Neurochemical Alterations in Sudden Unexplained Perinatal Deaths—A Review

Sudden unexpected perinatal collapse is a major trauma for the parents of victims. Sudden infant death syndrome (SIDS) is unexpected and mysterious death of an apparently healthy neonate from birth till 1 year of age without any known causes, even after thorough postmortem investigations. However, the incidence of sudden intrauterine unexplained death syndrome (SIUDS) is seven times higher as compared with SIDS. This observation is approximated 40–80%. Stillbirth is defined as death of a fetus after 20th week of gestation or just before delivery at full term without a known reason. Pakistan has the highest burden of stillbirth in the world. This basis of SIDS, SIUDS, and stillbirths eludes specialists. The purpose of this study is to investigate factors behind failure in control of these unexplained deaths and how research may go ahead with improved prospects. Animal models and physiological data demonstrate that sleep, arousal, and cardiorespiratory malfunctioning are abnormal mechanisms in SIUDS risk factors or in newborn children who subsequently die from SIDS. This review focuses on insights in neuropathology and mechanisms of SIDS and SIUDS in terms of different receptors involved in this major perinatal demise. Several studies conducted in the past decade have confirmed neuropathological and neurochemical anomalies related to serotonin transporter, substance P, acetylcholine α7 nicotine receptors, etc., in sudden unexplained fetal and infant deaths. There is need to focus more on research in this area to unveil the major curtain to neuroprotection by underlying mechanisms leading to such deaths.

Sudden unexpected perinatal collapse is a major trauma for the parents of victims. Sudden infant death syndrome (SIDS) is unexpected and mysterious death of an apparently healthy neonate from birth till 1 year of age without any known causes, even after thorough postmortem investigations. However, the incidence of sudden intrauterine unexplained death syndrome (SIUDS) is seven times higher as compared with SIDS. This observation is approximated 40-80%. Stillbirth is defined as death of a fetus after 20th week of gestation or just before delivery at full term without a known reason. Pakistan has the highest burden of stillbirth in the world. This basis of SIDS, SIUDS, and stillbirths eludes specialists. The purpose of this study is to investigate factors behind failure in control of these unexplained deaths and how research may go ahead with improved prospects. Animal models and physiological data demonstrate that sleep, arousal, and cardiorespiratory malfunctioning are abnormal mechanisms in SIUDS risk factors or in newborn children who subsequently die from SIDS. This review focuses on insights in neuropathology and mechanisms of SIDS and SIUDS in terms of different receptors involved in this major perinatal demise. Several studies conducted in the past decade have confirmed neuropathological and neurochemical anomalies related to serotonin transporter, substance P, acetylcholine α7 nicotine receptors, etc., in sudden unexplained fetal and infant deaths. There is need to focus more on research in this area to unveil the major curtain to neuroprotection by underlying mechanisms leading to such deaths.
Keywords: sudden infant death, sudden perinatal death, stillbirth, neuropathology, sudden intrauterine death, neurochemicals iNTRODUCTiON In the first year of life, the most frequent type of death is "Crib death, " "Cot death" commonly termed as "sudden infant death syndrome" (SIDS). Among every 1,700-2,000 births approximately, one baby gets affected (1). Numerous inherited abnormalities, such as morphological substrates for SIDS-sudden intrauterine unexplained death syndrome (SIUDS), were detected, mainly represented by variations of cardiac conduction system just like accessory pathway, abnormal resorptive degeneration, and hypoplasia/agenesis of the vital brainstem structures. The National  Child Health Institute and Human Development has expressed that SIDS is a developmental issue and it takes its root from the fetal development (2). The neuropathological examination plays a significant role in the death investigation procedure. However, just some limited reviews have sufficiently analyzed the neurological substrates, albeit even subtle anomalies of the autonomic nervous system can measure the dysfunctions in the fundamental functions, prompting sudden and unexpected death (3,4). In-depth examination results, performed at the University of Milan, Lino Rossi Research Center, have added to recognize the area and the nature of these anomalies, normally observed in both SIUDS and SIDS. External risk factors, for example, alcohol, maternal smoking, and drug abuse are identified to be the potential contributors of SIUDS and SIDS (5) while environmental pollution such as insecticides and pesticides has also been reported recently (6).

SUDDeN iNFANT DeATH SYNDROMe
In a number of these infants, the cerebrum portion that controls the arousal and breathing from sleep is not yet mature enough to work appropriately. Preterm births and intrauterine growth restrictions can cause repressed cognitive development and chronic infarctions. Low immunological development and postnatal sleeping positions are responsible for major respiratory distress. In this section, we have discussed the risk factors for SIDS. Figure 1 is derived from Filiano and Kinney hypothesis (7) and shows risk factors contributing to SIDS.

