Apnoea of prematurity and arousal from sleep

Physiology during sleep in preterm infants: Implications for increased risk for the sudden infant death syndrome

Approximately 15 million babies are born preterm (<37 weeks of completed gestation) worldwide annually. Although neonatal and perinatal medicine have contributed to the increased survival rate of preterm newborn infants, premature infants are at increased risk of mortality in the first years of life. Infants born preterm are at four times the risk of Sudden Infant Death Syndrome (SIDS) compared to infants born at term. SIDS is believed to be multifactorial in origin. The Triple Risk hypothesis has been proposed to explain this. The model suggests that when a vulnerable infant, such as one born preterm, is at a critical but unstable developmental period in homeostatic control, death may occur if exposed to an exogenous stressor, such as being placed prone for sleep. The highest risk period is at ages 2-4 months, with 90 % of deaths occurring before 6 months. The final pathway to SIDS is widely believed to involve some combination of immature cardiorespiratory control and a failure of arousal from sleep. This review will focus on the physiological factors which increase the risk for SIDS in preterm infants and how these factors may be identified and potentially lead to effective preventative strategies.

Maturation of Arousals during Day and Night in Preterm Infants

The objective of this study was to compare the maturation of spontaneous arousals during day and night sleep in preterm and term infants. From the Autonomic Baby Evaluation study, the sleep and arousal characteristics of 12 preterm (35.1 ± 2.1 weeks’ gestational age, GA) and 21 term (39.8 ± 0.8 weeks GA) newborns were compared between diurnal and nocturnal sleep periods at birth (M0) and 6 months (M6) of age. Models were adjusted for time (night/day), maturation (M0/M6), prematurity (yes/no). We found that preterm infants had less active sleep (AS)% than term infants with maturation during both day and night sleep, which may reflect accelerated brain maturation secondary to stress or environmental exposure after birth. Moreover, there was a difference in arousal maturation during day and night sleep in the preterm infants, as shown previously for term infants, which suggests the emergence of a circadian rhythm during the earliest postnatal period. We also showed that compared to term infants, these moderate preterm infants had fewer total arousals and, more specifically, fewer arousals in AS during day and night sleep, exposing them to a higher risk of sudden infant death syndrome.

Spontaneous and apnea arousals from sleep in preterm infants

BACKGROUND

The significance of arousal in apnea termination in preterm infants is not known.

METHODS

We investigated the appearance of arousals from sleep with polysomnography for 21 preterm infants at a median age of 36 gestational weeks.

RESULTS

The polysomnographic appearance of sleep was fragmented by frequent arousals. The number of spontaneous arousals unrelated to apneas was 18 per hour in sleep; higher in rapid eye movement (REM) sleep than in non-REM sleep (p < 0.001). Eighty-two percent of arousals were regarded as spontaneous, and 18% were related to apneas. In turn, arousal followed 5% of all apneas; 30% of mixed, 2% of central, and 20% of long apneas defined as apnea of prematurity. Apneas without an arousal led to lower oxygen saturation levels than those followed by an arousal (p < 0.001). Mixed apneas with an arousal had stronger breathing effort and a higher number of breaths compared with apneas without an arousal (p < 0.05).

CONCLUSIONS

In preterm infants, frequent spontaneous arousals or arousal-type phenomena make the polysomnographic appearance of sleep fragmented. However, even long apneas or hypoxia commonly fail to elicit arousals or any sign of sleep interruption. Our findings suggest that arousal appears not to be the main mechanism for apnea termination in preterm infants.

IMPACT

Polysomnographic appearance of sleep in preterm infants is fragmented by arousals. Contrary to older children and adults, arousal to apnea is uncommon in preterm infants. Even long mixed apneas with desaturation mostly fail to elicit an arousal response. In preterm infants, apnea termination appears not to depend on an arousal. Low arousability is suggested to be caused by a low ventilation response to hypoxia.

Long-range temporal organisation of limb movement kinematics in human neonates

OBJECTIVE

Movement provides crucial sensorimotor information to the developing brain, evoking somatotopic cortical EEG activity. Indeed, temporal-spatial organisation of these movements, including a diverse repertoire of accelerations and limb combinations (e.g. unilateral progressing to bilateral), predicts positive sensorimotor outcomes. However, in current clinical practice, movements in human neonates are qualitatively characterised only during brief periods (a few minutes) of wakefulness, meaning that the vast majority of sensorimotor experience remains unsampled. Here our objective was to quantitatively characterise the long-range temporal organisation of the full repertoire of newborn movements, over multi-hour recordings.

