Diagnostic ultrasound ( PDFDrive )

1498 PART IV Obstetric and Fetal Sonography
Standard Technique for Cervical
Measurement
1. Place the probe in the anterior fornix of the vagina.
2. Obtain a sagittal view of the cervix, with the long-axis
view of echogenic endocervical mucosa along the
length of the canal.
3. Withdraw the probe until the image is blurred, and
reapply just enough pressure to restore the image (to
avoid excessive pressure on the cervix, which can
elongate it).
4. Enlarge the image so that the cervix occupies at least
two-thirds of the image and external and internal os are
well seen.
5. Measure the cervical length from the internal to the
external os along the endocervical canal.
6. Obtain at least three measurements, and record the
shortest best measurement in millimeters.
With permission from Berghella V, Bega G, Tolosa JE, Berghella M.
Ultrasound assessment of the cervix. Clin Obstet Gynecol. 2003;
46(4):947-962. 21
A symmetrical image of the external os should be obtained,
and the distance from the surface of the anterior lip to the cervical
canal should be equal to the distance from the surface of the
posterior lip to the cervical canal. If the anterior lip appears
thinner than the posterior lip, there may be undue pressure
placed on the cervix by the transvaginal probe. An additional
sign of excessive pressure is increased echogenicity of the cervix 21
(Fig. 44.5).
When the cervix appears curved (deviation of the central
canal of >5 mm from a straight line connecting the external and
internal os), the cervical length can be either traced or the sum
of two straight lines that follow the curve can be used. 21,28 Using
these standard criteria, the interobserver coeicient of variation
can be improved to 3.3%. 27
TVS is superior to the TAS technique. Higher-frequency
transducers and closer proximity to the structures studied allow
for better resolution. Potential complications of TVS include an
increased risk of bleeding in the presence of placenta previa,
induction of uterine activity in women with cervical shortening
caused by cervical stimulation, and chorioamnionitis in the
presence of ruptured membranes. However, increased risk of
chorioamnionitis or neonatal sepsis with TVS ater preterm
premature rupture of membranes (PPROM) has not been
demonstrated. 29,30 TVS has also been deemed safe to use in patients
with placenta previa with no increased risk for bleeding; however,
caution is advised to ensure that the probe is always carefully
inserted under real-time visualization. 31
To date, TVS assessment of cervical length by three-dimensional
sonography has been limited to the development of a normal
distribution curve of cervical length through gestation. Overall,
mean cervical length appears to be longer than the measurement
by traditional two-dimensional scanning. However, there is to
be high intra/interobserver variability. 32 Currently, there are no
reported studies of the relationship between three-dimensional
TVS and SPTB prediction.
Technical Limitations and Pitfalls
As mentioned previously, one limitation of TAS, especially in
advanced gestation, is that the cervix may be obscured by the
presenting part, in particular with a cephalic presentation. In
addition, a short cervix or empty bladder may reduce the quality
of the measurement obtained, whereas a full bladder may artiicially
elongate the cervix.
here are a number of technical limitations and pitfalls associated
with TVS of the cervix. Large maternal body habitus can
limit visualization. Bowel gas can obscure visualization.
A lower uterine segment myometrial contraction, immediately
superior to the cervix, may result in a pseudoelongation of the
cervix (Fig. 44.6). he classic tips to recognize this appearance
is the artiicially elongated length of the cervix (>5 cm), the
thicker diameter of the “cervix” at the proximal extent, which
actually incorporates the lower uterine muscle so that it appears
thicker than at the external os. he thickness of the internal and
external cervical os should be similar. Lower uterine segment
contractions are transient and rarely persist beyond 15 minutes.
A second pitfall associated with the lower uterine segment
contraction has been termed “pseudodilation” of the cervix. It
A
B
FIG. 44.5 Example of Poor Technique During TVS of Cervix. (A) Excessive pressure on the cervix with anterior lip appearing thinner than
the posterior lip causing false elongation and increased echogenicity of the cervix. (B) Removal of pressure with equal anterior and posterior lips.
Calipers measure appropriate cervical length.