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
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