Definition & Pathophysiology

Clinical syndrome characterized by increased serum concentration of methemoglobin (MetHb).
Iron molecule in hemoglobin oxidizes from normal ferrous ($Fe^{2+}$) state to abnormal ferric ($Fe^{3+}$) state.
Ferric heme loses oxygen-binding capacity.
Presence of ferriheme causes leftward shift of oxygen dissociation curve for remaining ferrous hemes.
Results in severely impaired oxygen delivery to tissues.
Normal physiological MetHb maintained at ~1%.
Primary physiological reduction mechanism: NADH-dependent reaction catalyzed by cytochrome b5 reductase.
Alternate reduction pathway: NADPH generated by hexose monophosphate shunt via Glucose-6-phosphate dehydrogenase (G6PD), requiring extrinsic electron acceptors (methylene blue, ascorbic acid, riboflavin).
High infant vulnerability: Erythrocytes possess 50% of adult cytochrome b5 reductase levels; fetal hemoglobin (HbF) highly susceptible to oxidation; alkaline infant gastrointestinal tract promotes nitrite-producing gram-negative bacteria.

Etiology

Disorder	Inheritance	Pathogenesis & Features
Cytochrome b5 reductase deficiency (Type 1)	Autosomal Recessive	Enzyme deficiency restricted to erythrocytes. Chronic, ranges from asymptomatic to mildly symptomatic. Often associated with compensatory erythrocytosis.
Cytochrome b5 reductase deficiency (Type 2)	Autosomal Recessive	Generalized enzyme deficiency in all tissues. Severe symptoms beginning in infancy: encephalopathy, intellectual impairment, spasticity, microcephaly. Usually fatal by 2 years of age.
Hemoglobin M Disease	Autosomal Dominant	Point mutations in $\alpha$, $\beta$, or $\gamma$ globin chains within the heme pocket. Causes cyanosis but little to no cardiorespiratory distress.

Vastly more common than hereditary forms.
Triggered by exposure to oxidizing substances, chemicals, or drugs.
Medications: Benzocaine, dapsone, local anesthetics (EMLA, lidocaine, prilocaine), antimalarials (chloroquine, primaquine), sulfonamides, nitric oxide, nitroprusside.
Chemicals/Toxins: Aniline dyes, nitrates, nitrites, herbicides, well water, automobile exhaust.
Medical Conditions: Pediatric gastrointestinal infections, severe gastroenteritis with acidosis, sepsis.

Cyanosis unresponsive to 100% oxygen therapy.
Blood characteristically chocolate-brown in appearance.
Cyanosis visibly apparent at MetHb levels >1.5 g/dL (approximately 15% MetHb).
Lethal at MetHb levels >70%.
Arterial oxygen tension ($PaO_2$) remains normal or elevated despite severe cyanosis.
Asymptomatic at lower levels; exertional cardiorespiratory distress noted in congenital forms.

Diagnostic Modality	Findings / Utility
Bedside Test	Blood sample exposed to 100% oxygen and shaken remains chocolate-brown instead of turning bright red.
Co-oximetry	Multiwavelength co-oximetry definitively measures low blood oxygen saturation and exact MetHb percentage.
Pulse Oximetry	Highly inaccurate; yields falsely low readings failing to reflect true degree of desaturation.
Arterial Blood Gas	Normal $PaO_2$; calculated oxygen saturation misleadingly normal.

Primary Therapy: Intravenous Methylene Blue (1-2 mg/kg initially).
Mechanism: Utilizes alternate NADPH-dependent reduction pathway.
Contraindications: Strict avoidance of methylene blue in patients with G6PD deficiency. Ineffective and provokes severe, life-threatening oxidative hemolysis within 24 hours.
Ascorbic acid contraindicated for acute toxic events.
Remove offending oxidizing agent immediately.

Primary Therapy: Daily oral Ascorbic Acid (200-500 mg/day in divided doses).
Mechanism: Gradually reduces MetHb to ~10%, alleviating cyanosis.
Adverse Effects: Chronic high-dose ascorbic acid risks hyperoxaluria and renal stone formation.
Alternate Therapy: High-dose Riboflavin.
Preventive Care: Strict avoidance of oxidant drugs. Normal life expectancy for Hemoglobin M disease and Type 1 Cytochrome b5 reductase deficiency.