Regional cerebral blood flow during seizures in neonates

Cerebral vascular regulation and brain injury in preterm infants

Cerebrovascular lesions, mainly germinal matrix hemorrhage and ischemic injury to the periventricular white matter, are major causes of adverse neurodevelopmental outcome in preterm infants. Cerebrovascular lesions and neuromorbidity increase with decreasing gestational age, with the white matter predominantly affected. Developmental immaturity in the cerebral circulation, including ongoing angiogenesis and vasoregulatory immaturity, plays a major role in the severity and pattern of preterm brain injury. Prevention of this injury requires insight into pathogenesis. Cerebral blood flow (CBF) is low in the preterm white matter, which also has blunted vasoreactivity compared with other brain regions. Vasoreactivity in the preterm brain to cerebral perfusion pressure, oxygen, carbon dioxide, and neuronal metabolism is also immature. This could be related to immaturity of both the vasculature and vasoactive signaling. Other pathologies arising from preterm birth and the neonatal intensive care environment itself may contribute to impaired vasoreactivity and ineffective CBF regulation, resulting in the marked variations in cerebral hemodynamics reported both within and between infants depending on their clinical condition. Many gaps exist in our understanding of how neonatal treatment procedures and medications have an impact on cerebral hemodynamics and preterm brain injury. Future research directions for neuroprotective strategies include establishing cotside, real-time clinical reference values for cerebral hemodynamics and vasoregulatory capacity and to demonstrate that these thresholds improve long-term outcomes for the preterm infant. In addition, stimulation of vascular development and repair with growth factor and cell-based therapies also hold promise.

Epileptic seizures increase circulating endothelial cells in peripheral blood as early indicators of cerebral vascular damage

Circulating endothelial cells (CECs) are nonhematopoetic mononuclear cells in peripheral blood that are dislodged from injured vessels during cardiovascular disease, systemic vascular disease, and inflammation. Their occurrence during cerebrovascular insults has not been previously described. Epileptic seizures cause the long-term loss of cerebrovascular endothelial dilator function. We hypothesized that seizures cause endothelial sloughing from cerebral vessels and the appearance of brain-derived CECs (BCECs), possible early indicators of cerebral vascular damage. Epileptic seizures were induced by bicuculline in newborn pigs; venous blood was then sampled during a 4-h period. CECs were identified in the fraction of peripheral blood mononuclear cells by the expression of endothelial antigens (CD146, CD31, and endothelial nitric oxide synthase) and by Ulex europeaus lectin binding. In control animals, few CECs were detected. Seizures caused a time-dependent increase in CECs 2-4 h after seizure onset. Seizure-induced CECs coexpress glucose transporter-1, a blood-brain barrier-specific glucose transporter, indicating that these cells originate in the brain vasculature and are thus BCECs. Seizure-induced BCECs cultured in EC media exhibited low proliferative potential and abnormal cell contacts. BCEC appearance during seizures was blocked by a CO-releasing molecule (CORM-A1) or cobalt protoporphyrin (heme oxygenase-1 inducer), which prevented apoptosis in cerebral arterioles and the loss of cerebral vascular endothelial function during the late postictal period. These findings suggest that seizure-induced BCECs are injured ECs dislodged from cerebral microvessels during seizures. The correlation between the appearance of BCECs in peripheral blood, apoptosis in cerebral vessels, and the loss of postictal cerebral vascular function suggests that BCECs are early indicators of late cerebral vascular damage.

