If one sees opaque contrast material around the kidney of an infant with ascites, usually a urinary tract blockade is present. "Ascites" develops because urine fills the abdomen after escaping through tears in the hydronephrotic collecting systems.
On roentgenograms, the extravasating urine, made visible with intravenously administered or retrograde contrast material, takes the shape of the letter P (Fig 1, left). The rounded, upper part of the letter is roughly formed by urine trapped around the kidney and lying beneath Gerota's fascia (Fig 2). A translucency in the center of the urine pool is produced by the kidney itself and the dilated ureter adds a tortuous leg to the P.
This paper describes a new case of urinary ascites that showed the P sign and considers eight more like it that were found in the literature.1-4 From this review, it appears that the correct therapy for neonatal ascites caused
Dockray KT. The Perirenal P Sign: A New Roentgenogram Index to the Cause and Treatment of Urinary Ascites in Babies. Am J Dis Child. 1970;119(2):179–181. doi:10.1001/archpedi.1970.02100050181021
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