Felicia L. Trachtenberg

Fluctuating Pressure-Passivity Is Common in the Cerebral Circulation of Sick Premature Infants

Cerebral blood flow pressure-passivity results when pressure autoregulation is impaired, or overwhelmed, and is thought to underlie cerebrovascular injury in the premature infant. Earlier bedside observations suggested that transient periods of cerebral pressure-passivity occurred in premature infants. However, these transient events cannot be detected reliably by intermittent static measurements of pressure autoregulation. We therefore used continuous bedside recordings of mean arterial pressure (MAP; from an indwelling arterial catheter) and cerebral perfusion [using the near-infrared spectroscopy (NIRS) Hb difference (HbD) signal) to detect cerebral pressure-passivity in the first 5 d after birth in infants with birth weight <1500 g. Because the Hb difference (HbD) signal [HbD = oxyhemoglobin (HbO2) − Hb] correlates with cerebral blood flow (CBF), we used coherence between MAP and HbD to define pressure-passivity. We measured the prevalence of pressure-passivity using a pressure-passive index (PPI), defined as the percentage of 10-min epochs with significant low-frequency coherence between the MAP and HbD signals. Pressure-passivity occurred in 87 of 90 premature infants, with a mean PPI of 20.3%. Cerebral pressure-passivity was significantly associated with low gestational age and birth weight, systemic hypotension, and maternal hemodynamic factors, but not with markers of maternal infection. Future studies using consistent serial brain imaging are needed to define the relationship between PPI and cerebrovascular injury in the sick premature infant.

Gray Matter Injury Associated with Periventricular Leukomalacia in the Premature Infant

Neuroimaging studies indicate reduced volumes of certain gray matter regions in survivors of prematurity with periventricular leukomalacia (PVL). We hypothesized that subacute and/or chronic gray matter lesions are increased in incidence and severity in PVL cases compared to non-PVL cases at autopsy. Forty-one cases of premature infants were divided based on cerebral white matter histology: PVL (nxa0=xa017) with cerebral white matter gliosis and focal periventricular necrosis; diffuse white matter gliosis (DWMG) (nxa0=xa017) without necrosis; and “Negative” group (nxa0=xa07) with no abnormalities. Neuronal loss was found almost exclusively in PVL, with significantly increased incidence and severity in the thalamus (38%), globus pallidus (33%), and cerebellar dentate nucleus (29%) compared to DWMG cases. The incidence of gliosis was significantly increased in PVL compared to DWMG cases in the deep gray nuclei (thalamus/basal ganglia; 50–60% of PVL cases), and basis pontis (100% of PVL cases). Thalamic and basal ganglionic lesions occur almost exclusively in infants with PVL. Gray matter lesions occur in a third or more of PVL cases suggesting that white matter injury generally does not occur in isolation, and that the term “perinatal panencephalopathy” may better describe the scope of the neuropathology.

Myelin Abnormalities without Oligodendrocyte Loss in Periventricular Leukomalacia

The cellular basis of myelin deficits detected by neuroimaging in long‐term survivors of periventricular leukomalacia (PVL) is poorly understood. We tested the hypothesis that oligodendrocyte lineage (OL) cell density is reduced in PVL, thereby contributing to subsequent myelin deficits. Using computer‐based methods, we determined OL cell density in sections from 18 PVL and 18 age‐adjusted control cases, immunostained with the OL‐lineage marker Olig2. Myelination was assessed with myelin basic protein (MBP) immunostaining. We found no significant difference between PVL and control cases in Olig2 cell density in the periventricular or intragyral white matter. We did find, however, a significant increase in Olig2 cell density at the necrotic foci, compared with distant areas. Although no significant difference was found in the degree of MBP immunostaining, we observed qualitative abnormalities of MBP immunostaining in both the diffuse and necrotic components of PVL. Abnormal MBP immunostaining in PVL despite preserved Olig2 cell density may be secondary to arrested OL maturation, damage to OL processes, and/or impaired axonal‐OL signaling. OL migration toward the “core” of injury may occur to replenish OL cell number. This study provides new insight into the cellular basis of the myelin deficits observed in survivors of PVL.

Development of microglia in the cerebral white matter of the human fetus and infant

Although microglial activation may be an initial beneficial response to a variety of insults, prolonged activation can release toxic substances and lead to cell death. Microglial activation secondary to hypoxia‐ischemia and/or infection in immature cerebral white matter is important in the pathogenesis of periventricular leukomalacia (PVL), the major pathological substrate of cerebral palsy in the premature infant. We hypothesize that a transient overexpression in activated microglial density occurs normally in the cerebral white matter of the human fetus during the peak window of vulnerability for PVL. Such an increase could render this region susceptible to insults that cause prolonged microglial activation, as conceptualized in PVL. To examine the developmental profile of microglia in the human fetus and infant brain, immunocytochemistry with microglial specific markers were used in 23 control (non‐PVL) cases ranging from 20 to 183 postconceptional (PC) weeks. Tomato lectin, used to identify microglial morphology, revealed that the cerebral white matter of the human fetus and infant is densely populated with intermediate and amoeboid microglia; the latter is indicative of an activated state. Quantitative analysis with CD68 showed increased density of activated microglia in the cerebral white matter of the fetus (<37 PC weeks) relative to the neonate/infant (≥37 PC weeks) and to the overlying cortex of either age group (P = 0.01). The primary finding of a transient, developmental‐dependent overabundance of CD68‐activated microglia in the cerebral white matter of the fetus suggests a potential “priming” of this area for diverse brain insults characterized by activation of microglia, particularly PVL. J. Comp. Neurol. 497:199–208, 2006.

