Diagnostic ultrasound ( PDFDrive )

1094 PART IV Obstetric and Fetal Sonography
A
B
FIG. 31.4 First-Trimester Nasal Bone Assessment. (A) Midsagittal proile of a fetus shows the correct method of assessing the nasal bone.
Note that the nasal bone is more echogenic than the overlying skin and they form an equal (=) sign. (B) Midsagittal proile of a fetus with trisomy
21 shows the echogenic overlying skin but absent nasal bone. See also Video 31.3.
Criteria for Nasal Bone (NB) Evaluation
ALARA: Thermal Index BONE < 0.7 40
Margins of fetal anatomy clear without ambiguity in nasal
anatomy
Fetus in midsagittal plane
Fetal spine midsagittal in thoracic and cervical region
Tip of nose clearly seen in proile and the skin edge
over the nasal bridge is identiied
Care must be taken to demonstrate the skin edge
separately from the NB so that an equal (=) sign is
apparent
Third and fourth ventricle are identiied in the brain
Fetus occupies majority of image
Head, neck, and upper thorax ill image
Fetus occupies more than 50% of width and length of
image
Angle of insonation 45 degrees to fetal proile,
perpendicular to NB
Echogenicity of NB is comparable to other bony structures
and similar or brighter than the overlying skin.
Nonvisualization of the nasal bone increases with thickened
NT measurements. In fetuses with an NT below the 95th percentile,
1.6% had a nonvisualized nasal bone, compared with
2.7% for NT above the 95th percentile of 3.4 mm, 5.4% for NT
3.5 to 4.4 mm, 6% for NT 4.5 to 5.4 mm, and 15% for NT of
5.5 mm or greater. In this same study, an absent nasal bone was
seen in 69% of trisomy 21 fetuses and in 32% of fetuses with
other chromosomal defects. 54
No signiicant association exists between the biochemical
markers free β-hCG and PAPP-A and the fetal nasal bone;
therefore these can be combined to reine the irst-trimester
risk assessment for trisomy 21. 55 Cicero et al. 56 prospectively
evaluated 20,418 singleton fetuses between 11 and 14 weeks.
he fetal nasal bone was absent in 238 (1.2%), was present in
19,937 (97.6%), and could not be evaluated in 243 (1.2%). A
fetal nasal bone was absent in 113 in 20,165 (0.6%) of chromosomally
normal fetuses and in 87 in 140 (62.1%) of fetuses with
trisomy 21. he combination of NT and biochemical markers
in the irst trimester (combined irst-trimester screening) resulted
in the identiication of 90% of fetuses with trisomy 21 at an FPR
of 5%. Inclusion of the nasal bone resulted in the same rate of
detection of trisomy 21 but with the FPR decreasing to 2.5%.
hese statistics held true if all fetuses underwent screening with
a nasal bone evaluation performed at the time the NT was
measured or if the nasal bone was evaluated in a two-stage
approach in which only those at intermediate risk based on
combined screening results underwent nasal bone evaluation.
Orlandi et al. 57 used a simpliied method for assessing the presence
or absence of the nasal bone in which the nasal bone was
considered absent only if there was no line below the nasal bridge
at all. hey found that using this strategy resulted in fewer cases
being classiied as absent while maintaining a high detection
rate for aneuploidy.
Not all investigators have been as successful using the fetal
nasal bone to assess for aneuploidy. Sepulveda et al. 58 reported
on 1287 consecutive fetuses being evaluated for NT and presence
or absence of a nasal bone. Overall, 110 fetuses (8.5%) had an
NT at or above 95% and 25 (1.9%) had an absent nasal bone.
Of the 31 trisomy 21 fetuses, 28 had an NT above 95% and 13
had an absent nasal bone. he detection rate of trisomy 21 was
90.3% using NT and 41.9% using an absent nasal bone. All but
one fetus with an absent nasal bone had a thickened NT, and
only two normal fetuses had an absent nasal bone. hese authors
concluded that although an absent nasal bone is highly predictive
of trisomy 21, it is less useful as a sonographic marker than the
NT. Malone et al. 59 reported that the nasal bone evaluation was
not a useful test for population screening.
It is evident that the issues surrounding nasal bone screening
in the irst trimester are complex. he identiication of the nasal
bone as present or absent is a specialized skill that is attained