Local anaesthesia very often needs to be given during dental therapy in order for the agreed treatment to be successfully completed.

The local anaesthetic will be given by injections into the nerve plexus, the nerve trunk or by intraligamentary injection to cover all the nerve supply to the affected part. Local anaesthetics, like any other drug, can have side effects, so the clinical situation must be properly assessed and the most suitable compound chosen. It is therefore essential that the information patients provide reflects their current state of health, including all the drugs they have recently taken and are taking at the time of the visit; adverse reactions and interactions between the drugs taken and the anaesthetic can then be predicted. Temporary side effects, such as sweating, tremors, arrhythmias, nausea and vomiting are possible; if these symptoms become severe we may be dealing with vasovagal syncope, which is characterised by brief fainting due to cerebral hypoperfusion. This symptom appears and passes very quickly; recovery is complete and spontaneous.

Another complication to fear is methaemoglobinaemia, caused by the accumulation of methaemoglobin in the blood. The risk of this situation occurring is greatly increased in patients who produce an abnormal haemoglobin variant in which the iron atom of the 4 heme groups is oxidised from Fe2+ to Fe3+, and this is a contraindication to the administration of articaine and prilocaine.

The symptoms of the onset of methaemoglobinaemia are drowsiness and respiratory failure, shortness of breath, pale skin and cyanosis which is irreversible during oxygen therapy.

Allergic and toxic reactions caused by the use of local anaesthetics in dentistry

There may also be some rare cases of allergy. Predisposed individuals may have cutaneous symptoms: angio-edema may sometimes occur on one or more sites (eyelids, lips, tongue etc.) and the respiratory and cardiovascular systems may also be involved. The most rare and feared event is anaphylactic shock.

Allergic reactions occur within a few minutes or within a few hours of administration of the drug. However, in a small percentage of cases the clinical symptoms recur a few hours after the primary event, or they may be prolonged, lasting more than 24 hours.

In addition to allergies, toxic reactions may occur due to the use of local anaesthetics. However, the risk is greatly reduced by keeping within the safety parameters as regards the amount of drug administered and by using a correct injection technique. Cases of complications related to toxicity almost exclusively occur in patients who have other serious systemic diseases (liver disease, kidney disease etc.). Finally, cardiovascular reactions are rare and are most likely in the case of pre-existing cardiovascular disease (it is thus important that the patient informs the dentist of its existence). Signs of toxicity may be significant and include agitation, tremors, convulsions and bradycardia leading to myocardial and respiratory depression.

The onset of such reactions and complications is rare, due to the high safety profile of the active ingredients and the techniques used, but it is essential that the dentist is capable of recognising the symptoms in order to initiate the correct therapeutic protocol in case of need

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