Renal Artery Stenosis

Renal artery stenosis (RAS) is an uncommon but significant cause of secondary hypertension in children, accounting for 5–10% of such cases.
It is characterized by severe, often resistant elevation in blood pressure and a high propensity for target organ damage at presentation.

The underlying cause of RAS in children varies significantly based on geographic location and the presence of underlying genetic conditions.

Etiology Category	Characteristics & Associated Conditions
Fibromuscular Dysplasia (FMD)	The most common cause in Europe and North America. It is an idiopathic, nonatherosclerotic, and noninflammatory disease. The classic adult "string of beads" angiographic pattern is seen in only a minority of children.
Takayasu Arteritis	The most common cause of RAS in Asia and Africa. An inflammatory arteritis affecting the aorta and its major branches, often presenting with bruits, claudication, or pulse discrepancies.
Syndromic RAS	Accounts for 15–25% of cases; most notably associated with Neurofibromatosis type 1 (NF1) and Williams syndrome.
Other Acquired Causes	Includes midaortic syndrome, post-radiation therapy, trauma, umbilical artery catheterization complications, and transplant renal artery stenosis.

Stenosis of the renal artery leads to glomerular underperfusion, which stimulates the juxtaglomerular cells to release renin.
Renin converts angiotensinogen to angiotensin I, which is subsequently converted to angiotensin II by angiotensin-converting enzyme (ACE).
Angiotensin II acts as a potent vasoconstrictor and stimulates aldosterone release, leading to profound sodium and water retention.
In unilateral RAS, the contralateral non-stenotic kidney is exposed to high systemic blood pressures, resulting in a compensatory pressure natriuresis that may cause subclinical dehydration and potentially exacerbate renin release.
In bilateral RAS, both kidneys are underperfused, leading to massive volume expansion and sodium retention, which blunts the degree of hyperaldosteronism.

Infants and Young Children: Often asymptomatic or present with nonspecific symptoms such as failure to thrive, loss of appetite, and unexplained lethargy.
Older Children: Headache is the most common symptom (17–37%), often accompanied by vomiting.
Hypertensive Crisis: Manifestations include seizures, ischemic or hemorrhagic stroke, facial nerve palsy, and congestive heart failure.
Hyponatremic Hypertensive Syndrome (HHS): Occurs in unilateral RAS due to severe pressure natriuresis and polyuria driven by the contralateral normal kidney.
Target Organ Damage: Left ventricular hypertrophy is observed in 56–67% of patients, and hypertensive retinopathy (narrowing, stiffening, or exudates) is seen in 50–78% at presentation.
Laboratory Findings: Characterized by hypokalemia, metabolic alkalosis, elevated plasma renin activity, and high aldosterone levels.

Diagnostic imaging aims to localize the stenosis, assess hemodynamic significance, and plan for potential interventions.

Imaging Modality	Diagnostic Role and Key Findings
Doppler Ultrasonography	The first-line screening tool. Direct signs include peak systolic velocity (PSV) > 180–200 cm/s and a renal-aortic ratio > 2–3.5. Indirect signs in the renal parenchyma include a tardus et parvus (slow and small) waveform and a resistive index (RI) < 0.55.
Computed Tomography Angiography (CTA)	A highly sensitive (88%) and specific (81%) noninvasive modality that provides excellent spatial resolution for small intrarenal branches, though it involves radiation and contrast exposure.
Magnetic Resonance Angiography (MRA)	An alternative to CTA that avoids radiation, though it requires longer acquisition times (often needing sedation) and has slightly lower sensitivity compared to DSA.
Digital Subtraction Angiography (DSA)	The gold standard for diagnosing RAS. Allows for exact anatomical localization and intravascular pressure measurements, and importantly, facilitates concurrent therapeutic angioplasty.
Renal Scintigraphy (MAG3/DMSA)	While not recommended for primary diagnosis due to low sensitivity, it is highly valuable for assessing differential renal function pre- and post-intervention.

Medical Therapy: Antihypertensive medications serve primarily as a bridge to definitive invasive procedures.
- For unilateral RAS, first-line therapy typically includes calcium channel blockers, beta-blockers, and alpha-blockers.
- ACE inhibitors and angiotensin receptor blockers (ARBs) are generally avoided or used with extreme caution, as they dilate the efferent arteriole, acutely drop glomerular filtration pressure, and can induce ischemia in the stenotic kidney. They are absolutely contraindicated in bilateral RAS.
- Diuretics are highly effective in bilateral RAS or when secondary sodium retention occurs due to contralateral hypertensive nephropathy.
Percutaneous Transluminal Angioplasty (PTA): The primary and preferred intervention, leading to improved or normalized blood pressure in 50–100% of cases.
- Balloon angioplasty is the procedure of choice.
- Stents are generally avoided in growing children due to the risk of in-stent stenosis and somatic growth constraints, but they may be utilized as a rescue therapy for complications like vessel dissection.
Surgical Revascularization: Indicated for tight ostial stenoses, failed endovascular interventions, or complex midaortic syndrome.
- Procedures include kidney autotransplantation, aortorenal bypass, and renal artery reimplantation.
- Nephrectomy is strictly reserved as a last resort for severely ischemic, poorly functioning kidneys (split GFR < 10%) that are not amenable to revascularization.