METHODS

We monitored motor activity across 2-4 h in 11 healthy newborn infants (median 1 day old), who wore limb sensors containing synchronised tri-axial accelerometers and gyroscopes. Movements were identified using acceleration and angular velocity, and their organisation across the recording was characterised using cluster analysis and spectral estimation.

RESULTS

Movement occurrence was periodic, with a 1-hour cycle. Peaks in movement occurrence were associated with higher acceleration, and a higher proportion of movements being bilateral.

CONCLUSIONS

Neonatal movement occurrence is cyclical, with periods consistent with sleep-wake behavioural architecture. Movement kinematics are organised by these fluctuations in movement occurrence. Recordings that exceed 1-hour are necessary to capture the long-range temporal organisation of the full repertoire of newborn limb movements.

SIGNIFICANCE

Future work should investigate the prognostic value of combining these movement recordings with synchronised EEG, in at-risk infants.

Maternal dietary supplementation with omega-3 polyunsaturated fatty acids confers neuroprotection to the newborn against hypoxia-induced dopamine dysfunction

INTRODUCTION

Up to 84% of prematurely born infants suffer hypoxic, anoxic, and ischemic insults. Those infants with subsequent behavioral, motor or cognitive dysfunction represent 8-11% of all live births. Yet, no interventions employed during pregnancy attenuate risk of morbidity in those at-risk infants. Dietary supplementation with omega-3 polyunsaturated fatty acids (ω-3 PUFAs) has been shown to reduce stroke-induced neuropathology in rat models emulating this adverse clinical event. To extend those studies we sought to determine whether maternal dietary supplementation with ω-3 PUFAs would confer neuroprotection against hypoxia-induced neurochemical dysfunction in newborn rat pups exposed to repetitive hypoxic insults.

METHODS

We provided pregnant rats with either a ω-3 PUFA enriched diet or else a standard rat chow diet. At postnatal day 7, pups were assigned randomly to either repetitive hypoxic insults or repetitive bursts of room air. On postnatal day 12, pups were sacrificed and brain dopamine levels characterized.

RESULTS

Baseline brain dopamine levels did not differ between rat pups born to dams who received ω-3 PUFA enriched versus standard rat chow diets. Rat pups born to dams maintained on normal diets, who were exposed to five days of repetitive hypoxic insults, experienced a 57% reduction in striatal dopamine levels accompanied by significant apoptosis. In contrast, ω-3 PUFA-enriched newborn pups experienced no loss in striatal dopamine levels, and only minimal apoptosis.

CONCLUSIONS

Our findings suggest that it may be feasible to confer neuroprotection against hypoxia-induced dopamine dysfunction to newborns likely to experience hypoxic insults. This could significantly improve the outcomes of those 8-11% of newborns who would otherwise experience hypoxia-induced behavioral, motor and cognitive dysfunction.

Current definitions for neonatal apnoea: are they evidence based?

Apnoea is defined as cessation of breathing with implicit pathophysiology. This review considers definitions of neonatal apnoea currently available and explores the evidence to support their use. For preterm and term infants, apnoea definitions appear arbitrary, are not supported by guidelines and vary from study to study. Although most alarms on infant breathing monitors are set to alert after a respiratory pause >20s duration is detected, this time period is the equivalent of 17 missed breaths in a preterm infant. Apnoea is likely to be better defined by associated consequence than by pause duration alone in this age group; however, the degree of change in heart rate or oxygen saturation that defines a respiratory pause as pathological is yet to be defined. Further research is required to determine the characteristics that differentiate respiratory events of clinical consequence from normal respiratory variability in term and preterm infants.

