Renal arteriovenous malformation mimicking hydronephrosis-hidden danger.

Renal arteriovenous malformations (AVMs) are rare, with an incidence of approximately 0.04%. Diagnosis is often challenging due to mimics of AVMs. We report a case of renal AVM mimicking hydronephrosis on ultrasound and unenhanced computed tomography (CT). A 24-year-old female with background of recurrent urinary tract infections (UTIs) presented to the Accident and Emergency department with 1 day history of bilateral flank pain, dysuria, rigors and pyrexia. Urine dipstick showed microscopic haematuria and blood tests showed mild neutrophilia. Dilated right renal pelvis was seen on ultrasound. Unenhanced CT of the urinary tract demonstrated right hydronephrosis with no evidence of calculi. Subsequent Uro-radiology meeting discussion concluded that renal pelvis might be pus-filled and recommended an urgent nephrostomy. However, ultrasound Doppler scan performed at the time of the planned nephrostomy demonstrated colour flow within dilated renal pelvis suggestive of an AVM. Nephrostomy was abandoned and subsequent CT angiogram confirmed a large congenital AVM. The patient was referred for embolization.Colour flow ultrasound imaging is a simple and quick technique to diagnose AVMs. However, as in our case, when colour flow Doppler imaging was not used at the initial ultrasound, the opportunity to obtain an accurate diagnosis was missed. If the subsequently planned nephrostomy had taken place, this may have led to potentially serious outcomes. We suggest that colour flow imaging should be used prior to nephrostomy insertion to differentiate hydronephrosis from vascular abnormalities.