Neuropathology of SiDS
The major focus on cerebrum anomalies in SIDS victims for a physiological investigation demonstrates cardiopulmonary abnormalities and sleep arousal dysfunction. A typical pathway of these abnormalities at the level of brainstem, where these control functions including ventilation pathways, cardiac rhythm, and pathways for sleep/arousal. Neuropathological basis of SIDS, as proposed to be the major risk factor and needs more neurochemical investigation (7). Research on the neurochemical abnormalities of SIDS victims was started in the 1980s (8). Some neurotransmitters and their functions in a normal infant or fetus are shown in Figure 2.
For instance, acetylcholine (ACh) and serotonin [5-hydroxytryptamines (5-HT)] were found to facilitate breathing (9) while epinephrine (Epi) and norepinephrine (NE) depressed breathing (10). Moreover, it was demonstrated that Epi, NE, and 5-HT were additionally required in the organization of sleep (11). Dopamine was observed to be required in stimulating breath while the peptide neuromodulator substance P (SP) (12,13), endogenous opioids (14), and derived brain growth neurotrophic factor (BDNF) (15) were observed to be required in the focal control of breath. The neuropathology abnormalities identified in the SIDS brainstem (16) are summarized in Table 1.

ACh Receptor
Smoking in pregnancy fundamentally increases morbidity and perinatal mortality. It is presently the vital autonomous and modifiable risk factor adding to the sudden newborn child death disorder (SIDS) (27). The more convincing hypothesis for the connection among SIDS and smoking is that nicotine alters the vital breathing patterns and defensive reactions to hypoxia in sleeping (28). A lessened anxiety reaction intensifies hypoxia and apnea (29). The impacts of nicotine are interceded via its activation of very particular nicotinic cholinergic receptors (nAChRs) that are available in the carotid physiques and in  The animals used were an adult male, age-matched nAChRs with β2 contribute activity in REMS, NREMS, and the promoting effect of stress the serious brainstem cores, for example, the core of single tract and locus coeruleus (30). At these locales, nAChRs add to the cholinergic adjustment of arousal and breathing. Interference with the nAChRs functions on the presumed basis of negative nicotine reactions (31). Disturbing equilibrium among arousal and ventilatory responses could intensify respiratory failure in sleeping duration. Postnatal exposure to smoke tobacco during early stages is related to increase in the number of sicknesses in repository, pulmonary impaired function, and SIDS events. It is additionally connected through reduced (32) cognitive

Serotonin 5-HT Neurotransmitter
In the brain development, serotonin 5-HT neurotransmitter performs a central role in stress reactivity, mood regulation disorders of psychiatric risk factors and subsequently signaling in 5-HT in the early stage have complicated implications on mental health and behavior over the life span. It takes part in the intercession of cognition, arousal, mood, cerebral blood flow and motor activity. It regulates cardiovascular and cardiorespiratory function, chemosensitivity, thermoregulation, arousal, and pain (38). Figure 3 shows the role of serotonin 5-HT. SIDS victims have been found to have reduced levels of brainstem serotonin (5-HT) and tryptophan hydroxylase 2 (TPH2) but retain producing 5-HT neurons. TPH2 is cerebrum particular enzyme that translates tryptophan into 5-HTP, which is then transformed over into 5-HT via DOPA decarboxylase. Due to the disturbance in 5-HT medullary levels that outcomes in deadly dysfunction of these dependent sodium-pacemaker neurons regulated via projections of 5-HT from the Raphe and additional Raphe cores (39). We assumed that alteration might be brought about by contrasts in serotonin transporter mRNA expression and 5-HT1A receptor in mind regions included in the working and attention deficits in youth. Nicotine, the main neurotoxic segment of tobacco smoke, actuates its activities via binding to nicotinic acetylcholine receptors (nAChR). The immunohistochemical expression of nAChR subunits α2, α3, α4, α5, α7, α9, α1, and α2 in medulla brainstem was analyzed in a piglet model after postnatal nicotine exposure (33). Table 2 describes the ACh receptor abnormalities identified in the SIDS brainstem. control of feelings, memory, and additionally in areas controlling the focal serotonergic tone (40). Table 3 summarizes all the serotonin 5-HT neurotransmitter abnormalities identified in the SIDS brainstem so far.

low birthweight and Respiratory Tract infection
The sudden infant deaths are multifactorial, where low birthweight has been reported major risk factor for SIDS (47). Viral respiratory infections are mainly responsible for the occurrence of sudden death. Mild level of respiratory viral infection was observed by investigators in cases of sudden death infants up to 80% (48).

