Purpose or Case Report: Despite being recognized as a secondary sign of appendicitis, no clear definition exists about the amount of intra-abdominal fluid considered physiologic in children. The purpose is (1) to assess the amount of intra-abdominal free fluid by US and MRI in children with suspected appendicitis, (2) to detect the performance characteristics of US for detecting free fluid, and (3) to determine the association between fluid volume and perforated appendicitis. An additional ex-vivo study was done to assess the validity of US measurement abdominal free fluid volume using phantoms.
Methods & Materials: Ex-vivo: Tissue-mimicking phantoms filled with distilled water and put inside a chicken were used to simulate abdominal fluid. In-vivo: In children with suspected appendicitis and abdominal fluid in US the formula for ellipsoid volume was used [4/3 x width x height x length x pi]. Measurements were independently performed by two readers.
Results: Ex-vivo: no difference was noted between actual phantom fluid amount and measured volume using the ellipsoid formula for both US (P=0.19) and MRI (P=0.08). In-vivo: Abdominal fluid was present in 212/591(35.9%) children with suspected appendicitis; 75/212 (35.4%) of these patients had final diagnosis of appendicitis and 77/212 (36.3%) had a final diagnosis of physiologic abdominal fluid with no appendicitis or other abdominal pathology. Good inter-rater agreement was noted between the measurements made by the two readers with an intraclass correlation coefficient of ICC = 0.79 (p<0.001, 95%CI = 0.62 - 0.88) on US and ICC = 0.75 (p<0.0001, 95%CI = 0.56 - 0.86) on MRI. In patients with proven appendicitis, US had a sensitivity of 84% (95%CI,71%-93%) and specificity of 65%(95%CI,52%–77%) for detecting fluid; the volume of fluid in this subgroup of patients ranged from 0.7 ml to 1.1 liters. No association between increasing fluid volume and perforated appendicitis (Odds ratio,1.01, P=0.01).
Conclusions: The ellipsoid volume formula is a good method for quantifying abdominal fluid. Abdominal fluid is frequent in US of patients with suspected appendicitis although it might be present in patients with no final proven diagnosis of appendicitis. There is no association between greater volume of fluid and presence of perforated appendicitis.