Sudden infant death syndrome (SIDS) is a multifactorial disorder influenced by developmental, environmental, and biological risk factors and it is defined as the sudden death of an infant that is unexpected by history and unexplained by postmortem examinations. SIDS is a leading cause of infant mortality, accounting for 8% of all infant deaths
 and it is the most common cause of post neonatal infant mortality accounting for 40-50% of all deaths between one month and one year of age. Cases in which instead the cause is identified should be diagnosed not as SIDS but as SUDI, such as in specific genetic conditions. At present, long QT syndrome and fatty acid β-oxidation disorders represent the most frequent inherited causes of SUDI
Long QT syndrome is caused by mutations in genes encoding cardiac ion channels (such as KVLQT1, HERG, KCNE1, and KCNE2) leading to prolonged cardiac action potential by either increasing depolarization or decreasing repolarization current and so causing syncope, seizures or sudden death
Fatty acid β-oxidation disorders encompass almost 20 different inborn enzymopathies. Among them, medium chain acyl CoA dehydrogenase (MCAD) deficiency is largely the most frequent condition with incidence around 1: 8,000
. The enzymatic defect results in a blockade of medium chain fatty acids breakdown and of ketone production. The enzymatic deficiency compromises the availability of ketones during prolonged fasting or acute illness. This energy shortage primarily affects function of skeletal and cardiac muscle as well as the brain, and can lead to death. MCAD deficiency is usually silent and generally affects individuals clinically normal until an intercurrent illness episode triggers hypoglycemia and fatty acids discharge
The most common clinical presentations of the disease include hypoketotic hypoglycemia, Reye syndrome and, more dramatically, sudden unexpected death, generally during the first year of life. Surviving patients who comply with a treatment regimen, which essentially relies on avoidance of fasting, either dramatically reduce or completely eliminate recurrent life-threatening disease episodes
. The biochemical diagnosis is based on acylcarnitine profile, characterized by accumulation of hexanoyl-carnitine (C6) to decanoyl-carnitine (C10) species, with prominent octanoyl-carnitine (C8). Patients also show a characteristic urinary organic acids pattern, with typical dicarbossilic aciduria and elevated hexanoyl and suberylglycine. The diagnosis can be confirmed by molecular analysis of ACADM gene (1p31.1) which encodes for MCAD protein
. To date more then 90 mutations have been reported in the ACADM gene (HGMD - Human Genome Mutation Database)
The c.985A>G missense mutation is the most frequent molecular lesion and may account up to 90% of alleles of clinically diagnosed MCAD deficient patients, but is lower in cases ascertained by screening
. This suggests that this mutation may be more likely to lead to a severe clinical phenotype
[9, 10]. Although there is no “safe” genotype or metabolite profile for MCAD deficiency, profoundly elevated C8 on blood spot or the presence of severe mutations may reflect impaired ability to tolerate metabolic stress potentially leading to more severe or lethal phenotypes as confirmed in the MCAD deficient patient here reported who presented with neonatal SUDI.
Sudden neonatal death represents an unusual clinical presentation of MCAD deficiency, possibly related to specific severe genotypes, such as the homozygosity for the frame-shift c.244dup1(p.Trp82LeufsX23) mutation
. Unfortunately, neonatal death due to MCAD deficiency could be not prevented by routine newborn screening programs, as sampling for blood spot acylcarnitine analysis is generally carried out after 48 hours of life and results might not be available before the onset of acute clinical events. Nevertheless, expecially in those States in which expanded metabolic newborn screening is not yet available, to provide definite diagnosis and accurate genetic counseling in any sudden neonatal death, fatty acid oxidation inborn disorders must be routinely explored by collecting simple biological material as blood spot, that may be easily sent to specialized laboratories for acylcarnitine and/or DNA analysis.