Subtle autonomic and respiratory dysfunction in sudden infant death syndrome associated with serotonergic brainstem abnormalities: a case report.

Sudden infant death syndrome (SIDS) is characterized by a sleep-related death in a seemingly healthy infant. Previously, we reported abnormalities in the serotonergic (5-HT) system of the medulla in SIDS cases in 2 independent datasets, including in the Northern Plains American Indians. The medullary 5-HT system is composed of 5-HT neurons in the raphé, extra-raphé, and arcuate nucleus at the ventral surface. This system is thought to modulate respiratory and autonomic function, and thus abnormalities within it could potentially lead to imbalances in sympathetic and parasympathetic tone. We report the case of a full-term American Indian boy who died of SIDS at 2 postnatal weeks, and who had subtle respiratory and autonomic dysfunction measured prospectively on the second postnatal day. Cardiorespiratory assessment of heart rate variability suggested that the ratio of parasympathetic to sympathetic tone was higher than normal in active sleep and lower than normal in quiet sleep in this case. At autopsy, arcuate nucleus hypoplasia and 5-HT receptor-binding abnormalities in the arcuate nucleus and other components of the medullary 5-HT system were found. This case suggests that medullary 5-HT system abnormalities may be able to be identified by such physiological tests before death. Replication of these findings in a large population may lead to the development of predictive cardiorespiratory assessment tools for future screening to identify infants with medullary 5-HT abnormalities and SIDS risk.

The glutamate transporter EAAT2 is transiently expressed in developing human cerebral white matter

The major brain abnormality underlying cerebral palsy in premature infants is periventricular leukomalacia (PVL), a lesion of the immature cerebral white matter. Oligodendrocyte precursors (pre‐OLs; O4+O1−) predominate in human cerebral white matter during the peak time frame for PVL (24–32 gestational weeks) and are vulnerable to excitotoxicity. We hypothesize that PVL reflects, in part, excitotoxicity to pre‐OLs resulting from cerebral ischemia/reperfusion. Reversal of glutamate transport in the setting of energy failure is a major source of pathologic accumulation of extracellular glutamate. Here, we identify and localize the glutamate transporters in human cerebral white matter during the age range of PVL. In situ hybridization was performed with digoxigenin‐labeled probes directed against the full‐length coding regions of EAAT1, EAAT2, and EAAT3. EAAT2 mRNA was abundant in human fetal white matter during the period of peak incidence of PVL and virtually disappeared by 2 postnatal months. Its developmental profile differed significantly from that of both EAAT1 and EAAT3 mRNA. Immunoblotting demonstrated that EAAT2 protein was highly expressed in early development relative to adult values. Double‐label immunocytochemistry detected EAAT2 in OLs but not astrocytes or axons in the human fetal white matter. We conclude that transient expression of EAAT2 occurs during the window of peak vulnerability for PVL, suggesting that this developmentally up‐regulated transporter may be a major source of extracellular glutamate in ischemic injury to the cerebral white matter of the preterm infant. J. Comp. Neurol. 501:879–890, 2007.

The development of the medullary serotonergic system in early human life

The serotonergic (5-HT) neurons of the medulla oblongata are postulated to comprise a system that modulates homeostatic function in response to metabolic imbalances in the internal milieu in a state-dependent manner. In this study, we define the baseline development of the topography of the human medullary 5-HT system in 30 cases ranging from the embryonic period through infancy. We used immunocytochemical techniques with the PH8 antibody which recognizes the key 5-HT synthetic enzyme, tryptophan hydroxylase, and computer-based methods of cell quantitation. In the infant medulla, 5-HT neurons were distributed in raphé, extra-raphé, and ventral positions that place these neurons adjacent to, or intermingled with, the neurons in the lower cranial nerve nuclei and reticular formation that directly mediate respiration, upper airway reflexes, and autonomic function. Along the ventral and ventrolateral surface, 5-HT neurons formed two lateral and one midline columns in the rostrocaudal axis that are homologous in position to chemosensitive 5-HT neurons in rats, and that correspond in part to the classic respiratory chemosensitive fields. Serotonergic neurons comprised a subpopulation of the arcuate nucleus along the ventral surface; their short processes directly abutted the surface, suggesting a role for them in monitoring carbon dioxide levels in the cerebrospinal fluid. The medullary 5-HT system began to form in the embryo, with the raphé primordia appearing as early as 7 weeks (the earliest time-point available). By 10-12 weeks, the lateral tegmental 5-HT neurons clustered into the early primordia of extra-raphé subnuclei. By 20 weeks, the adult-like topography of the medullary 5-HT system was in place, with subtle (quantitative) changes occurring thereafter. Thus, protracted changes occur from the prenatal period through infancy. These data provide a foundation for 5-HT neuronal analysis in pediatric brainstem disorders, as proposed in the sudden infant death syndrome.

