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THE GENETIC
SONOGRAM |
Ultrasound cannot be used to diagnose or exclude chromosomal aneuploidy or Down syndrome:
- It is only a non invasive method of altering a patients priori risk (based on maternal age and MSS) by assessing a variety of sonographic signs and their likelihood ratios.
- The likelihood ratios adjust the priori risk either decreasing the risk (normal scan) or increasing the risk (presence of markers).
- Need your own local LR based on the frequency of a marker in the population and the risk of aneuploidy.
- This allows the patient the balance the risk of pregnancy loss due to invasive testing versus the risk of aneuploidy in that fetus.
- The performance of an amniocentesis for maternal age alone is “probably not” indicated in 2003.
Down Syndrome:
- Structural abnormalities that have clinical consequences despite the abnormal karyotype:
- Endocardial cushion defects.
- Duodenal atresia.
- Ventriculomegaly / Borderline ventriculomegaly..
- Cystic hygroma.
- Fetal hydrops.
- Markers or traits that have no serious structural consequence except their relationship to aneuploidy:
· Although each marker has a sensitivity for Down syndrome, most are found frequently as an isolated finding in normal fetuses.
· Many are transient and resolve in the third trimester.
· What are their roles in the low risk patient????? controversial.
· In high-risk patients, they form the basis for the genetic sonogram as they can increase the risk or likelihood ratio if a marker is present, but can also lower the risk in their absence.
|
Marker |
All Cases |
Isolated Marker |
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|
DS (%) |
Control (%) |
LR (%) |
DS (%) |
Control (%) |
LR (%) |
|
|
Nuchal fold –
6mm or greater |
42.3 |
0.5 |
94.7 (30.2-296.7) |
3.7 |
0 |
NC |
|
Nuchal fold
–
5 mm or greater |
47 |
0.8 |
61.6 (25.3-149.7) |
ND |
ND |
ND |
|
Short humerus |
48.7 |
2.1 |
23.5 (13.1 – 42.1) |
2 |
0.4 |
5.8 |
|
Short femur |
53.7 |
5.3 |
10.1 (7.1 – 14.3) |
4.3 |
3.7 |
1.2 (0.51 – 2.7) |
|
Hyperechogenic
bowel |
13 |
0.9 |
14.4 (5.4 – 38.1) |
0 |
0.7 |
NC |
|
EIF |
34.2 |
4.3 |
8 (4.8-13.3) |
5.4 |
3.8 |
1.4 (0.6 – 4.3) |
|
Pyelectasis |
21.3 |
2.4 |
8.8 (5.0 – 15.4) |
3.1 |
2.0 |
1.5 (0.6 – 4.3) |
|
Anomaly |
26.8 |
1.2 |
22 (10.6 – 45.8) |
3.1 |
0.9 |
3.3 (1.0 – 10.8) |
|
Any marker |
80.5 |
12.4 |
6.5 (5.3 – 8.1) |
ND |
ND |
ND |
|
No abnormality |
19.5 |
87.7 |
0.22 (0.16 – 0.3) |
ND |
ND |
ND |
- Normal scan – can reduce the risk of DS in high-risk patients.
- 80.5% of fetuses with DS had at least one sonographic marker (compared with 12.4% of control fetuses).
- 19.5% of fetuses with DS had a normal scan (no markers) compared with 87.7% of controls).
- Normal scan had a LR ratio of 0.2 (80% reduction in the priori risk of carrying a fetus with DS).
- Negative likelihood ratio:
- Nyberg et.al. 0.36 for DS
- DeVore 0.11 – 0.42 (depending on which sonographic marker is used).
- Smith-Bindman et.al. disagreed and reported that the negative LR was not low enough to decrease the risk of a fetus having DS.
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Priori risk of carrying a fetus with DS (based on
maternal risk and serum screening) X LR of the presence of a sonographic finding = New risk estimate for having a fetus with DS |
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|
Patients Age (yrs) |
Risk of DS based on maternal age |
Pyelectasis |
EIF |
Echogenic bowel |
Humerus |
Nuchal Thickening |
LR |
|
LR = 1.5 |
LR = 2 |
LR = 6.1 |
LR = 5.2 |
LR = 15.5 |
|
20 |
1/1521 |
1/1014 |
1/760 |
1/250 |
1/295 |
1/98 |
|
30 |
1/910 |
1/606 |
1/380 |
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1/58 |
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35 |
1/385 |
1/256 |
1/178 |
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1/24 |
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40 |
1/113 |
1/75 |
1/57 |
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1/7.2 |
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Relationship
between maternal age and the presence of ultrasound markers |
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REFERENCES |