Anaphylactic and histaminoid reactions are rare, but potentially fatal, accompaniments of all anaesthetic techniques. True allergy to the modern, amide-linked local anaesthetic agents is even rarer than to the other agents in the anaesthetic armamentarium, but the possibility cannot be totally excluded.
Unfortunately, the classical concept of an antigen-antibody reaction producing massive histamine release, typical features of hypotension, bronchospasm and peripheral oedema, accompanied by measurable changes in plasma markers of the reaction is not the only manifestation of "allergy". Reactions may occur without previous drug exposure by activation of complement or by direct pharmacological effect. The picture may be complicated further by the occurrence of "partial" syndromes. An immunologically mediated reaction may not produce all the classic symptoms and signs. The release of other mediators and/or the influence of concurrent drug administration on the actions of histamine and these other mediators may further modify the response. Yet another difficulty is that totally different aetiologies, particularly anaesthetic misadventure, may lead to similar signs and symptoms. Human nature being what it is, it may be easier to attribute "blame" to the patient by diagnosing an immunological reaction!
Against this complicated background, the clinician must be constantly on the alert for physiological changes in response to drug administration and seek to diagnose rapidly the cause of a particular reaction. The range of alternative diagnoses is wide and includes:
1. drug interactions,
2. adverse physiological response to anaesthetic depression or surgical stimulation,
3. complications (eg acid aspiration) of anaesthesia, and
4. vagal overactivity in the conscious patient.
A wide range of precipitants may produce a true immunological reaction, including the active agent, the solvent or any preservative in the drug solutions administered. Agents administered coincidentally, such as antibiotics, skin preparation solutions and latex rubber should also be considered.
Rapid diagnosis needs to be accompanied by equally rapid response. There is increasing evidence that immunologically based reactions must be treated with adrenaline before any other agent. Fortunately, this drug will correct many (but not all) of the adverse effects of other pathological states which produce a similar physiological picture. The preferred method of administration is small (1ml) incremental doses of adrenaline 1:10,000, titrated to effect. Subsequent management will very much depend on the nature and severity of the reaction, but serial blood samples should be taken for complement, immunoglobulin and other assays. Prolonged intensive care may be necessary.
Once the acute event, is over it is necessary to try and establish the true cause of the reaction so that definitive information may be given to the patient and incorporated in the patient's records. This requires careful consideration of all possible causes of the reaction and thoughtful analysis of blood results. In some circumstances, there may only be circumstantial evidence that a particular agent was involved, but it may be unnecessary to proceed further if alternative chemical entities can be used to achieve the same effect in a subsequent procedure. However, in the case of local anaesthetic drugs this is not possible and it is certainly appropriate to offer to the patient a "challenge" testing exercise involving intradermal and subcutaneous injection of progressively increasing doses of local anaesthetic to see what happens. There is some risk, but true allergy to local anaesthetics is rare and subsequent patient management can be very difficult if this diagnosis has been made without genuine evidence. Finally, if a diagnosis is reached of aIlergy a report should be made through the appropriate regulatory body.