The urographic signs of acute on chronic obstruction of the kidney.

The signs of acute obstruction of the kidney and of chronic obstruction are well recognised and the combination of these signs enables a group of patients with acute-on-chronic obstruction to be recognised. The signs of acute obstruction are entirely nephrographic. They are manifested by an increasingly dense nephrogram, which may become striated, followed by an anatomically normal pyelogram. The signs of chronic obstruction are both pyelographic and nephrographic. The pyelographic signs are a dilated pelvicalyceal system. The nephrographic signs are a normal or low density nephrogram. Crescents may appear; these are due to opacification of the distorted medulla which retains its ability to concentrate the urine. In acute-on-chronic obstruction the signs of both conditions are present. Thus there is an increasingly dense nephrogram with a negative pyelogram. Crescents may appear. There is a slow flow in the dilated pelvicalyceal system.