[Sulfhemoglobinemia secondary to the use of zopiclone. Report of two cases].

Sulfhemoglobin (SulfHb) is formed by hemoglobin (Hb) oxidation by sulfur compounds. Sulfhemoglobinemia is mainly associated with drugs or intestinal bacterial overgrowth. Patients present with central cyanosis, an abnormal pulse oximetry and normal arterial oxygen partial pressure. These features are shared with methemoglobinemia (MetHb) whose diagnosis requires an arterial co-oximetry. Depending on the device used, SulfHb may produce interference with this technique. We report two females aged 31 and 43 years, consulting at the emergency room with cyanosis. Both had a history of acute and chronic, high dose zopiclone ingestion. Pulse oximetry showed desaturation but with normal arterial oxygen partial pressure. Cardiac and pulmonary diseases were ruled out. Co-oximetry in two different analyzers showed interference or normal MetHb percentages. No other complications ensued, and cyanosis decreased over days. Since MetHb was discarded among other causes of cyanosis in a compatible clinical context, the diagnosis of sulfhemoglobinemia was made. The confirmatory method is not available in Chile. The presence of SulfHb is difficult to diagnose, confirmatory tests are not readily available, and it frequently interferes with arterial co-oximetry. This is attributed to a similar absorbance peak of both pigments in arterial blood. Venous co-oximetry can be useful in this context. SulfHb is a self-limited condition in most cases, however it must be differentiated from methemoglobinemia to avoid inappropriate treatments like methylene blue.