**6. Clinical studies**

Although many experimental studies have been conducted, this alone is not sufficient evidence to conclude that general anesthetics have a neurotoxic effect on the developing human brain. Even within mammals, species vary widely in the rate and timing of brain development. Total maturation of the brain takes only a few weeks in the rat, while maturation of the human brain occurs over many years. In addition, the dose and duration of anesthetics used in experimental models is not directly proportional to the procedures used in patients. In some cases, experimental doses may be as much as 20 times the standard clinical dose. Adjusted for the life span of a rat, 6 hours of anesthesia may correspond to 1 month of a human life span. Again, some observations from these studies, such as lactic acidosis, hypercarbia, and hypoglycemia, have mostly been ignored. Learning ability is also disturbed in subjects that are fasted for the duration of the anesthesia treatment [43, 65, 66].

cell. However, in conditions where plasmin release is reduced or blocked, such as when anesthesia is applied, proBDNF cannot be converted to the mature form, and it stimulates p75NTR instead of the TrkB receptor. Activation of p75NTR receptor, also called the "death receptor," leads to actin depolymerization and apoptosis. Head et al. [50] demonstrated that isoflurane

Apoptosis is a programmed cell death that can occur in both physiological and pathological conditions. Apoptosis is physiologically present in the developing brain, occurring at a rate of approximately 1%. However, apoptosis that occurs following pathological processes like hypoxia and ischemia is typically problematic. Several experimental studies have shown that apoptosis is increased following anesthesia exposure. However, it is not possible to conduct such studies in humans. Therefore, it is difficult to estimate the rate of apoptosis following anesthesia exposure in humans to what extent this apoptosis affects maturation of the developing brain. Experimental studies have shown that anesthesia induces apoptosis via intrinsic and extrinsic pathways. Anesthesia application causes leakage of cytochrome c and translocation of Bax protein to the mitochondria, leading to activation of the apaf-1 and caspase pathways, respectively. This in turn results in lipid peroxidation via release of free oxygen radicals. Apoptosis occurs not only in intrinsic pathway but also in extrinsic pathway which

There are three publications that demonstrate the relationship between microRNA and anesthetic-induced developmental neurotoxicity; according to these publications, while propofol downregulates microRNA-21, ketamine upregulates microRNA-34a, microRNA-34c, and

In cell culture models, it has been demonstrated that neuron development is highly dependent on the actin cytoskeleton, and anesthetics are dangerous for actin regulation [57–59].

Tau protein hyperphosphorylation at serine 404 demonstrates neurodegeneration and is

Translocator protein (TSPO, 18 kDa) is a biomarker that could be used for evaluation of reactive gliosis and microglia activity and has the potential for use in noninvasive imaging using positron emission tomography and single photon emission computed tomography [61]. The relationship between anesthesia-associated neurotoxicity and DNA methylation and gene

Treatment strategies to reduce neurodegeneration induced by anesthetics have also been widely investigated. Lithium, melatonin, estradiol, pilocarpine, dexmedetomidine, xenon, erythropoietin, L-carnitine, hydrogen gas, and pramipexole are among the leading candi-

Although many experimental studies have been conducted, this alone is not sufficient evidence to conclude that general anesthetics have a neurotoxic effect on the developing human

induced by ketamine. Therefore, microtubules are disrupted and damaged [60].

causes apoptosis in the neonatal mice brain through this pathway.

microRNA-124 and downregulates microRNA-137 [54–56].

activates Fas protein [51–53].

108 Current Topics in Anesthesiology

expression has been investigated [62].

dates for this emerging therapy [63, 64].

**6. Clinical studies**

In one retrospective birth cohort study that used New York State Medicaid data collected between the years 1999 and 2002, 383 children who underwent inguinal hernia repair with anesthesia before the age of 3 were evaluated along with 5050 children who did not undergo an operation. Hazard ratios regarding behavioral and developmental disorders were reported to be 2.3 with exposure to anesthesia, 1.0 for age, 2.7 for gender, 1.2 for race, and 1.6 for birth complications [6]. Considering that elective surgeries can be postponed, exposure to anesthesia is an avoidable risk for most infants.

In another report, patients that had been overexposed to anesthesia had more learning difficulties than those who were treated with appropriate doses. The risk of learning difficulties was progressively increased with repeated exposure to anesthesia [67, 68]. The effects of anesthesia used during cesarean procedures were examined in children. Infants born under regional anesthesia exhibited fewer learning difficulties in the later stages of their life [69, 70].

One retrospective study examined 10,450 siblings born between the years 1999 and 2005 and evaluated developmental and behavioral disorders among those who did and did not receive anesthesia prior to the age of 3. The incidence of developmental and behavioral disorder was 128.2/1000/year among those who were exposed to anesthesia and 56.3/1000/year among those who were not exposed to anesthesia. Therefore, behavioral disorders were 60% more frequent among those who received anesthesia in comparison to those who did not. The estimated hazard ratio for developmental and behavioral disorders was 1:1 for those who received anesthesia once before the age of 3, 2:9 for those exposed twice, and 4 for those who had been exposed to anesthesia three or more times [71].

