Figure 1: Image shows a flat external fundal contour (< 1 cm fundal cleft), with a fibrous septum separating the two uterine horns.

Figure 2: Coronal MRI T2-weighted HASTE image of the pelvis.

Septate uterus. (See companion case)

Müllerian duct anomalies are a rare (0.1-0.5%) but are often treatable cause of infertility. Patients with müllerian duct anomalies are known to have a higher incidence of infertility, repeated first trimester spontaneous abortions, fetal intrauterine growth retardation, fetal malposition, preterm labor, and retained placenta. There is a 20-25% association with renal anomalies. The two paired müllerian ducts in the embryo ultimately develop into the fallopian tubes, uterus, cervix and upper two thirds of the vagina in the female. The ovaries and lower one third of the vagina have separate embryologic development. The development of the müllerian ducts depend on the proper completion of organogenesis, fusion, and septal resorption. Failure of any of these processes can lead to anomalies of the female reproductive tract.

A septate uterus is formed as a result of failure of resorption of the septum between the two uterine horns. The septum can be partial or complete, in which case it extends to the internal cervical os. The other diagnostic consideration is a bicornuate uterus which results from partial non-fusion of the Müllerian ducts. In the septate uterus, the un-resorbed septum is predominantly composed of fibrous tissue and demonstrates low signal intensity on T2-weighted images. However, the composition of the septum can vary between the muscle and fibrous tissue and therefore is an unreliable imaging sign for distinguishing between a septate uterus and other uterine anomalies such as a bicornuate uterus. The uterine outer fundal contour is typically convex, flat, or minimally indented (<1 cm fundal cleft). There is a normal intercornual distance (2-4 cm) and each uterine cavity is usually small.

The most important imaging finding is a concave fundus with a fundal cleft > 1 cm. Examining the uterine outer fundal contour has been shown to be a more reliable means of distinguishing bicornuate uterus from septate uterus than determining the composition of the septum (fibrous vs. myometrial tissue). In the bicornuate uterus, an increased intercornual distance (>4 cm) may also be observed. The tissue separating the two horns demonstrates signal intensity identical to myometrium on all pulse sequences. However, the inferior portion of the septum which can extend for a variable length inferiorly may be fibrous, with low T1 and T2 signal intensity. Differentiation between a septate and a bicornuate uterus is important because septate uteri are treated with transvaginal hysteroscopic resection of the septum, while if surgery is possible and/or indicated for the bicornuate uterus, an abdominal approach is required for metroplasty.

References:

1. Dunnick NR, Sandler CM, Newhouse JH, Amis ES. Textbook of Uroradiology, 3rd Edition. Philadelphia: Lippincott Williams & Wilkins, c2000. PP.45-48, 66-67.
2. Thurmond, Amy S. Imaging of Female Infertility. Radiologic Clinics of North America. Women’s Imaging: Obstretics & Gynecology. 2003 July; 41: 757-767.
3. Syed, I.S., Hussain, H., Weadock, W., and Ellis, J. Uterus, Mullerian Duct Abnormalities. (2002). eMedicine (08/2/03).

Location and Date of Scan: New York University Medical Center, New York, Sep 2003.