Despite the high diagnostic accuracy of CT for appendicitis, numerous pitfalls exist that may result in a misdiagnosis. This pictorial review outlines the potential pitfalls in the CT diagnosis of appendicitis that includes atypical position of the appendix and coexisting pathologies. Various mimickers of appendicitis and clinical dilemmas will be highlighted. Upon completion, the reviewer should have an improved ability to recognise appendicitis mimickers and identify equivocal or atypical findings.
Acute appendicitis is the most common cause of acute abdominal pain that requires surgery in adult. Typical presentation includes right lower quadrant abdominal pain, anorexia, nausea and vomiting. Various overlapping clinical features exist between appendicitis and other diseases that may result in a clinical misdiagnosis. Atypical presentation of appendicitis due to different positions of the appendix or coexisting pathologies can also complicate the diagnosis.
Worldwide, CT and ultrasound of the abdomen are the two most commonly used imaging modalities for diagnosing acute appendicitis. The sensitivity and specificity of CT for acute appendicitis is reported between 91 and 98 and 75 and 93 percent, respectively.[4, 5] Ultrasound is widely used in paediatric and young female patients to avoid radiation exposure. MRI is an alternative modality in pregnant patients when ultrasound is equivocal.
The sensitivity and specificity of the individual CT signs of appendicitis have been studied in proven cases, with the most reliable sign being the maximal cross-sectional appendiceal diameter. Other helpful predictive signs include maximal luminal diameter of the appendix, peri-appendiceal inflammatory changes, maximal wall thickness of the appendix and presence of peri-appendiceal fluid (Table 1). A number of inflammatory or neoplastic processes involving the appendix or adjacent structures mimic appendicitis or its complications on CT scan. Such incorrect diagnoses may lead to a delayed management of the existing disease, unnecessary intervention or hospitalisation. This pictorial review categorises these pitfalls that can render the diagnosis of appendicitis challenging on CT scan.
Table 1. Sensitivity and specificity of individual CT signs of appendicitis
Maximal cross-sectional diameter
Maximal luminal diameter
Maximal wall thickness
Although CT has high accuracy for diagnosing appendicitis, many pitfalls exist. For the purpose of this pictorial review, these are divided into appendicitis mimickers on CT, diagnostic dilemmas, atypical clinical presentations due to anatomical variations and concurrent pathologies.
Appendicitis mimickers on CT
Assuming a normal appendix is not found in CT, various conditions may be misdiagnosed as acute appendicitis.
Right-sided colonic diverticulitis is a common clinical mimicker of acute appendicitis that may also be mistaken with appendicitis on CT scan by causing peri-colic inflammatory changes and adjacent colonic wall thickening. The presence of normal appendix makes appendicitis unlikely (Fig. 1a); however, when the appendix is not visualised, appendicitis cannot be ruled out (Fig. 1b).
Meckel's diverticulum is a tubular structure, containing air and fluid that is connected with the adjacent small bowel. In addition to morphologic similarities with the appendix, inflammation of the Meckel's diverticulum may occur by obstruction of the orifice. With incomplete or no visualisation of appendix, Meckel's diverticulitis is a potential mimicker for acute appendicitis (Fig. 2).
Cecal carcinoma can manifest as a discrete mass that may lead to appendiceal dilatation, peri-appendiceal fluid collection and colonic wall thickening on CT (Fig. 3). Obstructive appendicitis may occur as a result of cecal tumour. However, in some cases of colon cancer, appendiceal changes such as appendiceal dilation and wall thickening, without appendicitis, may be the dominant CT finding. Although obstruction of appendix is a predisposition for acute inflammation, without clinical signs and symptoms, these incidental CT findings could be followed-up without the need for surgery.
An appendiceal mucocele represents an appendix distended with intra-luminal mucus, secondary to chronic obstruction of the appendix, mucosal hyperplasia and benign or malignant neoplasms of the appendix. A substantial proportion of mucoceles and mucinous adenomas present clinically with obstructive appendicitis. The typical CT finding of a mucocele is a low attenuation, encapsulated cystic mass without wall thickening or stranding. Appendiceal dilation with surrounding stranding can mimic features of acute appendicitis (Fig. 4).
