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definition and the specific classification of SIDS as a distinct syndrome has facilitated identification of such cases, permitting the emergence of the descriptive epidemiology of SIDS in the 1970s and 1980s.

DESCRIPTIVE EPIDEMIOLOGY

SIDS occurs almost exclusively in infants between the ages of 2 weeks and 1 year. In industrialized countries, it is the most common diagnosis in infants who die between the ages of 1 month and 1 year (Thach, 1986). The age distribution of cases peaks at age 2 to 3 months and then gradually subsides, with only a small percentage of cases occurring after age 6 months. In the words of Peterson (1980, p. 100), "This [age} pattern has been documented time after time and constitutes the single most consistent, provocative and unique characteristic of SIDS yet identified."

Crude mortality as a result of SIDS reported from throughout the world has ranged from 0.3 to 5.2 per 1,000 live births (Golding et al., 1985). Although these differences in reported rates may be explained partly by differences in classification of deaths caused by SIDS, most of the variation in rates is probably due to real differences in the occurrence of SIDS in diverse populations. The great majority of SIDS deaths occur at home or en route to a hospital (Golding et al., 1985). A number of investigators have reported seasonal variations in SIDS mortality rates, with a relative increase in frequency in winter months (Golding et al., 1985).

Predictors of SIDS include individual characteristics (male sex, low birth weight, multiple births, and black race), maternal characteristics (young age, low education, and cigarette smoking), and low family income (Haglund and Cnattingius, 1990; Hoffman et al., 1987; Kraus et al., 1989). Rates in blacks have consistently been reported to be higher than those in whites; however, in one analysis (Kraus et al., 1989), this difference disappeared after controlling for maternal education and family income.

It has been postulated that apnea during sleep is a mechanism of SIDS, and evidence concerning this hypothesis has recently been reviewed (Sullivan, 1988). Ventilatory patterns during sleep have been studied (Keens et al., 1985), and home apnea monitors have been used for infants thought to be at risk for SIDS (Bryan, 1988). However, it remains uncertain whether there is a relationship between abnormal ventilatory patterns or recurrent apnea episodes and SIDS. In the National Institute of Child Health and Human Development (NICHD) SIDS Cooperative Epidemiologic Study (reviewed below), only 6 of the first 400 SIDS cases (1.5 percent) studied and 1 (0.3 percent) of the matched controls had medically documented apnea (Damus et al., 1988).

Although deaths from SIDS are, by definition, unexpected, children who



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