What is renal artery stenosis - causes, diagnosis and treatment

Our kidneys are the powerhouses that remove waste and excess fluid from our bodies and like all our organs, they need blood to function correctly. However, when renal artery stenosis affects the vessels supplying one or both of our kidneys there may not be sufficient blood delivered to the kidneys to keep them healthy. Dr Christopher Lawrence, a highly trained nephrologist, explains what this condition is, how it’s diagnosed and the treatment options available.

What is renal artery stenosis?

Renal artery stenosis (RAS) is a narrowing of the main artery that supplies the kidney with blood. This narrowing can result in a reduction of blood flow to the kidney(s) and as a result, a decline in kidney function.

What are the causes of renal artery stenosis?

The main cause of renal artery stenosis is atheromatous disease, which is when the arteries become furred up and eventually blocked by cholesterol and calcium plaques. This can occur in exactly the same way that coronary arteries (arteries of the heart) might become narrowed, although whilst coronary artery disease causes chest pain, it’s rare for renal artery stenosis to cause kidney pain (unless the artery becomes blocked suddenly).

What are the risk factors?

The risk factors for renal artery stenosis are:

• Ageing
• Being male
• Having a history of smoking
• Having a history of arterial disease in other areas of the body
• High cholesterol

What are the symptoms of RAS?

Renal artery stenosis can be entirely asymptomatic, meaning that it can show absolutely no symptoms at all. In some people, renal artery stenosis results in hypertension (high blood pressure) and fluid retention, which is characterised by swollen ankles and breathlessness due to fluid in the lungs.

If the artery that carries blood to the kidney blocks suddenly, it can result in loin pain but this is rare. Usually, RAS does not cause any pain.

Sometimes, a symptom is a decline in kidney function. This decline may be caused by the addition (or an increased dose) of certain blood pressure tablets, such as angiotensin-converting-enzyme inhibitors (e.g. ramipril) or angiotensin receptor blockers (ARBs). Whilst these drugs are thought to protect the kidneys, they work by dilating the blood vessels taking the blood away from the kidneys which can cause a drop in pressure in the blood reaching the kidney. Consequently, this can mean that too little blood flow reaches the kidney and as a result, the kidney can’t function fully.

How is Renal artery stenosis diagnosed?

RAS is diagnosed by talking to and examining the patient.

• A urine dipstick is usually unreliable in the sense that blood and protein levels may look normal.

• A renal ultrasound may offer results that appear to be normal, or it may show slightly small or shrunken kidneys, or classically ‘renal asymmetry’ which is where one kidney is smaller than the other (when only one kidney has been affected by RAS, or when one is more affected than the other).

• It is possible to do a doppler ultrasound of the renal arteries to try to detect any changes in blood flow that result from narrowed arteries, but this test is highly specialised and dependent on the expertise of the sonographer; it should probably only be used, if at all, as a guide.

• A magnetic resonance angiogram (MRA) or a CT renal angiogram (CTRA) are the best non-invasive tests used to look for renal artery stenosis, despite their potential to over-estimate the extent that the arteries have narrowed to. Each test has its pros and cons; both are relatively expensive tests, the MRA may not be appropriate for those who suffer from claustrophobia and the CT involves a dose of X-rays and usually requires an injection of iodinated contrast (the contrast can cause concern when renal function is poor).

• A direct renal angiogram (direct visualisation of the artery by injecting contrast dye directly in to the aorta just above the renal arteries) is the definitive test for renal artery stenosis. An angiogram is, of course, an invasive test, and the operator needs to be an experienced interventional radiologist. Another advantage of an angiogram is that it is possible to do ‘pressure wire studies’ in order to prove that any narrowing seen is indeed causing a reduction in blood flow to the kidneys. It is conducted by placing an extremely small pressure monitor into the coronary artery

How is RAS treated?

Another advantage of direct renal angiograms is that if found, the renal artery stenosis may be treated at the same time without the need for any further tests. Alternatively, it may be necessary for the radiologist and the nephrologist to discuss the case and agree on the best course of action (which might mean the patient has to come back another day for definitive treatment).

Renal artery stenosis is treated by using a balloon and usually a stent to open up the artery, in exactly the same way that cardiologists have treated chest pain for years. While in cardiology there is usually direct evidence of benefit, in renal artery stenosis it is less clear. Some people use this as an excuse not to treat renal artery stenosis with intervention but it is quite clear that some patients, especially those with a significant narrowing to the early part of the renal artery can benefit, and especially when it is to a single functioning kidney of preserved size (as in the case where the renal artery stenosis has caused one kidney to shrink already and the remaining, functioning, kidney is at imminent risk).

An experienced nephrologist working with an equally experienced radiologist is best placed to present the available options to people with renal artery stenosis.

It is rare for a renal artery angioplasty (usually combined with stenting) to result in an improvement in kidney function (although I have certainly encountered this); usually we hope to reduce the rate of decline of kidney function, and sometimes improve blood pressure and symptoms of salt and water retention.

Medical management of renal artery stenosis includes reducing or stopping the use of angiotensin-converting-enzyme inhibitors (ACEi) and angiotensin receptor blockers (ARBs), and instead giving the patient a low dose aspirin, a high dose of statins, and controlling the blood pressure by other means.

Dr Christopher Lawrence is a leading nephrologist and general physician who has a wealth of experience in treating kidney problems. If you’re concerned about your kidneys, don’t hesitate to book an appointment with him via his profile.