Download Dental Nerve Blocks

Dental Nerve Blocks
Krista Mendoza, DVM, Diplomate AVDC
Animal Dental Clinic of Pittsburgh, LLC
Pain management is a very important consideration when performing dental extractions. A
multi-modal approach incorporating oral regional anesthesia along with opioids, NSAIDS, alpha2 agonists, and sometimes steroids allows for a lower concentration of inhalant anesthetic
during the procedure, as well as continued local analgesia in the post-operative period. Local
anesthetics act mainly by blocking sodium channels, preventing depolarization of the neuronal
cell membrane so there is no generation of an electrical impulse after stimulation. Local
anesthetics can also work by blocking calcium channels, inhibiting reuptake of GABA which
enhances its effect 1. The drug’s lipid solubility at pKa (pH at which the drug exists in equal
amounts of charged and non-charged molecules) are inversely proportional to speed of onset.
Duration increases as lipid solubility increases, and decreases with higher rates of systemic
absorption as seen with drugs that cause vasodilation (lidocaine).
Drug
Lidocaine
pKa
7.9
Onset
Duration
Clinical dose
Toxic dose
Minutes
Hours
mg/kg
mg/kg
1-3
Dog 0.5-2.0
Dog 6.0
Cat 0.5-1.5
Cat 3.0
Dog up to 3.0
Dog 6.0
Cat up to 1.5
Cat 3.0
Dog 1.0-1.5
Dog 3.0
Cat 1.0
Cat 2.0
5-10
(Xylocaine)
Mepivacaine 7.6
5-10
1.5-3
(Horses-lower
Neurotoxicity)
(Carbocaine, Vetacaine)
Bupivacaine 8.1
20-30
(Marcaine)
3-8
Ropivacaine like bupivacaine, pure S-enantiomer Dog up to 3.0
Dog 5.0
(Naropin)
not established
less cardio toxic
Cat 1.5
Easy Rule of Thumb – keep total body dose under 2mg/kg
Typical nerve blocks in cat = 0.1-0.3ml per site Typical nerve blocks in dog = 0.1-0.5ml per site
Chart derived from 1 Handbook of Small Animal Regional Anesthesia and Analgesia Techniques
by Lerche, Aarnes, Covey-Crump, and Taboada 2016 and Veterinary Anesthesia and Analgesia,
5th Ed. Lumb and Jones 2015
Drug combinations (lidocaine and bupivacaine) can be mixed in 1:1 ratio to take advantage of
shorter onset/longer duration, but could lead to unpredictable blocks depending on pH of the
final mixture. Remember that local anesthetic toxicity of combinations is additive.
Other drugs can be added to local anesthetics to achieve various goals.
Epinephrine – 5ug/ml added to local anesthetic for vasoconstriction, which decreases systemic
absorption and prolongs the blockade. Fewer systemic side effects are seen with decreased
absorption and lower plasma concentration of local anesthetic. If systemic absorption of
epinephrine, will see increased heart rate, stroke volume, cardiac output, decreased peripheral
vascular resistance (with excessive absorption = tachycardia and arrhythmias).
Opioids – classically preservative-free morphine for epidural/spinal analgesia to give additive
effects up to 24hours. Complications could include respiratory depression (high volumes/doses),
urinary retention, vomiting, pruritis. Rare – myoclonus, paresis, hyperesthesia.
Alpha 2 adrenergic receptor agonists – xylazine (0.25mg/kg), medetomidine (15ug/kg),
dexmedetomidine (recent). Side effects bradycardia, hypertension. Increased duration of
analgesia by adding dexmedetomidine to local anesthetics due to hyperpolarization of C fibers
thru blockade. Alpha-adrenergic receptors do not seem to be involved: giving antagonists does
not reverse the conduction block nor decrease duration of the block.
Ketamine – 1-3mg/kg. Analgesic effects by blocking NMDA channels, also blocks NA/K channels
which decreases propagation of nocioceptive signals. Side effects – increased heart rate, blood
pressure, and myocardial work.
Sodium bicarbonate – 1mEq per 10ml (0.1mEq per 1ml) local anesthetic decreases pain
experienced on injection. Do not add to bupivacaine or ropivacaine = precipitation. Increases
the amount of active drug present which increases diffusion across cell membranes, leading to
shorter onset and longer duration of blockade. Can precipitate with time, use immediately.
Hyaluronidase – 3.75IU per ml local anesthetic results in better spread of block by improving
permeability of tissue secondary to depolymerizing hyaluronic acid. Duration may be decreased
and toxicity increased due to increased systemic absorption.
It is important to understand the complications of using local anesthetics and how to treat
them, as well as to know when their use is contraindicated. There are both local and systemic
complications. *For all nerve blocks, aspirate prior to injection to decrease the changes of
intravascular administration.
