Renal Artery Stenosis for ABIM

Renal Artery Stenosis for the American Board of Internal Medicine Exam

Renal artery stenosis (RAS) is the narrowing of one or both renal arteries, leading to decreased renal perfusion. This condition commonly results in secondary hypertension and can progress to ischemic nephropathy or chronic kidney disease (CKD).

Atherosclerosis:
The most common cause of RAS, accounting for approximately 90% of cases. It typically affects older patients, especially those with traditional cardiovascular risk factors such as hypertension, diabetes mellitus, hyperlipidemia, and smoking. Atherosclerotic plaques form at the origin or proximal segment of the renal artery, progressively narrowing the lumen and reducing blood flow.
Fibromuscular Dysplasia (FMD):
A non-atherosclerotic, non-inflammatory condition, typically affecting women under 50 years of age. FMD causes abnormal cellular growth in the arterial wall, leading to a characteristic “string of beads” appearance on angiography. FMD tends to affect the mid to distal renal artery segments and can be bilateral.
Other Causes:
Rare causes of RAS include vasculitis (e.g., Takayasu arteritis), neurofibromatosis, and external compression by tumors or other masses.

Decreased Renal Perfusion:
Narrowing of the renal artery leads to reduced blood flow to the kidney, which activates the renin-angiotensin-aldosterone system (RAAS) in response to perceived hypoperfusion. This results in systemic vasoconstriction, sodium retention, and increased blood pressure, causing renovascular hypertension.
Ischemic Nephropathy:
Chronic hypoperfusion due to severe or bilateral stenosis can lead to progressive renal ischemia, atrophy, and eventual loss of renal function. Over time, this can result in CKD or end-stage renal disease (ESRD).
Hypertension:
Activation of the RAAS causes volume expansion and vasoconstriction, driving systemic hypertension. In unilateral RAS, the unaffected kidney compensates by excreting sodium and water, but in bilateral RAS or a solitary functioning kidney, this compensation is inadequate, leading to volume overload.

Hypertension:
The hallmark of RAS is hypertension that is resistant to treatment, requiring multiple antihypertensive medications. It may present as sudden-onset hypertension in older adults, or worsening hypertension in those with previously controlled blood pressure.
Abdominal Bruit:
A systolic-diastolic bruit may be auscultated over the epigastrium or flank in patients with RAS, though this is not always present.
Renal Dysfunction:
In patients with bilateral RAS or severe unilateral RAS, progressive loss of renal function can occur, leading to AKI or CKD. An acute rise in serum creatinine after starting an ACE inhibitor or angiotensin receptor blocker (ARB) is a classic sign of bilateral RAS or RAS in a solitary kidney.
Flash Pulmonary Edema:
Sudden, recurrent pulmonary edema can occur in patients with severe bilateral RAS or unilateral RAS with a solitary kidney, often triggered by volume overload and poorly controlled hypertension.

Clinical Suspicion:
RAS should be suspected in patients with resistant hypertension (requiring ≥3 antihypertensive medications), worsening renal function after ACE inhibitor/ARB therapy, unexplained renal atrophy, or recurrent flash pulmonary edema.
Imaging Studies:
Duplex Ultrasonography: A non-invasive test that measures blood flow velocity in the renal arteries. Increased velocity suggests stenosis. It is operator-dependent but commonly used as an initial screening tool.
CT Angiography (CTA): Offers high-resolution images of the renal arteries and can visualize the extent of stenosis. However, it requires contrast administration, which may be contraindicated in patients with impaired renal function.
Magnetic Resonance Angiography (MRA): Provides detailed images of the renal arteries and does not require ionizing radiation. MRA is useful for patients with chronic kidney disease but should be used cautiously due to the risk of nephrogenic systemic fibrosis with gadolinium contrast in those with advanced kidney disease.
Renal Arteriography: The gold standard for diagnosing RAS. It allows direct visualization of the stenosis and also permits therapeutic intervention (angioplasty). However, it is invasive and involves the use of contrast.

Medical Therapy:
Antihypertensive Treatment: The goal is to control blood pressure and prevent further renal damage. First-line agents include ACE inhibitors or ARBs, but caution is required in bilateral RAS or a solitary functioning kidney due to the risk of acute kidney injury. Thiazide or loop diuretics may be added for volume control, especially in patients with significant edema.
Lipid-lowering Therapy: Statins are indicated for all patients with atherosclerotic RAS to manage underlying cardiovascular disease.
Antiplatelet Therapy: Aspirin is recommended to reduce the risk of cardiovascular events in patients with atherosclerotic disease.
Revascularization:
Percutaneous Transluminal Angioplasty (PTA) with or without Stenting: This is the preferred revascularization technique for patients with fibromuscular dysplasia and selected patients with atherosclerotic RAS who are symptomatic (e.g., worsening renal function, flash pulmonary edema). PTA can improve blood flow, control hypertension, and preserve renal function.
Surgical Revascularization: Rarely performed but may be considered in patients with complex vascular anatomy or those unsuitable for PTA. Options include renal artery bypass or endarterectomy.
Indications for Revascularization:
Progressive renal dysfunction, especially in bilateral RAS or solitary kidney.
Recurrent flash pulmonary edema or refractory heart failure.
Resistant hypertension unresponsive to medical therapy.

Atherosclerotic RAS: Progressive in nature, often leading to CKD and increased cardiovascular risk. Early identification and management can slow progression and improve outcomes, especially when combined with aggressive risk factor modification.
Fibromuscular Dysplasia: Has a better prognosis compared to atherosclerotic RAS, especially after successful angioplasty. Patients often achieve better blood pressure control and renal function preservation after revascularization.

Key Points

Renal artery stenosis is most commonly caused by atherosclerosis, but fibromuscular dysplasia is an important cause in younger patients.
Clinical features include resistant hypertension, renal dysfunction, and recurrent flash pulmonary edema.
Diagnosis is confirmed with imaging studies like duplex ultrasound, CT angiography, or MRA, with renal arteriography being the gold standard.
Management involves medical therapy to control blood pressure and prevent cardiovascular events, with revascularization (angioplasty or surgery) indicated in select cases.
Early diagnosis and intervention, especially in patients with fibromuscular dysplasia, can improve outcomes and preserve renal function.