Robert Hartman
Department of Diagnostic Radiology, Mayo clinic, Rochester MN
3.1 Introduction
Currently it is estimated that approximately 50 million or more Americans suffer from hyper- tension. The worldwide figures may include as many as 1 billion individuals with over 7 million deaths per year attributable to the disease. The vast majority of these individuals have essential hypertension but a significant subset have a secondary cause for the hyper- tension. Within this group of patients the largest single cause of secondary hypertension is renal mediated hypertension, of which renal artery stenosis is the leading pathology. It is estimated that as many as 5% of all cases of hypertension in the entire population is caused by renal artery stenosis equating to as many as 3 million cases in the USA of this poten- tially treatable form of hypertension. The detection of renal artery stenosis is of particular importance to ensure this group of patients with a potentially curable form of hypertension is identified and treated properly. Treatment of renal artery stenosis by angioplasty, with or without stenting, may cure or improve hypertension in many cases.
3.2 Clinical features
The initial evaluation of a patient with hypertension should include a comprehensive history and physical. Information from the examination should be focused on answering questions regarding:
• Lifestyle assessment and identification of other cardiovascular risk factors that may affect prognosis and help to guide treatment.
• Detection of identifiable causes of hypertension.
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44 CH 3 IMAGING OF RENAL ARTERY STENOSIS
• Assessment for the presence or absence of end-organ damage and cardiovascular disease.
Potential causes of secondary hypertension include:
• Renal artery stenosis
• Chronic kidney disease
• Primary hyperaldosteronism
• Coarctation of the aorta
• Cushing’s syndrome
• Pheochromocytoma
• Thyroid/parathyroid disorders
In certain cases a secondary cause of hypertension can be suspected. These situations include:
• Patients whose age, history, physical examination, severity of hypertension, or initial laboratory findings suggests such causes.
• Patients whose BP responds poorly to multidrug therapy.
• Patients whose BP begins to increase for uncertain reason after being well controlled.
• Patients with sudden onset of hypertension.
In particular, findings that suggest renal artery stenosis as the cause of secondary hyper- tension include:
• Rapid onset of hypertension or uncontrollable hypertension in a middle age female.
• Rapid onset of hypertension in patient over the age of 55.
• Abdominal bruit.
• Azotemia occurring on angiotensin converting enzyme (ACE) inhibitor or angiotensin receptor blocker (ARB).
• Previously well controlled hypertension becoming more resistant to drug therapy.
• Flash pulmonary edema.
• Renal failure of unknown cause.
• Hypokalemia.
In cases where there is a high clinical suspicion of renal artery stenosis and secondary hypertension radiological evaluation can often be helpful.
3.3 Pathology
Renal vascular hypertension, particularly hypertension secondary to renal artery stenosis, is due to activation of the renin-angiotensin cascade. The affected kidney experiences decreased blood flow and activates the renin-angiotensin cascade in order to establish a normotensive state. The cascade consists of:
• Renin released form the juxtaglomerular cell of the kidney.
• Circulating renin in the blood leads to conversion of angiotensinogen to angiotensin I.
• Angiotensin converting enzymes (ACE) in the lung convert angiotensin I to angiotensin II.
• Angiotensin II has effects on the adrenal glands stimulating release of aldosterone. It also has direct effect on the blood vessels causing vasoconstriction.
• Aldosterone affects the kidney resulting in Na+ and H2O retention leading to overall vascular volume increase.
• The combined effects of vasoconstriction and increased intravascular volume result in systemic hypertension.
3.4 Imaging of suspected renal artery stenosis
A number of exams utilizing a variety of modalities are available for the evaluation of suspected renal artery stenosis (RAS). These include:
• Grayscale and Doppler US
• ACE inhibitor Nuclear Medicine Renogram
• Computed tomography (CT) angiography
• Magnetic resonance (MR) angiography
• Digital subtraction arteriography
The choice of initial examination requires consideration of a number of factors including the availability of a particular exam, the invasiveness of a particular examination, the level of comfort of radiology personnel performing and interpreting individual examinations, as well as the suspected etiology of the RAS. The initial examination of choice will therefore vary from practice to practice in many instances.
In general catheter directed digital subtraction arteriography (DSA) is only used as the initial exam in cases where the clinical suspicion of RAS is very high and intervention is anticipated. Although DSA is the gold standard for the evaluation of RAS, allowing the performing Radiologist the ability to identify anatomic areas of stenosis as well as determine the presence or absence of a pressure gradient across the stenosis, the invasiveness of the exam has delegated it to use primarily in the treatment of stenoses identified via less invasive modalities (Fig. 3.1).
46 CH 3 IMAGING OF RENAL ARTERY STENOSIS
(a) (b)
Figure 3.1 Conventional catheter directed angiogram (a) demonstrating an eccentric high grade stenosis near the origin of the left renal artery (arrow) secondary to atherosclerotic disease.
Angiogram following angioplasty and stent placement demonstrating complete reduction of the stenotic segment (b).
The following sections describe the remaining modalities used for the evaluation of suspected RAS and particular typical findings.