How do you diagnose and rule out renal artery stenosis?

Diagnostic Approach to Renal Artery Stenosis

Duplex ultrasound should be used as the first-line imaging modality for diagnosing renal artery stenosis, followed by CTA or MRA if ultrasound is inconclusive or suspicious for stenosis. 1

Initial Diagnostic Evaluation

When renal artery stenosis (RAS) is suspected, a systematic diagnostic approach should be followed:

1. Clinical suspicion indicators:

   • Resistant hypertension (requiring >3 antihypertensive medications)
   • Unexplained renal failure
   • Flash pulmonary edema
   • Significant atherosclerotic disease elsewhere
   • Abdominal bruit
   • Young age of hypertension onset (especially in women, suggesting fibromuscular dysplasia)

2. First-line imaging: Duplex Ultrasound (DUS)

   • Recommended by guidelines as the initial screening test 1
   • Key diagnostic criteria:
     • Peak systolic velocity (PSV) >200-300 cm/s (sensitivity 85-91%, specificity 75-96%) 1
     • Renal-to-aortic ratio (RAR) >3.5 1
     • Parvus-tardus waveform in intrarenal vessels (acceleration time >70ms) 1
     • Renal resistive index (RI) assessment (<0.8 suggests viability) 1

3. Second-line imaging (if DUS is inconclusive or positive):

   • CT Angiography (CTA)

     • High sensitivity (64-100%) and specificity (92-98%) 1
     • Provides excellent spatial resolution
     • Caution with contrast in patients with renal impairment

   • MR Angiography (MRA)

     • High sensitivity (94-97%) and specificity (85-93%) 1
     • Excellent for characterizing renal arteries and surrounding structures
     • May overestimate stenosis severity
     • Limited use with renal artery stents due to artifacts

4. Gold standard: Digital Subtraction Angiography (DSA)

   • Reserved for:
     • Confirmation when non-invasive tests are inconclusive but clinical suspicion remains high 1
     • Measurement of pressure gradient across stenosis (significant if >20 mmHg systolic) 1
     • Planned intervention 1

Diagnostic Algorithm

1. Start with DUS in patients with clinical suspicion of RAS

   • If normal → rule out RAS
   • If inconclusive or technically limited → proceed to CTA or MRA
   • If positive (PSV >200 cm/s, RAR >3.5) → proceed to CTA or MRA for confirmation

2. CTA or MRA (selection based on patient factors)

   • If negative → rule out RAS
   • If positive → consider DSA if intervention is contemplated
   • If inconclusive but high clinical suspicion → consider DSA

3. DSA (reserved for confirmation and intervention)

   • Measure pressure gradient across stenosis
   • Consider intervention if gradient >20 mmHg or resting pressure ratio <0.90 1

Common Pitfalls and Limitations

1. Duplex Ultrasound:

   • Operator-dependent and time-consuming 1
   • Limited by patient body habitus and bowel gas 1
   • May miss accessory renal arteries 1
   • Different velocity thresholds used across institutions 1

2. CTA:

   • Contrast nephrotoxicity risk in patients with renal impairment
   • Radiation exposure

3. MRA:

   • Tends to overestimate stenosis severity 1
   • Limited use with stents due to artifacts 1
   • Gadolinium concerns in advanced renal failure

4. Outdated techniques:

   • Renal scintigraphy, plasma renin measurements, and venous renin measurements are no longer recommended for screening of atherosclerotic RAS 1

Assessment of Kidney Viability

When evaluating for potential revascularization, assess kidney viability using:

• Kidney size (>8 cm suggests viability) 1
• Distinct renal cortex (>0.5 cm) 1
• Albumin-creatinine ratio (<20 mg/mmol) 1
• Renal resistive index (<0.8) 1

By following this systematic approach, renal artery stenosis can be effectively diagnosed or ruled out, allowing for appropriate management decisions to reduce morbidity and mortality associated with this condition.