How is hydronephrosis severity in infants graded?

Updated: Dec 16, 2020
  • Author: Dennis G Lusaya, MD; Chief Editor: Bradley Fields Schwartz, DO, FACS  more...
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The following grading systems have been developed for evaluating hydronephrosis severity in infants [45] :

  • Anterior-posterior (AP) diameter of renal pelvis
  • Society for Fetal Urology (SFU)
  • Radiology
  • Urinary Tract Dilatation (UTD)
  • Onen

The Onen grading system applies to both prenatal and postnatal ureteropelvic junction–type hydronephrosis and includes two categories of findings: dilation of the pelvicalyceal system and, most important, the quality of the renal parenchyma (thickness and appearance). AP diameter is not important. [45]

The Onen system has four grades. Grade 1 criterion is renal pelvic dilatation. Grade 2 criteria are as follows:

  • Pelvis and caliceal dilatation
  • Renal parenchyma (medulla and cortex) are normal (> 7 mm)

Grade 3 criteria are as follows:

  • Pelvis and caliceal dilatation
  • Medulla is short and thin
  • Cortex is normal
  • Total parenchymal thickness 2-5 mm second trimester, 2.5-6 mm third trimester, 3-7 mm postnatal
  • Corticomedullary differentiation is normal

Grade 4 criteria are as follows:

  • Pelvis and caliceal dilatation
  • No medulla is present (total loss)
  • Cortex is thin (second trimester, < 2 mm; third trimester, < 2.5 mm; postnatal, < 3 mm)
  • No corticomedullary differentiation is visible
  • Recesses between calyxes are significantly short and thin

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