What Causes Free Peritoneal Fluid?

Patient Presentation
A pediatric resident was telling some other residents during their continuity clinic about some of the interesting radiology cases he had seen the previous week.
“Seems like I had a lot of abdominal cases. There was a CT case with blunt abdominal trauma where the spleen was ruptured and they had to take him to the operating room.
We had a teenage girl with pelvic pain who had a ruptured ovarian follicle and free fluid. And then there was the 4-year-old with nephrotic syndrome who had bad ascites.
I have a lot of good images to show in our pediatric case conference so everyone can learn.”

Peritoneal fluid is normal. It decreases the friction of the peritoneum covering abdominal and pelvic organs and helps to protect them and allow their movement. A normal amount of peritoneal fluid is expected on radiological evaluation. Increased peritoneal fluid is a continuum and is concerning as a wide variety of pathological causes are associated with it such as abdominal trauma and appendicitis. At the far end of the scale is ascites that is the accumulation of free fluid more than 25 ml. It is usually associated with abdominal distension but fluid must accumulate before distension can occur and therefore it may be diagnosed before distension.

One prospective study of prepubertal healthy children found the normal volume of free peritoneal fluid had a mean and standard deviation of 4.7 +/- 5.65 mL for females and 1.9 +/- 3.11 mL for males. Maximum volume was 25 mL for females and 17 mL for males. Fifteen percent of females and 3% of males had more than 10 mL of fluid. There are also normal variations with menstrual cycles in women.

Abdominal trauma is an obvious cause of increased free peritoneal fluid and can include blood or other abdominal organ fluids. Usually there is a trauma history, but some intra-abdominal injuries can be difficult to diagnose and may occur after the acute injury and therefore are unrecognized immediately. There is a mortality rate as high as 8.5% with abdominal trauma. With blunt trauma, the spleen is the most common organ injured followed by the liver and pancreas. Bowel perforation can occur acutely or a few days later due to bowel compression and possible devascularization of the mesentery. Seat belt injuries are common causes of small bowel injuries particularly the jejunum. Free fluid and free air on radiologic studies are red flags for a surgical abdomen.

Free fluid is very common in appendicitis and occurs in up to 90% in some studies.

Ascites usually is caused by chronic diseases especially of the hepatic system, but also the cardiac and renal system or multiple organ systems. Ascites is less common in the pediatric age group as cardiac and liver disease are less common. However, nephrotic syndrome is a common cause of ascites.

A history of trauma, abdominal or pelvic pain, and abdominal distension are common reasons for radiographic evaluation. Computed tomography and/or abdominal ultrasound are used to assess for intra-abdominal pathology, free peritoneal fluid and free air. Management obviously depends on the history and cause. Blunt abdominal trauma is often treated conservatively but patients who are hemodynamically unstable or have free air are usually surgically explored as well as these with penetrating trauma. Patients with appendicitis or intra-abdominal abscess may be treated surgically. Management of medical pathology depends on the acute or chronic cause and secondary problems.

Learning Point
The differential diagnosis of free peritoneal fluid includes:

  • Appendicitis
  • Trauma – Solid organ
    • Spleen
    • Liver
    • Pancreatic injuries
    • Renal
  • Trauma – hollow viscous
    • Appendix
    • Bowel
    • Gall bladder
    • Meckel’s diverticulum
  • Genitourinary
    • Ectopic pregnancy
    • Tubo-ovarian abscess rupture
    • Follicular cyst rupture
    • Ovarian torsion
  • Infectious/inflammatory
    • Perihepatitis
    • Pelvic inflammatory disease
  • Mid-gut torsion

The differential diagnosis of ascites includes:

  • Liver
    • Presinusoidal
      • Portal vein thrombosis
    • Sinusoidal
      • Cirrhosis
      • Liver failure
      • Vitamin A toxicity
    • Post sinusoidal
      • Budd-Chiari syndrome
      • Cardiac
    • Cardiac
      • Congestive heart failure
      • Pericarditis
      • Venoocclusive disease
    • Renal
      • Hepatorenal syndrome
      • Nephrotic syndrome
      • Dialysis
    • Neoplasms
      • Hepatocellular cancer
      • Lymphoma
      • Others
    • Infectious Disease
      • Tuberculosis
      • Viral hepatitis
      • Sepsis
      • Whipple disease
    • Inflammatory
      • Allergic
        • Eosinophilic gastroenteritis
      • Chemical
      • Systemic lupus erythematosus
      • Vasculitis
    • Miscellaneous
      • Protein losing enteropathy
      • Thoracic duct obstruction
    • Fetal ascites
      • Hydrops fetalis
      • Genetic disorders
      • Prune Belly syndrome

Questions for Further Discussion
1. What are the causes of abdominal pain? A review can be found here
2. What is the ROME criteria? A review can be found here
3. What causes pelvic pain? A review can be found here
4. What causes abdominal distension? A review can be found here
5. What are indications for computed tomography or ultrasound for potential abdominal pathology?

Related Cases

To Learn More
To view pediatric review articles on this topic from the past year check PubMed.

Evidence-based medicine information on this topic can be found at SearchingPediatrics.com and the Cochrane Database of Systematic Reviews.

Information prescriptions for patients can be found at MedlinePlus for this topic: Peritoneal Disorders

To view current news articles on this topic check Google News.

To view images related to this topic check Google Images.

To view videos related to this topic check YouTube Videos.

Hou W, Sanyal AJ. Ascites: diagnosis and management. Med Clin North Am. 2009;93(4):801-817, vii. doi:10.1016/j.mcna.2009.03.007

Lynch T, Kilgar J, Al Shibli A. Pediatric Abdominal Trauma. Curr Pediatr Rev. 2018;14(1):59-63. doi:10.2174/1573396313666170815100547

Held JM, McEvoy CS, Auten JD, Foster SL, Ricca RL. The non-visualized appendix and secondary signs on ultrasound for pediatric appendicitis in the community hospital setting. Pediatr Surg Int. 2018;34(12):1287-1292. doi:10.1007/s00383-018-4350-1

Stengel D, Leisterer J, Ferrada P, Ekkernkamp A, Mutze S, Hoenning A. Point-of-care ultrasonography for diagnosing thoracoabdominal injuries in patients with blunt trauma. Cochrane Database Syst Rev. 2018;12:CD012669. doi:10.1002/14651858.CD012669.pub2

Tadayoni A, Farhadi F, Mirmomen SM, et al. Evaluation of incidental pelvic fluid in relation to physiological changes in healthy pubescent children using pelvic magnetic resonance imaging. Pediatr Radiol. 2019;49(6):784-790. doi:10.1007/s00247-019-04355-y

Donna M. D’Alessandro, MD
Professor of Pediatrics, University of Iowa