Arousal from sleep mechanisms in infants

Arousals from sleep allow sleep to continue in the face of stimuli that normally elicit responses during wakefulness and also permit awakening. Such an adaptive mechanism implies that any malfunction may have clinical importance. Inadequate control of arousal in infants and children is associated with a variety of sleep-related problems. An excessive propensity to arouse from sleep favors the development of repeated sleep disruptions and insomnia, with impairment of daytime alertness and performance. A lack of an adequate arousal response to a noxious nocturnal stimulus reduces an infant's chances of autoresuscitation, and thus survival, increasing the risk for Sudden Infant Death Syndrome (SIDS). The study of arousability is complicated by many factors including the definition of an arousal; the scoring methodology; the techniques used (spontaneous arousability versus arousal responses to endogenous or exogenous stimuli); and the confounding factors that complicate the determination of arousal thresholds by changing the sleeper's responses to a given stimulus such as prenatal drug, alcohol, or cigarette use. Infant age and previous sleep deprivation also modify thresholds. Other confounding factors include time of night, sleep stages, the sleeper's body position, and sleeping conditions. In this paper, we will review these different aspects for the study of arousals in infants and also report the importance of these studies for the understanding of the pathophysiology of some clinical conditions, particularly SIDS.

Managing "healthy" late preterm infants

BACKGROUND

Late preterm infants are often managed in nursery rooms despite the risks associated with prematurity. The objective of this study was to determine the risks facing late preterm infants admitted to nursery rooms and to establish a management strategy.

METHODS

A total of 210 late preterm infants and 2648 mature infants were assessed. Infants born at 35 and 36 weeks' gestation weighing >or=2000 grams admitted to a nursery room and not requiring medical intervention at birth were of particular interest. The admission rates to the neonatal intensive care unit were evaluated according to the chart review.

RESULTS

Infants born at 35 and 36 weeks' gestation weighing >or=2000 grams had significantly higher admission rates than term infants at birth (Cochran-Mantel-Haenszel test, P < 0.001; common risk ratio, 4.27; 95% confidence interval, 2.41-7.55) and after birth (P < 0.001; common risk ratio, 3.57; 95% confidence interval, 2.40-5.33). More than 80% of admissions from the nursery room to the neonatal intensive care unit after birth were due to apnea or hypoglycemia in neonates born at 35 and 36 weeks' gestation. The admission rates due to apnea increased with decreasing gestational age. The admission rates due to hypoglycemia with no cause other than prematurity accounted for 24.3% of admissions for those born at 35 weeks' gestation and 14.1% of admissions for those born at 36 weeks' gestation; hypoglycemia due to other causes accounted for fewer admissions.

CONCLUSION

The management strategy for late preterm infants should be individualized, based on apnea and hypoglycemia. The respiratory state of late preterm infants should be monitored for at least 2 days, and they should be screened for hypoglycemia on postnatal day 0.

Differential diagnosis of apneas in preterm infants

Clinically relevant apneas, which are common in preterm infants, may adversely affect later neuropsychological condition in this group of patients. Pharmacotherapy to stimulate respiratory functions may be unsuccessful. Polygraphic recording may help in the differential diagnosis of these clinically relevant events. Twenty-nine preterm neonates born before 36 weeks of gestational age were examined using polygraphic recording (respiration--two channels, perioral electromyography, oxygen saturation, heart rate, electroencephalography, electrocardiography, electrooculography). The examination was ordered by the attending physician after an unsuccessful treatment of apnea by Aminophylline, and it should contribute to the clarification of the causes of these events. In the course of the polygraphic examinations, altogether 63 episodes were recorded during which the pulse oximeter alarm signal was set off. In 42 cases, the alarm signal was set off in events during which SaO(2) fell below 85%. In the remaining 21 cases, the alarm signal was set off in episodes during which early bradycardia below 90/min occurred. The onset of apnea was very often associated with the phasic increase of the perioral electromyography and with electroencephalography arousal reaction. Because of suspicion that these apneas may be triggered by episodes of gastroesophageal reflux, the interruption of the Aminophylline treatment and setting up an antireflux regimen were recommended. These therapeutic measures had a positive effect: The frequency of alarm signals decreased within 48 h by a statistically significant 50%. In cases where the pharmacotherapy of apnea by stimulation of respiratory functions is not successful, differential diagnostic analysis should be performed. Polygraphy may contribute to the clarification of the causes underlying clinically relevant apneas in a view of newly described polygraphic signs. It is feasible to suspect, based on these signs, that gastroesophageal reflux is the cause for clinically significant apneas in that case.

