Sulfhemoglobin

Sulfhemoglobin is an abnormal form of hemoglobin in which a sulfur atom is incorporated into the heme moiety, producing sulfheme. This chemical modification irreversibly alters the molecule and markedly reduces its ability to bind and release oxygen, thereby decreasing the oxygen-carrying capacity of blood.

Sulfhemoglobinemia most often occurs after exposure to sulfur-containing drugs and chemicals, such as certain sulfonamides and acetazolamide, and to some antibiotics and industrial compounds. Hydrogen sulfide exposure can also contribute. The reaction converts functional hemoglobin to sulfhemoglobin, and unlike other hemoglobin abnormalities, sulfhemoglobin cannot be converted back to normal hemoglobin by standard redox reactions.

Clinical features center on hypoxia: patients may develop cyanosis, dyspnea, and fatigue. The blood can take on a greenish or dark tint. Pulse oximetry readings may be discordant with arterial oxygen content, and common co-oximetry methods may have difficulty accurately quantifying sulfhemoglobin.

Diagnosis relies on a history of exposure to sulfur-containing agents and persistent cyanosis despite oxygen therapy. Specialized spectrophotometric testing or advanced co-oximetry is typically required to quantify sulfhemoglobin, as routine devices may misread the relevant fractions. The condition is usually benign in mild cases but persists for as long as red blood cells remain in circulation.

Treatment focuses on removing the offending agent and providing supportive care. There is no specific antidote. In severe cases, therapeutic exchange transfusion has been reported to rapidly reduce sulfhemoglobin levels. Levels decline only with the turnover of red blood cells, roughly matching the lifespan of circulating erythrocytes.