Several groups of inherited metabolic disorders, most notably the organic acidemias, urea cycle defects, and certain disorders of amino acid metabolism, typically present with acute life-threatening symptoms of an encephalopathy. These symptoms are the result of toxic effects of accumulating metabolites on the central nervous system (CNS). Because most of these metabolites cross the placenta and are cleared by the mother during gestation, affected infants usually appear normal at birth. The interval between birth and onset of clinical symptoms ranges from hours to months

source= Barbara K. Burton,MD  http://www.pediatricsdigest.mobi/content/102/6/e69.full

That up to 20% of Sudden Infant Death Syndrome cases may be attributed to an undiagnosed inborn error of metabolism such a urea cycle disorder. Urea cycle disorders are included in the category of inborn errors of metabolism. There is no cure. Inborn errors of metabolism represent a substantial cause of brain damage and death among newborns and infants. Because many cases of urea cycle disorders remain undiagnosed and/or infants born with the disorders die without a definitive diagnosis, the exact incidence of these cases is unknown and underestimated. It is believed that up to 20% of Sudden Infant Death Syndrome cases may be attributed to an undiagnosed inborn error of metabolism such a urea cycle disorder. In April 2000, research experts at the Urea Cycle Consensus Conference estimated the incidence of the disorders at 1 in 10,000 births. This represents a significant increase in case identification and diagnosis in the last few years.  Research studies have now been initiated to more accurately determine the incidence and prevalence of UCDs

Urea cycle disorder

A urea cycle disorder is a genetic disorder caused by a deficiency of one of the six enzymes in the urea cycle which is responsible for removing ammonia from the blood stream. The urea cycle involves a series of biochemical steps in which nitrogen, a waste product of protein metabolism, is removed from the blood and converted to urea. Normally, the urea is transferred into the urine and removed from the body. In urea cycle disorders, the nitrogen accumulates in the form of ammonia, a highly toxic substance, and is not removed from the body resulting in hyperammonemia (elevated blood ammonia). Ammonia then reaches the brain through the blood, where it causes irreversible brain damage, coma and/or death.

source=The National Urea Cycle Disorders Foundation 

California ( usa)