Meyer et al. observed development of convulsion with similar clinical characteristics in three infants under the age of 2 months, occurring after 23–30 hours of anesthesia induced and maintained using propofol. They reported that the seizures did not recur; however, two infants had progressive microcephaly and cognitive and behavioral disorder. Magnetic resonance imaging also showed white matter abnormalities [72]. The manufacturer of propofol does not recommend the use of propofol as a general anesthetic agent for children under the age of 3 [73].

Clinical studies in the literature are often retrospective, and even strong correlations are not evidence of causality. Therefore, the Mayo Anesthesia Safety in Kids (MASK) study was launched by Mayo Clinic at the suggestion of the FDA to evaluate neurotoxicity in children exposed to anesthesia. The study included children born in Olmsted County between 1997 and 2007 and who still lived there when they reached 8 years old. Those who received general anesthesia before the age of 3 were excluded from the study. Children classified as having single, multiple, or no anesthesia exposure were evaluated between the years 2007 and 2016, when they were at the age of 8–12 or 15–19 with a single session that lasted for 4 hours using the National Center for Toxicological Research-Operant Test Battery (NCTR-OTB). The NCTR-OTB test evaluates processing speed; cognitive/intellectual memory; attention, language, motor and visual-spatial, and cognitive processing; and executive functions [74].

The Pediatric Anesthesia and Neurodevelopmental Assessment (PANDA), which was conducted by the University of Columbia and followed sibling pairs under the age of 3 who underwent inguinal operation up to the age of 8–15, published four symposiums in 2-year interval. The first meeting in 2008 established the goals of the study. The second meeting in 2010 was interdisciplinary. The third meeting in 2012 was attended by different disciplines, parents, clinicians, FDA workers, and patient's rights advocates. In this meeting, attendees agreed to collaborate on advanced preclinical, clinical, and translational studies [75, 76]. Additionally in 2012, pediatric anesthesiologists and pediatric surgeons met to discuss the neurotoxicity risk of some elective procedures and anesthesia applications performed in children and specifically to discuss questions and concerns of parents. Meeting attendees, including pediatric general surgeons, urologists, plastic surgeons, and ophthalmologists, reviewed inguinal hernia, hypospadias-undescended testis, cleft lip, craniosynostosis, cataracts, and strabismus applications in early childhood. They emphasized that the amount of volatile anesthetics and sedation levels could be reduced by using balanced anesthesia methods, regional anesthesia methods, and the use of opioid and non-opioid analgesics, but the group was unable to reach a consensus on best practices [77]. At the 2014 meeting, the existing clinical studies, General Anesthesia Study (GAS), MASK, and PANDA, were evaluated, and Strategies for Mitigating Anesthesia-Related neuroToxicity in Tots (SmartTots) was presented along with the future targets of this organization. SmartTots is a public-private partnership that investigates the effects of anesthetic agents on neural development in infants and children. All panelists evaluated their anesthesia and clinical practices with the following questions [78, 79]:


Ordering imaging studies with sedation/anesthesia.

A child requiring multiple procedures under GA overtime.

A child requiring multiple procedures from different subspecialties at the same time.


How will I discuss this with the parents?

Will I change my practice and how?

The 2014 report indicated that the collected data was insufficient to draw any conclusions. However, it stated 2 years later that the results would be considered as a public health problem, leading to greater awareness [78]. On the other hand, the General Anesthesia Study (GAS), which is currently ongoing and only investigates causality, investigated cases that were less than 60 weeks from conception and greater than 26 weeks gestational age and had undergone inguinal hernia operations with sevoflurane-based general anesthesia or awakeregional anesthesia. This study was conducted in 28 hospitals from Australia, Italy, the USA, the UK, and Canada. No opioids or nitrous oxide was used. Regional techniques and intravenous acetaminophen were used for postoperative analgesia. Protocols were applied in order to prevent development of adverse states that would contribute in neurotoxicity, such as hypoglycemia, hypotension, and hypoxia. Children were assessed using the composite cognitive score of the Bayley Scales of Infant and Toddler Development III test at the age of 2 and with the Wechsler Preschool and Primary Scale of Intelligence Third Edition (WPPSI-III) Full Scale Intelligence Quotient score at the age of 5. During 2007–2013, 363 infants were enrolled in the awake-regional group, and 359 infants were enrolled in the general anesthesia group. According to the study results, the median general anesthesia duration was 54 minutes. No significant difference was found between the groups regarding cognitive composite score at 2 years of age. This study provides strong evidence that sevoflurane anesthesia lasting <1 hour in infants does not produce more severe neurotoxicity at the second year of age than awake-regional treatment. Nonetheless, the primary outcome of this study is the evaluation of neurodevelopmental state at 5 years of age, and this result has not been published yet. It was also reported in this study that early-period apnea development (<30 minutes) was less frequent in the regional anesthesia group [80].

Other discussed topics are applied anesthesia techniques to mothers during childbirth. Flick et al. determined that neuraxial labor analgesia for vaginal delivery did not cause learning disabilities in childhood [69, 70].

Another topic of discussion was how parents should be informed and the need to establish a protocol. However, since it was not possible to reach a consensus based on the current data, it was concluded that it would not be appropriate to inform parents and establish a protocol yet [81].
