Intraperitoneal fluid in children: normal ultrasound findings depend on which scan head you use

The detection of small quantities of unsuspected free intraperitoneal fluid (IPF) at US examination in children has increased markedly in our department since linear high-frequency probes have been used routinely for gastrointestinal examinations. Its clinical significance is unknown. OBJECTIVE. This study attempted to evaluate the sonographic detection, quantification and location of IPF in an unselected paediatric population using both convex or sector scan heads and a linear US probe in all to compare the results obtained using different probes.

We prospectively scanned 272 unselected consecutive patients using a 7- or 5-MHz multifrequency sector or convex scan head and a 7-MHz linear US probe. IPF was searched for and recorded as 0=none, 1=minimal, 2=small, 3=moderate and 4=large amounts in the cul-de-sac, Morison's pouch and between bowel loops. The patient's position on arrival, indication for the sonogram, clinical expectation of IPF and result of the US examination were recorded until 100 children (46 girls, 54 boys; age 3 days to 15 years) with IPF had been seen. Statistical analysis was done comparing all variables one by one, two by two and three by three.

Of the 100 children with IPF, its presence was expected in 7 and clinically explainable in 5 others. Nine of these had moderate-to-large amounts of IPF, seen well with all US scan heads. In the other 88 asymptomatic subjects, the detection of unsuspected IPF varied significantly with different probes. Fluid in the cul-de-sac was detected with similar frequency with all probes, whereas fluid between bowel loops was seen much more frequently with the linear probe ( P>0.0000). Position on arrival, age and sex had no influence.

Minimal and small amounts of IPF can be seen between bowel loops in up to 22% of normal children. In this location, it is often seen using a linear scan head only. In the cul-de-sac, a sector scan probe is equally as able to detect IPF as a linear probe. When scanning the bowel with a linear high-frequency US probe, small pockets of mobile anechoic IPF should be considered a normal finding.