One blue man with mucositis

Severe methemoglobinemia from topical anesthetic spray: case report, discussion and qualitative systematic review

Few health care professionals realize that topical anesthetic spray can cause methemoglobinemia. We describe a 56-year-old woman who was transferred to our emergency department when severe cyanosis and chest pain developed after administration of topical oropharyngeal benzocaine and lidocaine during outpatient endoscopy. Investigations revealed a methemoglobin level of 51%. Despite rapid diagnosis and treatment with methylene blue, pulmonary edema consistent with adult respiratory distress syndrome developed, endotracheal intubation was required, and the patient suffered a lengthy course in the intensive care unit. This article presents a detailed discussion of the pathophysiology, diagnosis and treatment of methemoglobinemia, as well as a qualitative systematic review of the English literature on methemoglobinemia induced by topical anesthetic. The implications of this condition for emergency physicians are also outlined.

Occupational methaemoglobinaemia. Mechanisms of production, features, diagnosis and management including the use of methylene blue

Methaemoglobin is formed by oxidation of ferrous (FeII) haem to the ferric (FeIII) state and the mechanisms by which this occurs are complex. Most cases are due to one of three processes. Firstly, direct oxidation of ferrohaemoglobin, which involves the transfer of electrons from ferrous haem to the oxidising compound. This mechanism proceeds most readily in the absence of oxygen. Secondly, indirect oxidation, a process of co-oxidation which requires haemoglobin-bound oxygen and is involved, for example, in nitrite-induced methaemoglobinaemia. Thirdly, biotransformation of a chemical to an active intermediate that initiates methaemoglobin formation by a variety of mechanisms. This is the means by which most aromatic compounds, such as amino- and nitro-derivatives of benzene, produce methaemoglobin. Methaemoglobinaemia is an uncommon occupational occurrence. Aromatic compounds are responsible for most cases, their lipophilic nature and volatility facilitating absorption during dermal and inhalational exposure, the principal routes implicated in the workplace. Methaemoglobinaemia presents clinically with symptoms and signs of tissue hypoxia. Concentrations around 80% are life-threatening. Features of toxicity may develop over hours or even days when exposure, whether by inhalation or repeated skin contact, is to relatively low concentrations of inducing chemical(s). Not all features observed in patients with methaemoglobinaemia are due to methaemoglobin formation. For example, the intravascular haemolysis caused by oxidising chemicals such as chlorates poses more risk to life than the methaemoglobinaemia that such chemicals induce. If an occupational history is taken, the diagnosis of methaemoglobinaemia should be relatively straightforward. In addition, two clinical observations may help: firstly, the victim is often less unwell than one would expect from the severity of 'cyanosis' and, secondly, the 'cyanosis' is unresponsive to oxygen therapy. Pulse oximetry is unreliable in the presence of methaemoglobinaemia. Arterial blood gas analysis is mandatory in severe poisoning and reveals normal partial pressures of oxygen (pO2) and carbon dioxide (pCO2,), a normal 'calculated' haemoglobin oxygen saturation, an increased methaemoglobin concentration and possibly a metabolic acidosis. Following decontamination, high-flow oxygen should be given to maximise oxygen carriage by remaining ferrous haem. No controlled trial of the efficacy of methylene blue has been performed but clinical experience suggests that methylene blue can increase the rate of methaemoglobin conversion to haemoglobin some 6-fold. Patients with features and/or methaemoglobin concentrations of 30-50%, should be administered methylene blue 1-2 mg/kg/bodyweight intravenously (the dose depending on the severity of the features), whereas those with methaemoglobin concentrations exceeding 50% should be given methylene blue 2 mg/kg intravenously. Symptomatic improvement usually occurs within 30 minutes and a second dose of methylene blue will be required in only very severe cases or if there is evidence of ongoing methaemoglobin formation. Methylene blue is less effective or ineffective in the presence of glucose-6-phosphate dehydrogenase deficiency since its antidotal action is dependent on nicotinamide-adenine dinucleotide phosphate (NADP+). In addition, methylene blue is most effective in intact erythrocytes; efficacy is reduced in the presence of haemolysis. Moreover, in the presence of haemolysis, high dose methylene blue (20-30 mg/kg) can itself initiate methaemoglobin formation. Supplemental antioxidants such as ascorbic acid (vitamin C), N-acetylcysteine and tocopherol (vitamin E) have been used as adjuvants or alternatives to methylene blue with no confirmed benefit. Exchange transfusion may have a role in the management of severe haemolysis or in G-6-P-D deficiency associated with life-threatening methaemoglobinaemia where methylene blue is relatively contraindicated.

Toxic methemoglobinemia after topical anesthesia for transesophageal echocardiography

Topical benzocaine spray is an anesthetic agent that is commonly used during transesophageal echocardiography (TEE). This agent is believed to be relatively safe because of very low systemic absorption. We report a case of toxic methemoglobinemia induced by benzocaine use for pharyngeal anesthesia during TEE. Physicians who perform TEE and use benzocaine or other topical anesthetics need to be familiar with this potential complication and have the necessary treatment readily available.

Methemoglobinemia from topically applied anesthetic spray

Topically applied anesthetic spray is commonly used as part of premedication for general anesthesia and for endoscopic procedures; it is rarely associated with side effects. In this report, we describe two cases of toxic methemoglobinemia that resulted from topically applied anesthetic spray used before endoscopy. In both cases, standard doses were used; however, methemoglobin levels of 45% and 38% developed within 1 hour of the procedure. Both patients had normal levels of erythrocyte methemoglobin reductase, an indication that this rare but potentially fatal side effect can occur in persons who have no predisposing factors. Because toxic methemoglobinemia is easily treated, our report emphasizes the need to recognize this problem when topically applied anesthetic sprays are used.

Concise review: methemoglobinemia

The ferrous iron of hemoglobin is exposed continuously to high concentrations of oxygen and, thereby, is oxidized slowly to methemoglobin, a protein unable to carry oxygen. To restore hemoglobin function, methemoglobin (ferrihemoglobin) must be reduced to hemoglobin (ferrohemoglobin). Under physiological conditions, methemoglobin reduction is accomplished mainly by red cell NADH-cytochrome b5 reductase (NADH-methemoglobin reductase) so efficiently that there is insignificant amounts of methemoglobin in the circulating blood. However, should methemoglobin formation be increased--e.g., due to the presence of oxidant drugs, or an abnormal methemoglobin not amenable to reduction (hemoglobin M), or a deficiency in red cell cytochrome b5 reductase--methemoglobinemia will result. Most methemoglobinemias have no adverse clinical consequences and need not be treated. Under certain conditions, such as exposure to large amounts of oxidant or in young infants, rapid treatment is necessary. In hereditary cytochrome b5 deficiency, treatment is often directed at improving the poor cosmetic effect of persistent cyanosis with the minimum amount of drugs to give satisfactory clinical results.