Deconstructing Black Swans: An Introductory Approach to Inherited Metabolic Disorders in the Neonate

Glucose-6-Phosphate Dehydrogenase Deficiency and the Benefits of Early Screening

Glucose-6-phosphate dehydrogenase (G6PD) deficiency, the most common enzymopathy worldwide, is an insufficient amount of the G6PD enzyme, which is vital to the protection of the erythrocyte. Deficient enzyme levels lead to oxidative damage, hemolysis, and resultant severe hyperbilirubinemia. If not promptly recognized and treated, G6PD deficiency can potentially lead to bilirubin-induced neurologic dysfunction, acute bilirubin encephalopathy, and kernicterus. Glucose-6-phosphate dehydrogenase deficiency is one of the three most common causes for pathologic hyperbilirubinemia. A change in migration patterns and intercultural marriages have created an increased incidence of G6PD deficiency in the United States. Currently, there is no universally mandated metabolic screening or clinical risk assessment tool for G6PD deficiency in the United States. Mandatory universal screening for G6PD deficiency, which includes surveillance and hospital-based risk assessment tools, can identify the at-risk infant and foster early identification, diagnosis, and treatment to eliminate neurotoxicity.

Hyperammonaemia in classic organic acidaemias: a review of the literature and two case histories

Background

The ‘classic’ organic acidaemias (OAs) (propionic, methylmalonic and isovaleric) typically present in neonates or infants as acute metabolic decompensation with encephalopathy. This is frequently accompanied by severe hyperammonaemia and constitutes a metabolic emergency, as increased ammonia levels and accumulating toxic metabolites are associated with life-threatening neurological complications. Repeated and frequent episodes of hyperammonaemia (alongside metabolic decompensations) can result in impaired growth and intellectual disability, the severity of which increase with longer duration of hyperammonaemia. Due to the urgency required, diagnostic evaluation and initial management of patients with suspected OAs should proceed simultaneously. Paediatricians, who do not have specialist knowledge of metabolic disorders, have the challenging task of facilitating a timely diagnosis and treatment. This article outlines how the underlying pathophysiology and biochemistry of the organic acidaemias are closely linked to their clinical presentation and management, and provides practical advice for decision-making during early, acute hyperammonaemia and metabolic decompensation in neonates and infants with organic acidaemias.

Clinical management

The acute management of hyperammonaemia in organic acidaemias requires administration of intravenous calories as glucose and lipids to promote anabolism, carnitine to promote urinary excretion of urinary organic acid esters, and correction of metabolic acidosis with the substitution of bicarbonate for chloride in intravenous fluids. It may also include the administration of ammonia scavengers such as sodium benzoate or sodium phenylbutyrate. Treatment with N-carbamyl-L-glutamate can rapidly normalise ammonia levels by stimulating the first step of the urea cycle.

Conclusions

Our understanding of optimal treatment strategies for organic acidaemias is still evolving. Timely diagnosis is essential and best achieved by the early identification of hyperammonaemia and metabolic acidosis. Correcting metabolic imbalance and hyperammonaemia are critical to prevent brain damage in affected patients.