In reviewing the images, it is important to consider the manner in which they were obtained: transabdominal or transvaginal. The clearest images of the heart were done via transvaginal ultrasound with the fetus in vertex presentation. The images obtained with the transabdominal probe are difficult to interpret as they are coronal rather than axial sections, which would provide better information. The gastric chamber and heart appear to be located on the opposite side of the longitudinal axis of the fetus. The four-chamber view of the fetal heart is normal, in addition to the left ventricular outflow tract arising from the left ventricle. On the transvaginal images, the fetus is in the vertex presentation with the back posterior, thus the cardiac axis points to the right side.
Situs refers to the arrangement of the viscera, atria, and vessels within the body. Situs solitus describes the normal arrangement. Situs inversus describes the inverted arrangement of the viscera and atria. Situs ambiguous (also known as heterotaxy) describes disturbances in arrangements that can neither be identified as solitus nor inversus. Prevalence of situs abnormalities is estimated at 1.1 per 10,000 live births.
Situs solitus is the normal arrangement of viscera, atria, and vessels within the body. The systemic venous atrium, defined as the atrium that receives blood from the inferior vena cava (IVC), is located on the right side. The remaining right-sided organs include the liver, gallbladder, IVC, and trilobed lung. The organs positioned on the anatomic left include the pulmonary venous atrium, stomach, single spleen, aorta, cardiac apex, and bilobed lung. The expected incidence of structural heart defect with correctly placed viscera is less than 1%.
Situs inversus is defined as inverted arrangement of viscera and atria. The totalis form, also referred to as situs inversus with dextrocardia, is a mirror image of situs solitus. This abnormality affects 0.01% of the general population. The expected incidence of structural heart defects with situs inversus totalis is 5% to 10%. Situs inversus with levocardia, also known as situs inversus incompletus, displays inverted position of chest and abdominal organs. The heart remains positioned in the left thorax while the abdominal visceral organs are reversed. Congenital heart disease is almost always present in these patients. Kartagener syndrome (which consists of situs inversus, nasal polyposis with chronic sinusitis, and bronchiectasis) is present in 20% of all patients with situs inversus.
Situs ambiguous is defined as a disturbance in the usual left and right distribution of visceral organs that does not entirely correspond to the complete or partial mirror image. It is commonly associated with complex congenital heart defects, abnormalities in venous drainage, bowel malrotation and obstruction, and splenic, biliary, and bronchial tree abnormalities. Their incidence has been estimated around 1 per 10,000 infants. Two types of heterotaxy exist: right isomerism and left isomerism. In right isomerism, also referred to as asplenia, both sides of the body show the right morphology. In left isomerism, also referred to as polysplenia, both sides of the body show the left morphology.
The sonographic definition of the fetal situs and determination of the left and right axes are important for performing fetal echocardiography. An abnormal relationship between abdominal and thoracic organs is a useful pointer for diagnosis of an abnormal situs or situs inversus. The primary indicators are a stomach located contralateral to the apex of the heart and a gallbladder located to the left of the umbilical vein. Despite these landmarks, it is not always simple to diagnose the malposition of an organ such as a right-sided stomach. In 2002, Bronshtein et al. presented the right-hand rule for transabdominal scanning and the left-hand rule for transvaginal scanning which enables the definition of the fetal situs. The rules apply in all cases where the direction of the ultrasound beam on the screen is from top to bottom. According to these authors, the sonographer should imagine that his left hand represents the fetus while performing a transvaginal scan. The dorsal forearm is the fetal back, the palmar forearm is the fetal abdomen, and the fist is the fetal head. The thumb will always point to the left side of the fetus. This is true for all fetal positions, vertex, breech, or oblique, as well as for cases of fetal back up or down.
Other terms important to this discussion describe the cardiac position. All cardiac positions can be described in terms of the location of the heart in the thorax and orientation of its axis. The orientation of the cardiac axis is independent from the position of the heart. Therefore, for each heart position, orientation of the axis can be to the left, to the midline, or to the right and must be described separately. Terms such as dextrocardia, dextroposition, or dextroversion are used interchangeably in the literature with no common agreement. For most of the authors, the heart location is given by the terms levoposition and dextroposition, while the orientation of its axis is referred to as levocardia, mesocardia, and dextrocardia. Dextroposition would be defined as a heart located on the right side of the fetal chest. However, Abuhamad and Chaoui use the term dextrocardia to describe a heart positioned in the right chest regardless of its axis. Dextroposition describes a heart positioned in the right chest with the axis pointing to the left. When the heart is in the right chest with the cardiac axis pointing to the right side, the term dextroversion is used. Therefore, for these authors, dextroposition and dextroversion are forms of dextrocardia. Dextrocardia with dextroversion can be found in the context of a situs inversus or can be isolated, occurring in conjunction with situs solitus and situs ambiguous. It is commonly associated with congenital heart defects, with the majority involving a discordant atrioventricular connection. In 1961, Arcilla and Gasul evaluated the clinical, angiographic, and autopsy results in 50 patients with congenital dextrocardia, a right-sided heart, and classified them into five major types which can be characterized as the intrinsic dextrocardia group (Type I-IV), caused by a developmental anomaly of the primitive heart tube, and the extrinsic dextrocardia group (Type V), caused by the displacement of the heart by congenital anomalies of the lungs, diaphragm, or thoracic cage.
