CT is the most common imaging modality used to evaluate adult patients with suspected appendicitis, with both high sensitivity and specificity for the diagnosis [12
]. In most cases, CT interpretation is straightforward with cases definitively falling into either positive or negative categories. When appendicitis is present, we typically find both appendiceal enlargement and secondary signs of inflammation. In normal cases, by contrast, the appendix may demonstrate a range of sizes, but no secondary signs of inflammation are usually identified. Unfortunately, as appendiceal enlargement is sometimes the only sign of appendicitis, up to 12% of cases may be equivocal at CT [4
As such, it is important for the radiologist to have a clear understanding of the size parameters of the normal appendix at CT. Similar to previous authors [9
], we found that 45% of normal appendices have outer wall-to-outer wall diameter greater than 6 mm at CT. However, our study also indicates that in the majority of cases (77%), the appendix had a recognizably normal morphology, either predominant luminal air or enteric contrast. Only 6.6% of normal appendices measured greater than 6 mm and also had a CT appearance that was ambiguous enough to be mistaken for appendicitis at CT, in other words, an isodense, collapsed appearance, or an airless fluid-filled lumen. None had diameter greater than 10 mm in combination with an ambiguous or equivocal morphology.
The pathophysiology of acute appendicitis usually involves appendiceal obstruction with continued mucinous fluid secretion and bacterial proliferation within the lumen of the appendix [23
]. As such, a fluid- or mucus-filled appendix is recognized as a sign of acute appendicitis at CT [24
]. To our knowledge, the incidence of this appearance in a normal population, however, has not been previously reported. In this study, only 0.67% or one of 150 of control patients demonstrated airless fluid in the appendiceal lumen. A recent study by Moteki and Horikoshi [24
] found that depth of the intraluminal appendiceal fluid greater than 2.6 mm had high sensitivity and specificity (>80%) for diagnosis of acute appendicitis. In our single normal appendix with a fluid-filled lumen, the maximal depth was only 2 mm. In clinical practice, airless fluid is at least occasionally identified in the appendiceal lumen in patients with no clinical evidence for appendicitis, particularly in the setting of a fluid-filled cecum. However, given the rarity of this appearance in the normal appendix and its known association with acute appendicitis, the finding of a fluid-filled appendix with diameter greater than 6 mm should prompt a clinical work up for appendicitis (including blood work analysis and surgical evaluation), even in the absence of secondary signs of inflammation at CT. If the diagnosis remains unclear following clinical correlation, patient observation and short-interval follow-up CT may be appropriate.
Our study has several limitations. The primary limitation is the lack of a standard of reference for proof of a normal appendix. However, pathological correlation is unavailable in this population, as none of the patients included in our study had any clinical evidence of acute appendicitis or colonic disease. Furthermore, the outer wall-to-outer wall diameter of the appendix in our study is concordant with values previously reported in the CT literature for normal appendices [9
]. Because the focus of our paper is on CT appearance of normal appendices, we also did not include a group of patients with clinically proven appendicitis to evaluate how many might have ambiguous CT findings of isodense or fluid-filled appendices. Another limitation is the retrospective nature of our study. Due to retrospective data collection, all patients were not scanned with intravenous contrast. It is possible that luminal contents may have been identified in more patients with an isodense-appearing appendix if intravenous contrast was administered to all patients. However, there was no statistical difference in the proportion of isodense appendices versus those with visible luminal contents between cases performed with versus without intravenous contrast.
In summary, our study found that while the outer wall-to-outer wall diameter of the normal appendix is frequently greater than 6 mm, none had diameter greater than 10 mm in combination with equivocal morphology. Furthermore, in the normal appendix, airless fluid within the lumen is rarely seen with a prevalence of less than 1%. Therefore, a fluid-filled appendix should alert radiologists and clinicians for the possibility of appendicitis, even in the absence of secondary signs of inflammation at CT. While appendicitis could undoubtedly occur in an isodense appendix measuring between 6 and 10 mm in diameter, such an equivocal appearance can occur in up to 6.6% of the normal population.