Critical rebound methemoglobinemia after methylene blue treatment: case report.

A 39-year-old man developed significant methemoglobinemia after receiving benzocaine spray; he was treated appropriately with intravenous methylene blue. The patient's methemoglobin levels decreased, but this was followed by a critical rebound phenomenon to levels frequently considered near fatal. After further treatment with methylene blue, the patient's methemoglobin levels returned to normal. Clinicians need to be aware that a decreasing level of methemoglobin does not necessarily indicate that a crisis has passed and that further monitoring and treatment may be indicated.