What are the key clinical features that suggest methaemoglobinaemia in a patient presenting with cyanosis?

Key clinical features suggesting methaemoglobinaemia in a patient presenting with cyanosis include:

• Central cyanosis unresponsive to oxygen therapy: Patients typically exhibit a slate-gray or blue discoloration of the skin and mucous membranes that does not improve with high-flow oxygen, distinguishing it from other causes of hypoxia NICE NG115 Hall et al. 1986.
• Chocolate-brown coloured blood: On sampling, the blood may appear dark brown rather than bright red, reflecting the presence of oxidized haemoglobin NICE NG115 Bradberry 2003.
• Symptoms of hypoxia despite normal or near-normal partial pressure of oxygen (PaO2): This paradox occurs because methaemoglobin cannot bind oxygen effectively, leading to tissue hypoxia despite adequate oxygen tension NICE NG115 Hall et al. 1986.
• Signs of hypoxia such as headache, fatigue, dizziness, and tachycardia: These systemic symptoms often accompany cyanosis and reflect impaired oxygen delivery NICE NG115 Bradberry 2003.
• History of exposure to oxidizing agents or drugs: Occupational or accidental exposure to chemicals like nitrates, aniline dyes, or certain medications (e.g., dapsone, local anaesthetics) is a common precipitant NICE NG115 Bradberry 2003.
• In severe cases, neurological symptoms and metabolic acidosis may develop: These indicate significant tissue hypoxia and systemic toxicity Hall et al. 1986.

Thus, the combination of persistent cyanosis unresponsive to oxygen, characteristic blood appearance, systemic hypoxia symptoms, and relevant exposure history strongly suggests methaemoglobinaemia in a cyanotic patient NICE NG115 Hall et al. 1986Bradberry 2003Ewenczyk et al. 2008.