Diagnostic ultrasound ( PDFDrive )

1496 PART IV Obstetric and Fetal Sonography
fetal membranes in the absence of uterine activity and cervical
change), or cervical incompetence (cervical dilation in the
absence of uterine activity). 2,9,10 Cervical incompetence can be
further deined as either a mechanical failure of the cervix to
remain closed against the increasing intrauterine expansion or
as a functional failure, with premature activation of the events
of cervical ripening (dilation and efacement) that normally occur
at term. 9 Although categorized in this manner, spontaneous
premature delivery is best described along a continuum of biologic
events leading to early delivery, given that the biochemical
mediators that efect uterine contractions, fetal membrane disruption,
and cervical ripening are similar: prostaglandins, the
metallomatrix proteases and their inhibitors, and the families
of proinlammatory and antiinlammatory cytokines. 2,10 As such,
the events of prematurity oten overlap; in particular, cervical
change can precede fetal membrane rupture and uterine contractions,
although the cervix may not be functionally or mechanically
incompetent.
Evaluation of the cervix has been used as a tool to predict
SPTB based on the concept that the cervix acts as an anatomic
marker of the underlying pathologic processes leading to a
premature delivery. Digital examination of the cervix measures
only the length from the external cervical os to the cervical-vaginal
junction, not the intrapelvic cervical-isthmus portion of the
cervix. herefore digital examination underestimates cervical
length by a mean diference of 12 mm in more than 80% of
women in the second and third trimesters compared with
sonography. 11 Moreover, dilation of the cervix starts proximally
so the distal portion may still appear normal on digital exam,
even when the cervix has started to shorten and/or dilate. he
current approach to evaluation of the cervix is by ultrasound,
preferably via the transvaginal route. his chapter reviews the
techniques of uterine cervix sonography and its relationship with
PTB prediction.
FIG. 44.1 Normal Cervix. TAS full-bladder technique. Longitudinal
midline image of the cervix. The cervical canal is seen from the internal
os (arrowhead) to the external os (arrow). B, Bladder.
B
SONOGRAPHY OF THE UTERINE
CERVIX
here are three approaches to scanning the cervix: transabdominal
sonography (TAS), transvaginal sonography (TVS) and transperineal
(translabial) sonography. Each approach has advantages
and limitations for diferent clinical scenarios. he closed length
of the cervix is the single most important parameter to report,
as it is most closely linked to the risk of PTB. Additional comments
include any evidence of funneling.
Transabdominal Approach
TAS of the cervix is typically performed during the standard
second- and third-trimester obstetric ultrasound examinations
and is used as a routine screening tool for measurement of closed
cervical length.
Longitudinal scanning is initiated in the midline of the lower
abdomen, just above the symphysis pubis, using a transducer
with a frequency of 3 MHz or higher. 12,13 Landmarks include the
internal cervical os, the external cervical os, the outline of the
cervical canal, and the outline of the cervical corpus 14 (Fig. 44.1).
FIG. 44.2 Normal Cervix. TAS empty-bladder technique. Longitudinal
midline image of the cervix obtained by scanning through the amniotic
luid. The cervical canal is indicated by calipers.
Measurement of cervical length is afected by overdistention of
the urinary bladder. Increased bladder pressure can compress
the lower uterine segment and falsely elongate the cervix or
mask cervical dilation. 12 Cervices less than 2 cm in length cannot
be easily visualized against the vaginal and bladder tissue. 12
In the second trimester, if the urinary bladder is empty,
amniotic luid can be used as an acoustic window to scan the
cervix. Longitudinal scans are obtained with the transducer angled
downward from just below the umbilicus. he cervix may assume
a more vertical orientation and appear bulkier (Fig. 44.2) when
the bladder is empty. Diiculty in identifying the external os
can contribute to error in cervical length measurement.
Visualization of the cervix can be diicult because of large
maternal habitus or an engaged position of the fetal head. 12,13,15
Regardless of the gestational age, there is relatively poor