Ultrasound Techniques For Fetal Cardiac Abnormalities
Case History :
The prenatal cardiac diagnosis has greatly benefited from advances in ultrasound technology in an ability to detect abnormalities.It has also enabled to detect single gene disorders, leading to substantive improvements in detection of such congenital anomalies.An early detection of these anomalies enables early referral to tertiary care centers with adequate expertise.Further comprehensive evaluation can be performed with a dedicated fetal echocardiography, particularly in high-risk pregnancies and in cases with extracardiac anomalies. The congenital cardiac disease is a major cause of morbidity and mortality, with half of these cases being lethal or requiring surgical correction. The causes for congenital cardiac defects include environmental, genetic, and chromosomal abnormalities with a higher incidence among infants with affected siblings or mother. Detection of cardiac anomalies can be challenging and is typically done by fetal cardiac ultrasound performed between 18 and 22 weeks. A transvaginal scan can detect anomalies even at 12–13 weeks. Some of the imaging techniques used for various fetal cardiac anomalies seen are described below Fetal cardiac ultrasound The first step in fetal cardiac ultrasound is to evaluate the orientation of the fetus within the maternal abdomen that is, fetal laterality (presentation and lie). Orientation is assessed from a transverse section of the fetal abdomen. For atrial situs, the relationship of the atria with respect to an organ is conventionally established by noting the arrangement of the aorta and inferior vena cava (IVC) at the level of the diaphragm. In situs solitus, the IVC is anterior and to the right of the aorta.In situs inversus, there is a mirror image pattern, with the aorta to the right of the IVC. In situs ambiguous, the aorta and IVC are located on the same side of the spine in right isomerism and the aorta is centrally located, with an interrupted IVC in left isomerism. M-mode ultrasound M-mode ultrasound is a 2D image of motion over time that is used for evaluation of fetal heart motion, heart rate, wall thickness, chamber size, and motion of the valves or myocardium. Fetal heart rate and rhythm can be evaluated by M-mode ultrasound through the atrial and ventricular wall, above and below the AV valve, respectively. Chamber size and function are evaluated by focusing at the level of AV valves. Doppler tissue imaging Color Doppler tissue imaging has been used recently for evaluating high-amplitude low-velocity signals, such as within the moving myocardium. This can be used to encode the direction of myocardial motion, which is particularly useful in the assessment of arrhythmias. Color Doppler can be used to detect vascular flow through cardiac chambers, vascular structures, and septal defects. It also significantly reduces the time required for Doppler examination of the heart. Color Doppler is also used in the evaluation of pulmonary and systemic venous connections and small septal defects. Ventricular septal defect Ventricular septal defect (VSD) is the most common congenital heart disease, accounting for about 40% of all cardiac anomalies. The defect is most commonly seen in the membranous septum and less commonly in the muscular, outlet, or inlet portions.Defects can be variable in size. VSD is best seen in a four chamber view as a discontinuity in the ventricular septum, particularly the inlet defects.The ventricular septum is ideally evaluated in images acquired perpendicular to the interventricular septum since a pseudoVSD, as a result of signal drop-out, can be seen in the superior aspect of images parallel to the ultrasound beam. Tetralogy of Fallot Tetralogy of Fallot is characterized by narrowing of the right ventricular outflow tract (RVOT), VSD, overriding aorta, and right ventricular hypertrophy.On ultrasound, the aorta is seen straddling a large membranous VSD. Depending on the size of the Pulmonary artery (PA), it may not be easily seen and the normal crossing of an aorta and pulmonary arteries is not seen. The aorta may be dilated, and the pulmonary valve is stenosed or atretic with a dilatedPA. Cardiomyopathies Cardiomyopathies account for 8–11% of fetal cardiovascular abnormalities. Extrinsic causes are intrauterine infections, maternal diseases (autoantibodies and diabetes), and twin-twin transfusion syndrome. In hypertrophic cardiomyopathy, the LV-RV myocardial thickness is increased, without an underlying structural abnormality. It has been associated with maternal diabetes and often regresses during the first 6 months of life. Ventricular hypertrophy can also be seen because of increased afterload. Restrictive cardiomyopathy is characterized typically by normal ventricular size and systolic function, but abnormal diastolic function and elevated filling pressure. Arrhythmia The premature atrial and ventricular contractions account for 75% and 8% of fetal arrhythmias, respectively. Arrhythmias are significant only when they are sustained (< 10%), resulting in supraventricular tachycardia or severe bradyarrhythmia causing complete heart block. Fetal tachycardia is diagnosed when the heart rate is greater than 180 beats/ min. Fetal bradycardia is diagnosed when the heart rate is less than 100 beats/min, lasting more than 10 seconds and is often associated with fetal hypoxia or asphyxia. MRI Unlike ultrasound imaging, the role of this modality is not affected by maternal and fetal conditions such as obesity and oligohydramnios, which particularly impair sonographic visualization of the fetal heart; maternal obesity increases the rate of suboptimal ultrasound visualization of the fetal cardiac structures. Recent studies have described the potential role of fetal MRI in the evaluation of the anatomy and pathology of the cardiovascular system. Thus, regular fetal cardiac ultrasound enables the detection and characterization of most of the cardiac anomalies. A further comprehensive evaluation can be performed with fetal echocardiography, particularly in high-risk pregnancies and extracardiac anomalies.
Investigation :
Which ultrasound technique for detection of fetal cardiac abnormalities do you prefer in your practice?