The cases of sudden, unexpected child death against the background of relative clinical well-being, i.e., in the absence of apparent reasons take a special place in the structure of infant mortality. Sudden Infant Death Syndrome (SIDS), which is recognized as one of the leading causes of postnatal infant mortality in most developed countries, is the most common cause of unexpected out-ofhospital death of a child. Today SIDS remains one of the most mysterious problems in medicine. The lack of identifiable mechanisms causing SIDS has led to a large number of theories about the mechanisms responsible for death due to this syndrome. Also, sudden unexplained death in childhood (SUDC), which is the unexplained death of children over one- year-old, is currently distinguished among cases of unexpected death. The main clinical features of SUDC include: more common in boys; death occurs at night time, children are found in the early morning in a prone position, face down; children often have convulsions, including febrile ones in the clinical symptom complex during life. Several authors have noted that in some cases, the death due to SUDC resembles Sudden Death in Epilepsy (SUDEP), which is the leading cause of death in epilepsy. To date, it has already been shown that SUDEP is one of the forms of canalopathies characteristic of young children and it is associated with a high risk of sudden death. The mechanisms of thanatogenesis in SUDEP remain unknown. SIDS, SUDC, and SUDEP are a series of fatal syndromes united by multifactorial pathophysiological mechanisms, the causes of which are not fully understood. In fact, these syndromes represent a catastrophic multisystem failure, which is caused by an extremely unfavorable combination of autonomic, respiratory and cardiogenic disorders.

Sudden Infant Death Syndrome (SIDS) is an unexpected non-violent death of an apparently healthy child in the first year of life, with no rational explanations for the death cause, anamnesis and pathomorphological studies. Risk factors of SIDS are divided into two main groups – modifiable and non-modifiable. Modifiable risk factors include: sleeping in the prone position, infant and adult sleeping together, tobacco smoke, soft bedding, and prematurity. Non-modifiable risk factors include: vulnerability due to morphofunctional dismaturity; critical period of lifespan development; exogenous stress factor. The cumulative impact of all non-modifiable risk factors was used to develop a «triple risk» model.Today, the majority of modern concepts of the sudden infant death syndrome pathogenesis are based on genetic mechanisms. The main hypotheses of thanatogenesis in SIDS are congenital metabolic defects, immunologic incompetence, serotonergic dysfunction, SIDS-critical diaphragmatic failure, cardiogenic mechanisms. The main success in reducing the SIDS rate has been achieved by controlling modifiable risk factors underlying the syndrome development.

Sudden infant death syndrome (SIDS) is sudden non-violent death of apparently healthy infant with absence of adequate reasons of the death according to the medical history and pathomorphological examination. At the present times SIDS is one of the most significant causes of postneonatal infants’ mortality in most developed countries in the world. The main and stable characteristics of SIDS are: death during the sleep and, mainly, in prone position; the peak of SIDS cases is in 2–4 months old; it frequently occurs in premature children and infants with low inborn body weight and in boys (60%); symptoms of mild respiratory infection precede SIDS. Clinical data, analysis of heart rate and breathing in children dead by SIDS afterwards, and experiments with animal models show that the leading pathologic mechanism of SIDS are cardio-respiratory disorders. The program of prophylaxis of ominous states and SIDS includes definition of risk groups with typical clinical and instrumental criteria and observation of children from risk groups. Key words: sudden infant death syndrome, mechanisms, prophylaxis.

The case report is presented for the first time in Russia of Timothy syndrome: the combination of the long QT syndrome, atrioventricular block, and syndactyly associated with a high risk of the sudden cardiac death in a 3 year old patient referred for examination because of recurrent syncope.