Prevalence and clinical significance of the medullary rim sign identified on ultrasound of feline kidneys

The medullary rim sign (MRS) is an ultrasonographic (US) feature identified in normal and diseased feline kidneys. The prevalence and potential clinical significance of the MRS in a referral hospital cat population was investigated.Retrospective case-control study. US images from 661 cats were reviewed. Cats with an MRS were identified and compared with equal number of time-matched control cats. Medical data and MRS features, including thickness, intensity and symmetry, were collected. Associations between independent variables and the MRS were examined with conditional and unconditional logistic regression, with initial univariable, and subsequent multivariable analysis.Of the 661 reviewed cats, 243 (36.8 per cent) showed a variation of the MRS. A thin MRS (133 cats) was not associated with azotaemic renal disease (P=0.87). A thick MRS (110 cats) was associated with azotaemic renal disease (P=0.001). There was an association between the presence of MRS and a final diagnosis of feline infectious peritonitis (FIP) (P=0.028).The MRS is a common finding in cats. In this cat population, a thick MRS was associated with azotaemic renal disease, while a thin MRS was not. In cases with a clinical suspicion of FIP, the MRS may be related to the underlying disease process and not be an incidental finding.