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Neuromonitoring and Neurodevelopmental Outcomes in Congenital Heart Surgery
History of Intraoperative Neuromonitoring
Brain injury is the most common and potentially disabling complication following congenital heart surgery. With improved survival, the focus has shifted to optimizing functional outcomes. An important goal of therapy for every congenital heart surgical patient is to reduce the risk of brain injury as much as possible. Along with updated perfusion, anesthetic, and surgical strategies, techniques for neuromonitoring have been refined and adopted by many centers performing pediatric cardiothoracic surgery.
Penfield and Boldrey first reported intraoperative neurophysiologic monitoring in 1937. They used direct cortical stimulation during operations for epilepsy. Recording an electroencephalogram directly from the cerebral cortex, also for epilepsy surgery, was first done in 1949 by Jasper and Marshall and Walker. Routine scalp electroencephalography (EEG), used first during carotid endarterectomy, was introduced in the late 1960s. Doppler ultrasound evaluation of the extracranial cerebral arteries was first reported in 1965 by Miyazaki and Kato. Transcranial Doppler (TCD) was reported by Aaslid in 1982 after measuring the flow of the middle cerebral artery with a probe placed on the scalp over the temporal bone. Near-infrared spectroscopy (NIRS) was introduced clinically in 1985 for monitoring cerebral oxygenation in preterm infants. Spinal intraoperative monitoring, specifically somatosensory evoked potentials, was developed in the 1970s. The first intraoperative neuromonitoring clinical service was established at the University of California at Los Angeles in 1979, and commercial neurophysiologic monitoring equipment became available in the early 1980s.
Neurodevelopmental Outcomes
Introduction
Survival after congenital heart surgery has improved, and the focus has shifted to optimizing neurodevelopmental outcomes. Survivors of repair of congenital heart disease in the neonatal period demonstrate cognitive, motor, speech, visual, and learning abnormalities.
Determining causation in abnormal neurodevelopment and the occurrence of neurodevelopmental disability is a challenging endeavor. Genetic predisposition and many other nonmodifiable patient factors, including prematurity, socioeconomic status, and maternal education, have been shown to be risk factors for worse neurodevelopmental outcomes. In addition, there is increasing evidence that congenital heart disease alters fetal brain growth and development.
In addition to nonmodifiable factors, operative management factors have been implicated in altering neurodevelopmental outcome. This includes preoperative/postoperative hypoxemia and arrest, cardiopulmonary bypass and circulatory arrest strategies, hematocrit levels during cardiopulmonary bypass, and blood gas management during bypass.
Nonmodifiable Factors Associated with Adverse Neurodevelopmental Outcomes
Fetal Brain Development and Preoperative Brain Abnormalities
Beginning in the third trimester of fetal life, patients with congenital heart disease are known to have smaller gestational age- and weight-adjusted brain volumes and impaired neuroaxonal development and metabolism. These abnormalities are most pronounced in patients with complex types of heart defects such as hypoplastic left heart syndrome and transposition of the great arteries. Structural malformations of the brain occur at a much higher rate in patients with congenital heart disease than in the general population. Microcephaly, microencephaly, and other malformations, including absence of the corpus callosum, have been documented in necropsy series of patients with hypoplastic left heart syndrome. Neonates with congenital heart disease have altered cerebral hemodynamics often with lower than normal cerebral blood flow and/or oxygen delivery. There is evidence of delayed white matter development, which can lead to an increased risk of white matter injury (periventricular leukomalacia [PVL]) and microcephaly. One study of neonates with complex congenital heart disease used preoperative pulsed arterial spin-label perfusion magnetic resonance imaging (MRI) to quantitate cerebral blood flow. More than half of the cohort had developmental or acquired lesions, and the cerebral blood flow was less than half of that reported in normal, term neonates. The same study also examined cerebrovascular responsiveness to CO 2 , and found that PVL was associated with decreased CO 2 responsiveness. Abnormal cerebrovascular reactivity to CO 2 has been associated with increased mortality and worse neurodevelopmental outcomes. White matter injury characterized by PVL is the most common pattern of injury. The mechanism of this injury is thought to be the result of effects of hypoxia and/or ischemia on pre-myelinating oligodendrocyte precursors during their most vulnerable time of 24 to 34 weeks gestation. Up to 40% of neonates with congenital heart disease have PVL on preoperative MRI, and PVL has been shown to be associated with poor neurodevelopmental outcomes. The incidence of PVL is highest in neonates undergoing cardiopulmonary bypass. A study by Galli and colleagues revealed a 54% incidence of PVL in neonates compared to 4% in infants. PVL is the neurologic lesion associated with cerebral palsy in infants born prematurely. This pattern of brain injury is seen not only in preterm newborns but also in term neonates with congenital heart disease. A comparison of newborns with congenital heart disease and a control cohort without heart defects revealed that almost one third of those with congenital heart disease had white matter injury. The injury pattern was not seen in those without heart defects.
Prematurity
Preterm birth, even in infants without congenital heart disease, is a powerful predictor of worse neurodevelopmental outcome. Along with low birth weight, preterm birth has been associated with long-term behavioral and learning issues. In a study of 125 very low-birth-weight preterm infants who were evaluated with the Bayley Scales of Infant and Toddler Development III at 24 months, later gestational age was associated with better neurodevelopmental outcome.
Socioeconomic Factors
Other nonmodifiable patient factors that adversely affect neurodevelopmental outcomes are socioeconomic status and maternal education. In a study of neurodevelopmental outcomes after repair of total anomalous pulmonary venous connection, lower socioeconomic status was predictive of lower scores on the Mental Developmental Index (MDI) of the Bayley Scales of Infant Development II. The Single Ventricle Reconstruction trial is the randomized, prospective trial comparing shunt types in the Norwood procedure. Evaluation of this cohort at 14 months demonstrated that lower maternal education was associated with lower MDI scores.
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