What Is Kidney Angiomyolipoma?

Medically reviewed by Kashif J. Piracha, MD

An angiomyolipoma is a type of tumor that usually occurs in your kidneys. It is the most common benign (noncancerous) solid renal (kidney) tumor type.

This tumor comprises thick-walled blood vessels, adipose (fat) tissue, and smooth muscle tissue. The proportions of these parts can vary by tumor type. While these tumors are likely to develop without a known cause, they often occur in those with a certain type of genetic disease.

In most cases, symptoms don't appear until this type of tumor has grown and spread. The problem can lead to a severe type of hemorrhage (bleeding) that can threaten your life without treatment.

This article describes angiomyolipoma symptoms, causes, diagnosis, treatment, and outlook for the problem.

Types of Angiomyolipoma

Types of angiomyolipomas are commonly classified according to the makeup of the tumor. Most angiomyolipomas contain dilated blood vessels, smooth muscle cells, and mature adipocytes (certain fat cells.)

Generally, the classification of an angiomyolipoma considers these three components in identifying a tumor as one of the following types:

Typical (triphasic) angiomyolipomas: These tumors make up the majority of angiomyolipomas. They contain all three components in various proportions.

Atypical (monophasic or epithelioid) angiomyolipomas: These tumors are less common. They are further divided according to the following criteria:

• Monophasic variant: Monophasic variants primarily contain one of the three tumor elements, with slight amounts of the other components

• Epithelioid (soft tissue) variant: Epithelioid variants contain numerous epithelioid muscle cells with few or no fat cells. These tumors have the highest tendency to become malignant (cancerous).

Learn More:What Is Kidney Disease?

Angiomyolipoma Symptoms

It is common to have an angiomyolipoma without any symptoms. More than 80% of these tumors are found by chance during imaging tests for other issues.

Though rare, symptoms typically occur when the tumor grows to 40 millimeters (mm) (a little over 1.5 inches) or larger and begins to spread. In tumors of this size, symptoms can occur in 80% of cases and can include the following:

• Flank pain (pain between your back and abdomen on one or both sides of your body)

• A palpable mass (a mass that can be felt)

• Hematuria (blood in your urine)

• Newly diagnosed high blood pressure

• Anemia

• Urinary tract infection (UTI)

• Renal (kidney) failure

A 40 mm or larger angiomyolipoma can cause spontaneous retroperitoneal hemorrhage, also called Wunderlich syndrome. This rare but potentially life-threatening condition involves bleeding in the area behind your abdominal cavity. It affects fewer than 15% of people who have these tumors.

Learn More:When to See a Kidney Doctor

Causes

The specific causes of angiomyolipomas are unknown. About 80% of these tumors occur as isolated masses. These tumors have a higher prevalence in women. Most are found when people are in their late 40s or early 50s, though they can occur at any age.

The remaining 20% of angiomyolipomas typically occur in patients with tuberous sclerosis complex (TSC), a rare genetic disease that causes benign tumors in organ systems like the kidneys. Angiomyolipomas occur in up to 75% of patients with tuberous sclerosis complex. There usually is more than one tumor and they're bilaterally located (found on both sides).

How Is Angiomyolipoma Diagnosed?

Most angiomyolipomas contain fat tissue that is visible in imaging studies. Since the detection of adipose tissue is considered the primary criterion for the diagnosis of classic angiomyolipomas, diagnosis is usually based on one of the following types of imaging:

• Computed tomography (CT) scan: A CT scan uses multiple X-rays to create a three-dimensional (3D) image of a targeted area of your body. While contrast-enhanced CT is not usually necessary for diagnosing this type of tumor, it may be used when there is a potential for rupture or bleeding.

• Magnetic resonance imaging (MRI): MRI uses radio waves and a strong magnetic field to create two-dimensional (2D) or 3D images of your organs, tissues, and skeleton.

While imaging studies are usually sufficient to diagnose an angiomyolipoma, the findings may be inconclusive in cases of low-fat or fat-invisible angiomyolipomas. For these tumors, a percutaneous renal biopsy is advised. This procedure uses a thin needle inserted through the skin to extract kidney tissue samples for analysis.

The findings from a percutaneous renal biopsy can distinguish a low-fat or fat-invisible tumor from papillary renal cell carcinoma, which may be easily confused with a fat-invisible angiomyolipoma.

Treatment Options for Angiomyolipoma

Treatment of angiomyolipomas is usually limited to a small number of patients who have this condition. Most tumors are small, benign, and exist without symptoms.

Watchful waiting, which involves observing the condition for signs of worsening, is usually advised for these cases. Annual or semiannual imaging studies may be used to track the risk of rupture.

The primary indications for treatment involve preventing the rupture of the tumor. Certain factors increase the risk of rupture. Treatment may be advised if you have an angiomyolipoma and any of the following risk factors for rupture:

• Presence of symptoms

• Pregnancy or being a woman of childbearing age

• Suspicion of a malignant tumor

• Tumor size larger than 40 mm

When necessary, treatment involves one of the following:

• Transarterial embolization: Transarterial embolization is regarded as the first line of treatment for bleeding and symptomatic tumors. The procedure involves inserting a catheter (a thin, flexible tube) into an artery near the angiomyolipoma and adding chemical agents to block the blood supply to the tumor.

• Medication: A group of immunosuppressive drugs classified as mTOR inhibitors can help control tumor cell division. This can help reduce tumor growth and the risk of tumor rupture without surgery. Research using Afinitor (everolimus), an oral mTOR inhibitor, has proven effective.

• Surgery: Surgery can be useful in treating large, symptomatic angiomyolipomas. Nephron-sparing surgery, or partial nephrectomy, is the optimal surgical approach for these types of tumors. It involves removing the tumor while preserving as much of your kidney as possible.
  

Prognosis

Research indicates that up to 91% of angiomyolipomas are asymptomatic with slow growth, averaging a rate of 0.02 centimeters (cm) a year. However, angiomyolipomas with tuberous sclerosis complex tend to grow faster and have a higher transformation rate than malignant tumors.

The frequency of symptoms increases with the size of an angiomyolipoma. As these tumors grow to 40 mm and larger, treatment is usually advised to counter the risk of tumor rupture. The spontaneous rupture of angiomyolipomas is the most common cause of retroperitoneal bleeding, which can become life-threatening.

Coping

Small, slow-growing angiomyolipomas typically don't cause symptoms, so you may not know the problem exists until it is discovered during an imaging study for another reason. If you are diagnosed with this type of tumor, you may be advised to continue to follow up with your healthcare provider so your tumor can be monitored for changes that could increase the risk of rupture.

An angiomyolipoma that causes symptoms or increases in size may require treatment, though it may be difficult to determine the source of your pain without imaging studies. Consult a healthcare provider if you notice any changes in the appearance and sensation of your abdominal area. This can help you identify problems with undiagnosed or diagnosed angiomyolipoma as early as possible.

Summary

An angiomyolipoma is one of the most common benign kidney tumors. It usually occurs without symptoms. It is often found during a CT scan or MRI for another health problem.

The most common type is small and grows slowly. Treatment is usually unnecessary, though the tumor may be observed for growth and changes.

There are no known causes for this type of tumor. A rare genetic disease called tuberous sclerosis complex raises your risk of having it. Tumors, in these cases, are more likely to grow faster and become cancerous.

Larger tumors have a higher risk of rupture in the abdominal cavity. Treatment is used to reduce this risk or to treat bleeding when it occurs. Without treatment, bleeding can have severe outcomes.