M–N

megakaryocyte See BONE MARROW.

methemoglobinemia A BLOODoxygenation dis-
order in which methemoglobin, a structure of
HEMOGLOBIN molecules that prevents iron from
binding with oxygen, accumulates in the BLOOD.
The result is diminished or insufficient oxygen
delivery to the body’s cells. Methemoglobin repre-
sents excessive iron molecule structures that are
in a ferric state, in which they are unable to bind
with oxygen. Normal iron molecules in the hemo-
globin are ferrous. Methemoglobin forms natu-
rally in the blood as a process of oxidation
(cellular METABOLISM), though an enzyme system
that converts ferric iron to ferrous iron continu-
ously restores methemoglobin to hemoglobin.
Methemoglobin is normally present in the blood
in minute quantities, accounting for less than 1
percent of the total hemoglobin forms. Levels
above 10 percent begin to cause symptoms, and
levels above 70 percent are fatal.

A potential cause of methemoglobine-

mia is the illicit use of “nitrite pop-

pers,” inhaled isobutyl nitrite, butyl

nitrite, and amyl nitrate products that

are popular among some recreational

DRUGusers.

Methemoglobinemia most commonly results
from toxic exposure to oxidizing chemicals or
drugs. Dozens of industrial chemicals can cause
methemoglobinemia, as can numerous medica-
tions in the nitrate, chlorate, and sulfonamide
families of drugs, as well as topical anesthetics
such as benzocaine and lidocaine. Exposure is
usually chronic. Other causes of methemoglobine-
mia include hemoglobin disorders that allow

excessive methemoglobin formulation and abnor-

malities in the blood enzyme system that normally

removes methemoglobin from the blood.

Symptoms and Diagnostic Path

Symptoms of methemoglobinemia may mimic

those of ANEMIA, such as fatigue and shortness of

breath (DYSPNEA) especially with exertion or exer-

cise, though important differences are present to

help distinguish methemoglobinemia from other

hemoglobin disorders. The most significant is a

characteristic CYANOSISthat gives the SKINa bluish

brown color and does not improve with the

administration of OXYGEN THERAPY. Blood tests that

analyze hemoglobin composition determine the

amount of methemoglobinemia in the blood; lev-

els higher than 1 or 2 percent confirm the diagno-

sis though most people who show symptoms have

much higher levels.

Treatment Options and Outlook

Most often, removing exposure to the causative

substance allows the blood to recover on its own,

usually within 72 hours. The doctor may choose

to hospitalize the person until hemoglobin levels

return to normal, to make sure the person

receives adequate oxygenation. When symptoms

are severe or persist, treatment may include meth-

ylene blue, a chemical that converts the hemoglo-

bin’s iron from ferric to ferrous. The typical

therapeutic approach is to administer an intra-

venous injection of methylene blue to rapidly con-

vert enough methemoglobin to hemoglobin to

relieve symptoms, then switch to oral methylene

blue until hemoglobin returns to normal. Rarely, a

person who is having severe symptoms may

require hyperbaric treatment, in which oxygen

under pressure can enter the body through the

skin to deliver oxygen to the tissues. Hyperbaric

156