RESUMO
Cyanosis is a bluish discoloration of the tissues due to increased levels of deoxygenated hemoglobin in capillaries. It is a common finding in newborn infants that can be caused by different diseases, including pulmonary, cardiac, infectious, and hematological disorders. Methemoglobinemia is a rare cause of cyanosis, in which hemoglobin is oxidized, changing its heme iron configuration from the ferrous (Fe2 +) to the ferric (Fe3 +) state, creating methemoglobin (Met-Hb), a form that does not bind oxygen, leading to decreased oxygen delivery to the tissues and cyanosis. We report a rare case of a preterm newborn, who developed cyanosis and worsening hypoxemia on day ten of life, she was found to have elevated Met-Hb percentage in blood gas analysis that required treatment with intravenous methylene blue. Her symptoms resolved after a period of maintenance treatment with oral methylene blue and ascorbic acid, and the etiology of her disease remains unclear.