**4. Experimental studies**

newborns, premature infants, infants, children, and adolescents. In particular, neonates carry 10 times more mortality and morbidity risk compared to other pediatric age groups. The most common complications in this age group involve the cardiovascular and respiratory system [1]. Holzman [2] noted that the practitioner's experience and the presence of existing respiratory, cardiac, or muscular disease are the key factors that determine the risk of mortality and morbidity. Hemodynamic disturbances due to hypotension, hypertension, tachycardia, bradycardia, asystole, or other arrhythmias arising in the cardiovascular system and respiratory system issues such as hypopnea, apnea, hypoxia, hypocapnia, or hypercapnia can lead to disturbances in microcirculation to the central nervous system (CNS). Although the rate of complications has been reduced through improved understanding of the anatomical, physiological, and pharmacological characteristics of pediatric patients, advances in monitoring methods, and practitioner specialization, the risks are

Despite recent advances in the field of pediatric anesthesia, an increasing number of recent reports point to the adverse effects of anesthetics on the developing brain, raising concerns about the application of anesthesia in pediatric patients. As early as 1965, Sir Austin Bradford Hill recognized this issue at a meeting of the Royal Society of Medicine, stating: "How do we determine what are physical, chemical and psychologic hazards of occupation and in particular those that are rare and not easily recognized?" and "… the available human studies … cannot exclude the possibility that the anesthesia- induced neurotoxicity observed in many animal studies may also occur in children" [3]. Although it has been nearly 50 years from that meeting of the Royal Society of Medicine, the short- and long-term effects of anesthesia applications in pediatric patients remain poorly understood. In this chapter, the acute and long-term effects of anesthesia and anesthetics on the developing

Neurotoxicity of anesthetic substances on the developing brain is determined by a reduction in neural density and apoptosis in experimental studies and by disturbances in memory, attention, learning, and motor activity in clinical studies [4–6]. Although anesthetic agents used in neonates have known neurotoxic effects, there are valid reasons for using these agents even in vulnerable patients. Because pain itself has a neurotoxic effect, anesthesia-analgesia application in painful conditions may have a net neuroprotective effect [7, 8]. It should also be noted that in cases of hypoxia-ischemia or trauma, administration of anesthetics reduces the infarct volume by reducing the metabolic rate, decreasing intracranial pressure, eliminating free oxygen radicals, and reducing secondary injury [9–11]. Another positive effect is neuroplasticities. These are described as the neurophysical and neurochemical ability to improve compliance against environmental changes and damage when used in depressive disorders and diseases. Neuroplasticity refers to the increase in intercellular connections. Agents that enhance neuroplasticity have raised new hope for the treatment of neurodegenerative dis-

never completely eliminated.

104 Current Topics in Anesthesiology

brain are summarized.

**2. Definitions**

eases [12–14].

#### **4.1. Inhalation anesthetics**

In an experimental study by Shen et al. [4], sevoflurane was applied to neonatal (PND3, PND7, and PND14) and adult rats (PNW7) at concentrations ranging from 1% to 4%. Spatial memory was then assessed in adulthood using the Morris water maze (MWM) test. The PNW7 rats were less sensitive to sevoflurane than neonatal rats. Memory defects were apparent in groups treated with repeated low doses or a single high-dose anesthetic. The authors concluded that neonatal exposure to sevoflurane can result in memory defects in adulthood, with greater deficits seen in animals treated with multiple doses in a short period of time. As a result, the authors recommend that exposure to anesthesia during the neonatal period should be limited in dose and duration. Another study has shown that 4-hour sevoflurane exposure (2.5%) resulted in reduced hippocampal postsynaptic density protein-95 expression without causing any neuronal loss and was associated with learning and memory disturbances [19].

Another experimental study reported that 0.5% minimum alveolar concentration (MAC) sevoflurane applied for 6 hours had no significant effect on apoptosis and S100β levels. Conversely, isoflurane, which is given in the same circumstances, was shown to increase the level of apoptosis and S100β levels [20]. In another study, which evaluated the effects of inhalation anesthetics in neonatal rats, it was demonstrated that sevoflurane, isoflurane, and desflurane increased caspase-3 levels. Interestingly, nitrous oxide application (up to 150% concentration) for 6 hours did not cause neuroapoptosis; however, apoptosis was increased when nitrous oxide was applied with isoflurane [21]. Halothane administered during the prenatal period was associated with neurodegeneration and behavioral changes [22, 23]. Xenon, the currently preferred anesthetic, does not cause neuroapoptosis when used alone; on the contrary, xenon reduced the effects of other inhalation anesthetics when administered first [24].

#### **4.2. Intravenous anesthetics**

Zou et al. [5] have examined the effect of ketamine anesthesia duration in newborn rhesus monkeys (PND5, PND6) through silver and Fluoro-Jade C stains and caspase-3 immunostain. Three hours exposure to ketamine did not produce any significant histochemical change, whereas profound brain cell death was observed in the frontal cortex among subjects that were under the effect of ketamine for 9 or 24 hours. In cell culture study of Bosnjak et al. [25], they demonstrated that ketamine decreases neuronal viability time and dose dependently, leads to neuronal ultrastructural abnormalities, causes depolarization of mitochondrial membrane potential, induces apoptotic pathway, causes cytochrome c release from mitochondria into cytosol, and induces free oxygen radical production.

Yu et al. examined neuroapoptosis and long-term behavioral changes in PND7 rats that were given single and repetitive doses of propofol. Their findings included reduction in neuron density, morphological changes in pyramidal cells, apoptosis, and suppressed release of excitatory neurotransmitters. Additionally, these effects were more pronounced among the group that was subject to repeated doses of propofol [26].

Benzodiazepines (clonazepam, diazepam, and midazolam), which are intravenous anesthetics, have controversial effects on apoptosis; however, barbiturates (pentobarbital, phenobarbital) clearly increase apoptosis. The few studies that have examined the effects of sodium thiopental reported that exposure did not result in increased apoptosis [27–33]. Thompson [34] has suggested high-dose narcotic anesthetic for neonatal and infant. But, fetal and neonatal chronic exposure to opioids has been associated with neuronal changes. Although opioid-based anesthesia and opioids coadministered with inhalation anesthetics have been shown to reduce apoptosis, safety has not been demonstrated with these preparations [35, 36]. However, these studies are controversial and their safety has been in question. Another study has demonstrated that dexmedetomidine, the current intravenous anesthetic, reduces prenatal toxicity caused by propofol [37].