SUDDeN iNTRAUTeRiNe UNeXPlAiNeD DeATH SYNDROMe
Risk factors for SIUDS are shown in Figure 4. Perinatal brain injuries may occur due to trauma during pregnancy, birth asphyxia, and postnatal accident (48). Fetal growth restriction (FGR) is a significant difficulty of pregnancy showing a fetus that does not develop to maximum capacity because of pathological compromise. FGR influences 3-9% of pregnancies in high-salary nations and is the main source of perinatal mortality. Placental insufficiency is the key reason for FGR, bringing about chronic fetal hypoxia. This initiates hypoxia of an adaptive fetal reaction of cardiovascular yield redistribution to support indispensable organs, involving the mind and is in result called cerebrum sparing. In spite of this, it is currently apparent that cerebrum sparing does not guarantee normal cerebrum growth in limited development fetuses (49). A brief summary of SIUDS is mentioned in Table 4.
Numerous neurodevelopmental issues of cognitive and motor function have their origins in the antenatal period. Fetal suboptimal growth is probably a key variable underlying altered cerebrum growth. FGR is related with perinatal death, preterm birth and, for survivors, an expanded risk of sensory and motor neurodevelopmental deficits, learning and cognitive impairments, and cerebral palsy. The implementation of the neuroprotective treatments can just happen in light of careful characterization of the abnormalities in brain growth that increases because of FGR, first require the identification of newborn children at most serious risk for the impairment of neurodevelopmental secondary to fetal poor development. Eighty pregnancies end up in termination following detection of an abnormal fetal, neonatal death, or stillbirth, describing no less than eight thousand cases per annum, and there are more than 500 unexplained baby and youth deaths every year. In these circumstances, the posthumous examination is frequently required to decide reason for death, set up implications for relatives, and direct administration of future pregnancies (55).
Fetal growth restriction is generally viewed as a risk for perinatal cerebrum injury with intraventricular hemorrhage (IVH), yet clinical reviews record altered outcomes with elevated, decreased, or unaltered rates of IVH in FGR newborn children contrasted with suitably developed counterparts. Placental insufficiency with anomalous umbilical artery Doppler was connected to the occurrence of IVH. While considering that cerebrum sparing is a characteristic reaction to placental chronic hypoxia, it is not amazing that changes in blood flow to the cerebrum might be both characteristics of the clinical seriousness of FGR, and related with impairments of neurodevelopment. The adaptive reaction of cerebrum sparing requires remodeling of the fetal cerebrum flow that can be diagnosed via Doppler ultrasound as a reduced pulsatility record in the cerebral arteries. At the point when a vast cohort of children was isolated into weekly birth interims, it was found that rates of IVH in FGR were significantly lower versus non-FGR newborn children born at 28 weeks, proposing a defensive impact of development limitation, however, that IVH rates elevated significantly in late-FGR preterm births >34 weeks. This outcome has been confirmed by a recent review demonstrating that IVH was common in late-FGR preterm babies contrasted with suitably developed newborn children. That concern the finding of late preterm births, >34 and <37 weeks, represent most preterm births, and occurrence of preterm births is expanding (59,60).
Sudden intrauterine unexplained death syndrome is multifactorial and polygenic condition. Although several genetic factors have been reported as cause of SIUDS but defining a specific genetic aberration at this stage is often a challenging issue due to limited phenotype-genotype correlation (61). In addition, genetic anomalies in under developed phenotypes are rarely investigated. Several studies have reported through whole genome sequencing the importance of neurodevelopmental and ion exchange pathway genes (ARHGAP35, BBS7, CASZ1, COL2A1, CRIM1, DHCR7, HADHB, HAPLN3, HSPG2, MYO18B, RYR1, and SRGAP2).

CONClUSiON
A brainstem abnormality is suggested to be the main underlying etiological factor in SIUDS and SIDS victims. Alterations in certain neurotransmitters such as ACh receptor, serotonin 5-HT neurotransmitter, SP, and brain-derived neurotrophic growth factor (BDNF) are identified in the SIDS and SIUDS, which have vital roles in chemosensation and cardiorespiratory control leading to these sudden deaths. However, further studies are suggested to investigate more into this serious life threatening events.

AUTHOR CONTRibUTiONS
All the authors have contributed equally in writing the manuscript.