Gynaecologic pathologies such as pelvic inflammatory disease and haemorrhagic functional ovarian cyst can manifest as appendicitis by causing acute right lower quadrant abdominal and pelvic pain.[1, 4]Ultrasound is preferred as the first-line modality in young females with lower abdominal or pelvic pain. When ultrasound is negative or equivocal, a CT can be performed for further evaluation. Occasionally, pelvic MRI can be helpful as a problem-solving tool in difficult cases. In exceptional circumstances, diagnostic laparoscopy is performed when imaging and clinical picture are inconsistent.
Another cause of thickened appendix mimicking appendicitis is Crohn's disease. Appendiceal changes are seen in approximately 20% of patients undergoing surgery for Crohn's disease. Appendiceal changes occur due to surrounding inflammatory changes or as a direct extension of Crohn's disease. Ileal, cecal and appendiceal wall thickening with associated inflammatory changes or fibro fatty proliferation of the adjacent mesentery is seen in both conditions[12, 13] (Fig. 5). It is important to note that appendiceal abnormalities seen in these circumstances are most commonly secondary changes of Crohn's disease, which on their own, are not an indication for surgery.
Epiploic appendagitis is a self-limited disease that results from venous occlusion of epiploic appendages. CT features include a low-attenuation oval fatty mass, and when it occurs in the right lower quadrant is accompanied with stranding mimicking CT features of appendicitis (Fig. 6).
Amyand hernia is a rare type of inguinal hernia defined by presence of appendix in inguinal canal. It occurs in 1% of all inguinal hernias, and acute appendicitis with appendix incarceration is a recognised complication. Assessment of the appendix can be challenging due to lack of surrounding fat (Fig. 7).
Equivocal cases of appendicitis, where only one feature is present, are observed frequently. This makes the diagnosis more challenging. Therefore, clinical correlation is essential in these cases. Appendicitis is encountered in about 30% of patients with equivocal CT findings. Non-visualisation of appendix is present in 5 to 10% of CT examinations and can create a unique diagnostic challenge in patients with suspected appendicitis. It is important to know that up to 40–50% of patients with right lower quadrant stranding or fluid collection, but non-visualisation of the appendix, end up with pathologic diagnosis of appendicitis.[7, 15] Early appendicitis can present with mild abdominal pain with normal appearing appendix on CT.
Tip appendicitis is inflammation of distal appendix with sparing of the proximal portion. Following the whole length of the appendix until identification of the blind end can prevent a missed diagnosis (Fig. 8).
Atypical clinical presentations due to anatomical variations
Due to anatomical variations of appendix, approximately one-third of patients with acute appendicitis present with abdominal pain, which is not localised in right lower quadrant. Common atypical locations of the appendix include right upper and left lower quadrant pain. Variable location of the appendix is usually due to both inconsistent position of the cecum and free end of appendix in relation to cecum (Fig. 9a). Some of the congenital anomalies, such as malrotation, are also responsible for atypical positioning of the appendix (Fig. 9b). Atypical location of the appendix can also be a problem in pregnancy due to rotation of the cecum with an expanding uterus.
Diagnosis of acute appendicitis may be difficult when another disease is also present. The coexisting disease process can be acute, such as diverticulitis (Fig. 10) or cholecystitis (Fig. 11), or longstanding process such as primary neoplasm of appendix (Figs 12, 13). It is often not possible to identify appendix or cecal tumour that is the underlying cause of appendicitis.
Various pitfalls exist in the CT diagnosis of appendicitis that may lead to a misdiagnosis. In addition, atypical location of the appendix or coexisting pathologies makes precise diagnosis more challenging. Familiarity with these conditions, as presented in this pictorial review, is important for an accurate diagnostic approach, prompt management and avoidance of unnecessary procedures.