Sonographic detection of subtle pelvic fluid collections.

The sonographic demonstration of small quantities of free intraperitoneal fluid often indicates significant pelvic pathology. In a review of pelvic fluid collections in 146 female patients, however, it became apparent that an overly distended urinary bladder may mask small quantities of free intraperitoneal fluid. The "mass effect" of a distended bladder may cause fluid in the pouch of Douglas to migrate to other parts of the peritoneal cavity, such as the peritoneal reflection over the fundus of the uterus. Fluid in this location produces a characteristic triangular "cap" and was present in 42 patients (29% of the study group). In 10 patients (6.9%) this was the only visible fluid collection. In addition, sonograms obtained after partial voiding demonstrated small quantities of free pelvic fluid in 14 patients (9.6% of the study group) that were not detected on routine full bladder scans. The sonographic appearance of small amounts of intraperitoneal fluid seen over the uterine fundus and the value of post-void scans are stressed in the demonstration of small quantities of intraperitoneal fluid.