Cerebroprotective functions of HO-2

The constitutive isoform of heme oxygenase, HO-2, is highly expressed in the brain and in cerebral vessels. HO-2 functions in the brain have been evaluated using pharmacological inhibitors of the enzyme and HO-2 gene deletion in in vivo animal models and in cultured cells (neurons, astrocytes, cerebral vascular endothelial cells). Rapid activation of HO-2 via post-translational modifications without upregulation of HO-2 expression or HO-1 induction coincides with the increase in cerebral blood flow aimed at maintaining brain homeostasis and neuronal survival during seizures, hypoxia, and hypotension. Pharmacological inhibition or gene deletion of brain HO-2 exacerbates oxidative stress induced by seizures, glutamate, and inflammatory cytokines, and causes cerebral vascular injury. Carbon monoxide (CO) and bilirubin, the end products of HO-catalyzed heme degradation, have distinct cytoprotective functions. CO, by binding to a heme prosthetic group, regulates the key components of cell signaling, including BK(Ca) channels, guanylyl cyclase, NADPH oxidase, and the mitochondria respiratory chain. Cerebral vasodilator effects of CO are mediated via activation of BK(Ca) channels and guanylyl cyclase. CO, by inhibiting the major components of endogenous oxidant-generating machinery, NADPH oxidase and the cytochrome C oxidase of the mitochondrial respiratory chain, blocks formation of reactive oxygen species. Bilirubin, via redox cycling with biliverdin, is a potent oxidant scavenger that removes preformed oxidants. Overall, HO-2 has dual housekeeping cerebroprotective functions by maintaining autoregulation of cerebral blood flow aimed at improving neuronal survival in a changing environment, and by providing an effective defense mechanism that blocks oxidant formation and prevents cell death caused by oxidative stress.

Treatment of neonatal seizures

Newborn babies with unusual movements thought to represent seizures are usually given a loading dose of phenobarbitone without electroencephalography being performed. Antiepileptic drugs (AEDs) are then continued, with the outcome determined by clinical observation alone. AED treatment, often involving multiple drugs for long periods, is undesirable at a time when the brain is developing rapidly and likely to be especially vulnerable to any toxic effects. Despite considerable advances in the pharmacology of AEDs, continuous EEG monitoring using compact digital systems with simultaneous videorecording allowing off-line analysis, automated seizure detection, neuroimaging, and basic science research on cellular mechanisms of brain injury, treatment of such babies has progressed little. A change in practice is long overdue to allow affected babies to benefit from the advances made.

Cerebral oxygenation during postasphyxial seizures in near-term fetal sheep

After exposure to asphyxia, infants may develop both prolonged, clinically evident seizures and shorter, clinically silent seizures; however, their effect on cerebral tissue oxygenation is unclear. We therefore examined the hypothesis that the increase in oxygen delivery during postasphyxial seizures might be insufficient to meet the needs of increased metabolism, thus causing a fall in tissue oxygenation, in unanesthetized near-term fetal sheep in utero (gestational age 125+/-1 days). Fetuses were administered an infusion of the specific adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine, followed by 10 mins of asphyxia induced by complete umbilical cord occlusion. The fetuses then recovered for 3 days. Sixty-one episodes of electrophysiologically defined seizures were identified in five fetuses. Tissue PO(2) (tPO(2)) did not change significantly during short seizures (<3.5 mins), 5.2+/-0.2 versus baseline 5.6+/-0.1 mm Hg (NS), but fell to 2.2+/-0.2 mm Hg during seizures lasting more than 3.5 mins (P<0.001). During prolonged seizures, cortical blood flow did not begin to increase until tPO(2) had begun to fall, and then rose more slowly than the increase in metabolism, with a widening of the brain to blood temperature gradient. In conclusion, in the immature brain, during prolonged, but not short seizures, there is a transient mismatch between cerebral blood flow and metabolism leading to significant cerebral deoxygenation.

Epileptic seizures cause extended postictal cerebral vascular dysfunction that is prevented by HO-1 overexpression

The extended postictal state is characterized by neurological problems in patients. Inadequate blood supply to the brain and impaired cerebral autoregulation may contribute to seizure-induced neuronal damage. Recent evidence in newborn pigs indicates that activation of the antioxidative enzyme heme oxygenase (HO) at the onset of seizures is necessary for increased cerebral blood flow during the ictal episode and for normal cerebral vascular functioning during the immediate postictal period. We hypothesized that seizures cause prolonged postictal cerebral vascular dysfunction that can be accentuated by HO inhibition and rescued by HO overexpression. Cerebral vascular responses to endothelium-dependent (hypercapnia, bradykinin) and -independent (isoproterenol, sodium nitroprusside) stimuli were assessed 48 h after bicuculline-induced seizures in: 1) saline-control newborn piglets, 2) HO-inhibited animals (HO was inhibited by tin protoporphyrin, SnPP, 3 mg/kg iv), and 3) HO-overexpressing piglets (HO-1 was upregulated by cobalt protoporphyrin, CoPP, 50 mg/kg ip). Extended alterations of HO expression in cerebral microvessels were confirmed by measuring CO production and inducible HO (HO-1) and constitutive HO (HO-2) proteins. Our data provide evidence that seizures cause a severe, sustained, postictal cerebral vascular dysfunction as reflected by impaired vascular reactivity to physiologically relevant dilators. During the delayed postictal state, vascular reactivity to all dilator stimuli was reduced in saline control and, to a greater extent, in HO-inhibited animals. In CoPP-treated piglets, no reduction in postictal cerebral vascular reactivity was observed. These findings may indicate that CoPP prevents postictal cerebral vascular dysfunction by upregulating HO-1, a finding that might have implications for preventing postictal neurological complications.

