From the Guidelines
The appendix can be visualized on ultrasonography, but the sensitivity of ultrasound decreases to 82% when including equivocal or indeterminate results, as noted in the 2024 clinical practice guideline update by the Infectious Diseases Society of America 1. When performing an ultrasound to evaluate for appendicitis, several key factors influence the success of appendix visualization. These include:
- The use of graded compression technique to displace overlying bowel gas and bring the appendix closer to the transducer
- Patient factors such as body habitus, appendix location, and the presence of overlying bowel gas
- The experience of the practitioner, as visualization rates can range from 50-85% in experienced hands The normal appendix appears as a blind-ending, tubular structure measuring less than 6mm in diameter, with a thin wall and compressible lumen, typically originating from the base of the cecum, about 2-3cm below the ileocecal valve 1. Ultrasound is particularly valuable in certain populations, such as pediatric and pregnant patients, to avoid radiation exposure, though further imaging with CT or MRI may be necessary when ultrasound findings are equivocal or the appendix cannot be adequately visualized, as suggested by the guideline update 1. Key considerations in the choice of imaging modality include the availability of MRI, the need for sedation in young children, and the concern about radiation exposure associated with CT, especially in children 1.
From the Research
Visibility of the Appendix on Ultrasonography
- The visibility of the appendix on ultrasonography (ultrasound) can be affected by various factors, including the skill of the sonographer and the patient's body mass index (BMI) 2, 3, 4.
- Studies have shown that ultrasound can be an effective initial imaging modality for acute appendicitis in both adult and pediatric populations, with sensitivity and specificity ranging from 67.65% to 89.6% and 53.0% to 98.41%, respectively 2, 3, 5.
- However, the appendix may not always be visible on ultrasound, with inconclusive results ranging from 29% to 49% of cases 2, 3, 6.
- Predictors of nondiagnostic ultrasound results include older age, male sex, and higher BMI 3, 4.
- In cases where the ultrasound is inconclusive, further imaging or a follow-up visit may be necessary to confirm or rule out appendicitis 2, 6.
Diagnostic Accuracy of Ultrasound
- The diagnostic accuracy of ultrasound for appendicitis can be improved by combining it with clinical scoring systems, such as the Alvarado score 6.
- A study found that the negative predictive value of inconclusive ultrasound findings and low Alvarado score combined was 99.6% 6.
- Another study found that bedside ultrasound performed by trained residents with attending supervision had a sensitivity of 67.65% and a specificity of 98.41% for diagnosing acute appendicitis 5.
Clinical Implications
- Ultrasound can be a useful initial imaging modality for suspected appendicitis, especially in patients with low BMI and low clinical suspicion 2, 3, 4.
- However, clinical follow-up is mandatory for patients with negative ultrasound results, as false negatives can occur 2.
- The use of ultrasound as an initial imaging modality can help reduce the utilization of computed tomography (CT) scans and associated radiation exposure 5.