Sudden Infant Death Syndrome (SIDS)

It has been suggested that SIDS victims have some inherent weakness due to fetal influences or genetic make-up, which may only become obvious when he or she is subjected to stress during a vulnerable developmental period (Rognum 1995). Many consider SIDS to be due to many factors, not a specific disease process, but a lethal situation in which an infant succumbs from the additive effects of several factors (Byard 2001).

SIDS is an entity with no pathological findings at autopsy, that is, a diagnosis by exclusion. A few similarities are often found at autopsy, but these findings do not provide an explanation for death (Guntheroth 1995; Byard 2001). Externally, the infant appears well developed, and all the pathologist may find is a small amount of mucoid, watery, or bloody fluid in the nostrils. Internally, minute hemorrhagic spots (petecchiae) are seen on the surface of the thymus, lungs, and heart in approximately 75 percent of the cases. There is often evidence of a slight infection in the upper airways, as well as increased amounts of fluid (congestion and edema) and numerous cells (macrophages) in the air sacks of the lungs. Several conditions involving all organ systems may be responsible for sudden death in infants and small children that appear reasonably well prior to death. In investigating cases of suspected SIDS, the possibility of underlying illness, accident, or even homicide must be considered. If an adequate postmortem examination, including a review of the history and circumstances is not performed, the possibility of determining other causes of death may be lost.

In the 1990s, researchers have focused on the role of the immune system in SIDS. Many SIDS victims have shown signs of a slight infection prior to death, and there is often evidence of a subacute infection in the upper airways, or a slight cold. This has led to several studies of the involvement of infections and regulatory immune mechanisms in SIDS. The immune system undergoes rapid development during the first weeks and months of life and can trigger a reduction of oxygen to the blood (hypoxemia), resulting in a self-amplifying vicious circle that can result in death. A possible biochemical marker for hypoxic (insufficient oxygen reaching the infant) episodes prior to death from SIDS, hypoxanthine, has previously been identified (Rognum 1995).

The pathophysiology of SIDS remains unknown. Several studies have suggested possible abnormalities, such as respiratory pattern, arousal responses, temperature regulation, cardiac control, and autonomic function. Abnormalities in the way the nervous system regulates cardiorespiratory control or other autonomic functions provide perhaps the most compelling hypothesis (Hauck 2000).

Epidemiological research has shown modifiable and nonmodifiable factors to be associated with increased or decreased SIDS risk (Guntheroth 1995; Rognum 1995; Byard 2001). From the middle of the 1980s, several studies began to report an increased risk of SIDS attributed to prone sleeping (sleeping on the stomach). Back to sleep campaigns were launched in several countries, including Australia, the United States, Germany, France, and Italy, resulting in an immediate decrease in the SIDS rate. Prone sleeping is still a major risk factor for SIDS, as is side sleeping. Other sleep environment factors, such as soft bedding and the use of pillows, covering of the head or face, the use of duvets, and overheating, have also, alone or together with prone sleeping, been associated with increased risk of SIDS. At the beginning of the 1990s there was an increase reported in the risk of SIDS associated with bedsharing or co-sleeping with an adult. Such an association is still controversial.

Sociodemographic factors, such as lower socioeconomic status (measured by low income, unemployment, low education, and young maternal age) have consistently been shown to be associated with greater risk of SIDS. Risk differences are found among different races, with African Americans and indigenous populations in the United States, Australia, and New Zealand having the highest rates, and most Asian communities around the world the lowest. An age peak has been seen between two to four months of age; more males than females are affected;, and SIDS has been more common during the colder months of the year. Factors related to pregnancy have been shown to increase the risk of SIDS, such as higher birth order, lower birth weight, and short gestation period. Maternal smoking during pregnancy is consistently associated with risk of SIDS, often showing a dose-response effect; that is, the more a mother smokes, the greater the risk of SIDS for her infant. Smoking is perhaps the most important maternal risk factor and is viewed as the most important modifiable risk factor of SIDS altogether, after the reduction in prone sleeping. The use of illegal drugs is associated with a somewhat increased risk.

Several epidemiological studies into possible risk and/or protective factors of SIDS have concluded that pacifier use may protect against SIDS. As to the role of breastfeeding as a potential preventive measure against SIDS, studies have been inconclusive. At the end of the 1990s however, comparison of epidemiological characteristics before and after the decline in SIDS rate due to "back-to-sleep" campaigns disclosed significant changes in variables such as a reduction in the two to four month age peak and in the winter peak as well as increased risk with young maternal age, low socioeconomic status, and maternal smoking during pregnancy (Byard 2001). Understanding these changes, coupled with the effects of reducing modifiable factors, will probably reduce the SIDS rate further, and, it is hoped, eventually lead to an understanding of both the etiology and pathogenesis of SIDS.

Genetic factors have also been thought to play a role in SIDS with findings of a modest, but significantly increased, recurrence rate of SIDS in families (Guntheroth 1995; Hauck 2000; Byard 2001). Death from inherited metabolic disorders has been proposed as the cause of death in a small number of SIDS cases. The long-QT syndrome, a cardiac arrhythmia that can cause sudden death, is another inherited disorder proposed as the cause of death in some cases of SIDS.