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The administration of local anesthesia is a daily routine for most dental practitioners. Local anesthetics work by blocking the travel of the pain signal to the brain. Besides delivering pain control, local anesthetics can provide vasoconstriction with the addition of vasoconstricting drugs into the anesthetic. Local anesthetics are generally safe and effective as long as proper administrative techniques and dosing are observed. Adequate preparation is essential for a dentist to mitigate against any risk and understand how to best manage any complications that may occur.
Administration and Formula
All drugs have side effects, and their risk may or may not be proportional to the route of administration. For example, even a medication taken orally can be lethal. An intramuscular injection reaches the central circulation faster than an oral medication, and an intravenous drug goes directly into the central circulation. Similarly, drugs can also be inhaled into the lungs, a robust vasculature. They can be administered into connective tissue, such as in local anesthetics in the oral cavity; in deep connective tissue, such as the case of a mandibular block; or superficially, as with the infiltration for a maxillary tooth.
A few basic complications can arise specifically in relation to the injection itself, known as perioperative complications.1 One problem is pain on injection. Pain during administration of a local anesthetic can be due to many factors, including the pH value of the anesthetic, the temperature of the solution, or the technique of administration. Another potential problem is the dentist not giving enough or alternatively too much of the anesthetic. Sometimes the injection is given in the wrong place, into blood vessels, nerves, and muscle, or it can diffuse to an unintended site. There is the potential for nerve injury when the needle penetrates the neurobundle. A chemical injury can also occur when the local anesthetic is toxic.
When vasopressors such as epinephrine or levonordefrin are used, the pH has to be adjusted to maintain stability, causing an acidic solution that will likely hurt slightly more than a physiologic injection. Furthermore, because an acidic solution favors hydrophilic, cationic moiety, it will not diffuse well into the nerve. The key formula: when pH is equal to pKa in the local anesthetic, the ionized and un-ionized moiety are in equilibrium. Both types of moiety are needed for local anesthesia. The water-soluble ionized moiety is seen in the carpule, and it is what enables the local anesthetic to diffuse to the nerve. The un-ionized part is fat-soluble and diffuses into the nerve. When in the nerve, it blocks the sodium channels as it once again turns into a water-soluble moiety. Notably, the low pH favors an ionized form, so if a reinjection is needed, or the site is infected, one would choose a drug with low pKa.
Vasoconstrictors narrow blood vessels and delay the absorption of local anesthetic. Epinephrine and levonordefrin are examples of vasoconstrictors. The delay reduces the risk for systemic toxicity, but it also prolongs the duration of the anesthesia.2 Epinephrine causes central vasodilation and peripheral vasoconstriction. Levonordefrin is unlikely to cause tachycardia because it is not a strong beta agonist. However, there may be negative side effects to mitigate against; interactions with vasoconstrictors may have detrimental effects. People who are on tricyclic antidepressants may suffer an exaggerated pressor effect. Patients on a nonspecific beta-blocker could experience hypertension and reflex bradycardia. Toxicity can also be produced by vasoconstrictors, which may, for example, cause the heart to beat faster temporarily. The pharmacologic principle of using the lowest dosage possible to produce the desired action, and to minimize the potential for toxicity, should be applied to the use of vasoconstrictors in local anesthetic agents.2
Dentists should keep in mind the maximum safe doses of anesthetics in order to avoid toxicity. However, the amount will vary depending on a variety of factors, including but not limited to the patient's size/age/health, the duration of the procedure, and the patient's sensitivity to the anesthetic. Some of the risks can be mitigated by following general rules. In children and elderly patients, safe levels of anesthetics are lower than in the remaining population.
The toxic effect is primarily directed at the central nervous system and cardiovascular system. Typical symptoms are restlessness, dizziness, dysphoria, dysarthria, convulsion, and loss of consciousness. More severe symptoms that can occur are coma, respiratory arrest, increase in blood pressure and heart rate, and even vascular collapse and cardiac arrest.3,4 The reaction to toxicity depends on the amount of the dosage, the individual, and the drug itself.
Methemoglobinemia is a rare but serious complication of local anesthetic administration in dentistry. An elevated methemoglobin level in the blood reduces its ability to carry oxygen, resulting in cyanosis and the potential for significant morbidity.5 Similarly, cardiovascular collapse from accidental local anesthetic toxicity is a rare but catastrophic complication of regional anesthesia, capable of causing death with accidental overdose.6
Bupivacaine, for example, will hit the heart relatively quickly. In some isolated cases, it has been inadvertently injected intravascularly or an overdose has been given, causing the patient to go into asystole. Articaine provides rapid onset with a tendency to infiltrate through the cortex of the mandible; 4% articaine solutions achieve high levels of anesthetic potency and low systemic toxicity in all clinical situations.7 Articaine solutions must not be used in persons who are allergic or hypersensitive to sulfite, due to content of sodium metabisulfite as vasoconstrictor's antioxidant in it. The incidence of serious adverse effects related to dental anesthesia with articaine is very low.7 Again, toxic reactions are usually due to an inadvertent intravascular injection or use of excessive dose.
