In a high proportion of fetuses with trisomy 21 and other chromosomal abnormalities the nasal bone is hypoplastic or not visible at 11-13 weeks' gestation. Assessment of the nasal bone at 11-13 weeks improves the performance of combined screening for trisomy 21 by maternal age, fetal nuchal translucency (NT) and serum biochemistry. The difficulty is when the gestation is 11 weeks or the beginning of the 12th week and the nasal bone is absent but the NT, the other ultrasound markers and the serum biochemistry are normal. Our advice is that in such cases the scan should be repeated in one week and if there is persistence of absence of the nasal bone then the risk for trisomies is increased.
Requirements for certification
The requirements for obtaining the FMF certificate of competence in assessment of the nasal bone are:
- Attendance of the internet based course on the 11-13 weeks scan.
- FMF certification in measurement of nuchal translucency.
- Successful completion of a short online test in which you will be asked to examine images on assessment of the nasal bone.
- Submission of a logbook of 3 images showing correct assessment of the nasal bone.
Protocol for assessment
- The gestational period must be 11 to 13 weeks and six days.
- The magnification of the image should be such that the fetal head and thorax occupy the whole image.
- A mid-sagittal view of the face should be obtained. This is defined by the presence of the echogenic tip of the nose and rectangular shape of the palate anteriorly, the translucent diencephalon in the centre and the nuchal membrane posteriorly. Minor deviations from the exact midline plane would cause non-visualization of the tip of the nose and visibility of the maxilla.
- The ultrasound transducer should be held parallel to the direction of the nose and should be gently tilted from side to side to ensure that the nasal bone is seen separate from the nasal skin.
- The echogenicity of the nasal bone should be greater that the skin overlying it. In this respect, the correct view of the nasal bone should demonstrate three distinct lines: the first two lines, which are proximal to the forehead, are horizontal and parallel to each other, resembling an "equal sign". The top line represents the skin and bottom one, which is thicker and more echogenic than the overlying skin, represents the nasal bone. A third line, almost in continuity with the skin, but at a higher level, represents the tip of the nose.
- When the nasal bone line appears as a thin line, less echogenic than the overlying skin, it suggests that the nasal bone is not yet ossified, and it is therefore classified as being absent.
Quality review and ongoing certification
- Sonographers who have obtained the Certificate of competence in the assessment of the nasal bone need to pass an audit on an annual basis.
- The audit involves (a) successful completion of a short online test in which you will be asked to examine images on assessment of the nasal bone and (b) submission of a logbook of 3 images showing correct assessment of the nasal bone.
- The date and time of the scan should always be clearly visible on each submitted image. If possible, please also include the name of your hospital. Cases older than the last audit will not be accepted.
- Sonographers who pass their audit will receive renewal of their licence for one year and their name will be published on the FMF website list of holders of the Certificate of competence in assessment of the nasal bone.
- Sonographers who fail their audit will not receive renewal of their licence and their name will be removed from the FMF website list of holders of the Certificate of competence in assessment of the nasal bone.