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Dx: Massive Hydronephrosis Causes False-Positive FAST Exam

altThe FAST exam done during the initial evaluation shows a large amount of anechoic fluid in the left upper quadrant (LUQ) in the area of the splenorenal space (fig. 2). The remainder of the FAST exam was normal. This fluid was later better delineated on CT scan to be due to hydronephrosis rather than due to trauma. The patient also had a chest X-ray that was unremarkable, and an X-ray of the right femur that revealed a mid-shaft femur fracture. He remained hemodynamically stable. CT scans preformed of the brain, cervical spine and chest were all normal. The abdomen/pelvis CT showed a comminuted pelvic fracture of the sacrum, a right inferior ramus fracture, and right L5 and S1 transverse process fractures. Also noted was severe left hydronephrosis extending down to the level of the bladder with severe renal cortical thinning (fig. 2). There was no intra-abdominal injury or free fluid noted. All laboratory results were unremarkable.

The patient was taken to the operating room and underwent an open reduction and internal fixation (ORIF) of his right femur. He was then transferred to a tertiary care facility for fixation of his pelvic fractures. At 3-week follow-up he was still admitted to the hospital and no further intervention was performed for his hydronephrosis.

This is a case of a patient with massive left hydronephrosis causing a false positive FAST exam. The hydronephrosis is localized to the upper pole of the left kidney, but because the renal cortex is so thin, it gives an appearance similar to a fluid collection at the splenorenal recess. Further testing showed that this patient had a duplicate collecting system in the left kidney, which was previously undiagnosed. The more superior system was obstructed causing hydronephrosis and hydroureter. This congenital abnormality is present in 0.2% of live births and has a 12% prevalence in first degree relatives. A duplicated urinary collecting system predisposes patients to obstruction, reflux and infection, though most remain asymptomatic.

Anechoic fluid detected on FAST exam, in the right clinical scenario, is suggestive for intra-abdominal bleeding. The FAST exam has become an integral part of the bedside assessment of patients with blunt or penetrating trauma. The results of this exam are often used to make critical decisions in patient management. At many institutions, a hemodynamically unstable patient with a positive FAST exam is taken to the OR for an immediate exploratory laparotomy. However, stable patients with a positive FAST exam can be usually be further evaluated with a computed tomography study.

The FAST exam is generally considered positive if there is detection of intraperitoneal fluid on any of the three abdominal windows (hepato-renal space, spleno-renal space or supra-pubic space) or if there is pericardial fluid detected on the cardiac window.  However, not all anechoic collections represent a hemoperitoneum. Fluid-filled bowel, fluid-filled stomach, ascites, free urine from an intraperitoneal bladder rupture, perinephric fat pads, subcapsular hematomas, renal cysts and fluid-filled gallbladders have all been reported as etiologies that resulted in false positive FAST studies.

Relevant to this case, distinguishing hydronephrosis from free intraperitoneal fluid, it is important to remember that free fluid in the LUQ typically lies in a dependent location forming an anechoic strip at the tip of the spleen or more commonly, between the spleen and the diaphragm since the phrenicocolic ligament restricts the amount of fluid that can collect in the splenorenal space.

On review of the literature, hydronephrosis has not been previously reported as a cause of a false positive FAST exam. Undoubtedly, a skilled ultrasonographer would be able to diagnose this anechoic collection as severe hydronephrosis. A neophyte may have more difficulty. It is imperative for the emergency physician who is incorporating the FAST exam into their trauma assessment to be able to recognize this and other causes of false positive studies to prevent unnecessary laparotomies and thereby avoid increased morbidity and mortality.

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