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RENOVASCULAR HYPERTENSION : Radiological Diagnosis and Treatment

Sundeep J Punamiya
Associate Consultant and Head, Department of Uroradiology and Vascular/Interventional Radiology, Bombay Hospital and Medical Research Centre,
12 Marine Lines, Mumbai 400 020.

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INTRODUCTION

Hypertension is a very common problem encountered in day to day clinical practice, with over 20% of the general adult population suffering from it. [1] If left uncontrolled, it is associated with a myriad of complications such as renal failure, myocardial infarction, stroke, intracranial haemorrhage, blindness and even death.

Renal artery stenosis (RAS) accounts for less than 5% of the cases of hypertension. Timely diagnosis and treatment of this condition is important because it carries a worse prognosis than essential hypertension. It seems to be less amenable to drug treatment, with greater risks of side effects associated with the high doses of drugs required to control the blood pressure. Even when blood pressure can be controlled with the latest anti-hypertensive medications, particularly beta-blockers and ACE inhibitors, the renal artery disease commonly progresses and can severely compromise renal function. Several studies have shown that almost 30-50% of patients with an insignificant stenosis of < 60% will progress to a significant stenosis of > 60% at the end of one year, and that a > 60% stenotic lesion will progress to occlusion of the renal artery with a frequency of 4-10%. [2,3] In addition it has been reported that as many as 30% of patients with RAS that are treated with anti-hypertensive drugs alone will show deterioration of renal function. [4]

Although the association of RAS with hypertension has been established since 1930, it was not until much more recently that its role in renal failure was recognised. The exact incidence of proven RAS causing end-stage renal disease remains unknown, although it is likely to occur in about 12-15% of patients on haemodialysis. Patients with renovascular disease on dialysis programmes do badly; there is a 2 year survival rate of 56%, the majority succumbing to either comorbid vascular disease or withdrawal death. [5]

Thus patients with RAS have a dual problem on hand : 1) control of hypertension and 2) prevention of renal failure and/or stabilisation of renal function. Unlike essential hypertension, these complications are potentially reversible, when blood flow to the affected kidney is restored at a treatable stage. Hence it is vital that this subgroup of patients are identified early by adequate screening and treated promptly to preserve renal function.

WHEN TO SUSPECT RENAL ARTERY STENOSIS?

The prevalence of renovascular disease is less than 1% in the general population of hypertensives, 5% in hospital-based population and up to 40% in patients referred to hypertension clinics. [6] In two-thirds of cases the cause of renovascular hypertension is atherosclerosis; less common causes are fibromuscular dysplasia, aortoarteritis, thrombosis, arterial dissection and stenosis in a transplanted kidney.

Because it has such a low prevalence in the large hypertensive population, one has to have an effective low-cost screening test for detecting RAS. However such a goal has been elusive, withno single diagnostic test being cheap, completely safe and highly accurate. Fortunately, several clinical clues can help determine which patients may be suffering from RAS (Table 1).

TABLE 1Clinical clues suggestive of renal artery stenosis

Age of onset < 25 or > 45 years Severe hypertension - Diastolic BP > 125 mm Hg Refractory hypertension - Diastolic BP > 100 mm Hg on 3 ormore anti-hypertensives Accelerated hypertension - 15% increase in BP within 6 months Sudden onset hypertension Abdominal bruit Grade III-IV retinopathy (along with diastolic pressure> 125 mm Hg) Renal insufficiency (along with age > 50 years andatheromatous disease) Deterioration of renal function after ACE inhibitor Negative family history of hypertension

If a patient with hypertension fulfils any of the above criteria, it would mandate confirmation of RAS, as the probability of RAS increases to about 10-40%, compared to the 1-5% seen in the general population. [7] To confirm the presence of RAS, several diagnostic imaging tests are available for assessing the location and severity of the stenotic lesion. These include angiography, Doppler sonography, MR angiography and CT angiography. The other category of tests are the functional diagnostic tests that are carried out to ascertain the pathophysiological importance of the stenosis. These tests are the rapid sequence intravenous urogram, plasma renin estimation and captopril renography. Some of these functional tests can also be used to predict cure or improvement of hypertension.

HOW TO DIAGNOSE RENAL ARTERY STENOSIS?

Various tests have been tried and improvised to achieve the status of an "ideal" screening test that would be non-invasive, safe, easy to perform, widely accessible, reproducible and highly accurate. Unfortunately to date no test has yet been devised that meets all these criteria. So how should one work up a patient with suspected RAS? Once the decision is made to pursue an evaluation for renovascular hypertension, one or more of the multiple imaging and physiological tests may be used. It is difficult to recommend a single rigid diagnostic algorithm for all patients, due to the variability of clinical conditions and with the imprecise studies presently available; the evaluation is usually tailor-made for each patient.

