Renal Arteriovenous Malformation

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Renal arteriovenous malformations (AVMs), first described in 1928 by Varela, are abnormal communications between the intrarenal arterial and venous systems. They cause hematuria and are associated with hypertension.

Renal AVMs may be either congenital or acquired (often by iatrogenic means). More commonly, the term refers to the congenital type of malformation. Two subtypes of congenital renal AVMs are described, as follows:

On the other hand, acquired renal arteriovenous anomalies are often termed renal arteriovenous fistulas (AVFs). Idiopathic renal AVFs have the radiographic characteristics of acquired fistulas, but no cause can be identified. They may be associated with intrarenal artery aneurysms that erode into a vein.

Renal AVMs are usually identified during the evaluation of gross hematuria. Treatment can be tailored to the individual patient. Angiographic embolization is the preferred treatment for symptomatic AVMs and has been used since the mid-1970s. Nephrectomy and partial nephrectomy are more invasive treatment options. The first planned nephrectomy was accomplished in 1869 by Simon for the treatment of ureterovaginal fistula. The first partial nephrectomy was performed for a nonmalignant renal mass by Wells in 1884.

Renal AVMs remain an uncommon clinical problem. However, the incidence may increase as the frequency of incidental renal masses increases. Small renal masses on abdominal imaging studies performed for other symptoms are becoming more common. Categorizing these masses as benign or malignant in an economic and safe manner has received much attention. Asymptomatic renal AVMs are a rare cause of the incidental mass, but several case reports describe clinical situations where a renal AVM was classified incorrectly as a malignant tumor or as hydronephrosis.

Specific computed tomography (CT) protocols seem especially promising as a minimally invasive way to improve the classification of renal masses. In addition, improvements in magnetic resonance imaging (MRI), magnetic resonance angiography (MRA), and Doppler ultrasonography (US) may decrease the need for the use iodinated contrast agents.

For patient education resources, see Blood in the Urine.

Knowledge of renal vascular anatomy is important in understanding diagnostic studies and planning therapy.

The renal artery is an end-organ branch from the aorta. Supernumerary renal arteries are common (≥25% of patients). The renal artery branches into four or five segmental renal arteries. The first branch is the posterior branch, which supplies the posterior segment of the kidney. The main artery then enters the renal hilum before dividing into the other segmental branches.

These branches of the renal artery supply minimal collateral circulation among the renal segments. The lobar renal arteries are located within the renal sinus and are branches of the segmental arteries.

The lobar arteries divide into the interlobar arteries, which are within the renal parenchyma. The interlobar arteries are in close proximity to the collecting system. The interlobar arteries divide into the arcuate arteries, which lead to the interlobular arteries.

The interlobular arteries lead to the afferent arterioles, which feed each glomerulus. Blood flows from the glomerulus to the efferent arteries, which lead to the vas recta, which, in turn, provides the network for venous drainage of the kidney.

The venous drainage follows the same pattern of branching as the arteries. However, unlike the arterial system, significant connections exist between the renal segments within the venous system.

Cirsoid AVMs are usually larger than 1 cm in diameter and are located adjacent to the collecting system. Cavernous AVMs are less than 1 cm in diameter and are usually located near the periphery. Aneurysmal AVMs are larger than 1 cm in diameter and are located near the renal hilum.^[1]

In the cirsoid congenital AVM, multiple communications exist between the arteries and veins. These communications develop multiple coiled channels, forming a mass within the renal parenchyma. The communicating vessels are tortuous, dilated, and located beneath the lamina propria of the renal urothelium. This cluster of vascular channels forms a mass, with the arterial supply arising from one or more segmental or interlobar renal arteries.

Microscopic features of the arteries and veins involved are identical to those of their normal soft-tissue counterparts. Occasionally, there may be some associated thromboses. Their nearness to the collecting system may explain the high prevalence of hematuria.

The less common cavernous congenital AVM is characterized by a single artery that feeds into a single cystic chamber, with a single draining vein.