Heme oxygenase inhibition reduces neuronal activation evoked by bicuculline in newborn pigs

Carbon monoxide (CO) is a product of heme degradation by heme oxygenase (HO) that is highly expressed in the brain. The present study addresses the hypothesis that CO can be involved in brain neuronal function. The effects of the HO inhibitor, tin protoporphyrin (SnPP), on brain electrical activity and pial arteriolar diameter were examined using quantitative electroencephalography (EEG) and cranial window techniques in the bicuculline model of sustained generalized seizures in newborn pigs. SnPP (3 mg/kg i.v.) inhibits brain HO as indicated by blocking cerebral vasodilation to heme and decreasing CO concentration in cortical periarachnoid cerebrospinal fluid. The quantitative spectral analysis of digitalized scalp EEG recordings was performed to determine the EEG amplitude and spectral power within a 1-15-Hz frequency range. SnPP did not affect basal brain EEG parameters. Bicuculline (3 mg/kg i.v.) immediately (in <1 min) evoked bursts of brain electrical activity characterized by four- to seven-fold increases in EEG amplitude and power in all analyzed frequency bands that occurred simultaneously with cerebral vasodilation. Increased EEG activity and cerebral vasodilation were sustained for a 2h period. SnPP inhibited cerebral vasodilation but did not affect the EEG amplitude evoked by bicuculline. However, 20-40% reductions of the power in 7.5 Hz (theta), 10 and 12.5 Hz (alpha), and a 15-Hz (beta) bands, the major evoked EEG spectral components, were observed for the duration of seizures in SnPP-treated animals. These findings suggest that endogenous CO can have proconvulsant action and affect neuronal activation during seizures.

Endogenous heme oxygenase prevents impairment of cerebral vascular functions caused by seizures

In newborn pigs, the mechanism of seizure-induced cerebral hyperemia involves carbon monoxide (CO), the vasodilator product of heme catabolism by heme oxygenase (HO). We hypothesized that seizures cause cerebral vascular dysfunction when HO activity is inhibited. With the use of cranial window techniques, we examined cerebral vascular responses to endothelium-dependent (hypercapnia and bradykinin) and endothelium-independent (isoproterenol and sodium nitroprusside) dilators during the recovery from bicuculline-induced seizures in saline controls and in animals pretreated with a HO inhibitor, tin protoporphyrin (SnPP). SnPP (3 mg/kg iv) blocked dilation to heme and reduced the CO level in cortical periarachnoid cerebrospinal fluid, indicating HO inhibition in the cerebral microcirculation. In saline control piglets, seizures increased the CO level, which correlated with the time-dependent cerebral vasodilation; during the recovery (2 h after seizure induction), responses to all vasodilators were preserved. In SnPP-treated animals, cerebral vasodilation and the CO responses to seizures were greatly reduced, and cerebral vascular reactivity was severely impaired during the recovery. These findings suggest that HO in the cerebral microcirculation is rapidly activated during seizures and provides endogenous protection against seizure-induced vascular injury.

Phenobarbitone, neonatal seizures, and video-EEG

AIMS

To evaluate the effectiveness of phenobarbitone as an anticonvulsant in neonates.

