Pouch of Douglas (Rectouterine Pouch) — Anatomy and Clinical Relevance

The pouch of Douglas — also called the rectouterine pouch, posterior cul-de-sac or rectovaginal pouch — is a deep, blind-ended extension of the peritoneal cavity that dips down between the uterus in front and the rectum behind. It is named after the Scottish anatomist James Douglas (1675–1742).

It forms because the pelvic peritoneum drapes over the pelvic organs like a sheet over furniture. In the midline, peritoneum runs down the back of the uterus and cervix, onto the upper posterior wall of the vagina next to the posterior fornix, and then turns back up onto the front and sides of the rectum. The fold it creates between uterus and rectum is the pouch of Douglas. In front of the uterus, a second, much shallower vesicouterine pouch lies between the uterus and the bladder.

The uterosacral ligaments are condensations of pelvic fascia running from the cervix to the posterolateral pelvic wall; the sharp, sickle-shaped rectouterine folds are the peritoneum lying over them. Together they form the side-walls of the pouch — which is why pain from the pouch, and tenderness on moving the cervix, are so characteristic of pelvic peritonitis and endometriosis.

Because it is the lowest recess of the peritoneal cavity in the pelvis, gravity drains free intraperitoneal fluid into it. In a woman who is sitting or standing, the pouch of Douglas is unquestionably the lowest point of the whole abdominopelvic cavity. It is also the most dependent part of the pelvis in the supine position, which is why the pelvic window of a trauma ultrasound looks there.

A small amount of free fluid in the pouch of Douglas is physiological in women of reproductive age — it accumulates around ovulation and during menstruation. Larger, echogenic (blood-containing) or complex collections are pathological.

The pouch of Douglas cannot be palpated through the abdomen, but it lies just behind the posterior vaginal fornix and in front of the rectum, so it can be felt on per-vaginal (bimanual) and per-rectal examination, sampled with a needle through the fornix, and imaged well with transvaginal ultrasound.

1. Culdocentesis — the cervix is steadied with a tenaculum and an 18-gauge spinal needle is passed through the posterior vaginal fornix, about 1 cm below the cervix, into the pouch of Douglas while aspirating.
2. Positive tap: non-clotting blood — blood that has pooled in the peritoneal cavity has been defibrinated, so it no longer clots. This indicates haemoperitoneum (classically a ruptured ectopic pregnancy).
3. Non-diagnostic tap: clotted blood (the needle probably entered a vessel) or no fluid at all. Serous fluid is a negative result.
4. Contraindication: a mass or a retroverted uterus occupying the pouch of Douglas.

In trauma, the FAST examination has four windows — right upper quadrant (hepatorenal recess), left upper quadrant (splenorenal recess), subxiphoid (pericardium) and suprapubic/pelvic. The pelvic window looks for fluid in the rectovesical pouch in men and the rectouterine and vesicouterine pouches in women. A full bladder acts as an acoustic window, and a few hundred mL of free intraperitoneal fluid is usually needed before ultrasound reliably picks it up.

Endometriosis deserves a closer look because it is asked every year. The ovaries are the most common site, followed by the posterior broad ligament, anterior and posterior cul-de-sac and the uterosacral ligaments. The classic triad is chronic pelvic pain/dysmenorrhoea, dyspareunia and infertility, and the classic examination is a fixed, retroverted uterus with tender nodularity of the uterosacral ligaments or cul-de-sac. Transvaginal ultrasound is the first-line imaging test; the definitive diagnosis rests on histology of lesions obtained at laparoscopy.

Surgery and procedures. A pelvic abscess in the pouch of Douglas can be drained through the vagina (posterior colpotomy) or the rectum without opening the abdomen, and image-guided transvaginal or transrectal drainage is usually successful. At vaginal hysterectomy, surgeons may approximate the uterosacral ligaments to close off (obliterate) the cul-de-sac to treat or prevent an enterocele. In peritoneal dialysis, the catheter tip is placed in this dependent space.

• Normal: the pouch is empty and impalpable; bowel slides freely behind the uterus.
• Tenderness / fullness in the posterior fornix — blood or pus in the pouch (ectopic pregnancy, PID, pelvic abscess). Cervical motion tenderness reflects irritated pelvic peritoneum.
• Nodularity of the uterosacral ligaments with a fixed retroverted uterus — endometriosis.
• Hard, shelf-like mass above the prostate or cervix felt through the anterior rectal wall — Blumer's shelf (peritoneal metastases).
• A mass occupying the pouch (e.g. retroverted gravid or fibroid uterus, ovarian tumour) — also the reason culdocentesis is contraindicated in that situation.

Examiners rarely ask 'define the pouch of Douglas'. Instead the pouch is the clue inside a clinical stem. Recognise the pattern and the answer follows.