Acquired AVMs result from traumatic disruption of renal vessels. A fistulous connection between the arterial and venous systems occurs as a result of the trauma.

Any renal AVM may result in renin-mediated hypertension.

The etiology of congenital AVMs is unknown. Conversely, the cause of acquired AVMs is usually known.

Percutaneous renal biopsy is the most common known cause of acquired renal AVF. An estimated 15-50% of biopsies result in some degree of fistula formation. In one study in which arteriograms were performed after every renal biopsy, radiographic evidence of fistula was identified in 15% of patients.

Trauma is another important, though uncommon, cause of acquired renal fistulas. In patients with hypertension following renal trauma, renal AVMs may occur in one third of patients. In patients with penetrating trauma, AVFs may affect as many as 80% of patients with posttraumatic hypertension. Trauma during ureteroscopy or percutaneous nephrostolithotomy or after partial nephrectomy has been described as a cause of intrarenal AVF.^[2]

Idiopathic AVFs are thought to arise from the spontaneous erosion or rupture of a renal artery into a nearby renal vein.

AVMs may also occur in the setting of malignancy. Renal cell carcinoma has a vascular predilection, with renal vein extension and parasitic tumor vessels both being relatively common. Angiogenic tumor factors have been implicated and may explain the development of AVMs within renal tumors.

Renal AVMs are uncommon. The estimated rate in large autopsy series is lower than 1 case per 30,000 patients. In clinical studies, which usually include patients undergoing evaluation with urologic or vascular imaging techniques, the incidence ranges from 1 case per 1000 patients to 1 per 2500. They account for fewer than 1% of all types of AVMs among the general population.

Congenital AVMs account for fewer than one third of renal AVMs. Most of these are the classic cirsoid type. Congenital cirsoid AVMs have a dilated, corkscrew appearance, much like a varicose vein. Cavernous AVMs, with single dilated vessels, account for the remainder of congenital malformations.

Acquired AVFs are the most common and represent as many as 75-80% of renal AVMs.

Idiopathic renal AVFs represent fewer than 3% of renal AVMs.

The international incidence of renal AVMs is influenced by the prevalence of percutaneous renal surgery and biopsies because these interventions cause most of the acquired renal fistulas.

Endovascular therapy with embolization is considered the treatment of choice for AVFs and AVMs because it allows preservation of the unaffected renal parenchyma. A study by Takebayashi et al successfully embolized 30 cases of congenital AVM.^[3]About 60% of patients responded to embolization; however, improvement of hypertension may take up to 2-3 months.

Eom et al retrospectively assessed technical and clinical success rates, radiologic and laboratory findings, and complications of renal artery embolization for 31 renal AVMs in 24 patients.^[4]The clinical success rate after initial embolization was 67%; the overall clinical success rate, 88%; and the technical success rate, 65%. There were 11 technical failures in 10 patients. In four, clinical success was attained without additional embolization; in three, a second embolization session yielded clinical success; and in three, recurrence necessitated nephrectomy. The authors noted that technical failure did not always result in clinical failure and that multiple embolizations may be effective for recurrence.

Nephrectomy remains an alternative option for treating renal AVMs. Hematuria due to an AVM resolves following nephrectomy, and hypertension is cured or improved in 60-85% of patients.

Further, with advances in available techniques, angiographic embolization treatment is the usual first line of therapy because it can be accomplished at the time of diagnosis, with little morbidity.

Most acquired renal fistulas resolve spontaneously.

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Mark R Wakefield, MD Associate Professor of Surgery/Urology, Chief, Division of Urology, University of Missouri-Columbia School of Medicine; Director, Renal Transplantation, University Missouri Health Care

Mark R Wakefield, MD is a member of the following medical societies: American College of Surgeons, American Society of Transplant Surgeons, American Society of Transplantation, American Urological Association, Missouri State Medical Association, Society of University Urologists