Suggested readings:
• Abuhamad A, Chaoui R. Fetal Situs. In: Abuhamad A and Chaoui R, ed. A practical guide to fetal echocardiography: normal and abnormal hearts, 3rd ed. Wolters Kluver, Philadelphia, PA, 2016; pages 66-77.
• Applegate KE, Goske MJ, Pierce G, Murphy D. Situs revisited: imaging of the heterotaxy syndrome. Radiographics. 1999 Jul-Aug;19(4):837-52; discussion 853-4. doi: 10.1148/radiographics.19.4.g99jl31837. PMID: 10464794.
• Arcilla RA, Gasul BM. Congenital dextrocardia. Clinical, angiocardiographic, and autopsy studies on 50 patients. J Pediatr. 1961 Feb;58:251-62. doi: 10.1016/s0022-3476(61)80165-0. PMID: 13684050.
• Bernasconi A, Azancot A, Simpson JM, Jones A, Sharland GK. Fetal dextrocardia: diagnosis and outcome in two tertiary centres. Heart. 2005 Dec;91(12):1590-4. doi: 10.1136/hrt.2004.048330. PMID: 16287744; PMCID: PMC1769217.
• Bronshtein M, Gover A, Zimmer EZ. Sonographic definition of the fetal situs. Obstet Gynecol. 2002 Jun;99(6):1129-30. doi: 10.1016/s0029-7844(02)02017-3. PMID: 12052611.
• Chiappa EM, Cook AC, Botta G, Silverman NH. The Cardiac Position and Axis Orientation. In: Chiappa EM, Cook AC, Botta G, Silverman NH, ed. Echocardiographic Anatomy in the Fetus. Springer-Verlag, Milan, Italia, 2008; pages 29-37.
• Lambert TE, Kuller J, Small M, Rhee E, Barker P. Abnormalities of Fetal Situs: An Overview and Literature Review. Obstet Gynecol Surv. 2016 Jan;71(1):33-8. doi: 10.1097/OGX.0000000000000262. PMID: 26819133.
• Yeo L, Luewan S, Markush D, Gill N, Romero R. Prenatal Diagnosis of Dextrocardia with Complex Congenital Heart Disease Using Fetal Intelligent Navigation Echocardiography (FINE) and a Literature Review. Fetal Diagn Ther. 2018;43(4):304-316. doi: 10.1159/000468929. Epub 2017 Jun 23. PMID: 28641300; PMCID: PMC6105280.
Situs refers to the arrangement of the viscera, atria, and vessels within the body. Situs solitus describes the normal arrangement. Situs inversus describes the inverted arrangement of the viscera and atria. Situs ambiguous (also known as heterotaxy) describes disturbances in arrangements that can neither be identified as solitus nor inversus. Prevalence of situs abnormalities is estimated at 1.1 per 10,000 live births.
Situs solitus is the normal arrangement of viscera, atria, and vessels within the body. The systemic venous atrium, defined as the atrium that receives blood from the inferior vena cava (IVC), is located on the right side. The remaining right-sided organs include the liver, gallbladder, IVC, and trilobed lung. The organs positioned on the anatomic left include the pulmonary venous atrium, stomach, single spleen, aorta, cardiac apex, and bilobed lung. The expected incidence of structural heart defect with correctly placed viscera is less than 1%.
Situs inversus is defined as inverted arrangement of viscera and atria. The totalis form, also referred to as situs inversus with dextrocardia, is a mirror image of situs solitus. This abnormality affects 0.01% of the general population. The expected incidence of structural heart defects with situs inversus totalis is 5% to 10%. Situs inversus with levocardia, also known as situs inversus incompletus, displays inverted position of chest and abdominal organs. The heart remains positioned in the left thorax while the abdominal visceral organs are reversed. Congenital heart disease is almost always present in these patients. Kartagener syndrome (which consists of situs inversus, nasal polyposis with chronic sinusitis, and bronchiectasis) is present in 20% of all patients with situs inversus.
Situs ambiguous is defined as a disturbance in the usual left and right distribution of visceral organs that does not entirely correspond to the complete or partial mirror image. It is commonly associated with complex congenital heart defects, abnormalities in venous drainage, bowel malrotation and obstruction, and splenic, biliary, and bronchial tree abnormalities. Their incidence has been estimated around 1 per 10,000 infants. Two types of heterotaxy exist: right isomerism and left isomerism. In right isomerism, also referred to as asplenia, both sides of the body show the right morphology. In left isomerism, also referred to as polysplenia, both sides of the body show the left morphology.