LOCAL
SKIN – Do not use local anesthetics where tissue at the site of injection is inflamed,
infected, or obviously neoplastic = transmission of infectious organisms or tumor cells into
adjacent tissue
NERVE – Direct trauma from injection =loss of sensation, pain, discomfort, weakness.
Can be directly toxic to nervous tissue. Damage to blood vessels close to nerves = ischemia.
TRAUMA- inadvertent block of lingual nerve with mandibular (inferior alveolar) block can
lead to self-mutilation on recovery. Sometimes self-trauma/pawing at face secondary to dental
blocks is seen (e-collar, sedation).
SYSTEMIC
CENTRAL NERVOUS SYSTEM – Excessive systemic uptake of local anesthetic can cause
seizures. Inhibitory neurons are more sensitive to Na+ channel blockade, and if they are
blocked = excitation.
CARDIOVASCULAR - At low concentrations, most local anesthetics have an
antiarrhythmic effect, but at higher concentrations, they produce cardiac toxicity. Local
anesthetic techniques should be avoided in patients that are hypotensive/shocky – inadvertent
drug administration into a vein or unexpected rapid systemic absorption can cause
cardiovascular collapse.
COAGULOPATHY – caution with techniques where hemorrhage is a possible complication
METHEMOGLOBINEMIA – rare, ester type benzocaine and amide type prilocaine,
oxidative damage to the hemoglobin molecule (cannot bind oxygen). Chocolate brown blood
ALLERGIC REACTIONS – more common with ester (Procaine). Very rare in dogs/cats.
Anaphylactic reactions – bronchospasm, upper airway edema, vasodilation, cutaneous wheals.
Tx – maintain airway, oxygen therapy, epinephrine, volume expansion.
Maxillary Nerve Block
Region anesthetized: Dorsal head and muzzle to midline. Maxilla, maxillary teeth, palate,
nares, upper labium and rostral nasal cavity.
Technique: Find depression formed by caudal maxilla, ventral edge of zygomatic arch,
and vertical ramus of mandible. Needle is directed perpendicular to the pterygoid fossa to also
block the nasal and palatal nerve branches. (B in photo below).
Can also palpate ventral aspect of zygomatic arch where it joins the maxilla, and direct the
needle dorsally adjacent to the bone, advancing to the level of the root tips of the last molar.
White arrow pointing to maxillary foramen, which is the caudal opening of the infraorbital canal.
Disadvantage is that it involves blind placement of the anesthetic into the retrobulbar space,
which houses a number of vital structures. Advantage is that may provide improved block for
the last premolar and molars.
Infraorbital Block
Region anesthetized: Rostral part of muzzle including skin, maxilla, maxillary teeth,
palate depending on level block is placed.
Technique: Lift the upper lip, and locate the infraorbital foramen dorsal to the 3rd
premolar tooth. Start needle thru the mucosa in a caudal direction into the entrance of the
foramen (caution on distance in cats, brachycephalic dogs). Medial canthus of eye is guide for
maximum needle advancement. Aspirate back, inject with thumb or finger over the opening and
keep firm digital pressure to help encourage posterior direction of flow (also helps with
hematoma formation).
Advantages over other techniques to block the maxillary nerve: 1) ensures needle is advanced
accurately to desired location 2) tip of needle parallel to nerves avoiding perpendicular contact,
decreasing risk of nerve damage 3) canal guides the needle, so damage to the zygomatic
salivary gland is unlikely 4) safer and more effective than lateral approach. Disadvantage is that
the infraorbital approach may not consistently anesthetize the maxillary molar area.
Mandibular (Inferior Alveolar)
Region anesthetized: Hemi-mandible up to the chin. Tissues between the mandibles
should not be anesthetized if performed correctly (tongue should NOT be blocked).
Technique: Can be done with intra-oral or extra-oral approach.
Extra-oral: Palpate the angular process of the mandible, then open the mouth and
palpate the mandibular foramen (1/2 way between last molar tooth and condylar process).
Introduce the needle thru the skin dorsally along the medial aspect of the mandible (stay
directly on the bone to avoid lingual block). Direct tip of needle toward foramen (your finger is
on this intra-orally), aspirate back, and diffuse the area (very difficult to get into the foramen).
Intra-oral: Thumb on coronoid process and forefinger on angular process of mandible.
Imaginary line ½ way between (notch on mandible) – needle introduced medial aspect and
advanced on the line
Desensitization of the tongue can occur with bilateral blocks using large volume or poor
technique (not locating and injecting the foramen/as close as possible). Can lead to selfmutilation of the tongue.
Mental (Middle Mental) Block
Region anesthetized: Chin with bilateral, up to 4th premolar if inside the foramen.
Technique: Palpate the foramen ventral to the 2nd premolar tooth from the inside
of the lower lip (near lip frenulum). Introduce the needle, advancing caudally at the entrance
(or place around outside of foramen – cats, small dogs). Aspirate back, digital pressure to force
the agent to diffuse caudally into the mandibular canal).