Nitrosative stress and inducible nitric oxide synthase expression in periventricular leukomalacia

Periventricular leukomalacia (PVL) is a lesion of the immature cerebral white matter in the perinatal period and associated predominantly with prematurity and cerebral ischemia/reperfusion as well as inflammation due to maternofetal infection. It consists of focal necrosis in the periventricular region and diffuse gliosis with microglial activation and premyelinating oligodendrocyte (pre-OL) injury in the surrounding white matter. We previously showed nitrotyrosine in pre-OLs in PVL, suggesting involvement of nitrosative stress in this disorder. Here we hypothesize that inducible nitric oxide synthase (iNOS) expression is increased in PVL relative to controls. Using immunocytochemistry in human archival tissue, the density of iNOS-expressing cells was determined in the cerebral white matter of 15 PVL cases [29–51 postconceptional (PC) weeks] and 16 control cases (20–144 PC weeks). Using a standardization score of 0–3, the density of iNOS-positive cells was significantly increased in the diffuse component of PVL (score of 1.8xa0±xa00.3) cases compared to controls (score of 0.7xa0±xa00.3) (Pxa0=xa00.01). Intense iNOS expression occurred in reactive astrocytes in acute through chronic stages and in activated microglia primarily in the acute stage, suggesting an early role for microglial iNOS in PVL’s pathogenesis. This study supports an important role for iNOS-induced nitrosative stress in the reactive/inflammatory component of PVL.

Sudden Death, Febrile Seizures, and Hippocampal and Temporal Lobe Maldevelopment in Toddlers: A New Entity

Recently, we reported hippocampal and temporal lobe abnormalities in 5 toddlers with sudden unexplained death in childhood (SUDC). The association of these anomalies with a high incidence (40%) of individual/family histories of simple febrile seizures in the cases raised concern that febrile seizures can be associated with death. In a series of 64 toddlers with sudden death, we tested the hypothesis that an SUDC subset is characterized by hippocampal and temporal lobe maldevelopment and an individual and/or family history of simple familial seizures. Cases of sudden and unexplained death in children aged 1.0 to 5.9 years (median 1.7 years) were divided into groups based upon a history of febrile or nonfebrile seizures, familial febrile seizures, and autopsy classification of cause of death. Forty-nine of the 64 cases (77%) were classified as SUDC, of which 40% had an individual/family history of febrile seizures. Of the 26 SUDC cases with available hippocampal sections, 62% (16/26) had hippocampal and temporal lobe anomalies, including 82% (9/11) of cases with an individual/family history of febrile seizures. Cases with these anomalies were all found dead during a sleep period, typically in the prone (87%) position. We conclude that a potential new entity may account for the majority of SUDC in toddlers, defined by sleep-related death in the prone position, individual/family history of febrile seizures, and hippocampal and temporal lobe anomalies. The mechanism of death appears analogous to sudden death in (temporal lobe) epilepsy, with a putative unwitnessed seizure during sleep leading to airway occlusion and death. This study mandates further research into the potential link between simple febrile seizures and death.

Serotonin Transporter Abnormality in the Dorsal Motor Nucleus of the Vagus in Rett Syndrome: Potential Implications for Clinical Autonomic Dysfunction

Autonomic dysfunction is prevalent in girls with Rett syndrome, an X-chromosome-linked disorder of mental retardation resulting from mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2). This gene plays a role in regulating neuronal activity-dependent gene expression, including brain-derived neurotrophic factor (BDNF), which is a potent serotonergic (5-HT) neuronal growth factor. We analyzed selected parameters of the 5-HT system of the medulla in autopsied patients with Rett syndrome because of the role of BDNF in 5-HT cell development and because 5-HT plays a key role in modulating autonomic control. 5-HT neurons were identified by immunostaining for tryptophan hydroxylase, the biosynthetic enzyme for 5-HT. We quantitated the number of 5-HT cells in the medulla at 2 standardized levels in 11 Rett and 7 control cases. There was no significant difference in 5-HT cell number between the groups. We analyzed binding to the serotonin transporter (SERT) using the radioligand [125I]-RTI-55 with tissue autoradiography in 7 Rett and 5 controls in 9 cardiorespiratory-related nuclei. In the dorsal motor nucleus of the vagus (DMX) (preganglionic parasympathetic outflow), SERT binding for the control cases decreased significantly over time (p = 0.049) but did not change in the Rett cases (p = 0.513). Adjusting for age, binding between the Rett and control cases differed significantly in this nucleus (p = 0.022). There was a marginally significant age versus diagnosis interaction (p = 0.06). Thus, altered 5-HT innervation and/or uptake in the DMX may contribute to abnormal 5-HT modulation of this major autonomic nucleus in patients with Rett syndrome. These data suggest hypotheses concerning 5-HT modulation of vagal function for testing in MeCP2 knockout mice to understand mechanisms underlying autonomic dysfunction in patients with Rett syndrome.