Metabolic acidosis in the neonate can have several causes, including increased acid intake from an exogenous source, increased endogenous production of an acid such as seen in an inborn error of metabolism (IEM), inadequate excretion of acid by the kidneys or excessive loss of bicarbonate in urine or stool. Presence or absence of an anion gap (AG) can help to distinguish the underlying etiology. The AG can be calculated using the following equation: AG = ([Na+]) − ([Cl−] + [HCO3−]). A normal AG is typically less than 16 mEq/L.1 Common anions that result in an elevated AG include lactate and the ketone bodies β-hydroxybutyrate and acetoacetate, as well as the accumulation of the toxic organic acids typically found in individuals with IEMs.
Clues that increase the likelihood of identifying an IEM include a family history of consanguinity or history of a neonatal or sibling death. Occasionally, maternal history of acute fatty liver of pregnancy may be solicited.
Infants with IEMs are usually normal at birth. They develop nonspecific findings similar to those for sepsis, such as poor feeding, vomiting, lethargy, hypotonia, hypothermia, seizures, and coma. Infants will try to correct metabolic acidosis by a reflex respiratory alkalosis using hyperventilation and Kussmaul respirations. More severe uncompensated acidosis can decrease peripheral vascular resistance and cardiac ventricular function, leading to hypotension, pulmonary edema, and tissue hypoxia, which will further complicate the picture by increasing lactate production because of hypoxia and poor perfusion.
The IEMs are enzyme deficiencies that result from amino acid, carbohydrate, or fatty acid metabolism (Figure 107-1). Most of these disorders can be diagnosed by routine metabolic testing (Table 107-1), including plasma amino acid, plasma acylcarnitine profile, and urine organic acid analyses. Please see Table 107-2 for details about IEMs and the specific abnormalities detected by laboratory evaluations.
Table 107-1Laboratory Tests for Any Patient With a Known or Suspected Inborn Error of Metabolism |Favorite Table|Download (.pdf) Table 107-1Laboratory Tests for Any Patient With a Known or Suspected Inborn Error of Metabolism
Venous/arterial blood gas
Comprehensive metabolic panel
Anion gap ([Na+] − ([Cl−] + [HCO3−]))
Complete blood cell count
Plasma amino acids
Plasma acylcarnitine profile
Urinalysis to assess ketones
Urine organic acids
Table 107-2Summary of Inborn Errors of Metabolism (IEMs) and Their Findings |Favorite Table|Download (.pdf) Table 107-2Summary of Inborn Errors of Metabolism (IEMs) and Their Findings
|IEM ||Enzyme ||Gene(s) ||Newborn Screen ||Plasma Amino Acids ||Plasma Acylcarnitines ||Urine Organic Acids |
|Amino Acid Metabolism |
|Propionic acidemia ||Propionyl CoA carboxylase ||PCCA, PCCB ||C3 propionylcarnitine ||Glycine ||C3 propionylcarnitine ||3-Hydroxypropionic and methylcitric acids |