From the Guidelines
Free fluid in the cul-de-sac can result from various physiological and pathological conditions, including normal physiological processes, pelvic inflammatory disease, ruptured ovarian cysts, endometriosis, ectopic pregnancy, appendicitis, malignancies, trauma, and systemic conditions causing ascites. The most common causes of free fluid in the cul-de-sac include:
- Normal physiological processes such as ovulation and menstruation, where a small amount of fluid is released or flows into the peritoneal cavity 1
- Pathological causes such as pelvic inflammatory disease (PID), ruptured ovarian cysts, endometriosis, ectopic pregnancy, particularly if ruptured, appendicitis with peritoneal irritation, and malignancies such as ovarian cancer that can produce ascites
- Trauma to pelvic organs or recent pelvic surgery may also result in blood or serous fluid collection
- Systemic conditions causing ascites, like liver cirrhosis, heart failure, or hypoalbuminemia, can lead to fluid accumulation in dependent areas including the cul-de-sac It is essential to note that the clinical significance of free fluid depends on its quantity, characteristics (serous, bloody, purulent), and associated symptoms, with small amounts of fluid (less than 10 ml) often being physiological in reproductive-age women, while larger volumes or fluid with concerning characteristics warrant further investigation 1. In cases where ectopic pregnancy is suspected, the presence of fluid in the pouch of Douglas, along with serum hCG levels, can aid in diagnosis, with a serum hCG level greater than 1,500 mIU/mL indicating ectopic pregnancy with virtual certainty in patients with sonographic abnormalities such as fluid in the pouch of Douglas or an ectopic mass 1.
From the Research
Causes of Free Fluid in the Cul-de-sac
- Ectopic pregnancy: Free fluid in the cul-de-sac can be a sign of ectopic pregnancy, especially if the patient is experiencing vaginal bleeding or lower abdominal pain 2.
- Pelvic inflammatory disease: Pelvic inflammatory disease can cause free fluid in the cul-de-sac, and it is associated with complications such as infertility, chronic pelvic pain, and ruptured tubo-ovarian abscess 3.
- Ovarian hyperstimulation syndrome: Ovarian hyperstimulation syndrome can cause free fluid in the cul-de-sac, especially in patients undergoing fertility treatment 4.
- Idiopathic theca lutein cysts: Idiopathic theca lutein cysts can cause free fluid in the cul-de-sac, although the exact mechanism is not well understood 4.
- Normal variant: A small amount of free fluid in the cul-de-sac can be a normal finding in some individuals, especially in children and pregnant women 4, 5.
Quantification of Free Fluid
- Transvaginal ultrasonography can be used to quantify the amount of free fluid in the cul-de-sac, although amounts less than 35 ml may not be visualized 6.
- The presence of more than 2-4 mm of free fluid in the cul-de-sac may not be physiologic, especially in pregnant patients without antecedent trauma 4.