Renal extracapsular hypoechoic rim and kidney cortical thickness

A renal extracapsular hypoechoic rim was previously recognized and interpreted as a typical sonographic finding of renal failure. Subsequently it was hypothesized that the hypoechoic rim could be produced by a state of sodium retention and oedema caused by nephropathy but not necessarily associated with renal failure. In order to get this cleared we performed a retrospective analysis of 80 renal ultrasound examinations, carried out at our center, in 41 of which it was found a renal extracapsular hypoechoic rim.

For each patient we recorded the glomerular filtration rate, the diameters in the longitudinal axis, the resistive indexes and the cortical thickness of each kidney, the possible presence and thickness of the hypoechoic rim and yet the possible coexistence of diabetes mellitus, proteinuria and clinical signs of fluid overload as peripheral oedema, distended jugular veins, ascites, increased caliber and reduced respiratory excursion of the vena cava.

The F value calculated to assess the weight/influence on the hypoechoic rim of each of the variables showed as all variables, except the sex, significantly weighed on the hypoechoic rim although the greatest weight was reached by a glomerular filtration rate < 60 ml/min/1.73 m2 and a renal cortical thickness between 7 and 12 mm. The hypoechoic rim was found only when cortical thickness was between 7 and 12 mm while it was absent if the cortical thickness was less than 7 or greater than 12 mm. We also found numerous cases of sidedness of the hypoechoic rim.

It is our opinion that in case of unilateral finding of an hypoechoic rim, the association between the hypoechoic rim and the cortical thinning is consistent and therefore more accurate than the correlation between the presence of the hypoechoic rim and the reduction of the glomerular filtration rate.