Ultrasound is a tool which, through the use of high frequency sound waves, enables the physician to examine fetal body organs, as well as the flow of blood within the cardiovascular system. For adequate imaging to occur, the sound waves need to be transmitted through the maternal abdominal tissues which have a high water content. Because the fetus floats in a sac of water (amniotic fluid), ultrasound can be used to examine the anatomy of the fetus. Ultrasound has been used in obstetrics for over three decades, and has not been demonstrated to be harmful. Although there are several ultrasound modalities, the most commonly used is real-time B-mode ultrasound. This is the type that most obstetricians will use in the office setting. Real-time B-mode displays an image slice in a two-dimensional format in various shades of gray. Typically dense objects such as bone are displayed as white and the least dense material, fluid, is displayed as black. Tissues with varying densities are displayed in various shades of gray. The following image illustrates this:
This is a B-Mode image of the fetal brain. The bone is white, the amniotic fluid surrounding the fetus is black, and the tissue is gray.
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Recently, manufacturers of ultrasound equipment have added a feature in which the gray scale is replaced with color. This improves the recognition of subtle tissue differences that can be helpful in the interpretation of the ultrasound image. This is the modality which I currently prefer to use to examine the fetus.
This is a B-Mode image of the fetal brain. The grayscale is now displayed in B-Mode color.
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Ultrasound Abnormalities Associated with an Increased Risk for Down Syndrome
A number of ultrasound studies have documented specific types of ultrasound abnormalities that are associated with an increased risk for Down syndrome. The following is a list of defects that are commonly seen in fetuses with Down syndrome:
Head
Dilated Ventricles of the brain. When the ventricles measure 10 mm or greater, this is abnormal.
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Dilated Posterior Fossa of the brain.
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Increased nuchal skin fold behind the fetal neck. The increased thickness of the skin can also be seen on the side of the head. The nuchal skin fold is abnormal if it measures more than 5 or 6 mm, depending upon the study that is cited.
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Heart
Atrial and/or ventricular chamber disproportion. When this is present, the right atrium and/or ventricle is larger than the left atrium and/or ventricle.
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Ventricular and atrial septal defects. This is a hole in the wall that separates the two ventricular and atrial chambers. This is also known as an endocardial cushion defect or an AV canal defect. When this is present the risk for Down syndrome is 50%.
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Pericardial Effusion. This is a collection of fluid along the side of the heart. In fetuses with Down syndrome, this is usually located along the right ventricle of the heart.
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Echogenic focus. This is a white dot that appears in the left ventricle, occasionally in the right ventricle, and rarely in both ventricles. There is debate as to whether it increases the risk for Down syndrome when observed in isolation.
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Abdomen
Hyperechoic bowel. This occurs when the bowel appears to be brighter than the surrounding tissue. Besides Down syndrome it has been associated with an increased risk for cystic fibrosis, cytomegalovirus infection, parvovirus infection, and growth restriction that occurs later in pregnancy.
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Pyelectasis. This is a dilated kidney that measures 4 or more millimeters in diameter. This often resolves as the pregnancy continues. Rarely, it develops into hydronephrosis that must be treated following birth.
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Extremities
Short femur and humerus. These are the upper bones of the leg and arm. Both have been shown to be shorter in fetuses with Down syndrome. However, because there is so much variation among ethnic groups, these markers are less helpful than the other markers described above.
Detection of Down Syndrome and Computation of Risk Following Genetic Ultrasound
If you would like to see how the risk for Down syndrome is computed following a Genetic Ultrasound, CLICK HERE.
60% Detection Rate for Down Syndrome
In a study published by Dr. DeVore, he reported that the detection rate for Down syndrome was dependent upon which of the above ultrasound abnormalities were detected by the physician. For example, if the physician unable to evaluate the fetal heart, but could examine the fetal head and abdomen, 60% of fetuses with Down syndrome would be detected, with a screen -positive rate of 5.9%.
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*Likelihood ratio is the number that the risk for Down syndrome is increased when an abnormal ultrasound finding is present. To compute the risk for Down syndrome the Likelihood Ratio is multiplied by the risk for Down syndrome prior to the test.
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75% Detection Rate for Down Syndrome
If, however, the physician could examine the heart and was able to identify the presence or absence of atrial and/or ventricular disproportion, then the detection rate would increase to 75% with a screen-positive rate of 6.4%.
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*Likelihood ratio is the number that the risk for Down syndrome is increased when an abnormal ultrasound finding is present. To compute the risk for Down syndrome the Likelihood Ratio is multiplied by the risk for Down syndrome prior to the test.
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