STRUCTURAL abnormalities in a baby鈥檚 brainstem may lie behind around half the cases of sudden infant death syndrome.
Hannah Kinney and David Paterson at the Children鈥檚 Hospital Boston examined the brains of 31 infants who had died of SIDS and compared them with the brains of 10 infants who had died of other causes. They found abnormalities in the medulla, the part of the brainstem that regulates breathing, blood pressure, body heat and arousal. SIDS babies had more of the neurons that release serotonin, but fewer receptors for the neurotransmitter (see Diagram).
Kinney has previously recorded low numbers of serotonin receptors in around 50 per cent of SIDS babies, while studies in mice have shown that 鈥減acemaker鈥� cells in the medulla, which prompt gasping and recovery, don鈥檛 fire when serotonin is taken away (快猫短视频, 18 March, p 21).
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鈥淚t might be that a defect in the medulla鈥檚 serotonin system is inhibiting a baby鈥檚 ability to gasp,鈥� says Paterson. Only further tests will establish whether pacemaker cells are simply not responding to serotonin, or whether the ability of neurons to release it is turned off, he says.
鈥淚t might be that a defect in the medulla鈥檚 serotonin system is inhibiting a baby鈥檚 ability to gasp and recover鈥�
No one yet knows what causes these abnormalities in infants鈥� brains, although Paterson suspects they begin early in fetal development and are down to a combination of genetic and environmental factors, such as the mother smoking or drinking.
SIDS is the leading cause of death in infants under one year in the US, affecting around 0.67 in every 1000 live births. According to the 鈥渢riple risk hypothesis鈥�, proposed by Kinney in 1994, a baby who is at risk of SIDS has some genetic or inborn susceptibility, is at a vulnerable stage of development at around six months and is exposed to one or more external stress factors such as infection, a lack of oxygen or overheating.
A serotonin defect would fall into the first category, says Kinney, adding that the first six months are risky time for babies because they have to adjust to breathing on their own and maintaining their blood pressure.
鈥淚f the baby is then put through additional stresses such as overheating and overbundling and it is unable to meet those challenges then it may die,鈥� she says. Around 65 per cent of the SIDS infants in the study had been sleeping on their stomach or side 鈥� a known risk factor for SIDS.
George Haycock, scientific director of UK-based research organisation the Foundation for the Study of Infant Deaths, says the new findings are important and unlikely to be due to chance. 鈥淗owever, I doubt that this is the only inherited or non-modifiable risk factor,鈥� he says. Other studies have shown differences in genes for immune system signalling molecules and a growth factor critical for lung development in SIDS babies, for example.
Paterson agrees that brainstem abnormalities may be involved in around 50 per cent of SIDS cases, but adds, 鈥淭here are definitely other factors causing it as well.鈥�
