Internal Medicine

0521779407-14 CUNY1086/Karliner 0 521 77940 7 June 4, 2007 21:16

Methemoglobinemia 981

➣Inheritance

Autosomal recessive – type 1 (erythrocyte restricted) and 2

(ubiquitously expressed) cytochrome b5 reductase deficiency

Autosomal dominant – mutant hemoglobin (Hb M)

Signs & Symptoms

■Cyanosis – clinically apparent when methemoglobin exceeds 1.5

gm%

■(10–15% methemoglobin at normal hemoglobin concentration)

■Blood characteristic “chocolate” color

tests

■Methemoglobin detection:

➣Co-oximeter analysis at specific absorption spectrum

false positive from other substances and medications

➣Evelyn-Malloy assay – specific for methemoglobin

■cytochrome b5 reductase assay and assays for hemoglobin M

mutants in congenital cases

differential diagnosis

■Hypoxemia with unsaturated hemoglobin >4 gm% (low PaO2)

■Sulfhemoglobin >0.5 gm% (also has normal PaO2 and may be mis-

taken for methemoglobin by co-oximeter; distinguish by Evelyn-

Malloy assay)

management

What to Do First

■Withdraw offending agent in acquired (acute toxic)

■No other treatment may be necessary

specific therapy

Indications

■Symptomatic – usually the case in overdoses or poisoning

■Treatment not needed for chronic asymptomatic – cosmetic pur-

poses only

Treatment Options

■Methylene Blue

■Ascorbic acid

➣If methylene blue is contraindicated

➣May be useful in chronic, congenital b5R deficiency

■Riboflavin – reported use in b5R deficiency (chronic)

■No known beneficial treatment for type 2 b5R deficiency