Paresthesia and Hypesthesia
Hypesthesia is a diminished capacity for physical sensation, whereas paresthesia is more generally an altered or abnormal sensation. Paresthesia can result from nerve injury during needle insertion or withdrawal. Inferior alveolar nerve block is the second-most common cause of permanent altered sensation of the trigeminal nerve (the most common is third-molar removal).1 However, these kinds of injuries are rare, with a ratio of about 4:100,000.8,9
Prilocaine and articaine are more highly associated with paresthesia after a mandibular block.10 However, paresthesia in the lingual nerve is much greater than in the mandibular nerve because it is smaller. Phentolamine is a nonselective alpha-blocker that reverses the vasoconstrictor.11 The concept is to avoid a negative experience caused by numbness; for example, a child biting his or her lip after a dental procedure. A situation can also arise in which the lip is numb but the tooth is not, causing the patient to feel pain during treatment. This problem is most common with block anesthesia, especially in the lower jaw.1
Failures in dental local anesthesia can be classified as anatomical, pathologic, psychological, or due to poor technique, with the latter being the most common.1 However, sometimes the anesthetic goes into the wrong place even when injected into the proper location. Local anesthesia could also diffuse into nearby structures, resulting in a loss of function or numbness in those structures and areas. The effect typically dissipates when the drug wears off. If the drug were to go into muscle, patients may return a week later because they cannot open their mouth properly. This situation will usually resolve with the use of a nonsteroidal anti-inflammatory drug, or perhaps a steroid. If the local anesthetic were to go into an artery, it would be carried away from the heart, whereas in a vein, it would go toward the heart. Both interarterial and intravenous injections may produce overdose reactions. Intravascular injection is usually prevented by using an aspirating syringe. A return of blood into the dental cartridge is considered a positive aspiration. Even if there is no return of blood into the dental cartridge, there is still a risk of intravascular injection if the injection is administered rapidly and in close proximity to a capillary bed.12 Side effects to anesthesia in unintended locations range from complaint of a runny nose to temporary blindness.
Re-usage of needles among different patients should never happen in contemporary dentistry; yet, it can occur during the same appointment when giving additional dosages of anesthetics to the same patient. Repeated injections with the same needle can cause fatigue of the structure and increase the risk of needle fracture.13 The clinician should take care to always check all needles for any deformations before injection.
Metal alloys used currently in injection needles are flexible, stainless, and more durable than previously. This has decreased the number of needle breakages, although not entirely. Small gauges are easier to break, so 30-gauges are not recommended for a mandibular block. Flexibility and narrowness allow the needle to penetrate softly into the tissue; however, the needle can break more easily when bent or otherwise used incorrectly.14
Hypersensitivity or allergy to local anesthetic is very rare. It is estimated that less than 1% of all complications are caused by allergy.15,16 Dermatologic signs of allergic reactions include hives, itching, and redness. Allergic reactions vary from a mild skin irritation or rash to an anaphylactic shock. The latter can rapidly lead to a life-threatening condition due to airway passage obstruction in association with laryngeal edema.3
Esters are more prone to cause an allergic reaction due to one of their metabolites, para-amino benzoic acid, which is structurally similar to methyl-paraben. If the patient is allergic to ester local anesthetics, the practitioner may want to try an amide local anesthetic, and vice versa. If the patient is allergic to both amide and ester local anesthetics, the practitioner may want to try a methyl-paraben–free product.
Dentists should keep a basic medical emergency kit and epinephrine in their office. If a patient experiences an allergic reaction, the safest choice is usually to call emergency medical services (EMS) or refer the patient to an emergency room.
Pain can be considered one of the oldest of all dental problems; the control of pain during routine dental procedures is an important part of dental care delivery.17 Local anesthesia creates a numbing feeling that eliminates sensation in a specific area without loss of consciousness. Dental providers have the ability to control and alleviate pain during and after procedures.
It is crucial to patient care for a dentist not only to understand the risk factors of local anesthesia, but also to diagnose and subsequently treat any complications. The best way to avoid nearly all complications relating to the administration of local anesthetics is to use the right technique and have a comprehensive knowledge of anatomy as well as the patient's medical history.
About the Author
Robert C. Bosack, DDS
Orland Park, Illinois
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