Renal Vascular Diseases

Renal vascular disorders can generally be due to large-vessel occlusions, fibromuscular diseases, and embolic disorders. The kidney has a rich vasculature to meet the high metabolic needs associated with filtering the blood; when blood flow is impaired, ischemia and necrosis can occur. Characterized by narrowing of the renal artery. Be aware that mild stenosis is normal and causes only minor hemodynamic effect. Significant renal artery stenosis is the most common cause of secondary hypertension. Stenosis reduces renal blood flow, which triggers the juxtaglomerular apparatus (JGA) in the kidney and activates the renin-angiotensin-aldosterone system as follows:

• JGA activation triggers the release of renin, which converts angiotensinogen to angiotensin I
• Angiotensin I is converted to Angiotensin II by Angiotensin-Converting-Enzyme (ACE)
• Angiotensin II increases the retention of sodium and water in the renal tubules (which increases water volume) and activates sympathetic and hormonal vasoconstriction of the peripheral arterial system.

Inappropriate systemic vasoconstriction induces hypertension, ischemic nephropathy, and in severe cases, heart failure and acute coronary syndromes. Renal arterial stenosis can be unilateral or bilateral. The top causes of stenosis are: Atherosclerosis and fibromuscular dysplasia. Atherosclerosis is common in older people, especially those with a history of cigarette smoking. Fibromuscular dysplasia, which is a systemic, noninflammatory, nonatherosclerotic disease of the vascular walls; abnormal cellular proliferation in the arterial walls produces stenosis, which can be unifocal or multifocal with a "string of beads" appearance. Fibromuscular dysplasia is more often diagnosed in women, who are 30-50 years old, and, in addition to arterial stenosis, can also cause aneurysms, dissection, and tortuosity. We suspect renal artery stenosis in patients who have sudden onset of hypertension without a family history. In these patients, hypertension is severe and resistant to antihypertensive agents. We may hear abdominal bruits. Patients with bilateral renal artery stenosis (or unilateral stenosis in single-kidney patients) may present with Pickering syndrome, which is characterized by "flash" pulmonary edema. Diagnosis: Renal arteriogram (contraindicated in patients with renal failure, as the contrast dye is nephrotoxic); MRA (magnetic resonance angiography), and Duplex Doppler ultrasonography. If these non-invasive tests are not conclusive, we can use catheter angiograph. Treatment: We can use antihypertensives drugs, diuretics, statins, and antiplatelet therapies. Be aware that we need to monitor patients for acute kidney injury. ACE inhibitors are contraindicated in patients with bilateral renal artery stenosis or unilateral stenosis in single-kidney patients. Percutaneous renal artery stenting may be necessary in severe cases of atherosclerotic renal artery stenosis, and it is often curative in fibromuscular dysplasia. Patients are often asymptomatic but may present with flank pain, hypertension, hematuria, or proteinuria. When occlusion is severe, blood clots can impair renal function and cause chronic kidney injury and kidney failure. Renal thrombi in the arteries and veins are especially likely in hypercoagulable states. Renal artery thrombosis is most often the result of emboli that break free from the heart and larger systemic arteries. Renal vein thrombosis is most often due to the hypercoagulable states induced by nephrotic syndrome and COVID-19. Atheroembolic renal disease is a complication of severe atherosclerosis, in which "showers" of cholesterol embolize from larger arteries and lodge in the renal vessels. Weak, bulging portions of the renal artery wall; they are most often the saccular form. Recall that true aneurysms include all layers of the vessel wall. Renal artery aneurysm is especially common in patients with fibromuscular dysplasia, and, though generally benign, they can rupture and cause massive gross hematuria. Risk of rupture is elevated in patients who are pregnant, have inflammatory conditions, or when the aneurysms are larger. Caused by systemic hypertension (as opposed to renal artery stenosis, which causes systemic hypertension). This disorder is particularly common in African American males. When systemic blood pressure is chronically elevated, glomerular hydrostatic capillary pressure is also elevated, which induces vascular sclerosis and hyalinosis with interstitial fibrosis. Hypertensive nephrosclerosis is a common cause of end-stage renal disease. Signs/Symptoms: We suspect hypertensive nephrosclerosis in patients with hypertension, increased serum creatinine, proteinuria, hematuria, and red and white blood cell casts in their urine. Diagnosis: hypertensive nephrosclerosis is a diagnosis of exclusion, and some authors argue that it is a poorly defined disorder that requires reconsideration (particularly because it is often hard to tell which came first – the hypertension or the renal injury). Treatments: In any case, we need to treat the systemic hypertension to protect the kidney from further damage. In sickle cell nephropathy, the sickled red blood cells cause microvascular infarctions; as we discuss elsewhere, this can lead to papillary necrosis, acute tubular necrosis, and nephrotic syndrome. In diabetic nephropathy, we see hyalinosis of the afferent and efferent arterioles, and the small arteries of the kidneys (in addition to other glomerular changes). Scleroderma:

• Kidney involved in 60-70% of patients.
• Can cause malignant HTN. Renal crisis in 20%
• Various changes (fibroid necrosis) in arterioles, arteries, including glomeruli

Thrombotic microangiopathy

• Heterogenous group of disorders
• Hemolytic uremic syndrome (HUS): typical (Shiga toxin) and atypical (complement-mediated). HUS is characterized by microangiopathic hemolytic anemia, thrombocytopenia, acute kidney injury, possible systemic involvement (may see petechiae, purpura, internal bleeding, aphasia, dysphasia, seizures, coma).

Thrombotic thrombocytopenic purpura (TTP): Dysfunction of von Willebrand multimer cleavage. Sporadic forms are associated with drugs, pregnancy, organ transplantation.