Effects of prematurity on heart rate control: implications for sudden infant death syndrome

In Western countries, 5-11% of all infants are born before 37 weeks of gestation, and with improvements in modern intensive care techniques the number of these preterm infants that survive continues to increase. Preterm birth is one of the leading causes of neonatal morbidity and mortality in developed countries, accounting for 60-80% of infant deaths in those without congenital anomalies. Furthermore, in the post-neonatal period, preterm infants are at four-times greater risk of sudden infant death syndrome. It has been suggested that this increased risk is due to immature autonomic control. This article provides an overview of studies assessing autonomic control of the cardiovascular system in preterm infants.

Massage therapy and sleep behaviour in infants born with low birth weight

This study attempts to evaluate the impact of massage therapy on sleep behaviour in infants born with low birth weight (LBW) in St. Petersburg, Russia. Fifty infants (22 boys, 28 girls) who were born in St. Petersburg between 2000 and 2002 and defined as LBW babies (<2500g at birth) were enrolled onto the study at the age of 2 months. Of these, 41 (19 boys, 22 girls) were light and pre-term infants (gestational age < or =36 weeks), and 9 (3 boys, 6 girls) born light at term. The control group consisted of 50 healthy infants born with LBW who were cross-matched with an experimental group of babies and controlled for gender, gestational age, weight and date of birth. The groups were also matched for proximal geographical distribution in the city. Babies in the experimental group were assigned massage intervention therapy that include gentle rubbing, stroking, passive movements of the limbs and other means of kinaesthetic stimulation performed by professionals until the infant is 8 months old. The findings suggest that 8-month-old LBW infants who received massage intervention were less likely to snore during sleep, required less feeding on waking-up at night, and appeared more alert during the day. These apparent correlations remained significant after adjustment was made for major potential confounders. No statistically significant difference was found in sleep behaviour between LBW infants exposed to massage therapy who were either born pre-term or at term. It is suggested that massage may be a valuable approach to improve quality of sleep and reduce sleep-disordered breathing in infants born with LBW. It is acknowledged that whilst this study does not represent a large sample, it is felt that the findings suggest further investigation and offer an insight into an area previously relatively unexplored.

Lung-function tests in neonates and infants with chronic lung disease: tidal breathing and respiratory control

This paper is the fourth in a series of reviews that will summarize available data and critically discuss the potential role of lung-function testing in infants with acute neonatal respiratory disorders and chronic lung disease of infancy. The current paper addresses information derived from tidal breathing measurements within the framework outlined in the introductory paper of this series, with particular reference to how these measurements inform on control of breathing. Infants with acute and chronic respiratory illness demonstrate differences in tidal breathing and its control that are of clinical consequence and can be measured objectively. The increased incidence of significant apnea in preterm infants and infants with chronic lung disease, together with the reportedly increased risk of sudden unexplained death within the latter group, suggests that control of breathing is affected by both maturation and disease. Clinical observations are supported by formal comparison of tidal breathing parameters and control of breathing indices in the research setting.

Heart rate responses to non-arousing trigeminal stimulation in infants: effects of sleep position, sleep state and postnatal age

AIMS

The aim of this study was to examine the effects of maternal smoking, sleeping position, sleep state and postnatal age on heart rate changes following non-arousing trigeminal stimulation in infants.

SUBJECTS

We studied healthy term infants, 13 of whom were born to mothers who did not smoke and 11 to mothers who smoked during pregnancy. Each infant was studied using daytime polysomnography on 3 occasions: (a) 2-3 weeks, (b) 2-3 months and (c) 5-6 months after birth. Nasal air-jet stimulation was presented in both active sleep (AS) and quiet sleep (QS) when infants slept both prone and supine.

RESULTS

We found no difference between infants of smoking and non-smoking mothers in any of the parameters measured. Minimum HR (MinHR) following non-arousing trigeminal stimulation was significantly lower in the supine compared to the prone sleeping position at 2-3 weeks and 2-3 months of age (p<0.05) in AS, and at all 3 ages in QS (p<0.01). MinHR was significantly lower in QS compared to AS at 2-3 months when infants slept prone and at 5-6 months when sleeping supine (p<0.01). In QS, MinHR became lower with increasing postnatal age in both sleep positions (p<0.01). In AS, there was no maturational effect. The normalized bradycardia (DeltaHR%) was significantly greater in AS than in QS at 2-3 weeks of age (p<0.05) when infants slept supine.

