METHEMOGLOBINEMIA

Question 1

Methemoglobin is

Answer 1

oxidized form of hemoglobin.

Question 2

Causes of METHEMOGLOBINEMIA

Answer 2

1-Many oxidant chemicals and drugs are capable of inducing methemoglobinemia (eg. nitrites and nitrates, bromates and chlorates, aniline derivatives, antimalarial agents, dapsone, and local anesthetics).
2-An important environmental source for methemoglobinemia in infants is nitrate-contaminated well water.
3-Amyl nitrite and butyl nitrite are abused for their alleged sexual enhancement properties.
4-Oxides of nitrogen and other oxidant combustion products make smoke inhalation an important potential cause.

Question 3

Mechanism of formation:

Answer 3

Methemoglobin inducers act by oxidizing ferrous (Fe2+) to ferric (Fe3+) hemoglobin.

• This abnormal hemoglobin is incapable of carrying oxygen.
• In addition, it increases the affinity of the normal hemoglobin for oxygen and Shifts the oxygen hemoglobin dissociation curve to the left, which further impairs oxygen delivery.

Question 4

Normally, reduction of hemoglobin occurs through two enzymatic pathways in erythrocytes:

Answer 4

The most active is the NADH-dependent MetHb reductase system. (95% of MetHb reduction).

The second pathway is catalyzed by NADPH-dependent MetHb reductase(<5% of MetHb reduction.) (NADPH combines with MetHb in the presence of the cofactor methylene blue) and it depends on G6PD

Question 5

Clinical presentation of Methemoglobinemia are

Answer 5

The severity of symptoms usually correlates with measured methemoglobin levels.

Symptoms and signs are caused by hypoxia : 
 headache, 
dizziness, 
 nausea, 
dyspnea, 
confusion, 
seizures, 
 coma. 
skin discoloration ("chocolate cyanosis“ not responding to O2 treatment), especially of the nails, lips, and ears, can be striking.

Question 6

Investigations of Methemoglobinemia

Answer 6

• ABG: calculates falsely normal oxygen saturation.
• The diagnosis is suggested by the finding of “chocolate brown” blood (dry a drop of blood on filter paper and compare with normal blood)
• Specific levels. The co-oximeter identifies the different wavelengths of oxyhemoglobin, deoxyhemoglobin, methemoglobin, and carboxyhemoglobin and their concentrations can be determined.
• Pulse oximetry is not reliable; it reads absorbance of light at wavelengths of oxyhemoglobin and deoxyhemoglobin only. When methemoglobin is present, the readings become inaccurate.

Question 7

Treatment

Answer 7

A. Emergency and supportive measures (ABCD)

B. Antidotes
1. Methylene blue: It accelerates the enzymatic reduction of MetHb by NADPH-MetHb reductase
Indications:
Symptomatic patient with methemoglobin levels >20%
When even minimal compromise of oxygen-carrying capacity is potentially harmful (in pre-existing anemia, congestive heart failure, pneumonia, angina pectoris).
. Caution: Methylene blue can causes hemolysis in patients with G6PD deficiency.

C. Enhanced elimination:
Exchange transfusion Should be used for:
1-Infants.
2.Patients who do not respond to methylene blue.
3.Patients with G6PD deficiency.
4.Patients with MetHb levels of more than 70%.

Hyperbaric oxygen may be useful in extremely serious cases that do not respond rapidly to antidotal treatment