METHODS

An observational study using video-EEG telemetry. Video-EEG was obtained before treatment was started, for an hour after treatment was given, two hours after treatment was given, and again between 12 and 24 hours after treatment was given. Patients were recruited from all babies who required phenobarbitone (20-40 mg/kg intravenously over 20 minutes) for suspected clinical seizures and had EEG monitoring one hour before and up to 24 hours after the initial dose. An EEG seizure discharge was defined as a sudden repetitive stereotyped discharge lasting for at least 10 seconds. Neonatal status epilepticus was defined as continuous seizure activity for at least 30 minutes. Seizures were categorised as EEG seizure discharges only (electrographic), or as EEG seizure discharges with accompanying clinical manifestations (electroclinical). Surviving babies were assessed at one year using the Griffiths neurodevelopmental score.

RESULTS

Fourteen babies were studied. Four responded to phenobarbitone; these had normal or moderately abnormal EEG background abnormalities and outcome was good. In the other 10 babies electrographic seizures increased after treatment, whereas electroclinical seizures reduced. Three babies were treated with second line anticonvulsants, of whom two responded. One of these had a normal neurodevelopmental score at one year, but the outcome for the remainder of the whole group was poor.

CONCLUSION

Phenobarbitone is often ineffective as a first line anticonvulsant in neonates with seizures in whom the background EEG is significantly abnormal.

Dynamic cerebral autoregulation in sick newborn infants

The sick newborn infant is vulnerable to brain injury and impaired cerebral autoregulation is thought to contribute to this. Coherent averaging is a method of measuring the dynamic cerebral autoregulatory response that is particularly suitable for neonates. We used this method in combination with a measure of the gradient of the cerebral blood flow velocity (CBFV) response following transient blood pressure (BP) peaks to study dynamic autoregulation in infants undergoing intensive care. Term and preterm infants at high risk of neurologic injury were compared with a control group of infants, also undergoing intensive care. Simultaneous video-EEG, CBFV (using transcranial Doppler), and arterial blood pressure measurements were obtained intermittently during a study period of at least 2 h. Cerebral autoregulatory response curves were constructed for high risk and control groups. Intact cerebral autoregulation produces a characteristic response consisting of a brief period when CBFV follows arterial blood pressure but quickly returns to baseline value. An impaired autoregulatory response shows CBFV mirroring the arterial blood pressure curve closely. Thirteen high-risk infants, who also had seizures (10 term and 3 preterm) and 12 control infants (6 term and 6 preterm) were studied. Autoregulation was absent in high-risk term and preterm infants. It was also absent in preterm control infants. Term, neurologically healthy infants undergoing intensive care have an intact autoregulatory response. The constant passive response seen in high-risk infants may reflect the severity of the underlying neurologic disease.

Cerebral blood flow velocity during neonatal seizures

AIM

To determine if cerebral blood flow velocity increases during all types of neonatal seizure, and whether the effect is due solely to an increase in blood pressure, transmitted to the cerebral circulation when autoregulation is impaired.

METHODS

Seizures were diagnosed in 11 high risk neonates using cotside 16 channel video-EEG polygraphy. EEG, cerebral blood flow velocity (CBFV) using transcranial Doppler ultrasound, and arterial blood pressure (ABP) measurements were made. At least two 5-10 minute epochs of simultaneous measurements were performed on each infant. These epochs were then reviewed to eliminate artefacts, and one minute data periods containing a clear seizure onset were created. Each period contained 20 seconds before the seizure. Data periods without seizures from the same infants were also analysed and compared with seizure periods.

RESULTS

Four infants had purely electrographic seizures-without clinical manifestations. Six infants had electroclinical seizures. One infant displayed both seizure types. A random effects linear regression analysis was used to determine the effect of seizures on CBFV and ABP. A significant increase was found in mean CBFV in those periods containing seizures. The mean percentage change in velocity for all infants was 15.6%. Three infants showed a significant increase in mean ABP after seizures but the overall increase in ABP for all infants was not significant.

CONCLUSION

Electroclinical and electrographic neonatal seizures produce an increase in CBFV. In some infants the increase is not associated with an increase in blood pressure. These preliminary results suggest that electrographic seizures are associated with disturbed cerebral metabolism. Treatment of neonatal seizures until electrographic seizure activity is abolished may improve outcome for these infants.