Should you care about a medullary rim sign?

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

Amy Ferreira, Rachel Marwood, Daniel Batchelor, Thomas Maddox, Jeremy R. Mortier

Background
The medullary rim sign (MRS) is an ultrasonographic finding observed in feline kidneys, appearing as a hyperechoic band in the outer medulla. Its clinical significance is uncertain, as previous studies have linked it to both normal and diseased kidneys. Some reports associate MRS with chronic interstitial nephritis, feline infectious peritonitis (FIP), and hypercalcemic nephropathy, while others consider it an incidental finding. This study aimed to determine the prevalence of MRS in a feline referral population and investigate its association with renal disease and specific diagnoses.

Methods
A retrospective case-control study was conducted using ultrasound images from 661 cats examined at a veterinary referral hospital between 2010 and 2016. Cases with MRS were identified and compared to time-matched control cats without MRS. The thickness, intensity, and symmetry of MRS were assessed, and associations with azotemia, chronic kidney disease (CKD), and specific diagnoses were analyzed using logistic regression models.

Results
MRS was identified in 243 cats (36.8%). A thin MRS (<2.5 mm) was not associated with azotemic renal disease (P = 0.87), whereas a thick MRS (≥2.5 mm) was significantly associated with azotemic renal disease (P = 0.001). There was also a significant association between MRS and a final diagnosis of FIP (P = 0.028). No association was found between MRS and hypercalcemia. The ultrasound appearance of MRS remained stable over time in cats that underwent repeat scans.

Limitations
The retrospective design introduced variability due to different ultrasound machines, operators, and patient conditions. Histopathologic confirmation of MRS-related pathology was lacking, limiting definitive conclusions about its etiology. Additionally, the study focused on a referral hospital population, which may not represent the general feline population.

Conclusions
MRS is a common ultrasonographic finding in cats, but its clinical significance depends on its characteristics. A thick MRS is associated with azotemic renal disease, whereas a thin MRS is likely incidental. In cats with clinical suspicion of FIP, MRS may be related to the disease process rather than an incidental finding. Further studies are needed to clarify its pathophysiological implications.

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