CONCLUSION

Our study has shown that there was a decrease in heart rate (MinHR) following trigeminal stimulation in infants up to 6 months of age and this was affected by sleep position and sleep state, being larger in the supine sleeping position and the QS state.

Comparison of postnatal development of heart rate responses to trigeminal stimulation in sleeping preterm and term infants

Autonomic dysfunction has been regarded as a possible cause of the sudden infant death syndrome (SIDS) and it has been suggested that preterm infants, who are at a greater risk of SIDS than term infants, may have immature autonomic control. Our aim was to compare the maturation of cardiac autonomic control during sleep in preterm and term infants by examining heart rate responses to arousing and non-arousing trigeminal stimuli. Preterm infants (n = 15) and term infants (n = 24) were studied longitudinally with daytime polysomnography. Air-jet stimulation of the nares was delivered in both active sleep (AS) and quiet sleep (QS), and heart rate (HR) changes recorded for both arousal and non-arousal responses. Changes in HR (DeltaHR%) were calculated as the relative differences between baseline HR (BHR) and either MaxHR (arousal) or MinHR (non-arousal). Comparisons of HR changes between sleep states and postnatal ages were made with two-way anova for repeated measures and between groups with two-way anova. The increase in HR (DeltaHR%) was greater in term than preterm infants (P < 0.05), but only at 2-3 weeks corrected postnatal age (CPA). In preterm infants, there were no differences in BHR between sleep states, whereas in term infants, BHR was higher in AS than in QS at 2-3 weeks and 2-3 months of age. The smaller DeltaHR% to arousing stimuli in preterm infants compared with term infants at 2-3 weeks suggests that cardiac sympathetic activity in preterm infants may be lower than in term infants. This mechanism may account for the increased risk for SIDS of preterm infants.

Respiratory control and arousal in sleeping infants

Control of the cardiovascular and respiratory systems undergoes rapid maturation during infancy. Sleep is at a lifetime maximum during this period and has a marked influence on cardiorespiratory function. The mechanisms leading to sudden infant death syndrome (SIDS) may include a failure in the neural integration of the cardiovascular and respiratory systems, with a concomitant failure to arouse from sleep. Studies have shown that sleep states exert a marked influence on respiratory control and arousability. Infants are more arousable in active sleep compared with quiet sleep from both somatosensory and respiratory stimuli. Post-natal and gestational age at birth also have a marked influence on arousability. Arousability is depressed by the major risk factors for SIDS (prone sleeping, maternal smoking, prematurity and recent infection) and is increased by factors that decrease the risk for SIDS (e.g. use of dummies, breastfeeding).

Ontogeny of sleep and awake states in relation to breathing in preterm infants

This review will focus on the development of behavioural states and breathing during early developmental stages prior to term gestation. Although these behavioural states are immature during early development, their cyclicity is clearly seen. Preterm infants characteristically have a large proportion of indeterminate sleep and small amount of wakefulness. Whereas oxygenation is relatively stable during active and quiet sleep in ventilated preterm infants, indeterminate sleep and arousals are associated with hypoxaemic episodes. Arousals have also been linked to apnoea in spontaneously breathing infants. Since well-defined sleep cycles are beneficial for the oxygenation of preterm infants, we should explore ways to promote their natural sleep while they are exposed to neonatal intensive care. Care practices such as clustering procedures, kangaroo care and optimal positioning have been shown to improve the integrity of sleep. Optimizing the sleep cycling might improve the long-term outcome of preterm infants. More studies in this area are clearly needed.

Sudden unexpected deaths after discharge from the neonatal intensive care unit

The published evidence on the risk of sudden unexpected death in infants after discharge from the neonatal intensive care unit (NICU) is reviewed, together with the relevance of the various identified potentially modifiable post-natal risk factors, particularly sleeping position. Infants of low birthweight, short gestation, and those with adverse perinatal histories are at substantially increased risk of sudden infant death syndrome (SIDS), but the potential benefits from following the measures designed to reduce the risk of SIDS are proportionally greater than for term infants. The use of home apnoea monitors has not been shown to be of value in preventing SIDS, but the importance of maintaining adequate oxygenation in infants with bronchopulmonary dysplasia is emphasised. Evidence based recommendations for care of infants after discharge from the NICU with a view to reducing the risk of SIDS are presented, and do not differ significantly from those for low-risk infants.

