Medullary pyramid thickness: The optimal cut-off value associated with the need for pyeloplasty in infants

Introduction: The medullary pyramid compresses during the early phases of severe hydronephrosis and represents a promising ultrasound metric for the diagnosis and surveillance of PUJ obstruction. The aim of this study was to define the optimal cut-off value and utility of medullary pyramid thickness (MPT) associated with the need for pyeloplasty in infants being followed up for hydronephrosis.

Methods: A retrospective review was performed over a five-year period to identify patients that were under surveillance for hydronephrosis during infancy and underwent a MAG3 to monitor the possible need for pyeloplasty. Ultrasound images were retrospectively reviewed to measure the MPT of the affected kidney in a blinded fashion. 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 optimal cut-off value associated with the requirement for pyeloplasty.

Results: A total of 63 patient cases were included, of which 45 underwent pyeloplasty (70%). A significant difference was found in the median MPT measurement between the pyeloplasty and non-operative groups (1.7 mm vs. 3.8 mm, p < 0.001). The optimal cut-off value of MPT associated with pyeloplasty was 3.4 mm. An MPT threshold of ≤3.4 mm conferred a sensitivity of 98%, specificity of 63%, positive predictive value of 86%, and negative predictive value of 92%.

Conclusion: Thinning of the medullary pyramid is an important ultrasound sign of parenchymal deterioration in high-grade hydronephrosis. An optimal MPT cut-off value of ≤3.4 mm is associated with subsequent pyeloplasty in infants. MPT should be considered in future studies addressing the diagnosis and surveillance of PUJ obstruction.