PITFALLS IN THE DIAGNOSIS OF INCOMPETENT CERVIX

1. Pseudodilatation ("false hourglass membranes").
• Occurs a result of contractions involving the lower uterine segment, which has the appearance of dilatation of the endocervical canal.
• Suspect when the length of the cervical canal exceeds the maximum expected length of 5 cm when the maternal bladder is empty (1).
• The increase in length is due to incorporation of the lower uterine segment as part of the cervical length measurement.
• The normal cervix is caudal with respect to the pseudodilatation, with the pseudodilatation cephalad with respect to the angle of the bladder.
• Transient phenomenon.

Pseudodilation of the Cervix:
Myometrial contraction of the anterior and posterior wall of the lower segment (arrows on the left); Normal internal Os and canal (short arrows on the right); (p = Placenta)

2. Distended Bladder.
1. True cervical dilatation can be masked by a distended bladder that compresses the cervix and obliterates the fluid that is present in the cervix from the incompetence that is present. Transvaginal or translabial scans are recommended in high-risk patients as mild dilatation can be obscured by a distended urinary bladder (2).
• A full bladder may result in elongation of the cervical canal. In the example below a 2.6 cm cervix is elongated to 4.2 cm by a full urinary bladder.

3. Transient dilatation of the cervical canal (in some patients) (3).

4. Changes in uterine pressure (secondary to contractions or polyhydramnios) can alter the appearance of the internal os.

REFERENCES

1. Karis JP, Hertzberg BS, Bowie JD. Sonographic diagnosis of premature cervical dilatation: potential pitfall due to lower uterine segment contractions. J Ultrasound Med 1991;10:83-87.
2. Harris RD, Barth RA. Sonography of the gravid uterus and placenta: Current concepts. AJR 1993;160:455-465.
3. Parulekar SG, Kiwi R. Dynamic incompetent cervix uteri. J Ultrasound Med 1988;7:481-485.