Renovascular hypertension

Etiology and anatomical background

The main renal arteries are medium-sized vessels that branch off from the upper abdominal aorta at the level of the first lumbar vertebra (L1). Topographically, their ostia are located in close proximity to the origin of the celiac trunk and superior mesenteric artery (SMA). Normally, renal arteries can be single, but multiple anatomical variants are also common, usually with one distinct dominant artery.

The primary cause of renovascular hypertension is renal artery stenosis, most frequently resulting from two pathologies:

• Atherosclerosis (atherosclerosis): the most common cause. The atherosclerotic plaque is almost always located proximally, directly involving the artery ostium (ostium).
• Fibromuscular dysplasia (FMD): unlike atherosclerosis, this pathological process affects the vessel distal to its proximal portion, which creates certain diagnostic challenges.

Imaging modalities: CT and MRI

Two main modalities are used for the radiological diagnosis of stenosis: CT angiography (CTA) and MR angiography (MRA). The choice of a specific modality usually depends on institutional preferences and the radiologist's experience.

Imaging features:

1. The renal artery ostium is easily visualized on both CTA and MRA.
2. Evaluation of the distal segments, which is critically important to rule out fibromuscular dysplasia, is significantly hindered. This is due to the progressive decrease in vessel caliber and its continuous displacement during the cardiac cycle (motion artifacts persist even with perfect patient breath-holding).
3. Secondary signs of stenosis that can be assessed on scans include decreased perfusion of the affected kidney and a visible reduction in its size.

Advantages of MR angiography: MRA has the unique ability not only to visualize anatomy but also to measure blood flow using phase-contrast techniques (phase-contrast techniques). Renal parenchymal perfusion can be assessed directly: either by administering a contrast bolus or using arterial spin labeling (arterial spin labelling).

Important rule: renal artery imaging must always be accompanied by visualization of the adrenal glands. This is achieved using non-contrast CT or T1-weighted in-phase and opposed-phase MRI. Additionally, CTA and MRA are excellent for evaluating renal transplant arteries, although their variable location may require additional complex post-processing of images.

Evaluation of mesenteric vessels

Since atherosclerosis is a systemic disease, when examining the abdominal aorta, the radiologist simultaneously evaluates the mesenteric vasculature. The blood supply to the intestine comes from three main sources:

• Celiac trunk (coeliac trunk / coeliac axis): supplies derivatives of the foregut (foregut), which include the stomach and duodenum down to the duodenojejunal flexure (duodenojejunal flexure).
• Superior mesenteric artery (SMA): supplies derivatives of the midgut (midgut)—the area from the duodenojejunal flexure to a variable point in the colon (most commonly the splenic flexure).
• Inferior mesenteric artery (IMA): supplies derivatives of the hindgut (hindgut), including the remaining part of the colon down to the lower third of the rectum.

The celiac trunk and SMA have a predictable origin from the aorta at the T12/L1 level, making them easy to evaluate on CTA and MRA (stenosis here is also almost always ostial). The IMA has a significantly smaller caliber and is traditionally considered more challenging to analyze, but modern scanners allow for its stable and clear visualization.

Acute and chronic mesenteric ischemia

Risk factors leading to renovascular hypertension also predispose to mesenteric ischemia. It is important to understand that the acute and chronic forms are two completely different diseases connected only by a shared vascular network.

Chronic mesenteric ischemia (CMI), also known as "intestinal angina" (intestinal angina), results from prolonged atherosclerotic occlusion. To establish a definitive imaging diagnosis, severe stenosis or complete occlusion must be identified in at least two of the three major mesenteric arteries.

Acute mesenteric ischemia (AMI) is a life-threatening condition requiring emergency intervention. In most cases, it presents as a sudden embolic occlusion of the SMA, which rapidly leads to fatal small bowel infarction without active treatment.

Sources of thrombi and pathogenesis of AMI:

• Cardiogenic emboli: form in the left atrial appendage in atrial fibrillation or in a hypokinetic left ventricle after a myocardial infarction.
• Atheroma at the ostium of the SMA, fragments of which cause distal embolization.
• Acute thrombosis occurring over a pre-existing unstable atherosclerotic plaque.
• Rare causes: vasculitis (vasculitis), abnormally low blood flow states (low-flow state), or intestinal malrotation (malrotation).

Massive ischemia triggers the release of inflammatory mediators and early tissue necrosis. The primary clinical marker that raises suspicion for AMI is pain out of proportion to physical findings (pain out of proportion to physical findings). The diagnosis is confirmed on CTA by the detection of a filling defect in the SMA. An exception is bowel infarction occurring against the background of severe hypotension: in such cases, a proximal embolus within the vessel lumen is absent, and the diagnosis is based exclusively on secondary signs (for example, specific changes in the bowel wall itself).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Methods for assessing renal perfusion: phase-contrast MRI and arterial spin labeling
• Radiological semiotics of fibromuscular dysplasia of the renal arteries
• Diagnostic criteria for chronic mesenteric ischemia ("abdominal angina")
• CT presentation of secondary bowel wall changes in hypotensive infarction
• Scanning and post-processing technique for renal transplant arteries

Other topics in the "Interventional radiology" section

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