Convulsions associated with a low plasma level of local anaesthetics

Editor—We read with interest the case report by Satsumae and colleagues, 1 addressing the question of sensitivity to local anaesthetics (LAs) with toxicity occurring at a lower than expected dose. We encountered a similar case in a 51-yr-old woman who presented with generalized myoclonic movements after a popliteal-sciatic blockade with a non-stimulating catheter, inserted with a peripheral nerve stimulator. Ropivacaine 24 mg had been given 60 min after injection of lidocaine 300 mg. The dosages and time intervals between injections complied with guidelines of the French Society of Anesthesiology. 2 A venous blood sample drawn during the myoclonic movements showed non-toxic levels of LAs (0.12 and 0.1 mg litre 21 for ropivacaine and lidocaine, respectively). The blood glucose level was normal, and she had no significant medical history, such as neurological disease or high alcohol intake, and took no medication. The postoperative course was uneventful and an electroencephalogram performed 30 days later showed no abnormality. This case suggests that toxic complications may occur after administration of low dosages of LA in the absence of intravascular injection. In healthy volunteers, 3 a mean plasma level of ropivacaine 2.2 (0.8) mg ml –1 was found for the first signs of neurological toxicity, but myoclonic movements were considered as signs of systemic toxicity, as in our patient, and the lowest plasma level associated with this kind of symptoms was 0.5 mg ml –1 . 3 Two case reports have previously reported symptoms of toxicity of LA associated with the administration of a small dose. 45 In one case, symptoms occurred with low plasma levels of ropivacaine 4 suggesting that some patients may have a low tolerance to LA or that the threshold for toxicity varies with factors such as medication, hypercarbia, electrolytic abnormalities, and carnitin deficiency. In addition, general anaesthesia may influence the toxicity of ropivacaine by central nervous system effects and altered pharmacokinetics. This case suggests that these complications may occur with low dosages, despite careful attention to the needle and catheter placement, 6 fractionated dosing, and frequent aspirations, in the absence of intravascular injection. It must also be pointed out that a 1 mg litre 21 plasma level of lidocaine and 0.12 mg litre 21 of ropivacaine seem to be very low, especially when it was measured during the myoclonia suggesting that patients with a low tolerance to LAs may exist. Therefore, safety in regional anaesthesia cannot rely only on the use of ‘safe’ dose limits. Careful monitoring and preparation for managing complications throughout the course of regional anaesthesia is of paramount importance.