The Role of Preoperative Evaluation for Congenital Methemoglobinemia

Oral methylene blue treatment in a dog with cytochrome b5 reductase deficiency and 78, XX testicular disorder of sex development

A 6-month-old mixed breed dog was referred for evaluation of a potential disorder of sex development (DSD) and lower than expected energy level. Genitourinary examination revealed ambiguous external genitalia, hypospadias, and a subtle pouch of skin that resembled an empty scrotum. Corrective surgery was planned and subsequently aborted after cyanosis was identified preoperatively and an arterial blood gas analysis by co-oximetry identified increased methemoglobin (MetHb) concentration (35%, normal < 2%) with normal arterial oxygen tension. Ensuing investigations confirmed hereditary methemoglobinemia caused by cytochrome b₅ reductase (CYB5R) deficiency via molecular genetic (Arg219Pro homozygous variant in CYB5R3 gene) and biochemical (cytochrome b₅ reductase enzyme activity of 8% [normal, 100% activity] testing. Karyotyping and molecular analysis of sex chromosomes revealed the dog was genetically female with a normal female karyotype (78,XX), and was negative for the Y-linked SRY gene and positive for the X-linked androgen receptor gene. Methylene blue (MB, 3.3 mg/kg per os [PO] q24 h) was administered and the MetHb concentration decreased to 9% within 14 days. Urogenital revision surgery proceeded without complication and the dog was maintained on MB (3-4 mg/kg PO q24 h) long-term without adverse effects. This is the first report to describe the use of PO MB to decrease MetHb concentrations in a dog with CYB5R deficiency in preparation for anesthesia and highlights its potential as a viable alternative to the intravenous formulation for elective procedures. In addition, this report describes the clinical, molecular, imaging, surgical, and macroscopic and microscopic pathological features of a dog with SRY-negative, 78,XX testicular DSD.

Recommendations for diagnosis and treatment of methemoglobinemia

Methemoglobinemia is a rare disorder associated with oxidization of divalent ferro‐iron of hemoglobin (Hb) to ferri‐iron of methemoglobin (MetHb). Methemoglobinemia can result from either inherited or acquired processes. Acquired forms are the most common, mainly due to the exposure to substances that cause oxidation of the Hb both directly or indirectly. Inherited forms are due either to autosomal recessive variants in the CYB5R3 gene or to autosomal dominant variants in the globin genes, collectively known as HbM disease. Our recommendations are based on a systematic literature search. A series of questions regarding the key signs and symptoms, the methods for diagnosis, the clinical management in neonatal/childhood/adulthood period, and the therapeutic approach of methemoglobinemia were formulated and the relative recommendations were produced. An agreement was obtained using a Delphi‐like approach and the experts panel reached a final consensus >75% of agreement for all the questions.

Intraoperative Management of a Child Undergoing Cardiac Surgery With Congenital Methemoglobinemia Detected in the Operating Room: A Case Report

Congenital methemoglobinemia is a rare disease that is easily overlooked in its mild form. It can lead to tissue hypoxia as methemoglobin does not possess oxygen-carrying capacity. Its management approach depends on the severity of the symptoms, methemoglobin level, and associated medical conditions. The perioperative management of congenital methemoglobinemia is well described in the literature; however, its management in children with congenital heart disease and undergoing cardiac surgery using cardiopulmonary bypass has not been reported. We present a case and its management where congenital methemoglobinemia was detected in the operating room in a child scheduled for cardiac surgery.

Methemoglobinemia in the Operating Room and Intensive Care Unit: Early Recognition, Pathophysiology, and Management

The objectives of this review are to describe the acquired and hereditary causes of methemoglobinemia, to recommend the most sensitive diagnostic tests, and to enable critical care clinicians to rapidly detect and treat methemoglobinemia. To meet these objectives, Internet search engines were queried with the keywords to select articles for review that included case reports, case series, observational, longitudinal, and surveillance studies. The most common causes of methemoglobinemia include oxidizing reactions to cocaine-derived anesthetics, such as benzocaine and lidocaine, to antibiotics, such as dapsone and other sulfonamides, and to gases, such as nitric oxide. Additionally, CO-oximetry is superior to standard pulse oximetry in detecting methemoglobinemia. Finally, effective treatments for methemoglobinemia include intravenous administration of methylene blue, ascorbic acid, and riboflavin. In this manuscript we will discuss methemoglobinemia, how it occurs, and how to treat it.

Long-term Treatment with Methylene Blue in a Dog with Hereditary Methemoglobinemia Caused by Cytochrome b5 Reductase Deficiency

A juvenile male mixed breed dog was presented for lethargy, exercise intolerance, and aggression when touched on the head. Cyanosis, tachycardia, and tachypnea were observed and persisted during oxygen supplementation. Arterial blood gas analysis by co‐oximetry identified an increased methemoglobin concentration (27%; normal, <2%) with normal arterial oxygen tension. The methemoglobinemia and associated clinical signs resolved after administration of methylene blue (1 mg/kg) IV, and the dog was discharged. The affected dog's whole‐genome sequence contained 2 potentially causal heterozygous CYB5R3 missense mutations suggesting that cytochrome b5 reductase deficiency was responsible for the methemoglobinemia. This hypothesis was confirmed by enzyme analysis that identified cytochrome b5 reductase activity in the affected dog's erythrocytes to only approximately 6% of that in a control sample. Clinical signs recurred 11 days after discharge but normalized and the methemoglobin concentration decreased with methylene blue administration PO (1.5 mg/kg, initially daily and then every other day).