Accurate diagnosis of RVH is essential and involves a combination of clinical assessment and diagnostic tests.

Atherosclerotic renal artery stenosis (ARAS) is a condition characterized by the narrowing of the renal arteries due to the accumulation of atherosclerotic plaques. Typically affects older individuals with a history of systemic atherosclerosis. In general, atheromatous lesions involve the proximal segment of the renal artery, within 2 cm from the ostium, and appear as a progressive luminal stenosis (9).

Fibromuscular dysplasia (FMD) of the renal artery is a vascular disorder characterized by abnormal growth of fibrous and muscular tissues within the arterial walls. It affects the middle or distal segment of the renal arteries and is characterized by a “string of beads” appearance of the affected artery, representing alternating segments of small stenoses and areas of dilatation along the vessel. The exact cause of FMD remains unclear, but genetic and hormonal factors may contribute to its development (9).

Less common causes include renal artery dissection, encircling tumors, polyarteritis nodosa, etc. Uncovering these less common etiologies often requires a thorough diagnostic workup, incorporating imaging studies and clinical assessments.

Diagnostic Modalities:

Doppler Ultrasound:

Doppler ultrasound is a non-invasive imaging modality that plays a central role in the initial assessment of renovascular hypertension due to its accessibility, safety, and real-time imaging capabilities. By evaluating blood flow velocities in the renal arteries, Doppler ultrasound provides valuable information about potential stenosis or occlusion. 

The interpretation of ultrasound parameters for renovascular hypertension can vary among different institutions and guidelines. However, there are general threshold values that are commonly used to identify potential renal artery stenosis (RAS) (3,4,6,17,11):

• Renal artery peak systolic velocity (PSV): an elevated PSV suggests the presence of renal artery stenosis. However, the threshold may vary depending on factors such as patient age, body mass index, and the presence of collateral vessels.
  • Threshold value: PSV > 180 cm/s suggest the presence of a stenosis of more than 60%; some authors recommend a  threshold value of 200 cm/s
    

    Fig 1: Ultrasonography images of a 53-year-old woman with renal insufficiency and previously controlled hypertension. (a) Duplex Doppler ultrasonographic waveform from a segmental right renal artery of the right kidney shows an abnormal waveform with a delayed, weak systolic peak and decreased resistive index ("parvus tardus" waveform). (b) Duplex Doppler ultrasonography of the right renal artery (distal segment) shows a peak systolic velocity of 229 cm/s, indicating a a stenosis greater than 70%.
• Renal-to-aortic ratio (RAR): the renal aortic ratio is then calculated by dividing the PSV in the renal artery by the PSV in the prerenal abdominal aorta. The use of the RAR instead of the absolute PSV value is more advisable since hypertension itself can cause increased PSV velocities in all the vessels in hypertensive patients. A higher renal aortic ratio indicates increased blood flow resistance in the renal arteries, often associated with narrowing or blockage.
  • Threshold value: RAR > 3.5; some authors recommend a  threshold value of 3.
    

    Fig 4: Ultrasonography images of a 80-year-old man with persistent hypertension despite five different medications. (a) Spectral Doppler waveform near the ostium of the right renal artery shows increased peak systolic velocities (PSV 225.7 cm/s). (b) Spectral Doppler waveform in the prerenal abdominal demonstrates a a peak systolic velocity of 74.4 cm/s. In this case renal/aortic ratio is 3 (< 3.5), indicating a non significative arterial stenosis (around 50-60%). Not shown, normal waveform in segmental right renal arteries.
• Turbulent flow in the post-stenotic area.
• Resistance index (RI): decreased RI may suggest severe stenosis.
  • Threshold value: RI < 0.55
• Resistance index difference between kidneys.
  • Threshold value: > 5% 
• Peak systolic acceleration time (AT): prolonged AT may suggest proximal renal artery stenosis. The reasoning is that the flow distal to a hemodynamically significant stenosis should become damped and show a slow rise to the peak systole, in a “parvus tardus” waveform. However, although helpful, the absence of this waveform does not exclude renal artery stenosis. In patients with atherosclerosis, vessel compliance may be reduced, making the “tardus-parvus”  waveform less apparent. 
  •  Threshold value: > 0,07s.
    

    Fig 1: Ultrasonography images of a 53-year-old woman with renal insufficiency and previously controlled hypertension. (a) Duplex Doppler ultrasonographic waveform from a segmental right renal artery of the right kidney shows an abnormal waveform with a delayed, weak systolic peak and decreased resistive index ("parvus tardus" waveform). (b) Duplex Doppler ultrasonography of the right renal artery (distal segment) shows a peak systolic velocity of 229 cm/s, indicating a a stenosis greater than 70%.

Accessory renal arteries are common, seen in approximately 25–30% of patients. Nevertheless, the occurrence of hemodynamically significant stenosis isolated to an accessory RA has been reported in 1-1.5% of cases following the workup for renovascular hypertension (5) and are not routinely studied. 

It is crucial to recognize that these threshold values are not absolute, and clinical judgment should always be applied in conjunction with other diagnostic information. Diagnostic accuracy is optimized when ultrasound findings are integrated with other imaging modalities and clinical information to guide patient management decisions. If renal artery stenosis is suspected based on the ultrasound findings, further imaging studies, such as CTA or MRA, may be recommended for confirmation and detailed anatomical assessment.

Computed Tomography Angiography (CTA):

CTA has become a cornerstone in renovascular hypertension diagnosis, offering high-resolution three-dimensional images of the renal vasculature. With the ability to precisely visualize renal artery anatomy and identify stenosis, CTA provides essential information for treatment planning. Its rapid imaging capabilities and accessibility contribute to its widespread use in clinical practice (2, 9).   

