“The sudden and unexpected death of an infant under 1 year of age, with onset of the lethal episode apparently occurring during sleep, that remains unexplained after a thorough investigation including performance of a complete autopsy, and review of the circumstances of death and the clinical history.
Despite the reduction in incidence, SIDS still accounts for the deaths of more babies between the ages of one month and one year than any other cause. The peak incidence of SIDS is between the ages of two and four months. Overall, 60% of SIDS victims are male and 40% are female. There is a close temporal association of the death with sleep.
The aetiology of SIDS is still unknown. The cause may be multifactorial with an interaction between physiological, maternal, infant and environmental factors.
Some factors which have been identified with an increased risk of SIDS include:
Many hypotheses have been advanced regarding the aetiology of SIDS. It has become apparent that SIDS is not the result of a single disease entity but more likely to be the result of a heterogenous group of disorders leading to a fatal outcome.
One of the more significant hypotheses appears to be that of abnormal autonomic function leading to apnoea and death. Subtle neuropathological abnormalities have been found in SIDS infants involving the brain stem, cerebellum, limbicparalimbic systems, as well as peripheral receptors and autonomic nerve fibres. These pathological findings may indicate an abnormality in the autonomic nervous system function in infants susceptible to SIDS. These infants could be at risk through a deficit of arousal in response to a life threatening episode during sleep. This seems to occur during a vulnerable postnatal period i.e. often at age 24 months, when dramatic developmental changes are taking place in sleep/waking patterns and autonomic function. Other hypotheses have suggested there may be obstructive apnoea occurring in some of these infants as a consequence of airway obstruction. Others suggest prolonged expiratory apnoea may occur, leading to inhibition of inspiration and severe hypoxaemia. Abnormal surfactant may contribute to alveolar instability and hypoxaemia.
Other researchers claim that autonomic nervous system control of the heart may be abnormal e.g. conduction abnormalities may occur, such as prolongation of the QT interval and fatal arrhythmias.
Very rare cases of sudden infant death may be associated with anaphylaxis, malignant hyperpyrexia and inherited metabolic disorders such as enzyme deficiencies (postulated to lead to fatal hypoglycaemia at times of physiological stress).
An additional factor which has recently received attention is the perceived risk of SIDS when bedsharing. Research currently suggests that there is an increased risk of SIDS when mothers who smoke bedshare with their babies. Most studies suggest that there is no significantly increased risk of SIDS for babies of nonsmoking parents who bedshare. However, bed sharing is still unsafe if a baby slips under the bedding or into pillows, is trapped between the bed and a parent or the wall, falls out of bed, becomes too hot from too much bedding or is rolled on. Bed sharing is also inadvisable if parents have been drinking alcohol or are affected by other drugs.
For more detailed information on risk factors refer to "Reducing the Risk of Sudden Infant Death Syndrome (SIDS): Scientific Literature" available by contacting SIDS New Zealand.
Significant positive morphologic findings in SIDS may include:
None of these findings are specific for or diagnostic of SIDS and may occur in explained infant deaths. The diagnosis of SIDS remains one of exclusion.