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ARTICLES:
Ahmed Dahshan and G. Kevin Donovan
Severe Methemoglobinemia Complicating Topical Benzocaine Use During Endoscopy in a Toddler: A Case Report and Review of the Literature
Pediatrics 2006; 117: e806-e809 [Abstract] [Full text] [PDF]
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[Read P3R] Endoscopy Suite Methemoglobinemia - Not "Out of the Blue"
Kevin C Osterhoudt   (21 April 2006)

Endoscopy Suite Methemoglobinemia - Not "Out of the Blue" 21 April 2006
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Kevin C Osterhoudt,
Medical Director
The Poison Control Center, Philadelphia

Send letter to journal:
Re: Endoscopy Suite Methemoglobinemia - Not "Out of the Blue"

OsterhoudtK{at}email.chop.edu Kevin C Osterhoudt

Methemoglobinemia following mucosal application of benzocaine, in an endoscopy suite setting, should not be a surprise. Indeed, the authors point out several recent reports of this phenomenon, and benzocaine was already well-known to be a hemoglobin oxidant prior to the start of their 20-year literature review. The cited manufacturers of the benzocaine spray used in this case clearly list methemoglobinemia as a possible complication on their product information. That said, several points of this case report merit comment.

It is most remarkable that the authors' case description describes hypoxemia manifest by an arterial blood gas "PaO2 = 29%." It is unclear what this number represents, as the blood partial pressure of oxygen is typically recorded in units of mmHg. In any case, methemoglobinemia does not lower the blood partial pressure of oxygen - only the hemoglobin oxygen content. This physiology is the basis for the well-described calculated versus measured hemoglobin "saturation gap."

It would have been useful to provide the patient's hemoglobin concentration at the time of endoscopy, as methemoglobinemia (expressed as a percentage) is best interpreted in the context of the hemoglobin level. Also, the authors' try to estimate the benzocaine content of what is usually an unstandardized unit of measure - the "spray." An aerosol spray may deliver a different amount of drug based upon the concentration of drug in the can, the pressure in the can, the force with which the nozzle is depressed, and the duration of time for which the nozzle is depressed. More information about the administered spray is needed.

Although benzocaine-induced methemoglobinemia is well characterized, it seems to have a low incidence when considered in the context of the volume of endoscopies performed. The authors suggest their patient may have been more severely affected due to increased systemic drug absorption due to his tonsillectomy; however, local anesthetics are well-absorbed after normal mucosal application (for example, insufflation of cocaine). I would point out one other possibility for consideration - phenotypic variability in the expression of NADH-dependent methemoglobin reductase activity.

Methemoglobinemia following mucosal application of benzocaine, in an endoscopy suite setting, should not be considered to be from "out of the blue." Every gastroenterologist, pulmonologist, and surgeon should be taught this adverse drug effect prior to using an endoscope clinically.

Conflict of Interest:

None declared