A 63-year-old man with a history of nephrolithiasis and neurogenic bladder requiring chronic indwelling urinary catheter was admitted for the management of urinary tract infection and acute kidney injury (AKI) with a serum creatinine level of 3.6 mg/dL (baseline approximately 1.9-2.2). He was initially brought to the emergency room for altered mental status. The patient was afebrile, but laboratory studies were significant for leukocytosis and pyuria. Bedside renal sonogram did not demonstrate hydronephrosis or nephrolithiasis (Figures 1A and B). Non-contrast computed tomography (CT) scan of the abdomen also was negative for hydronephrosis but showed a 2.6-cm staghorn-appearing calculus in the right kidney (Figure 1C). There was no perinephric stranding. Because the patient appeared dehydrated, he was treated with intravenous hydration along with antibiotics. His mental status improved but renal function continued to worsen. Because of high index of suspicion for obstructive nephropathy, a radioisotope furosemide renogram was performed, which demonstrated complete obstruction on the right (Figure 2A). Moreover, the right kidney had a differential function of approximately 70%, which explains AKI with unilateral obstruction. Interestingly, a repeat renal sonogram demonstrated mild hydronephrosis and calculus in the right kidney (Figures 2B and C). Serum creatinine improved significantly after nephrostomy tube placement and reached baseline value in a few days.
Sensitivity of ultrasound for the detection of stones and hydronephrosis varies widely depending on stone size (low for those <3 cm), patient’s body habitus, and operator expertise.1, 2 On the contrary, sensitivity of CT for detecting renal stones is the highest of all the available imaging modalities and is estimated to be approximately 97%.3 The diagnosis of obstructive urinary tract stones on ultrasonography depends on identification of the offending stone and concomitant hydronephrosis, and false-negative findings may occur in settings such as volume depletion and retroperitoneal fibrosis because pelvis and calyces fail to dilate despite obstruction.4, 5 It is estimated that over 30% of acute obstructions will be missed by ultrasonography in patients who are not specifically hydrated for the procedure,3 and interestingly, in the study that reported highest sensitivity of ultrasound for the detection of hydronephrosis (97%), patients were administered 500 ml of intravenous fluid prior to the scan.6 Therefore, it is worthwhile to repeat renal imaging after rehydration in cases where the likelihood of obstructive nephropathy is high.
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-Abhilash Koratala, MD, Gajapathiraju Chamarthi, MD, Justin Lee Loy, MD, Olanrewaju A Olaoye, MD
This article originally appeared in the December issue of The American Journal of Medicine.
