time to onset of action is longer with bupivacaine than lidocaine, some texts referring to a 20-minute period necessary before the nerve impulses are effectively blocked. Lidocaine is labeled for veterinary use while bupivacaine is not. The most common concentration of bupivacaine is 0.5% (5mg/mL) while lidocaine is 2% (20mg/mL). Mixing these drugs should be discouraged until substantiated research is preformed determining that the drug remains active and what the concentration of active drug is. A recent publication demonstrated that mixing lidocaine and bupivacaine worked longer than lidocaine alone. That study’s results with bupivacaine mixed with lidocaine demonstrated a shorter duration of action than a study using bupivacaine alone. This begs the question whether mixing the two drugs results in a reduction in the prolonged effect of bupivacaine alone?

Key Clinical Diagnostic Points
Using the techniques covered in this presentation, it has been the experience of the author that small dosages are sufficient to achieve the desired result of local blockade. Using the techniques discussed, the entire mouth can be anesthetized through the administration of local anesthetic in only four locations.

Bupivacaine
0.1-0.15mL per site (cat or small dog)
0.2mL per site (medium dog)
0.3mL per site (large dog)
The various blocking locations are listed below.

Infraorbital Block
Location: immediately within the infraorbital canal above the distal roomy of PM3
What it blocks: maxillary incisors, canine tooth, premolars 1-2, +/- PM3, buccal mucosa, ipsilateral lip, ipsilateral soft tissue of that side of the face.
What it won’t block: palatal mucosa, PM4 (commonly extracted), may not completely anesthetize for extraction of the central incisors due to crossover innervation.

Caudal Maxillary Block
Location: advance the needle parallel with the hard palate trough the infraorbital canal and as far casually as to the last molar tooth. What it blocks: all the maxillary teeth in that quadrant, ipsilateral lip, ipsilateral hard/soft palatal mucosa, ipsilateral soft tissues on that side of the face. What it won’t block: may not completely anesthetize the central incisors.

Middle Mental Block
Location: ventral to the mesial root of the 2nd premolar (dog) or labial frenulum (cat). Enter through the mesial aspect of the labial frenulum and place the needle against periosteum half the height of the mandible and centered over the tip of mesial root of the second premolar. What it blocks: ipsilateral lip and rostral soft tissues, incisors? and canine tooth? What it won’t block: Questionable coverage for the ipsilateral mandibular incisors and canine tooth (probably due to diffusion into the mandibular canal).

Caudal Mandibular Block (Inferior Alveolar Block)
Location: two main approaches
1. Intraoral: half the distance between the angular process and the mucosa immediately caudal to the third molar (dog) or only molar (cat) (lingual side of the mandible).
2. Extraoral: palpate the ventral notch of the mandible, half the distance of the length of the notch, place needle perpendicular to the notch and immediately on the lingual surface, advance needle ½ to 1 cm.
What it blocks: all ipsilateral mandibular teeth, rostral mandibular soft tissues
What it won’t block: questionable coverage for caudal mandibular soft tissues, if applied correctly, should not risk anesthetizing the tissues of the tongue.

Note: It has been shown that intraoral administration of the caudal mandibular block is more accurate than the extraoral approach- this may be useful in helping to reduce the risk of inadvertent blocking of the sensory innervation to the tongue. Once the needle has been placed, it is important to aspirate, and re-aspirate, while rotating the needle 90o along its long axis to ensure the injection is not given intravascular. Medication should be administered with the needle being placed on periosteum for the middle mental, and caudal mandibular blocks. Even if the bevel is not directly over the nerve, by being deposited on periosteum, the local will cover more surface area and increase the chance that the nerve will be coated. Once the local has been administered, the needle should be withdrawn and digital pressure should be placed for 1 minute to provide adequate time to prevent hematoma formation.

