Harmless herbs: a cause for concern?

There is extensive use of the herbal medicine St John's Wort in many western countries. It is used to treat anxiety, anorexia and depression and is readily available over the counter in most High Street chemists. Its active ingredient, hypericin, causes irreversible inhibition of both monoamine oxidase (MAO) A and B with higher activity towards type B [1]. Therefore, hypericin (like MAOIs) may also have the potential to interact with anaesthetic drugs. A 37-year-old female was scheduled for removal of silicone breast implants. She was fit and generally well with no significant past medical history. On direct enquiry, she denied any regular medication. She was noted to be anxious pre-operatively and so was offered premedication. However, she declined this and instead took one of her St Johns Wort tablets that she had found to ease her anxiety in the past. It transpired that she had been taking St Johns Wort regularly and therefore her elective operation was postponed due to the potential risk of a serious reaction to opioids and sympathomimetics, as discussed below. There are no reported cases of interactions between St John's Wort and anaesthetic drugs, but there have been cases of adverse reactions in patients undergoing anaesthesia while taking MAOIs. For example, a 21-year-old on an MAOI, developed hypotension and bradycardia while under spinal anaesthesia. This was treated with ephedrine and atropine but she subsequently developed agitation, dyspnoea, severe headache, hypertension and tachycardia [2]. Churchill-Davidson advised checking patients' sensitivity to MAOIs and opioids by giving a test dose of 5 mg of pethidine and monitoring vital signs regularly [3]. There are also reports of MAOIs causing an increased predisposition to the development of a malignant hyperthermia syndrome [4] and prolongation of suxamethonium block [5]. Thus, the recommendation is that MAOIs should be stopped 2 weeks prior to anaesthesia because of the potential for serious drug interactions [6]. From 1983 to 1989, the National Poisons Unit in London received 1070 inquiries following exposures to herbal preparations of which 25.2% were symptomatic [7]. Increased public use has made the medical community begin to seek accurate information about their therapeutic safety and side-effects. Medicinal herbs are not considered as drugs, are not regulated by the FDA and are exempt from the stringent testing procedures that licensing under the Medicines Act 1968 requires. The Pharmaceutical opinion is that excessive use of St John's Wort should be avoided. It would be wise for patients taking this drug to observe precautions appropriate for conventional MAOIs and the same applies to anaesthetic practice. In addition, there should be an increased awareness of self-medication with herbal adjuncts in the anaesthetist's pre-operative assessment.