The sonographic definition of the fetal situs and determination of the left and right axes are important for performing fetal echocardiography. An abnormal relationship between abdominal and thoracic organs is a useful pointer for diagnosis of an abnormal situs or situs inversus. The primary indicators are a stomach located contralateral to the apex of the heart and a gallbladder located to the left of the umbilical vein. Despite these landmarks, it is not always simple to diagnose the malposition of an organ such as a right-sided stomach. In 2002, Bronshtein et al. presented the right-hand rule for transabdominal scanning and the left-hand rule for transvaginal scanning which enables the definition of the fetal situs. The rules apply in all cases where the direction of the ultrasound beam on the screen is from top to bottom. According to these authors, the sonographer should imagine that his left hand represents the fetus while performing a transvaginal scan. The dorsal forearm is the fetal back, the palmar forearm is the fetal abdomen, and the fist is the fetal head. The thumb will always point to the left side of the fetus. This is true for all fetal positions, vertex, breech, or oblique, as well as for cases of fetal back up or down.
Other terms important to this discussion describe the cardiac position. All cardiac positions can be described in terms of the location of the heart in the thorax and orientation of its axis. The orientation of the cardiac axis is independent from the position of the heart. Therefore, for each heart position, orientation of the axis can be to the left, to the midline, or to the right and must be described separately. Terms such as dextrocardia, dextroposition, or dextroversion are used interchangeably in the literature with no common agreement. For most of the authors, the heart location is given by the terms levoposition and dextroposition, while the orientation of its axis is referred to as levocardia, mesocardia, and dextrocardia. Dextroposition would be defined as a heart located on the right side of the fetal chest. However, Abuhamad and Chaoui use the term dextrocardia to describe a heart positioned in the right chest regardless of its axis. Dextroposition describes a heart positioned in the right chest with the axis pointing to the left. When the heart is in the right chest with the cardiac axis pointing to the right side, the term dextroversion is used. Therefore, for these authors, dextroposition and dextroversion are forms of dextrocardia. Dextrocardia with dextroversion can be found in the context of a situs inversus or can be isolated, occurring in conjunction with situs solitus and situs ambiguous. It is commonly associated with congenital heart defects, with the majority involving a discordant atrioventricular connection. In 1961, Arcilla and Gasul evaluated the clinical, angiographic, and autopsy results in 50 patients with congenital dextrocardia, a right-sided heart, and classified them into five major types which can be characterized as the intrinsic dextrocardia group (Type I-IV), caused by a developmental anomaly of the primitive heart tube, and the extrinsic dextrocardia group (Type V), caused by the displacement of the heart by congenital anomalies of the lungs, diaphragm, or thoracic cage.
Suggested readings:
• Abuhamad A, Chaoui R. Fetal Situs. In: Abuhamad A and Chaoui R, ed. A practical guide to fetal echocardiography: normal and abnormal hearts, 3rd ed. Wolters Kluver, Philadelphia, PA, 2016; pages 66-77.
• Applegate KE, Goske MJ, Pierce G, Murphy D. Situs revisited: imaging of the heterotaxy syndrome. Radiographics. 1999 Jul-Aug;19(4):837-52; discussion 853-4. doi: 10.1148/radiographics.19.4.g99jl31837. PMID: 10464794.
• Arcilla RA, Gasul BM. Congenital dextrocardia. Clinical, angiocardiographic, and autopsy studies on 50 patients. J Pediatr. 1961 Feb;58:251-62. doi: 10.1016/s0022-3476(61)80165-0. PMID: 13684050.
• Bernasconi A, Azancot A, Simpson JM, Jones A, Sharland GK. Fetal dextrocardia: diagnosis and outcome in two tertiary centres. Heart. 2005 Dec;91(12):1590-4. doi: 10.1136/hrt.2004.048330. PMID: 16287744; PMCID: PMC1769217.
• Bronshtein M, Gover A, Zimmer EZ. Sonographic definition of the fetal situs. Obstet Gynecol. 2002 Jun;99(6):1129-30. doi: 10.1016/s0029-7844(02)02017-3. PMID: 12052611.
• Chiappa EM, Cook AC, Botta G, Silverman NH. The Cardiac Position and Axis Orientation. In: Chiappa EM, Cook AC, Botta G, Silverman NH, ed. Echocardiographic Anatomy in the Fetus. Springer-Verlag, Milan, Italia, 2008; pages 29-37.
• Lambert TE, Kuller J, Small M, Rhee E, Barker P. Abnormalities of Fetal Situs: An Overview and Literature Review. Obstet Gynecol Surv. 2016 Jan;71(1):33-8. doi: 10.1097/OGX.0000000000000262. PMID: 26819133.
• Yeo L, Luewan S, Markush D, Gill N, Romero R. Prenatal Diagnosis of Dextrocardia with Complex Congenital Heart Disease Using Fetal Intelligent Navigation Echocardiography (FINE) and a Literature Review. Fetal Diagn Ther. 2018;43(4):304-316. doi: 10.1159/000468929. Epub 2017 Jun 23. PMID: 28641300; PMCID: PMC6105280.