There is reasonable expectation that the addition of opioids to a local block may improve postoperative analgesia long after the effects of the sodium channel blockade wear off. In a study performed in dogs comparing bupivacaine versus bupivacaine + buprenorphine (30mcg) it was shown that 3 of 8 dogs with the combination demonstrated analgesia 72-hours post administration while 2 of 8 dogs experienced analgesia 5 days following administration. It has been well established that mu receptors exist in the peripheral nervous system and are up-regulated when exposed to chronic noxious stimulation. Dentistry patients undergoing procedures for acute injuries, such as tooth fracture, are less likely to demonstrate the benefits of opioids in their local blocks as compared to cats with stomatitis or tooth resorption. Chronic conditions may make some drugs work better or last longer. There are several situations where long-term desensitization of a surgery site may be undesirable. Patients suffering from an oronasal fistula already have a loss of bone and a communication between the oral and nasal cavities. Repairing these defects and having the surgery site be completely numb may result in the animal becoming preoccupied with feeling the sutures on their tongue and subsequently tongue thrusting through the surgery site up into their nasal cavity. Similar potential situations exist with maxillectomy patients. It is this author’s experience and recommendation that using short acting local anesthetics like lidocaine followed by aggressive post-operative pain management will result in a comfortable patient after surgery with decreased risk of tongue thrusting. Procedures involving the tongue should never receive local block administration because these patients will be at very high risk of self-trauma and risk “chewing their tongue off.” Use of large volumes when performing local blocks has also been anecdotally reported in resulting in this form of self-mutilation. Sticking with the small volumes and accurate placement afford good results with decreased risk.

Whenever there is potential for the local block needle to traverse through an area of possible tumor, the local block should not be performed. Seeding tumor cells through the infraorbital canal may extremely complicate treatment options available for a maxillary tumor. Using a 25-gauge 1 inch to 27 gauge 1.5-inch needle helps reduce possible nerve injury.

Complications
Complications with local anesthetic blocks have been reported in the literature. Paresthesia, altered sensation and motor changes are occasionally reported anecdotally from practitioners. It is unclear as to where the origin of nerve injury associated with local anesthesia comes from. While histologic nerve changes associated with local anesthetic administration are reported in veterinary patients (Correspondence: J Anthony), true clinical significance should be considered since similar blocks have been performed in humans for decades with a low incidence of true complications. Peripheral nerve paresthesia is a rare complication reported in humans. One human dental textbook reports an occurrence of 1 case in 1 million injections. Peripheral nerve paresthesia and subsequent self-mutilation of the veterinary patients’ tongue has been only anecdotally reported. The technique for proper needle placement for local anesthetic placement is different than it is for venipuncture. After initial needle penetration, the needle should be guided into position for local administration. When these needles are guided through foramen (as in the infraorbital or caudal maxillary blocks) the needle should be advanced slowly and in most situations the needle bevel with help to displace the neurovascular bundle as the bevel is advanced. Nerves penetrated by needle placement can have variable effects- from no change to permanent sensory or motor dysfunction.

There is a school of thought that nerve injury associated with local blocks may not be directly related to physical damage by needle placement. Peripheral nerve ischemia associated with the addition of epinephrine to a local block may also be associated with nerve injury. The addition of epinephrine to long-acting local blocks has therefore been recommended against for that very reason. Beyond the delayed absorption of local anesthetics by the vasoconstriction associated with epinephrine, it has been shown that this catecholamine has some alpha-2 agonist analgesic activity.

The use of small doses in regional anesthesia and aspiration immediately after needle placement can help avoid inadvertent intravascular injection. The most common complications with intravascular injections of local anesthetics include seizures and cardiac toxicity. Bupivacaine has a high affinity for cardiac sodium channels and can cause brady-dysrhythmias as well as ventricular tachycardia and ventricular fibrillation in humans.

The complications of inadvertent anesthesia of the tongue and iatrogenic globe penetration with the needle while performing the maxillary nerve block should both be effectively prevented by close attention to careful needle placement. Iatrogenic perforation of the globe by a needle during local anesthetic placement has a high mortality rate to the eye.

Conclusions
Effective local blocks are not a replacement for safe, effective general anesthesia or multimodal postoperative pain management. Use of local anesthetic agents helps to reduce the amount of inhalant general anesthesia required to keep a veterinary patient anesthetized. The unwanted, most frequently seen complications associated with general anesthesia in veterinary patients who are anesthetized for any reason are hypotension, cardiac dysrhythmias, hypercapnia and hypoxemia. Multimodal analgesia anesthesia can help reduce these unwanted side effects by reducing the amount of gas required to keep the patient anesthetized.

Recommended Reading
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8. Pascoe, Peter J. “The effects of lidocaine or a lidocaine-bupivacaine mixture administered into the infraorbital canal in dogs.” Am J Vet Res 2016;77(7):682-687.
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