Medullary pyramid thickness: A novel ultrasound metric that predicts the need for pyeloplasty in infants.

Objectives: To evaluate the optimal cut-off value, and diagnostic characteristics of medullary pyramid thickness (MPT) in predicting the need for pyeloplasty in infants with hydronephrosis. Method(s): A retrospective review was performed over a five-year period to identify patients that were under surveillance for hydronephrosis during infancy to monitor the possible need for pyeloplasty. Ultrasound images were reviewed by one blinded investigator to measure the MPT of the affected kidney. The primary outcome measure was subsequent requirement for pyeloplasty before three years of age. The Mann-Whitney U test was used to determine statistically significant differences in the minimum MPT between the infant group requiring pyeloplasty and the non-operative group. Receiver operating characteristic analysis was performed to determine the cut-off values that most accurately predicted the requirement for pyeloplasty. Result(s): A total of 63 patient cases were included, of which n = 45 underwent pyeloplasty (70%). A significant difference was found in the median MPT measurement between the pyeloplasty and nonoperative groups (1.7 mm [IQR: 1.1 mm] vs. 3.8 mm [IQR: 4.1 mm], p <.001). The optimal cut-off value of MPT to predict pyeloplasty was 3.4 mm. An MPT threshold of <=3.4 mm conferred a sensitivity of 98% in predicting the need for pyeloplasty, specificity of 63%, positive predictive value of 86%, and a negative predictive value of 92%. Conclusion(s): MPT is the earliest sign of parenchymal deterioration in high-grade hydronephrosis. A cut off value of <=3.4 mm is optimal in predicting the need for subsequent pyeloplasty in infants. MPT should be considered in future studies addressing classification and stratification of hydronephrosis.