Arousal from sleep is thought to be essential for restoring an infant's airway integrity during life threatening situations like obstructive apnea or when bedding covers the face. Our past work indicates that the earliest stages of arousal, sigh and startle, are critically important for recovery in both cases and airway reopening or avoidance of bedding may depend on the magnitude of neck extension associated startle movements. Unlike sighs, infantile sleep startles are suppressed by 2-3 years of age. The time course for their disappearance is unclear. We hypothesised that maturational changes may occur in sigh and startle during the first six months of life that could be related to the period of vulnerability for Sudden Infant Death Syndrome (SIDS). We briefly occluded the face mask airway of sleeping infants to stimulate arousal. We studied 16 healthy infants (1-5 months old, 103 occlusions analysed) by recording EEG, ECG, O2-Saturation, diaphragmatic, nuchal and limb EMG, mask pressure, and startle intensity (I-III) scored by video taped limb and body movements. At onset of arousal a sigh occurred simultaneously with a startle, characterised by a sudden body movement with a pronounced neck extension. Startle intensity correlated positively with sigh intensity (peak negative mask pressure) (p<0.05). Initially startle intensity increased with age, peaked at three months and then fell (ANOVA p<0.005). The time for a sigh and startle to occur during occlusion decreased with age (ranging from 11 sec at 34 days to 2 sec at 134 days)(p<0.01).
We conclude both that the timing and the intensity of sigh and startle evoked by airway occlusion are age dependent with large maturational changes occurring during the peak age of risk for SIDS. Maturational deficits in these mechanisms could be a factor determining SIDS vulnerability.
