Introduction

Acute abdominal pain is a common reason for emergency department visits. Acute appendicitis constitutes the most common cause of lower abdominal pain and the most common diagnosis made in young patients admitted to the emergency department with an acute abdomen¹.

Appendectomy is one of the most common procedures performed by general surgeons^2,3. The lifetime risk of developing appendicitis is approximately 7%, but it is relatively rare at the extremes of age with a peak incidence between 15 and 30 years, mainly in men⁴.

Despite acute appendicitis is the leading cause of acute abdomen, its diagnosis is still difficult in some cases, particularly in those without typical symptoms. This difficulty causes delay in both diagnosis and treatment, increasing morbidity and mortality^5,2. The imaging tests most commonly used in cases of suspected acute appendicitis are abdominal ultrasound, computed tomography scan and magnetic resonance imaging⁶.

In case of diagnostic uncertainty abdominal ultrasound is a useful, accessible, reproducible, affordable, and low-cost bedside tool, easy to perform, with high specificity, sensitivity and cost-effectiveness^2,7. The findings suggestive of appendicitis described in the literature include an appendix with a maximum transverse diameter exceeding 6 mm, wall thickening, non-compressibility and hyperechogenic periappendiceal fat⁸. The fact that ultrasound is an operator dependent technique and its sensitivity and specificity may vary according to the operator is the disadvantage of the method⁹. When abdominal ultrasound is inconclusive, computed tomography should be considered, but its disadvantage is that it is not available in some centers, uses radiation and can produce side effects caused by contrast agent which may delay the start of treatment^6,8.

Traditionally, early diagnosis and surgery have been described as fundamental strategies to avoid certain complications, such as abscesses, peritonitis and sepsis. In turn, imaging tests are important because negative appendectomies are not free of complications⁸.

Imaging tests have also demonstrated a reduction in the frequency of unnecessary appendectomies and appendicitis progression, thus improving the diagnostic protocols and the quality of care^9,10. The aim of this study was to evaluate the performance of abdominal ultrasound in the diagnosis of acute appendicitis in a tertiary health care center.

Material and methods

We conducted longitudinal study to evaluate the performance of a diagnostic test using data retrospectively collected. The population was made up of patients between 3 and 90 years who consulted due to pain in the right iliac fossa and presented a typical clinical picture of acute appendicitis or an atypical presentation suggestive of acute appendicitis. All the patients underwent abdominal ultrasound and were followed up until the results of the pathological examination were available, in the period between February 2016 and February 2018.

We excluded patients with pain in the right iliac fossa without suspicious for appendicitis, previous appendectomy, those with suspicious symptoms who did not undergo abdominal ultrasound and those who underwent surgery without undergoing previous ultrasound. The ultrasound was performed by a specialist in diagnostic imaging particularly trained in the assessment of right iliac fossa pain.

Appendicitis was diagnosed by ultrasound when at least one of the following criteria was met: thickening of appendiceal wall with a diameter > 6 mm, appendicolith, non-compressibility and inflammation of the periappendicular fat (Fig. 1). The following were considered negative findings: failure to visualize the appendix, a visible and compressible appendix, and appendix diameter < 6 mm.

Figure 1 B-mode ultrasound sequence using linear array transducer showing the base of the normal appendix (black arrow) and inflamed appendix that is not compressible (arrowhead) distal to the appendicolith (white arrow). 

Patients with clinical and ultrasound diagnostic criteria of appendicitis underwent surgery. Those with equivocal presentation and absence of ultrasound findings suggestive of appendicitis were kept under observation. Surgery was indicated for those cases with persistent symptoms. The results of the pathological examination were available in all the patients operated on.

Quantitative continuous data were expressed as median and interquartile range. The analysis included sensitivity, specificity, positive predictive value and negative predictive value of ultrasound for the diagnosis of acute appendicitis. Two-way tables were used to analyze qualitative data of the presence or absence of the disease and positive or negative result of ultrasound. Proportions were compared using the chi-square test. A p value < 0.05 was considered statistically significant. All the calculations were performed using Open Epi^® software package (Emory University).

