Ultrasound Appearance of the Outer Medulla in Dogs Without Renal Dysfunction

Abstract

Ultrasound findings of the canine kidney include a hyperechoic cortex and a hypo to anechoic medulla. In this study, the sonographic appearance of the outer renal medulla in dogs without evidence of renal disease is described. Dogs that underwent abdominal ultrasound over a 6-month period were subjected to review and then divided into six groups based on body weight (kg): < 4.9, 5.0-9.9, 10-19.9, 20-29.9, 30-39.9, and ≥ 40. Chi-square analysis was used to determine if the frequency of a hyperechoic outer medulla was significantly different between weight groups, sex, and age (P-value < 0.05). Of the 145 dogs that met the inclusion criteria, 45 had a hyperechoic outer medulla relative to the cortex and inner medulla. In the remaining dogs, the outer medulla was isoechoic to the cortex. Dogs less than 5 kg had the highest frequency of a hyperechoic outer medulla (P < 0.0001) and dogs greater than 40 kg did not have a hyperechoic outer medulla (P < 0.0001). Sex had no influence on the presence or absence of the hyperechoic outer medulla; however, younger dogs were overrepresented (6.4 ± 0.6 years compared with 7.8 ± 0.4 years; P = 0.04). Ultrasound descriptions of the canine kidney have not taken into account the contributions of the renal cortex and outer medulla. Based on this study of dogs with no clinically significant renal disease, the outer medulla can be isoechoic or hyperechoic to the cortex and a hyperechoic outer medulla is more commonly seen in small breed dogs.

Renal ultrasonographic abnormalities are associated with low glomerular filtration rate calculated by scintigraphy in dogs

“…Subjective evaluation included the following variables: cortical echogenicity, medullary echogenicity, and kidney shape/contour . The subjective abnormalities detected were graded according to severity using a grading system created for the study, but based on the veterinary kidney ultrasound literature . Kidney severity grades were: grade 0 for normal appearance, grade 1 for mild abnormality, grade 2 for moderate abnormality, and grade 3 for severe abnormality.…”

“…For measurement of the medulla, the arcuate vessels were used to define the medullary borders . This method made it possible to distinguish the outer margin of the medulla from the cortex even when they were isoechoic . If the outer portion of the medulla was hyperechoic compared to the cortex, then the most peripheral limit of the hyperechoic region was used to place the calipers .…”

“…Subjective evaluation included the following variables: cortical echogenicity, medullary echogenicity, 13,16,17,32 and kidney shape/contour. 33 The subjective abnormalities detected were graded according to severity using a grading system created for the study, but based on the veterinary kidney ultrasound literature.…”

“…16 If the outer portion of the medulla was hyperechoic compared to the cortex, then the most peripheral limit of the hyperechoic region was used to place the calipers. 16 The kidney/medullary length was considered to be the greatest cranial to caudal dimension in either a dorsal ( Figure 1A and B) or sagittal ( Figure 1C and D) plane image. The kidney/medullary height was considered to be the shortest dorsal to ventral dimension in either a sagittal ( Figure 1C and D) plane image that divided the kidney in halves or in a transverse ( Figure 1E and F) plane image where the kidney shape was as round/oval as possible and included the renal pelvis region.…”

“…On account of its wide availability, low cost, and safety, ultrasound is often the first diagnostic imaging method used for investigation of kidney disease. 12 Several studies have analyzed the sonographic features of the kidneys both in normal dogs [13][14][15][16][17][18][19][20][21][22][23] and dogs with kidney disease. [24][25][26][27] It is stated in the literature that morphologically abnormal kidneys may function normally, and that morphological alterations may be absent from functionally impaired kidneys, 3,11 but studies on the association between structural and functional abnormalities are lacking.…”