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Asymptomatic Hyperammonemia in Low Birthweight Infants

Summary: At 0–3 days of age the plasma ammonium concentration in full term appropriate for gestational age (AGA) infants was (mean ± SEM) 27.5 ± 0.5 μM; a value similar to that reported in adults. Ammonium levels in low birthweight AGA and SGA groups were 47.0 ± 2.0 μM and 45.1 ± 3.3 μM respectively; significantly elevated (P < 0.001) as compared to the full term group. These increased ammonium levels persisted at 3–5 weeks of age. Associated with the hyperammonemia was a significant (P < 0.01) decrease in plasma α-ketoglutarate concentration: 11.8 ± 1.0 μM, in the low birthweight AGA as compared to 20.7 ± 0.6 μM in the full term AGA infants. There was an inverse linear correlation between plasma concentrations of ammonium and α-ketoglutarate r = −0.86, P < 0.001. Urinary orotate excretion was significantly elevated (P < 0.05) in low birthweight AGA infants. There was no difference in the plasma concentrations of glutamine, glutamate, or alanine among the various groups. Hyperammonemia was not associated with neurologic dysfunction.Speculation: There is a greater accumulation of ammonium in low birthweight as compared to full term infants. This may be a consequence of a developmental delay of one or more of the enzymes required for urea synthesis. Alternatively, the hyperammonemia may be related to arginine and/or ornithine diversion out of the cycle and incomplete repletion of ornithine from glutamate via the Δ′-pyrroline-5-carboxylate pathway.