Differential Diagnosis for Acute Onset of Cyanosis in a 15-Month-Old Male

Single Most Likely Diagnosis

• Pulmonary Embolism is unlikely in this age group, but given the acute onset of cyanosis without respiratory distress and lack of response to supplemental oxygen, a more plausible single most likely diagnosis is: Methemoglobinemia, possibly induced by an oxidizing agent such as those found in certain medications or foods, or more relevantly, benzocaine-containing teething gels or liquids, which the child might have been exposed to due to teething.

Other Likely Diagnoses

• Congenital heart disease with a previously undiagnosed condition that has become apparent due to changes in the child's physiology or a new onset of a condition affecting cardiac function.
• Pneumonia or other respiratory infections that could lead to severe hypoxemia, although the lack of respiratory distress and failure to improve with supplemental oxygen makes this less likely.
• Anaphylaxis or severe allergic reaction, though typically accompanied by other symptoms such as rash, swelling, or gastrointestinal symptoms.

Do Not Miss Diagnoses

• Cardiac Tamponade: Although less common in this age group without a history of cardiac disease, it could present with sudden onset of cyanosis and distress.
• Pulmonary Hypertension Crisis: Could present with sudden onset of cyanosis and might not respond to supplemental oxygen.
• Foreign Body Aspiration: Could cause sudden onset of cyanosis, especially if the object obstructs a major airway.

Rare Diagnoses

• Cyanide Poisoning: Could present with sudden onset of cyanosis without initial respiratory distress, though this would be extremely rare in a 15-month-old without a clear exposure history.
• Mitochondrial Disorders: Certain mitochondrial myopathies can present with episodes of cyanosis, though these are rare and typically have other associated symptoms.
• Congenital Methemoglobinemia: A rare condition where the body has a deficiency in the enzyme diaphorase, leading to persistent methemoglobinemia.

Each of these diagnoses requires careful consideration of the patient's history, physical examination, and response to initial treatments, as well as further diagnostic testing to determine the underlying cause of the cyanosis.