Neonatal hyperammonemia caused a defect of carnitine-acylcarnitine translocase by H. Ogier de Baulny, MD, A. Slama, PhD, G. Touati, MD, D. M. Turnbull, MD, M. Pourfarzam, Phb, and M. Brivet, PhD From the Centre d'investigation ctinique, H6pital Robert Debr6, Paris, France, the Depart- ment of Clinical Neuroscience and Child Health, The Medical School, Newcastle upon Tyne, United Kingdom, and the Laboratoire central de biochimie, centre hospitalier de Bic6tre, Le Kremlin Bic6tre, France Carnitine-acylcarnitine translocase deficiency is a newly recognized inborn error of metabolism that involves transport of long-chain fatty acids into mitochondria, which in turn impairs mitochondrial 13-oxidation, and ketogenesis. We report a new familial example; the affected twins had neonatal distress, hyperammone- mia, and transient intracardiac conduction defects.Clinical and biochemical analysis of both our patients and the two previously reported patients revealed that this inherited defect could be manifested during the neonatal period without any of the signs classically associated with fatty oxidation defects. In contrast, all four patients had sustained and "isolated" hyperammonemia, which could be misinterpreted as being caused by urea cycle defects. We conclude that carnitine-acylcarnitine translocase deficiency is a potential differential diagnosis in neonates with unexplained neonatal hyperammonemia. Cardiac and muscle involvement may represent further early pivotal symptoms. (J PEDIATR 1995; 127:723-8) Apart from the urea cycle disorders, a number of other in- born errors of metabolism, including organic acidurias, some congenital lactic acidemias, and defects in fatty acid oxida- tion, can cause hyperammonemia.1 In addition, transient hy- perammonemia in the neonate is a condition of unknown origin in which the diagnosis relies on the exclusion of the conditions noted above. The fatty acid oxidation defects are characterized by acidosis, hypoglycemia, and dicarboxylic aciduria, but none of these biochemical markers is a constant finding, and neonatal fatty acid oxidation defects may easily be misdiagnosed in the absence of a high level of suspicion. 2 We describe premature twins who had a long-chain fatty Supported in part by a grant from' 'recherche clihique' ' No. 930 608 and by Action Research for the Crippled Child. Reprint requests: Hrl~ne Ogier de Baulny, Centre d'investigation clihiqne, Hrpital Robert Debrr, 48 Boulevard Serurier, 75019 Paris, France. Submitted for publication Feb. 28, 1995; accepted Jtme 9, 1995. Copyright © 1995 by Mosby-Year Book, Inc. 0022-3476/95/$5.00 + 0 9/20/66928 oxidation defect caused by carnitine-acylcarnitine translo- case deficiency, with neonatal "isolated" hyperammonemia as the initial manifestation. CASE REPORT These twins, the first children of Algerian consanguineous par- ents, were born at 34 weeks of gestation with birth weights of 2050 and 2300 gm, respectively. Along with intravenous glucose CoA Coenzyme A CPT Camitine palmityl transferase MCT Medium-chain triglyceride NAGA N-Acetylglutamate [synthetase] infusions and antibiotics, continuous nasogastric feeding with a medium-chain triglyceride-supplemented formula was started within the first 24 hours of life. Both had neurologic examination appropriate for gestational age, and blood glucose levels were nor- mal on repeated testing. On the second day of life, twin A, a girl, had unexplained recur- rent episodes of tachypnea, cyanosis, and bradycardia. At 72 hours of life, abrupt deterioration with tracheal hemorrhage required 723