22.3 Cul-de-Sac, Vagina & Pelvic Floor

The Cul-de-Sac

The cul-de-sac — the rectouterine pouch, or pouch of Douglas — is the most dependent recess of the peritoneal cavity in an upright woman, so free intraperitoneal fluid collects here first and is readily seen on transvaginal scanning behind the cervix and upper vagina. A small amount of simple, anechoic fluid is physiologic at virtually any point in the menstrual cycle and is expected just after ovulation. Fluid becomes pathologic when it is large in volume, contains internal echoes, septations, or debris, or is accompanied by symptoms. Echogenic fluid implies blood or pus: hemoperitoneum from a ruptured hemorrhagic cyst or a ruptured ectopic pregnancy, purulent fluid from pelvic inflammatory disease, or particulate ascites from malignancy or cirrhosis. When significant pelvic fluid is found, extend the survey into the upper abdomen — the paracolic gutters and Morison pouch — to gauge the total burden and to search for a source.

The Vagina

On translabial or transvaginal imaging, the vagina is a collapsed, roughly H-shaped muscular tube posterior to the bladder and urethra and anterior to the rectum. After hysterectomy the vaginal cuff is routinely measured in the midline sagittal plane; a normal cuff should be under about 2.1 cm in anteroposterior dimension, and an enlarging cuff or a discrete cuff mass in a patient treated for malignancy raises concern for recurrent tumor and should be correlated with biopsy. A Gartner duct cyst is a remnant of the mesonephric (Wolffian) duct and appears as a simple anechoic cyst in the anterolateral vaginal wall — usually an incidental finding. A retained foreign body such as a tampon appears as an echogenic structure with posterior shadowing and can explain discharge or infection. In a pediatric patient with primary amenorrhea and cyclical pain, look for obstructive anomalies (imperforate hymen or a transverse vaginal septum) producing hematocolpos — a blood-distended vagina — or hematometrocolpos when the uterus is also obstructed.

Pelvic Varices and Congestion Syndrome

Pelvic congestion syndrome is a cause of chronic pelvic pain lasting more than six months, typically in multiparous premenopausal women, produced by valvular incompetence and reflux in the ovarian and internal iliac veins. Sonographic criteria include:

• Multiple tortuous periuterine and parauterine veins dilated beyond 5-6 mm
• Slow or bidirectional flow on spectral Doppler
• Reversed (caudad) flow that augments with the Valsalva maneuver

The left ovarian vein is usually the primary reflux source because it drains into the left renal vein, where it can be compressed between the aorta and the superior mesenteric artery (the nutcracker mechanism). Scanning with color Doppler during Valsalva, or with the patient upright, improves detection of the reflux.

Pelvic Floor Basics

The levator ani complex — puborectalis, pubococcygeus, and iliococcygeus — forms the muscular pelvic diaphragm and surrounds the levator hiatus, through which the urethra, vagina, and rectum pass. Translabial (perineal) ultrasound with 3D/4D volume acquisition evaluates the hiatus at rest, during pelvic floor contraction, and during Valsalva. The key assessments are:

A urethral diverticulum appears as a cystic or complex periurethral structure, often wrapping partly around the urethra and sometimes containing debris or calculi; patients classically report post-void dribbling, dysuria, and dyspareunia.

Peritoneal Inclusion Cysts and Para-Ovarian Cysts

A peritoneal inclusion cyst occurs in premenopausal women with peritoneal adhesions from prior surgery, endometriosis, or pelvic inflammatory disease. The normally functioning ovary keeps producing fluid, and the adhesions trap it; the result is a large, loculated fluid collection that conforms to peritoneal contours with the ovary suspended inside it or at its margin — the spider-in-a-web appearance. The ovary itself is normal, and mistaking the collection for a cystic ovarian neoplasm leads to unnecessary surgery. Para-ovarian cysts arise within the broad ligament from mesothelial or paramesonephric (Müllerian or Wolffian remnant) tissue and account for roughly 10% of adnexal masses. The decisive maneuver is demonstrating a normal ipsilateral ovary separate from the cyst — gentle probe pressure may displace one structure away from the other. The hydatid of Morgagni is a small, pedunculated para-ovarian cyst attached to the fimbrial end of the tube and is almost always incidental.

Technique and Documentation

For the cul-de-sac, document the greatest fluid depth behind the cervix and describe the echogenicity of any fluid. Measure the vaginal cuff in the AP dimension on a true midline sagittal view. For suspected varices, sweep transversely through the periuterine tissues with color Doppler at a low velocity scale, measure the largest vein, and record flow direction with and without Valsalva. Pelvic floor studies are performed translabially with the bladder empty or nearly empty, acquiring volumes at rest, at maximum squeeze, and at maximum Valsalva so that hiatal area and organ descent can be measured against the symphysis pubis reference line.