Perinephric stranding on CT means that there is edema or dirty hazy appearance of the normal fat that surrounds each kidney. This is a very common finding that can mean different things depending on if it’s on one side or both and whether the patient has symptoms referable to the urinary tract.
Most commonly, I see this finding on both sides. More commonly in older patients who are getting scans for unrelated reasons. In these patients, this finding has no definite significance. It can also vary from being barely visible to severe with small pockets of fluid and stranding.
Perinephric stranding is more concerning when it’s seen on one side only. Most commonly I see this when patients are passing a kidney stone. The stone blocking the ureter is seen and there is hydronephrosis which is a dilated kidney collecting system. The stranding around the kidney is reactive to the blockage and has no significance by itself in many cases.
Sometimes this can indicate a superimposed infection. The clinical doctors will need to determine if there is an infection occurring by correlating your symptoms and blood tests such as a white blood cell count.
A recently passed stone will also show stranding around the kidney. In these cases, a stone blocking the ureter will not be seen. The stone may instead be seen in the bladder as it has been recently passed. Hydronephrosis or dilated collecting system of the kidney may persist after the stone is passed for sometime.
Another common cause of perinephric stranding on one side is an infection of the kidney called pyelonephritis. In these cases, a CT done with contrast will commonly show patchy areas of decreased enhancement or infection in the kidney. There may also be stranding which is an associated and sometimes not the primary finding. A CT with contrast can also exclude an abscess in the kidney which is a collection of pus. Often patients will have flank pain and elevated white cell count. This is treated with antibiotics.
Less common causes include trauma to the kidney with bruising, laceration and bleeding. Often the bruising and tearing of the kidney will be seen on a CT with contrast. The stranding around the kidney in these cases may be blood. Sometimes a tumor will be associated with stranding around the kidney. In these cases, a mass will be visible.
CT and the radiologists interpretation therefore plays a crucial role in the significance of this finding. Everything from an insignificant finding all the way to life threatening. The clinical context is also important for the correct interpretation of this finding.