Renal pyramid echogenicity in ureteropelvic junction obstruction: correlation between altered echogenicity and differential renal function

Background Improvement in resolution and use of high-frequency transducers in US has enabled visualization of previously unreported changes in medullary pyramid echogenicity in children with obstructive hydronephrosis. Objective To determine whether these unreported changes in echogenicity and morphology of the renal pyramids in ureteropelvic junction (UPJ) obstruction correlate with differential renal function (DRF) of the kidney as determined by technetium-99m mercaptoacetyltriglycine (Tc-99m-MAG3) scan. Materials and methods Renal sonograms in 60 children with UPJ obstruction were retrospectively reviewed. Children were divided into three groups based on the echogenicity of the pyramids: (1) normal echogenicity of the pyramids, (2) increased echogenicity of the pyramids with maintained corticomedullary differentiation (CMD), and (3) loss of CMD. DRF, as determined by Tc-99m-MAG3 scan, of the obstructed kidney of >= 45% was considered normal and of <= 44% was considered abnormal based on a published study correlating histological changes with DRF. Fisher's exact test was performed for assessing the association between DRF and altered echogenicity of the pyramids. Results In group 1, which consisted of 13 patients with normal pyramids on US, DRF was normal in 11 and abnormal in two. In group 2, which consisted of 33 patients with echogenic pyramids and preserved CMD, DRF was normal in 15 and abnormal in 18. In group 3, which consisted of 14 patients with complete loss of CMD, DRF was normal in 2 and abnormal in 12. There was a strong correlation between abnormal pyramids and DRF (P=0.0009). The risk ratio (RR) of DRF becoming abnormal for those kidneys with abnormal echogenicity of the pyramids with preserved CMD (group 2) compared to normal pyramid echogenicity (group 1) was 1.56 (95% CI 1.088-2.236). The RR of DRF becoming abnormal for those kidneys with loss of CMD (group 3) compared to normal pyramid echogenicity (group 1) was 5.571 (95% CI 1.530-20.294). Conclusion We observed that in obstructed kidneys the echogenicity of the pyramids may be abnormal. Increased echogenicity of the pyramids correlated weakly with abnormal DRF and does not necessarily indicate poor renal function. However, loss of CMD strongly correlated with poor renal function.