Ultrasound Diagnosis of Acute Appendicitis: Maximum Outer Diameter and Graded Compression
Graded Compression Technique
Graded (gradual) compression is the primary method for visualizing the appendix. A normal appendix is often visualized with gradual compression; it compresses easily, and its lumen may be empty or filled with gas or fecal content. In contrast, an acutely inflamed appendix is non-compressible.
According to the literature, the graded compression ultrasound technique provides a sensitivity of 88% and specificity of 96% (Textbook of Diagnostic Sonography, 9th ed., 2023).
Diameter Measurement (MOD)
The appendix is measured from the outer-to-outer wall contour, using the maximum outer diameter (MOD), with and without compression. Measurements are optimally performed in the plane with the most clearly defined wall, as in acute appendicitis, the wall may be poorly differentiated or irregular, with loss of the normal 'gut signature'.
| Parameter | Normal | Sign of Appendicitis |
|---|---|---|
| Maximum Outer Diameter (MOD) | up to 6 mm | ≥7 mm under compression (in children and adults) |
| Compression | easily compressible | non-compressible |
| Wall Thickness | <1.7 mm (range 1.1–2.7 mm); in children <6 years <3 mm is normal | wall edema >2 mm |
Signs of Wall Inflammation
An inflamed appendix shows wall edema with a thickness greater than 2 mm. Asymmetric wall thickening may indicate perforation. In inflamed specimens, the integrity and stratification of the wall layers are disrupted: layer differentiation is impaired, and each layer is sonographically heterogeneous. The appearance of the wall should not be the sole criterion for confirming appendicitis.
According to Diagnostic Ultrasound: Abdomen & Pelvis (2nd ed., 2022), a sonographic sign is a blind-ending, non-compressible tubular structure with a caliber >7 mm, originating from the cecum, with surrounding hyperechoic fat; in the appropriate clinical context, a thickened and distended appendix combined with a calcified appendicolith is diagnostic.
Color Doppler Imaging
Color Doppler Imaging (CDI) can reveal increased blood flow, demonstrating hyperperfusion associated with inflammation; vessels are traced along the periphery of the enlarged appendix. Adding CDI itself does not increase the sensitivity of appendicitis detection compared to grayscale ultrasound but serves as a simple way to confirm B-mode findings.
Differential Diagnosis
Diagnosis is challenging because many conditions have a similar clinical presentation of acute abdomen. The differential diagnosis includes: acute gastroenteritis, mesenteric lymphadenitis in children, ruptured ectopic pregnancy, mittelschmerz, inflammation of Meckel's diverticulum, regional enteritis, and torsion of the right ovary.
Frequently asked questions
What is considered the normal maximum outer diameter of the appendix?
The MOD of a normal appendix is up to 6 mm; a value of ≥7 mm under compression corresponds to appendicitis in both children and adults.
How should the diameter of the appendix be measured?
From the outer-to-outer wall contour (maximum outer diameter, MOD), with and without compression, in the plane with the most clearly defined wall.
What is the diagnostic accuracy of graded compression?
Sensitivity is 88% and specificity is 96% (Textbook of Diagnostic Sonography, 9th ed., 2023).
What is the wall thickness in inflammation?
Wall edema is greater than 2 mm; normally, the maximum wall thickness is less than 1.7 mm (range 1.1–2.7 mm), and in children under 6 years, it is permissible to be less than 3 mm.
Does CDI increase diagnostic sensitivity?
Adding CDI itself does not increase sensitivity compared to grayscale ultrasound but serves as a simple confirmation of B-mode findings by detecting hyperperfusion.