Mesenteric adenitis is a self-limited inflammatory process that affects the mesenteric lymph nodes in the right lower quadrant. Its clinical presentation mimics that of acute appendicitis.  Cross-sectional imaging is routinely applied in the examination of patients and may help avoid unnecessary surgery in cases of mesenteric adenitis. Previously, the diagnosis was most frequently made when laparotomy performed to assess presumed appendicitis yielded negative findings. [2, 3, 4, 5]
See images of mesenteric adenitis below.
Ultrasonography (see the images below) of the right lower quadrant with graded compression has been the mainstay of diagnosis in children. [6, 7, 8, 9, 10] The American College of Radiology (ACR) recommends this procedure as the most appropriate for patients under the age of 14 years with fever, leukocytosis, and possible appendicitis but an atypical presentation. 
The ACR notes that abdominal CT (see the images below) with intravenous contrast may be useful in children with negative ultrasound studies and recommends this technique as the most appropriate for adolescents and adults with this presentation.  CT is especially useful for evaluation of possible appendicitis in men and in patients in whom visualization of the appendix may be compromised by their body habitus. Although the findings are frequently nonspecific, abdominal radiographs occasionally reveal findings and permit alternative diagnoses (eg, appendicoliths). [12, 13, 14]
In some centers, CT is performed for the evaluation of appendicitis without intravenous and/or oral contrast enhancement. This approach reduces the sensitivity of CT for bowel wall thickening and mesenteric adenopathy, especially in small children with a paucity of intraperitoneal fat.
The normal appendix is sonographically occult in a significant subset of patients. When lymph node enlargement is detected on sonographic examination in these patients, excluding appendicitis as a cause of reactive adenopathy is difficult. [15, 16]
With any imaging modality, the finding of lymph node enlargement as an isolated finding is nonspecific; it can be observed in association with several inflammatory processes. Occasionally, nodes exceeding the normal size threshold are observed in children who have no demonstrable disease. [8, 12, 17]
In the context of recent concerns about the radiation dose associated with pediatric CT, tailoring the examination protocol to the patient is prudent to minimize exposure during studies for benign and self-limited conditions such mesenteric adenitis. In this respect, sonography is favored as the initial diagnostic test.
Findings on supine and upright abdominal radiographs are often normal. Nonspecific findings include a regional ileus or evidence of bowel wall thickening in the right lower quadrant.
Plain radiographic findings can never indicate a specific diagnosis in mesenteric adenitis, but they can occasionally confirm an alternative diagnosis. Cross-sectional imaging may be indicated regardless of the plain radiographic findings.
Diagnostic features include enlarged mesenteric lymph nodes, with or without associated ileal or ileocecal wall thickening, in the setting of a normal appendix. 
Rao et al specified the criterion of 3 or more nodes with a short-axis diameter of at least 5 mm, clustered in the right lower quadrant (see the images below). In the prospective study by Rao et al, none of the 18 patients with the aforementioned CT criteria had surgical or clinical evidence of appendicitis, whereas appendicitis was correctly diagnosed in 56. 
Lymph nodes are generally larger, more numerous, and more widely distributed in mesenteric adenitis than in appendicitis. Ileal thickening is diagnosed when the wall is thicker than 3 mm over at least 5 cm of the bowel despite bowel lumen opacification and distention (see the image below).
Further imaging is generally not indicated, although a definitive diagnosis might not be made in a case with equivocal findings until laparotomy performed for the evaluation of appendicitis reveals negative findings. Nonopacified bowel may be mistaken for enlarged lymph nodes, especially in thin patients or small children. Bowel wall thickness is difficult to determine in this setting.
In patients with fever, abdominal tenderness, and a normal appendix, adenopathy that predominantly involves but is not limited to the right lower quadrant suggests the diagnosis (see the images below).
Usually, 5 or more nodes are present and are often clustered. Nodal tenderness in response to transducer pressure is typical. Nodes are more rounded and hypoechoic than normal. Abnormal nodes have a short-axis diameter of at least 5 mm, and the diameter can exceed 1 cm. The nodes are typically larger and more numerous with mesenteric adenitis than with appendicitis (see the images below). 
The demonstration of hyperemia within the node and surrounding mesentery with Doppler imaging is variably reported. Other findings include intestinal hyperperistalsis, which is seldom observed in appendicitis; nodular or circumferential thickening of the bowel wall; mesenteric thickening; fluid-filled loops; cecal involvement; and free fluid (see the images below).  Occasionally, a fluid-filled appendix is seen, but the lumen is readily compressible. 
Increased echogenicity of intra-abdominal fat has been reported as a nonspecific marker for right lower quadrant inflammatory disease, seen in appendicitis, mesenteric adenitis, and other illnesses.  Similarly, increased echogenicity of renal parenchyma has been noted in children with mesenteric adenitis, as well as appendicitis and other acute illness; it is a transient feature and does not necessarily indicate renal disease. 
Although nodal size and number are generally greater in mesenteric adenitis than in appendicitis, overlap does occur. In Simonovsky's large series of 609 patients, 426 had appendicitis, and 81 had mesenteric adenitis.  The inflamed appendix was missed in 2 of the latter cases.
Puylaert stated that in adult patients with right lower quadrant pain in whom the sole findings are enlarged mesenteric lymph nodes, the risk of appendicitis is 64%.  Therefore, the diagnosis of mesenteric adenitis is hazardous when the appendix is not visualized; correlative CT may be beneficial in this setting.
In mesenteric adenitis and lymphoma, the size, shape, distribution, and Doppler imaging characteristics of the lymph nodes overlap considerably, although the clinical context is useful in narrowing the differential diagnosis. 
Mesenteric lymph nodes that exceed the normal size threshold may be visualized in the absence of disease, although these are usually nontender.
Nuclear scintigraphy is seldom useful in this setting. A single case report describes abnormal white-cell localization in the nasopharynx, cervical lymph nodes, and right lower quadrant during imaging with indium-111–labeled white blood cells in a 4-year-old boy with fever, pharyngitis, and abdominal pain.