Organic acidaemias cause acidosis with a raised anion gap; urea cycle defects cause alkalosis
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Both present in a previously well neonate after a protein feed or catabolic stress, with poor feeding, vomiting, lethargy, seizures and coma.
| Organic acidaemia | Urea cycle defect | |
|---|---|---|
| Ammonia | Raised | Very high |
| Acid-base | Metabolic acidosis, raised anion gap | Respiratory alkalosis, normal anion gap |
| Ketones | Present | Absent |
| Examples | Methylmalonic, propionic, isovaleric acidaemia; maple syrup urine disease | Ornithine transcarbamylase deficiency (X-linked, commonest), citrullinaemia |
Maple syrup urine disease: branched chain ketoacid dehydrogenase deficiency, with the characteristic sweet-smelling urine.
Ornithine transcarbamylase deficiency shows a raised urinary orotic acid, because accumulated carbamoyl phosphate is diverted into pyrimidine synthesis.
Fatty acid oxidation defects form a third group, with hypoketotic hypoglycaemia and no acidosis.
Emergency management is common to all: stop protein, give high-dose intravenous glucose to reverse catabolism, treat ammonia with scavengers and consider dialysis, and take the critical samples (ammonia, gas, glucose, lactate, ketones, acylcarnitines, amino acids and urine organic acids) before treatment alters them.