Tetralogy of Fallot and Genetics Literature Review Chapter

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Tetralogy of Fallot: Literature Review

Tetralogy of Fallot is a congenital heart defect associated with systemic cyanosis, accounting for approximately 5 to 6% of all cases of congenital heart disease and is characterized by; ventricular septal defect, aortic override, pulmonary stenosis and right ventricular hypertrophy. It is the most common cause of blue baby syndrome with children diagnosed developing Tet spells. Sudden increases in cyanosis followed by syncope characterize Tet spells and may result in hypoxic brain injury and death. Environmental and genetic disorders are other causes of TOF; always associated with chromosome 22 deletions and DiGeorge syndrome and occurs slightly more often in males than in females. If left untreated, Tetralogy of Fallot rapidly results in progressive right ventricular hypertrophy due to the increased resistance on the right ventricle. This progresses to dilated cardiomyopathy which begins in the right heart chambers often leading to left heart failure. Actuarial survival for untreated Tetralogy of Fallot is approximately 75%, 60%, 30% and 5% after the first year of life, four years, ten years, and forty years respectively.

Review of the Literature

According to Digilio, et al. (2001), Transposition of the Great Arteries (TGA) has a prevalence rate of 0.2 per 1000 live births in the United States accounting for approximately 5 to 7% of all Congenital Heart Defects (CHDs). TGA is the most frequent cyanotic CHD diagnosed in the neonatal period and genetic contribution to the pathogenesis of TGA is not considered to be strong. This is due to few familial cases identified and that genetic syndromes are uncommonly associated with TGA. The study concludes that the mean precurrence risk for CHD among siblings of patients affected with TGA is 1.4%, and TGA is not always a sporadic occurrence in these families. Familial precurrence of concordant cardiac defects within affected family members' supports monogenic or oligogenic inheritance of TGA in selected pedigrees. In addition, TGA and congenitally corrected TGA can segregate in the same family due to a probable monogenic transmission supporting a pathogenetic link between some cases of complete TGA and looping abnormalities.

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In their publication, Nembhard, Salemi, Wang, Loscalzo, & Hauser (2010) argue that birth defects are the leading cause of infant morbidity and mortality in the United States and CHD is the most common of all birth defects. Despite having an annual prevalence of six to twelve affected infants per 1,000 live births, a meager 5% of CHDs are attributed to chromosomal abnormalities and single gene defects. They argue that though etiologies of most cases of CHDs are unclear, there are other modifiable factors that may result in increased risk of congenital heart defects. There are maternal illnesses such as phenylketonuria, diabetes, rubella, maternal febrile illnesses, and influenza that may increase the risks of CHD-affected pregnancy. In addition, the results of studies investigating the effect of environmental exposures on CHD risk are inconclusive, but, organic solvents have been identified as directly associated with increased risk of CHD among children. In contrast, the association between socio-demographic factors, such as maternal and paternal age, socioeconomic status, and pre-natal maternal stress are inconsistent.

The role of other socio-demographic factors such as maternal race and ethnicity, are also unclear. The publication outlines that most studies report excess risk of specific types of CHDs for Whites compared to Blacks and that very few studies have reported risk of specific types of CHD for Hispanics. Moreover, the report provides information on the prevalence of isolated, multiple heart and CHD with non-cardiac defects by maternal race and ethnicity. Hence, their publication's intent is to determine the prevalence of CHD at live birth and determine the prevalence of isolated CHD, multiple heart and CHD plus non-cardiac defects for non-Hispanic-White, non-Hispanic-Black and Hispanic infants.

As portrayed by Jing-bin, Ying-long, Pei-wu, Xiao-dong, Ming, & Xiang-ming (2010), CHD is the most common type of birth defect. Despite the many advances in understanding cardiac development and many genes related to cardiac development, the fundamental etiology for the majority of cases of congenital heart disease remains unknown. CHD is a multifactorial complex disease, with both environmental and genetic factors playing roles in its spread among children......

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