[Physiological hydronephrosis in pregnancy: Occurrence and possible causes. An MRI study]. / L'hydronéphrose physiologique durant la grossesse : prévalences et causes possibles. Une étude basée sur l'IRM.

INTRODUCTION: The etiology of the uretero-hydronephrosis in pregnancy is just hypothesis: hormonal or mechanical hypothesis, only investigated by echographic studies. MRI permits to visualize the entirety of the urinary tract, which can be helpful to find out a mechanical cause. METHODOLOGY: We have analysed the MRI of 100 asymptomatic pregnant women. We have determined the number and locations of the uretero-hydronephroses and researched whether there is any relationship between the uretero-hydronephrosis and certain abdominal structures. We focused on the psoas muscle and measured its depth, width and calculated its surface by a reproducible method. RESULTS: The analysis revealed that the uretero-hydronephrosis was predominantly at the right side (63%) and in the majority of the cases limited to the kidney (42%) and/or the proximal third of the ureter (42%). We were able to rule out some proposed etiologies: a compression of the ureter between the uterus and the iliac or ovarian vessels; a protective effect of the left intestinal structures. A link was observed between the psoas muscle and the physiological uretero-hydronephrosis: the ipsilateral psoas muscle seemed smaller in pregnant women presenting a uretero-hydronephrosis. CONCLUSION: We have highlighted a link between a physiological uretero-hydronephrosis during pregnancy and a lesser developped psoas muscle. The hypothesis proposed is that a smaller psoas muscle would have a less protective effect of the ureter due to a lesser development. This study offers a practical conclusion: a left sided uretero-hydronephrosis during pregnancy and/or including the entirety of the ureter is more probably a pathological hydronephrosis. LEVEL OF EVIDENCE: 4.