The effect of maternal smoking on respiratory and arousal patterns in preterm infants during sleep

Prenatal exposure to cigarette smoke is associated with an increased risk of sudden infant death syndrome. The effect of maternal smoking on apnea and arousal patterns in preterm infants is currently unknown. Multichannel polysomnographic studies were performed in preterm infants. Thirty infants were enrolled into the study: 16 exposed prenatally to cigarette smoke (S) and 14 control infants (C). There was no difference in the gestational and postconceptional ages at the time of study. Maternal smoking was associated with a significant increase in the apneic index in these infants (28.6 +/- 6.4/hour [S] vs. 13.2 +/- 3.9 [C]; p<0.05), and the difference was noted for obstructive events and only during active sleep. The arousal index was significantly decreased in the maternal smoking group (34.5 +/- 2.3/hour [S] vs. 46.3 +/- 5.6/hour [C]; p<0.05), with a specific decrease in percentage of arousal after respiratory events (10.7 +/- 2.1% [S] vs. 29.4 +/- 5.4% [C]; p<0.05). In conclusion, preterm infants exposed prenatally to cigarette smoke have increased respiratory events during active sleep, predominantly due to obstructive apnea, and possibly a higher arousal threshold during apneic events. These alterations in respiratory and arousal patterns in preterm infants born to smoking mothers may lead to significant vulnerability in this population.

Arousal responses and risk factors for sudden infant death syndrome

BACKGROUND

Failure to arouse from sleep has been postulated as a mechanism to explain the final pathway of sudden infant death syndrome (SIDS).

METHODS

We have reviewed the effects of the major risk factors for SIDS, prone sleep position, maternal smoking, prematurity and recent infection on arousability from sleep. In human infants it has been consistently demonstrated that arousal from sleep in response to a variety of stimuli is more difficult to induce from quiet sleep (QS) compared to active sleep (AS) over the first 6 months of life.

RESULTS

In the prone position both stimulus-induced and spontaneous arousability from both QS and AS were impaired at 2-3 weeks and 2-3 months, but not at 5-6 months of age in both term and preterm infants. In term infants exposed to maternal smoking during pregnancy both stimulus-induced and spontaneous arousability were impaired when infants slept supine in QS at 2-3 months of age. Healthy preterm infants showed no impairment in arousability compared with term infants at matched postconceptional ages. However, preterm infants with a history of apnoea and bradycardia of prematurity showed decreased arousal responses in both QS and AS and this impairment was positively correlated to their 'perinatal risk score'. Infants who had recently suffered an infection requiring hospitalization showed decreased arousability in QS on the day of discharge when compared to 2 weeks later when they were completely well.

CONCLUSIONS

In summary it has been found that the major risk factors for SIDS identified from epidemiological studies also decrease arousability from sleep in infants. We propose that this decreased arousability from sleep may be involved in the final pathway of SIDS.

Temperament ratings do not predict arousability in normal infants and infants at increased risk of sudden infant death syndrome

This study sought to determine whether temperament was an indicator of arousability from sleep in infants. We hypothesized that the "threshold" dimension would be the most predictive characteristic because it measures the stimulus intensity required to evoke a discernible response. Healthy term, healthy preterm, and preterm infants with a neonatal history of apnea underwent polysomnography at 2 to 3 months. Arousal was induced using air-jet stimulation of the nostrils in active (AS) and quiet sleep (QS). Temperament was assessed using the Early Infancy Temperament Questionnaire. Arousal thresholds were elevated in QS compared with AS in each group ( <.001), and preterm infants with a neonatal history of apnea were less arousable than healthy preterm infants ( <.05). Temperament was not a predictor of arousability in AS. "Adaptability" was the only significant predictor of arousability in QS. This study demonstrates that temperament characteristics as measured by questionnaire may not be reliable indicators of arousability from sleep.