Editorial Comment from Dr. Kato to Recurrent urinary retention due to clots caused by a congenital renal arteriovenous malformation that forms a complex vascular network: Report of two cases.

Renal arteriovenous malformations (AVMs) are a rare disease that is characterized by anastomotic and abnormal communications between renal arterial and venous systems. Among them, congenital renal AVMs are rare renal vascular abnormalities which is present in approximately 20% of all renal AVM cases with a prevalence of 0.04% in general population and usually remain asymptomatic until the age of 30 or 40. These are a rare but sometimes fatal cause of hematuria because about 30% of patients may present with signs of congestive heart failure from high‐output fistulas, and up to 50% with cardiomegaly and hypertension. So far, congenital AVMs are subdivided into three categories; type I, a single or few arteries (<4) shunt into a single draining vein; type II, multiple arterioles shunt into a single draining vein; and type III, multiple shunts are present between the arterioles and venules that form a complex vascular network. Especially, type III renal AVMs pose a diagnostic challenge in clinical settings.

In the present study, Ishii et al. reported two renal congenital renal AVM cases that were adequately diagnosed by renal angiography. Importantly, a contrast enhanced computed tomography was not enough to make a diagnosis of these AVM cases since type III AVMs consists of an arteriovenous fistula with a complex vascular network in a small branch of renal artery. In addition, type III AVMs are difficult to be diagnosed with ultrasonography because an arteriovenous fistula normally forms complex and abnormal vascular networks. Considering these features, although the study contained the limitation of a small number of cases, we physicians need to consider that type III AVMs can be one of the causes for persisting hematuria that is not diagnosed by several types of examination.

To promote better understanding of renal AVMs, future studies should investigate the recurrence rate of arteriovenous fistula after angiographic embolization. In addition, it is necessary to develop a new diagnostic method for patients with impaired renal function (e.g., magnetic resonance angiography) since contrast agent is hard to use in these patients.

Conflict of interest

The authors declare no conflict of interest.