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Diagnosing renal artery stenosis can be difficult. This article explores possible types of imaging for this, ranging from safe, inexpensive tests to more invasive procedures
A 79 year old hypertensive man was referred for investigation of impaired renal function. At the time of referral he was taking a single antihypertensive agent and his blood pressure measured 170/85 mm Hg. He smoked 50 g of pipe tobacco a week. He had no other medical history of note. No renal artery bruit was detected, and the remaining clinical examination was unremarkable. His serum creatinine concentration at presentation was 116 mmol/l (normal range 60-110 mmol/l), representing an estimated glomerular filtration rate of 55 ml/min. Urine analysis was normal. His serum cholesterol concentration was raised (5.9 mmol/l (normal range 3.0-5.2 mmol/l). Over two years, his blood pressure remained raised despite an increase in his antihypertensive treatment, and his serum creatinine concentration rose to 206 mmol/l.
Unexplained, progressive renal impairment, and hypertension that is poorly controlled despite the use of multiple drugs, warrants further investigation. The absence of blood and protein in the urine points away from intrinsic renal disease. Both a prerenal cause (such as renal artery stenosis) and obstructive uropathy are possible diagnoses. Renal artery stenosis is an important diagnosis to make as it is common, potentially treatable, and associated with a higher mortality from end stage renal failure than other causes.1 2
Ultrasonography is the usual first investigation as it is safe, inexpensive, and widely available. Obstruction is generally easily detected, but occasionally a calculus causing intermittent obstruction may be missed. Size difference of >1.5 cm or a solitary kidney are the potential ultrasound features of significance in renal artery stenosis. Doppler studies are conducted for renal artery evaluation in some centres. The reported sensitivity ranges from 60% to 97%, and specificity from 85% to 99%,3 but the results are highly dependent on operator expertise, and interpretation is consistently accurate only in non-atheromatous arteries. Visualisation is limited by adipose tissue and bowel gas; accessory renal arteries may be missed; and mild stenosis is difficult to detect. Given the technical difficulties and time consuming nature, Doppler studies may be replaced by other, non-invasive methods of imaging the renal arteries.
Conventional 99mtechnetium-mercaptoacetyltriglycine (99mTc-MAG3) renography allows evaluation of the split function of the kidneys (contribution of each kidney to overall function) and exclusion of functional obstruction by assessing renal excretion and drainage. In the absence of obstruction, a difference in split function may be indirect evidence of renal artery stenosis. The use of an angiotensin converting enzyme inhibitor in conjunction with renography (captopril challenge scintigraphy) removes angiotensin II mediated vasoconstriction, which increases the difference in glomerular filtration rates of the stenotic kidney and the contralateral kidney. Sensitivities of captopril renography have been reported at 90%, with specificity of 79%.4 A positive scan indicates the presence of renovascular hypertension with a haemodynamically significant renal artery stenosis. False negatives arise in the presence of bilateral stenosis, overhydration, and chronic use of angiotensin converting enzyme inhibitors, and impaired renal function reduces the specificity of the examination. Owing to these limitations, renography is not widely used for non-invasive diagnosis of renal artery stenosis.
Non-contrast enhanced computed tomography may show a small calculus not seen by other methods if the history is suggestive. The introduction of multislice computed tomography with the possibilities of three dimensional reconstruction has made computed tomography angiography an important non-invasive means of identifying renal artery stenosis. Image interpretation may be difficult in heavily calcified arteries.
However, computed tomography angiography involves the use of ionising radiation and iodinated contrast medium. The procedure is therefore contraindicated in patients with contrast allergy and may not be tolerated by claustrophobic patients. The use of contrast in those with impaired renal function can result in contrast induced nephropathy. The most effective means of preventing this is by prior hydration. The use of various pharmacoprophylaxis agents and the benefits of iso-osmolar radiographic contrast agents remain controversial, with more evidence required. Computed tomography angiography is reported to have sensitivities of 90-98% and specificities of 85-94%.3
Magnetic resonance angiography has similar diagnostic accuracy to computed tomography angiography, with reported sensitivities of 90-100% and specificities of 88-100%.3 Comparable information is obtained about the number of renal arteries, the size of the kidneys, and the presence of anatomical variants. It has the benefits of producing angiograms without using iodinated contrast or radiation. Enhancement with gadolinium based contrast has generally replaced non-contrast enhanced techniques, but contrast associated nephrogenic systemic fibrosis in patients with moderate to end stage renal failure has been reported.5 (For further information visit www.mhra.gov.uk.) Contraindications to magnetic resonance imaging include use in patients with implants such as pacemakers, defibrillators, cochlear implants, and spinal cord stimulators. Claustrophobic patients should also be excluded.
Arteriography with digital subtraction improves contrast resolution by removing background information. It is an invasive technique, carrying a small but definite risk that is related to the arterial puncture and to the manipulation of the catheter and wire. Although angiography uses iodinated contrast and entails radiation, carbon dioxide may be used as a non-nephrotoxic contrast agent in patients with renal impairment or contrast allergy. Pressure gradients can be measured across areas of stenosis to determine haemodynamic significance where there is doubt, and therapeutic procedures such as percutaneous transluminal angioplasty or stenting can be carried out at the same time, although how much such an intervention improves renal function is the subject of ongoing trials.
Our patient had an ultrasound scan, which showed normal sized kidneys. As creatinine concentration improved and then deteriorated, 99mTc-MAG3 renography was conducted to look for functional obstruction. The renogram showed asymmetric function. As 69% of the overall function was from the left kidney, this raised the suspicion of underlying renal artery stenosis on the right (fig 11).). Magnetic resonance angiography showed single renal arteries bilaterally with bilateral stenoses and a slightly smaller right kidney (fig 22).). Our patient was encouraged to stop smoking and was started on simvastatin as his cholesterol concentration remained raised.
A year later, his serum creatinine concentration had increased from 116 mmol/l to 206 mmol/l and his blood pressure remained difficult to control despite an increase in his antihypertensive treatment. In view of the rising creatinine concentration, a decision was made to proceed to angioplasty and stenting in an attempt to preserve his renal function (figs 33,, 44 and 55).). The left renal artery was stented, but a guidewire could not be passed across the tight stenosis in the right renal artery. His serum creatinine concentration fell from 206 mmol/l to 163 mmol/l. His blood pressure improved from 170/85 mm Hg to 130/58 mm Hg with no change in medication.
This series provides an update on the best use of different imaging methods for common or important clinical presentations. The series advisers are Fergus Gleeson, consultant radiologist, Churchill Hospital, Oxford, and Kamini Patel, consultant radiologist, Homerton University Hospital, London
Contributors: All three authors contributed to the selection of the patient and the preparation and editing of the manuscript. AMB is the guarantor.
Competing interest: AMB has been reimbursed expenses by Boston Scientific, Johnson & Johnson, and Abbott and Gore—all manufacturers of arterial stents—for attending scientific conferences.
Provenance and peer review: Commissioned and externally peer reviewed.