Dyshemoglobin

Dyshemoglobin refers to a group of conditions in which hemoglobin molecules are chemically altered to forms that do not function normally for oxygen transport and delivery. These abnormal hemoglobins, collectively called dyshemoglobins, can impair tissue oxygenation and present with varying degrees of cyanosis and hypoxia. They can be congenital, due to structural hemoglobin variants or redox enzyme defects, or acquired, resulting from exposure to certain drugs, chemicals, or toxins.

The two most common acquired forms are methemoglobinemia and sulfhemoglobinemia. In methemoglobinemia, iron in the heme is oxidized from Fe2+ to Fe3+, which cannot bind oxygen and shifts the dissociation curve, often causing chocolate-brown blood and cyanosis despite a normal arterial oxygen tension. Methemoglobinemia can be inherited from defects in NADH-cytochrome b5 reductase or occur with exposure to oxidant drugs such as nitrates, dapsone, benzocaine, or certain antibiotics. In sulfhemoglobinemia, a sulfur atom is incorporated into the heme, creating a molecule that carries little oxygen and is not readily reduced back, and it is typically linked to certain drugs or sulfonamides. A related situation is elevated carboxyhemoglobin from carbon monoxide exposure, which binds hemoglobin with high affinity and diminishes oxygen delivery.

Diagnosis relies on co-oximetry to quantify dyshemoglobins; pulse oximetry can be unreliable, and arterial blood gas may show normal PaO2. Treatment involves removing the offending agent and providing supportive care; methylene blue is the antidote for methemoglobinemia (when G6PD status is normal), with ascorbic acid as an alternative in some cases, while sulfhemoglobinemia lacks a specific antidote and improves after elimination of the cause. Prognosis is generally favorable with timely management, though hereditary forms require ongoing care.