Methemoglobin describes the form of hemoglobin (Hb) in which the heme moiety contains iron that has been oxidized from its ferrous (Fe2+) to ferric (Fe3+) state [1,2,3,4,5]. This oxidized Hb is then unable to bind and release oxygen to the tissues. In normal erythrocyte metabolism, methemoglobin is continuously produced and then reduced enzymatically by nicotinamide adenine dinucleotide (NADH) cytochrome B5 reductase , and usually comprises < 1% of total Hb [2,3,4]. Methemoglobinemia occurs when the balance between oxidation and reduction reactions are disrupted, and methemoglobin accumulates to > 3% [4, 5].
The congenital form of MetHb can be caused by an autosomal recessive inheritance of a deficient CYB5R gene but can also be due to hemoglobin variants called HbM [6,7,8,9,10]. HbM are a group of defects due to single amino acid substitutions in the normal globin chain, inherited in an autosomal dominant fashion. In HbM, heme iron is stabilized in the ferric state and resists reduction by the normal enzymatic pathway [1, 2, 11]. These can occur in α, β, and γ globins and thus can affect both fetal and adult hemoglobin. There are several known variants of HbM, including Boston, Fort Ripley, Hyde Park, Iwate, Kankakee, Osaka, and Saskatoon, but the true incidence is not known [12, 13]. Patients with HbM variants usually have MetHb levels in the 30–40% range yet have few symptoms due to physiologic compensation over time .
Hb F-M-Fort Ripley (HBG2: c.227 C > T) is a form of HbM caused by a missense mutation in the gamma chain, causing a histidine residue at codon 92 to be replaced with tyrosine . It is one of six known HbM γ-globin variants associated with neonatal cyanosis [4, 15, 16]. It was first described in a case report by Priest et al. in 1989 , and again by Molchanova et al. in 1992 . It demonstrates autosomal dominant inheritance and incomplete penetrance .
The approach to the neonate with persistent cyanosis without evidence of a cardiac or pulmonary etiology should raise suspicion for methemoglobinemia. Acquired or congenital cases of MetHb that result in deficiency of CYB5R can be treated with methylene blue at a dose of 0.5–2 mg/kg administered intravenously over 5 minutes and repeated after 1 hour if there is insufficient reduction in MetHb [2, 5]. Ascorbic acid is an alternative treatment, utilized in instances where methylene blue is unavailable . Treatment is typically reserved for symptomatic patients with methemoglobin levels > 10% [2, 5, 19]. While treatment with methylene blue is very effective in the setting of acquired or congenital methomoglobinemia, it does carry a risk of severe hemolysis in patients with Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency and therefore warrants a detailed family history to assess for risk of G6PD deficiency, as well as testing for G6PD deficiency in patients anticipated to receive methylene blue . It is not always feasible to obtain G6PD deficiency results prior to initiation of methylene blue treatment, which necessitates close post-treatment monitoring for signs of hemolysis, as this could be life-threatening.
While rarer, HbM is another congenital form of methemoglobinemia involving mutations in the α, β, and γ globin gene resulting in resistance to reduction by CYB5R. There is substantial variability in initial timing of cyanosis and persistence of cyanosis dictated by the specific globin gene mutated. In patients with mutations to the α globin gene, which is present in the predominant forms of hemoglobin at birth (fetal Hemoglobin F (HbF) [α2γ2) and throughout life (adult Hemoglobin A (HbA) [α2β2]), patients typically present with cyanosis in the neonatal period and can persist throughout life . Patients with mutations in β globin genes typically do not present in the neonatal period, but rather present around 6 months of life as HbA (α2β2) takes over as the predominant form of hemoglobin and can persist throughout life [1,2,3,4,5]. Although mutations in the γ globin gene are more rare, these forms of HbM typically result in transient cyanosis that presents during the initial neonatal period and resolves within the first 6 months of life as fetal hemoglobin declines and is replaced by adult hemoglobin over time; it is not expected to recur [5, 7, 17].
Initial evaluation for methemoglobinemia in the neonatal period involves obtaining a methemoglobin level, which can help dictate if and when treatment should be initiated. However, obtaining a MetHb level in a neonate can be a challenge. Routine pulse oximetry is unable to distinguish between MetHb and normal oxyhemoglobin and deoxyhemoglobin [21, 22]. Co-oximetry specialized pulse oximeters that use multiple wavelengths of light can detect absorption of MetHb at 630 nm (6.3e-7 m) [16,17,18]. The most accurate method of detecting MetHb is the Evelyn-Malloy assay, but this specialized method of methemoglobin detection is rarely available in a timely fashion [17, 18]. Accurate quantification of MetHb is difficult in the presence of other substances that absorb light at around 630 nm. High concentrations of HbF, bilirubin, and lipids in neonatal patients can obscure results [20, 23, 24]. In our patient, the unconjugated bilirubin level was > 5 mg/dL (442 μmol/L) at 4 hours of life, which invalidated all available local methods for determining the concentration of MetHb.
In instances where methemoglobin levels cannot be obtained, studies evaluating CYB5R activity and G6PD deficiency are recommended. If there is concern for a Hemoglobin M, hemoglobin electrophoresis can be helpful to identify a hemoglobinopathy. Hemoglobin electrophoresis can be helpful in identifying variants to the α and β globin genes, however Hb F-M-Fort Ripley is unstable and variants to the γ globin gene are often not detectable via hemoglobin electrophoresis [7, 10, 17]. Therefore, in cases with high suspicion of Hemoglobin M due to a mutation in the γ globin gene further genetic testing to confirm diagnosis is warranted.
Follow up with hematology is dictated by the expected persistence of cyanosis based on the specific HbM variant. In general, patients with congenital methemoglobinemia can compensate well, often with an adaptive polycythemia and can tolerate methemoglobin levels up to 40% without significant symptoms. However, these patients are at high risk of acute decompensation when challenged with oxidizing agents .
Because Hb F-M-Fort Ripley is due to a problem with γ globin, symptoms are transient as HbF production converts to HbA production over the first few months of life [7, 10]. Prior case reports of infants with Hb F-M-Fort Ripley have described resolution of cyanosis between four and 8 weeks of life, or 42 weeks corrected gestational age in the case of a premature infant with concurrent bronchopulmonary dysplasia [7, 10, 15]. Of our patient’s affected family members, including the father and aunt among others [7, 17], the individuals were asymptomatic at birth, but developed cyanosis by a few days of age with no other major medical concerns. They had prolonged NICU stays for cyanosis and hypoxia, but none required treatment with methylene blue and none have had recurrent issues with hypoxia to date. Their cyanosis resolved by 3–8 weeks of age [7, 17]. Our patient was successfully weaned off supplemental oxygen by 5 weeks of age, with no recurrence of his cyanosis. For this patient, given the expected transient nature of his methemoglobinemia and cyanosis, he was seen for follow up in hematology clinic upon discharge from the NICU and at 2 months of life with resolution of his cyanosis noted at that time. No further hematology follow-up has been necessary, and no further attempts at obtaining methemoglobin levels have been made.