This study was conducted to establish a radiographic reference range for the diameter of the colon in normal cats and to assess whether a numeric cutoff value for colon diameter could be defined. Radiographs of 50 cats with no history of GI disease were evaluated to establish a normal reference range for radiographic diameter of the colon. Thirteen cats with constipation and 26 with megacolon were compared with normal cats to characterize the accuracy of the reference range and identify a cutoff for differentiating constipation from megacolon. All radiographs were laterolateral abdominal views that included the lumbar vertebrae, and the measured portion of colon was distended with feces in all cats. There was a significant difference between abnormal and normal cats for maximum colon diameter, colon diameter at the cranial aspect of the pelvic inlet, and colon diameter ventral to L5. A ratio of maximal-colon diameter to L5 length <1.28 was proposed to be suggestive of a normal colon diameter; a ratio of 1.28:1.48 suggests constipation; and a ratio >1.48 is suggestive of megacolon. All cats with a maximal colon diameter >1.62 times the length of L5 had megacolon and subsequently underwent subtotal colectomy.

Commentary
Diagnosing constipation and megacolon is important but should not be achieved by radiographs alone. History, examination findings, and response to treatment are important factors in differentiating the diseases. Cats may have constipation that later progresses to megacolon, either from true disease progression or poor owner compliance in administrating medications. Using the measurements from this paper, there is an overlap between patients with constipation and megacolon; the sensitivity (77%) and specificity (85%) are not very high. Therefore, radiographic measurement of the degree of colonic distention may not be an important criterion in making the diagnosis. However, it may be more helpful for the less-experienced clinician.—Jean Reichle, DVM, MS, DACVR

Source
Radiographic diameter of the colon in normal and constipated cats with megacolon. Trevail T, Gunn-Moore D, Carrera I, et al. VET RADIOL ULTRASOUN 52:516-520, 2011.