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    Low Serotonin Levels May Be Key to SIDS

    Study Shows Link Between Sudden Infant Death Syndrome and Deficiency of Hormone Serotonin

    Explanations for SIDS

    SIDS research is a ''controversial area," Kinney says. Many experts look to the "triple risk" model to explain it, believing that SIDS results from an underlying vulnerability, a critical developmental period, and an outside stressor.

    ''The real risk period is the first six months," Kinney says of the critical period in which most deaths occur.

    But experts disagree on what the vulnerability is. Kinney's research suggests low levels of the neurotransmitter serotonin, and probably other brain chemicals yet to be identified, are what make infants vulnerable. Other experts suspect other vulnerabilities, such as infections.

    Yet others say SIDS is due simply to suffocation, she says. "We say, yes, some babies will die if they are severely asphyxiated," Kinney tells WebMD. But she adds, "what we are saying is in the majority of cases, the babies have an underlying defect that puts them at risk that makes them unable to respond to a stressor, such as having their face compressed [while sleeping on the stomach or becoming tangled in soft bedding]."

    In previous research, Kinney and her colleagues found defects in the serotonin system of SIDS babies, including defects in the serotonin receptors, which are crucial for serotonin to work.

    ''But we never knew if there was too little or too much serotonin," she says. "In this study, we actually measured the levels of serotonin and the enzyme that makes serotonin."

    Measuring Serotonin Levels

    Kinney and colleagues evaluated serotonin and tryptophan hydroxylase (TPH2), the enzyme that helps make serotonin, in 35 infants who died from SIDS.

    They compared these measurements with those from two groups -- five infants who died suddenly for whom a cause of death was established and five infants who died while hospitalized for having insufficient oxygen to the tissues.

    They retrieved tissue samples from the autopsies to measure the enzyme and hormone levels.

    They found that:

    • Serotonin levels were 26% lower in the SIDS babies than in the infants who died suddenly with a known cause of death, and the enzyme levels were 22% lower. The serotonin and enzyme levels were also lower in the SIDS babies than in the group of hospitalized infants.
    • Levels of binding to the serotonin receptors were also lower in the SIDS babies.

    Finding the deficiency in serotonin levels isn't the whole story, Kinney says. "We think there are probably several neurotransmitter systems involved in SIDS."

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