Background:

            - increase of hemoglobin with their iron oxidized to the ferric (Fe 3+) form, making it unable to carry oxygen

            - physiologic level of methemoglobin is 0 to 2%

            - suspect in patients presenting with cyanosis that does not improve with oxygen

            - children less than 4 months old lacks enzyme that normally reduces methemoglobin, making them more susceptible to acquired form

Causes:

            - Hereditary:

                        - rare

                        - deficiency of enzyme that normally converts methemoglobin to functional hemoglobin

                        - homozygous for mutation usually not compatible with life

                        - heterozygous mutations may not manifest signs until under the stress of oxidant drugs

            - Acquired from exposure to oxidizing drugs:

                        - benzocaine and other topical anesthetics (lidocaine rare)

                        - nitrites and nitrates (ex: amyl nitrite and sodium nitrite both found in cyanide antidote kits, “poppers” or butyl nitrite used to enhance sexual pleasure)                                   

                        - quinines 

                        - antimalarials

                        - dapsone

                        - aniline containing dyes

                        - phenazopyridine

Clinical Features:

            - pale/gray/blue discoloration of skin (levels 10-15%)

            - headache, nausea, fatigue, anxiety (levels 20-30%)

            - loss of consciousness, myocardial ischemia, dysrhythmia, seizures, metabolic acidosis (levels above 50%)

            - death (levels above 70%)

Evaluation:

            -  blood appears chocolate brown

            -  co-oximetry on arterial blood gas analyzer with either arterial or venous sample           

            -  pulse co-oximeters are available at some hospitals

            -  standard pulse oximetry will give reading of approximately 85% which does not change with administration of oxygen (falsely elevated)

Treatment:

            - Methylene Blue

                        - should be given to symptomatic patients or if levels above 25%

                        - initial recommended dose of 1-2 mg/kg given IV over 15 minutes

                        - if no improvement clinically after 20 minutes can repeat dose

                        - contraindicated with patients with G6PD deficiency, instead treated with ascorbic acid (vitamin C) or exchange transfusion

Sources:

Prchal, JT. “Clinical Features, Diagnosis, and Treatment of Methemoglobinemia.” UpToDate. 7 June 2017 < https://www.uptodate.com/contents/clinical-features-diagnosis-and-treatm...

Tintinalli, JE. Tintinalli's Emergency Medicine: A Comprehensive Study Guide. 8th ed. New York, NY: McGraw-Hill Education LLC, 2016.