Results

A total of 113 patients were analyzed; 63 (55.8%) were female and 50 (44.2%) were male. Median age was 19 years (IQR 12-30) with a minimum of 3 years and a maximum of 90 years. Ultrasound was positive for the diagnosis of appendicitis in 32 patients (28.3%) and was negative in 81 patients (71.7%).

Of the patients with positive ultrasound, 30 (93.7%) underwent surgery and the disease was confirmed by pathological examination; the remaining 2 patients (6.3%) did not undergo surgery (Table 1).

Table 1 Contingency table showing ultrasound findings in 113 patients according to the diagnosis of apendicitis 

Of the patients with negative ultrasound, 23 (28.4%) underwent surgery, and the disease was confirmed in 14 (17.2%). Fifty-three patients (46.9%) underwent surgery; 44 (38.9%) had confirmed appendicitis while the appendix was healthy in 9 patients (7.96%), and among these patients, the ultrasound had been negative for appendicitis. (Table 2)

Table 2 Contingency table showing ultrasound findings in the 53 patients undergoing surgery. 

Of the 44 patients with appendicitis, 35 were edematous appendicitis, 5 were phlegmonous, 3 were gangrenous and 1 had an appendiceal mass. According to the pathological stage of the disease, ultrasound was positive in 26 patients with edematous appendicitis, in 3 with phlegmonous appendicitis, negative in all the patients with gangrenous appendicitis and positive in the appendicular mass. There were no differences in sensitivity according to the pathological stage. The diagnostic performance of ultrasound in this series can be seen in Table 3.

Table 3 Performance of ultrasound to detect appendicitis. 

The global prevalence of appendicitis is 8% and this value was taken as reference to calculate the PPV/NPV. With such adjustment, the positive predictive value decreases to 66.3%. In addition, ultrasound the false positive rate was 6.3% (2/32) and the false negative rate was 17.2% (14/113).

Discussion

The diagnosis of acute appendicitis is still a challenge for emergency surgeons despite the increasing number of diagnostic tests, particularly imaging tests in cases where the physical examination and laboratory tests are inconclusive⁵.

Currently, the use of imaging tests has become of great help in the diagnosis of acute appendicitis, resulting in a significant reduction in the number of negative appendectomies and in complicated appendicitis, thus improving quality of care indicators^8,11. The strategy for using imaging tests may be controversial; ultrasound is used in some centers as the first imaging test in cases of suspected appendicitis, while others use computed tomography¹². Ultrasound is still a very useful diagnostic method in patients with suspected appendicitis, with a sensitivity between 59 and 96% and specificity between 83 and 98%, besides the advantage of its high availability and low cost¹³.

The results obtained in the present study in terms of sensitivity and specificity are similar to those published in the international literature; sensitivity has the lowest performance due to the wide confidence interval found in our series. The positive and negative predictive values are also in agreement with the literature, but we observed that their value decreased after adjusting for the worldwide prevalence of the disease. According to international guidelines, a correct combination of ultrasound findings, clinical indicators and laboratory tests may significantly increase the sensitivity and specificity of ultrasound and reduce the need for computed tomography¹⁴.

It should be noted that ultrasound is the initial imaging modality in the pediatric population, and its sensitivity and specificity are similar to those described for adults. In an effort to avoid the use of computed tomography in children and/or reduce unnecessary hospitalizations, some studies have demonstrated and described the presence of secondary signs of appendicitis to help in the diagnosis as fluid collections, free fluid, thickened periappendiceal fat, small bowel loops congestion and abnormal lymph nodes¹⁵. Timely diagnosis and treatment are the best tools to obtain good outcomes in the pediatric population also¹⁶.

Ultrasound is also useful in elderly patients in whom the diagnosis of acute appendicitis may be more difficult¹⁷. The sensitivity and specificity for the presence of any secondary sign in diagnosing appendicitis are about 40% and 90, respectively¹⁸.

Although the diagnosis of appendicitis was previously considered to be clinical, the usefulness of imaging tests for diagnosis has been demonstrated over time, thereby decreasing the number of negative appendectomies, especially in patients with equivocal or atypical symptoms^19,20.

In conclusion, we may state that abdominal ultrasound is a useful tool that can help in the diagnosis of appendicitis in equivocal cases and contribute to the decision-making process. It also provides additional information to clinical examination and can play a relevant role in those cases with suspected appendicitis and equivocal symptoms.