Renal vascular malformation with features suggestive of arteriovenous shunting presenting as hemorrhagic medullary cyst-like lesions: a clinicopathologic diagnostic pitfall

Renal arteriovenous malformations (AVMs) are uncommon vascular anomalies that may present with persistent hematuria and pose significant diagnostic challenges due to their variable clinical and radiologic manifestations. Although imaging plays a central role in diagnosis, some lesions remain unrecognized until surgical excision, particularly when they lack a discrete mass and mimic other renal lesions. We report the case of a 70-year-old male who presented with intermittent gross hematuria of three weeks' duration, occurring every 2–3 days, approximately two months before definitive diagnosis. The bleeding was initially attributed to benign prostatic hyperplasia based on ultrasonographic evidence of marked prostatic enlargement (>150 g). Baseline evaluation revealed mild anemia (hemoglobin 11 g/dL) with stable vital signs. The patient underwent transvesical prostatectomy; however, hematuria persisted postoperatively. Subsequently, he developed acute urinary retention requiring cystoscopic clot evacuation, and cystoscopy demonstrated no active bleeding from the prostatic fossa, making a lower urinary tract source less likely. Contrast-enhanced computed tomography demonstrated a non-excreting left kidney with an obstructing ureteral clot and blood within the pelvicalyceal system, suggesting a renal source of hemorrhage; however, no definitive vascular lesion was identified preoperatively. The patient developed profound anemia with a hemoglobin level of 4 g/dL and required transfusion of six units of whole blood. Due to persistent hemorrhage and suspicion of an underlying renal pathology, total nephrectomy was performed. Gross examination revealed blood clots within the calyceal system and multiple small hemorrhagic spaces localized to the medullary papillary region without a discrete renal mass. Histopathologic examination demonstrated extensive replacement of the renal medulla by a disorganized network of tortuous, variably sized vascular channels embedded within delicate fibrocollagenous stroma. The abnormal vessels showed marked heterogeneity in wall thickness, including thin-walled dilated vascular channels and larger muscular vessels demonstrating irregular asymmetric mural thickening suggestive of arterial-type differentiation, accompanied by ectatic dilatation, papillary-like luminal outpouchings, and focal thrombus formation. Overall, the lesion was interpreted as a renal vascular malformation with features suggestive of arteriovenous shunting; however, definitive classification as a fully characterized arteriovenous malformation was limited by the absence of angiographic correlation and elastic fiber staining. This case highlights a distinctive gross and histopathologic presentation of renal AVM and emphasizes the importance of considering vascular malformations in patients with persistent hematuria despite treatment for more common urologic causes.