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Allergy
Allergic reactions from local anaesthesia have been reported in some patients. However, this occurrence is rare even in patients who had a past history of adverse reactions to LA.

There are mainly 2 classes of local anaesthetic agents: Amide or Ester linkages, based on their chemical structure.


 * E.g. of amide LA: lidocaine, prilocaine, articaine, mepivacaine
 * E.g. of ester LA: benzocaine, procaine

Genuine allergic reactions of an amide LA is very uncommon. An ester LA is more possible to result in an allergic reaction because the compound will be broken down to para-aminobenzoic acid (PABA) which is a trigger for allergic reactions. In general dentistry, only topical applications of LA contains esters (benzocaine) when applied onto area before LA is administered.

If one is allergic to an ester LA, then the use of other types of ester LA should be avoided as the breakdown of all esters will produce PABA. However, patients allergic to ester LA will usually not be affected by amide LA because PABA is not produced upon breakdown of amide LA. Unlike ester LA, allergy to an amide LA will not eliminate the use of other types of amide LA.

Some reactions are caused by administration of too much drug, usually because of the route of entry of drug (intravenously) or the quick uptake of drug into the system, or the aftereffect of the vasoconstrictor. Unfavourable reactions to LA can be classified into 3 different groups: psychogenic, allergic, toxic.

Differential Diagnosis & management

 * Psychogenic reactions

Unfavourable reactions to LA are commonly be due to a hyperemotional response to a perceived danger within someone’s mind, and it could be demonstrated in several ways. Examples are temporal loss of consciousness, sweating, flush, change in heart rate or blood pressure, panic attack, hyperventilation, of which may be mistaken as allergic reactions.

When treating such patients, treat them with care and take into consideration their anxiety. During treatment if the patients feel faint or experiences a drop in blood pressure, lay them flat and keep their legs elevated in an attempt to restore their blood pressure. Loosen any tight clothing and keep the patients sugary food/drink after they regain consciousness. Reassure the patient.


 * Toxic reactions

This may occur when there are large amounts of anaesthetic within their vascular system, which may be due to them receiving repeated LA, intravenous entry of drug, or have underlying systemic conditions that does not metabolize or utilize the drug efficiently. Signs and symptoms mainly involve the nervous system e.g. aggressive behaviour, drowsiness, speech alteration, disorientation etc.

Their symptoms should usually resolve in a few hours, up to 12 hours, as the body will gradually rid the bloodstream of the drug. Assure the patient that their symptoms will improve after a few hours and that such a reaction should not recur, and that there is no need to abstain from that drug hereafter.

Such reactions can be minimized via practicing safe injection methods using an aspirating syringe to prevent injecting in blood vessels, slow administration of drug, and avoid overprescribing LA, keeping in mind the patient’s weight, age and medical history.

Signs & symptoms of allergic reactions to LA
Genuine allergy to LA will manifest either as Type 1 or Type 4 hypersensitivity. Signs and symptoms will vary depending on the type of allergy. Type 1 reactions have a rapid onset of symptoms which include swelling, redness, rashes, itchiness, chest tightness, breathing problems. A Type 4 reaction has a delayed onset of symptoms and is usually localized to the site of injection.

Management
If a genuine allergic reaction to LA should occur, the patient should be treated as an emergency for anaphylaxis, according to the guidelines in the respective areas. For the UK, the section on medical emergencies in dental practice in the “Prescribing in Dental Practice” part in the BNF should be referred to. The patient should be sent immediately to the hospital if their condition worsens.

The individual should undergo further tests to certify their allergy to the LA or for other possible causes of the adverse reaction.

Soft tissue Injury
A long-acting local anesthetic (i.e., bupivacaine) is not recommended for the child or the physically or mentally disabled patient due to its prolonged effect, which increases the risk of soft tissue injury. Bleeding and infections may occur in soft tissue injury which is usually caused by lip or cheek biting post-operative while numbness is still felt by patient. Individual who receive dental anesthesia or his caregiver should be informed regarding the duration of numbness, possibility of soft tissue injury and precautions way. Selection of type of anesthesia agents is important to minimized the residual soft tissue anesthesia especially to pediatric and special needs patient.

Methemoglobinemia
An end product of prilocaine metabolism can induce formation of methemoglobin, reducing the blood’s oxygen-carrying capacity may cause decreased in oxygen saturation in the body and unexplained cyanosis despite adequate ventilation. At higher concentration of methemoglobin symptoms like headache, dizziness, weakness, dyspnea, tachycardia followed by confusion, seizures and coma may develop.

Management
Immediate intravenous administration of methlene blue 1-2mg/kg over a 5 to 10 minutes period repeatable in 1 hour to a maximum dose of 7mg/kg.

Paresthesia
Risk of permanent paresthesia has been estimated to be 1:1,200,000 for 0.5 percent, two percent, and three percent local anesthetics and 1:500,000 for four percent local anesthetics. undefined the dubious reputation of articaine with regard to post-operative paresthesia and the discussion about it being manufactured as a 4% solution instead of 2% like lidocaine for dental local anesthesia.