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Renal sonography is frequently used to exclude obstruction in patients who develop an elevation in serum creatinine. However, non-obstructive causes such as heart failure are a more likely etiology for development of acute renal insufficiency. A recent study from Gottlieb et al was undertaken: 1) to assess the change in outcome resulting from the use of sonography, and 2) to determine relevant clinical parameters that would assist in better triaging those who would benefit from sonography. Patients who underwent sonography for stones or infection but without an increase in serum creatinine were excluded. Sixty patients (40 men, 20 women) who were referred for sonography because of an increased serum creatinine level (³ 1.3 mg/dL) were evaluated retrospectively.
By sonography, 21 of 60 patients (35%) were hydronephrotic. Obstruction was present in two-thirds (14 of 21) of the hydronephrotic patients. About half of the obstructed patients (8 of 14) were successfully treated with reversal of renal failure. Thus, sonography contributed significantly in changing outcome in 57% of obstructed patients. All obstructed patients had one of three clinical parameters: pelvic mass (8 patients), calculi (4 patients), and flank pain (1 patient).
In non-obstructed patients, sonography did not alter clinical management and outcome significantly, as compared to obstructed patients (7% vs 57%, respectively). Reversal of renal failure occurred in about one-fourth of non-obstructed patients, but it was related to volume replacement and discontinuation of nephrotoxic drugs and not to sonographic findings. All but two of 44 non- obstructed patients lacked the three clinical parameters (pelvic mass, stones, and flank pain) that characterized the obstructed patients. Serum creatinine levels were similar in both patient groups.
Renal size and echogenicity did not influence the outcome in obstructed or non-obstructed patients.
This study raises a valid question. When should sonography be used in patients who experience a rise in serum creatinine? There are two major indications that have diagnostic and therapeutic implications:
1. History suggestive of obstruction (i.e., stones, known pelvic masses or tumors, or flank pain).
2. Evaluation of renal size, in particular, for disparity in kidney size. New onset or refractory hypertension with renal insufficiency in an older patient may suggest renovascular hypertension, which may be treated with angioplasty.
Not all obstruction, of course, will be picked up by renal sonography. Bladder outlet obstruction should be kept in mind in patients with suspected prostatic masses. Functional obstruction from a neurogenic bladder in diabetics and in elderly women may also be associated with renal insufficiency. Bladder catheterization may be both diagnostic and therapeutic; it establishes the diagnosis of obstruction, and renal function improves with catheterization. When ordering sonography, it would be better to order a kidney and bladder sonogram. It provides information regarding renal anatomy without catheterization, is non-invasive, risk-free, and, of course, there is an additional charge.