A 67 year old male presents to the ER with a creatinine of 450 m/L. His JVP is low, his mucous membranes are dry and he has postural tachycardia and hypotension. He is felt to be volume depleted and IV normal saline is started. An abdomal ultrasound shows no evidence of hydronephrosis or hydroureter. 3 days later, the patient is clinically euvolemic but the serum creatinine is still 400 m/L.
Has obstruction been truly ruled out? Provide two explanations describing how the patient could be obstructed despite an unremarkable ultrasound on admission.
Obstruction is generally noted on ultrasound due to the accumulation of urine causing the physical distension of the collecting system. If a patient is obstructed but also severe volume contracted, they may not make enough urine to sufficiently distend the collection so that hydronephrosis and/or hydroureter is notable on ultrasound. However, a repeat ultrasound may show hydronephrosis and/or hydroureter after re-expansion of the extra-cellular fluid volume.
Some patients have renal insufficiency due to extrinsic compression of the collecting system, such as in retroperitoneal fibrosis or malignancy. In these cases, one will not see a dilated collecting system on routine ultrasonography.