Furthermore, CTA allows for the simultaneous assessment of surrounding structures. This holistic view aids in identifying potential secondary effects of renovascular hypertension, such as renal atrophy or other parenchymal changes, providing a more comprehensive understanding of the patient's condition.

Two notable considerations in using CTA are the exposure to ionizing radiation and the use of contrast agents to enhance vascular visibility, as patients with allergies or impaired renal function may experience adverse reactions to contrast agents. 

The interpretation of CTA findings involves evaluating various parameters (9):

• Cross-sectional area stenosis:  percentage of cross-sectional area reduction  of the renal artery is calculated.
• Threshold value: >75% cross-sectional area reduction indicates significant stenosis.
  

  Fig 5: Same patient as in figure 4. (a) VR image of angioCT shows severe atheroesclerotic of the abdominal aorta, including the ostium and proximal portion of the right renal artery. (b) CT arterial analysis software indicates a 60.7% stenosis of the right renal artery, confirming suspected non significative renal artery diameter reduction.
  

  Fig 6: Diagnostic computed tomography image of a 63-year-old man with previously controlled hypertension. (a) The maximum intensity projection image shows a focal stenosis of the main right renal artery near its origin (arrow). (b) CT arterial analysis software indicates a 92.4% stenosis of the right renal artery.
  

  Fig 8: CT images of a 17-year-old patient with hypertension. (a) CT angiography and (b) three-dimensional, volume-rendering reconstruction image show a preocclusive stenosis oft he right renal artery near its origin (arrow). Muscular hypertrophy of the right diaphragmatic crus was determined to be the cause (dashed arrows).
• Presence of collateral vessels may suggest chronicity and severity of stenosis.
• Presence of parenchymal changes

Magnetic Resonance Angiography (MRA):

MRA, utilizing magnetic fields and radio waves, provides detailed images of blood vessels similar to CTA without ionizing radiation. MRA can be performed in patients with some degree of renal insufficiency, though, not in patients with severe renal insufficiency (GFR ≤30 mL/minute/1.73 m2) due to the risk of nephrogenic systemic fibrosis secondary to gadolinium contrast. Additionally,  MRA cannot be used for patients with non compatible pacemakers or other metallic objects and is generally less available than CTA (2, 9).

When the clinical level of suspicion is high and the results of noninvasive tests are inconclusive, angiography is recommended (9).

Angiography (Conventional or Digital Subtraction Angiography - DSA):

Invasive angiography, either conventional or using digital subtraction techniques, remains the gold standard for diagnosing renovascular hypertension.  The procedure provides not only anatomical details but also dynamic information about blood flow and potential obstructions. This accuracy is crucial for determining the underlying cause of hypertension and guiding subsequent treatment decisions. 

The interpretation of arteriography findings involves evaluating the narrowing of the renal arteries. The degree of stenosis is often assessed visually by comparing the diameter of the narrowed segment to the normal adjacent vessel segment.

Fig 2: Same patient as in Figure 1. (a) Maximum intensity projection image from computed tomography angiography and (b) selective right renal artery injection image from an intra-arterial catheter-directed angiogram show irregularities of the distal third of the right renal artery, giving it a "string of beads" appearance, suggestive of fibromuscular dysplasia.

Beyond its diagnostic utility, renal angiography serves as an essential tool in guiding therapeutic interventions for renovascular hypertension (10). Revascularization is recommended when the degree of renal artery stenosis is greater than 80%, in patients with bilateral stenosis or stenosis in a solitary functioning kidney regardless of whether they have renal insufficiency or not (11).

Treatment:

Treatment strategies encompass lifestyle modifications as well as pharmacological therapy with antihypertensive medications. In cases of severe or refractory hypertension, revascularization procedures may be necessary to restore renal artery patency and improve blood pressure control:

• Angioplasty and stenting: percutaneous transluminal renal angioplasty (PTRA) is a common intervention facilitated by angiography. During PTRA, a balloon catheter is inflated at the site of stenosis, widening the narrowed artery and restoring blood flow. In some cases, a stent may be placed to maintain vessel patency.
  

  Fig 3: Same patient as in Figures 1 and 2. (a) Images taken during and (b) after angioplasty show a reduction in the severity of the stenoses from the fibromuscular dysplasia (arrow).
  

  Fig 7: Diagnostic angiography images of a 58-year-old woman with new onset hypertension. (a) Selective right renal artery injection image from an intra-arterial catheter-directed angiogram shows a focal stenosis of the main left renal artery near its origin (arrow). (b) An image taken after angioplasty and stent placement shows resolution of the stenosis.
  

  Fig 9: Same patient as in figure 8. Surgery of the right diaphragmatic crus was performed, but hypertension was persistent. CTA was performed. (a) The maximum intensity projection image shows the stenosis of the right renal artery (arrow). (b) The patient underwent an effective angioplasty of the right renal artery.
• Embolization: renal artery embolization is employed to treat conditions such as renal artery aneurysms or arteriovenous fistulas. By injecting embolic materials through the catheter, angiography helps visualize and guide the precise placement of these materials, effectively occluding abnormal vessels and restoring normal blood flow patterns.
• Surgical planning: in cases where surgical intervention is deemed necessary, angiography plays a crucial role in preoperative planning. It provides surgeons with a detailed map of the renal vasculature, helping them strategize the optimal approach for procedures such as renal artery bypass surgery.