1. Rapid sequence intravenous urogram

Rapid sequence urogram was widely accepted as a screening method for renovascular hypertension in the past. [8] This technical modification of the intravenous urogram uses films exposed at 1,2,3 and 5 minutes after contrast injection to evaluate the parenchymal and early excretory function. It must be emphasised that this test does not directly image renal arterial anatomy. Rather it relies on differences in excretory function between a normally perfused and an ischaemic kidney. The three major features of renal artery stenosis on this modality are 1) delayed appearance of contrast in the calyces of the involved kidney, 2) hyperconcentration of contrast within the pelvicalyceal system, and 3) reduced size of the ischaemic kidney (Fig. 1). Although this test is easy to perform, its predictive value is only 31%. [9] With the availability of more accurate newer modalities, rapid sequenceIVU has currently little or no role in the evaluation of RAS.

Fig. 1

2. Doppler sonography

Doppler sonography is a commonly used modality for assessing RAS because it is non-invasive, cheap and freely available. However, certain disadvantages preclude its routine use in the RAS screening programme. Renal Doppler is one of the most technically demanding tests done in the USG department. It involves a lot of time, with some patients taking up to 2 hours for completion, becoming more difficult with obesity and overlapping bowel gas. There is a learning curve which the technologist/radiologist has to undergo before obtaining consistent results. The earlier investigators used the peak systolic velocity in the renal artery (> 100 cm/sec) and the renal artery : aorta velocity ratio (> 3.5) to determine the presence of RAS. [10,11] However, the detection rate was very low, largely due to poor visibility of the main renal artery in up to 42% of patients. In addition accessory renal arteries exist in 14-24% of patients, which are extremely difficult to visualise. [12] Hence the emphasis shifted to assessing the intrarenal vessels. These arteries are easily identified in virtually all patients. In RAS, there is a change in wave form configuration in the segmental renal arteries and this pattern recognition is supposed to increase the sensitivity markedly. [13] According to one of the series, the sensitivity can be further increased by evaluating the vessels distal to the stenosis after captopril administration. [14] Being a highly operator-dependent procedure, the likelihood of producing comparable and consistent results would rely on the experience and dedication of the person doing the examination.

3. MR Angiography (MRA)

Several magnetic resonance imaging (MRI) and MR angiographic techniques have been developed for imaging the kidneys and renal arteries. T1-weighted images demonstrate the kidney size, parenchymal thickness, corticomedullary differentiation and location. Time-of-flight and phase-contrast MR angiography are flow-sensitive techniques that depict the motion of flowing blood and do not require any contrast injection. [15] They are accurate for normal renal arteries, but the slowing of blood flow associated with RAS introduces image degrading artefacts (Fig. 2). Three-dimensional (3D) MR angiography requires an injection of gadolinium and produces true anatomic pictures similar to conventional angiograms. [16] Several advantages of MRA exist. It is a non-invasive method that does not require injection of any nephrotoxic contrast and can provide physiological information regarding functional significance of RAS. However, the drawbacks are plenty. It is an expensive examination and is not available in rural areas and many cities of India. In fact, the equipment required for 3D-MRA, the most ideal MR technique, is not available in majority of the MR centres in the country. MRA requires substantial expertise for appropriate image acquisition and interpretation and is contraindicated in patients with pacemakers, cerebral aneurysm clips, metallic implants and intra-ocular metallic foreign bodies.

4. CT Angiography (CTA)

CTA is an attractive alternative for evaluating patients with suspected RAS. The images are obtained using a spiral CT scan equipment afterintravenous injection of a bolus of contrast. The images are then processed by the computer to give images almost identical to the angiographic films (Fig. 3). An advantage of CTA is that the vessel lumen and the pathological vessel wall are both visualised, which can help in determining the choice of interventional therapy, such as angioplasty or stenting. [17] However, visualisation of small intrarenal vessels is difficult and accurate assessment of peripheral stenoses beyond the level of the renal hilum may be difficult. Another limitation is that mural calcification may interfere with CT evaluation of RAS, by underestimation of the stenosis. A distinct drawback of CTA is the large volume of contrast is required, almost 120-150 ml per scan. Use of this large amount of contrast increases the risk of contrast-induced nephropathy, which in the setting of RAS, where renal function is already at risk, could cause renal shutdown. It has also been reported that in patients with pre-existing impaired renal function, CTA may be unreliable in assessment of RAS. [18]

Fig. 2

5. Captopril renogram

This test relies on the assessment of renal function using radio-isotope such as technetium-99m DTPA or MAG3. When ACE inhibitors such as captopril are administered, the glomerular filtration rate in the ischaemic kidney drops precipitously and RAS is implied. The renal handling of a radio-isotope is thus compared before and immediately after captopril ingestion; a positive test is indicative of a functionally significant stenosis. The captopril renogram is a fairly accurate investigation having a sensitivity and specificity of > 90%. [19] Also, it provides a means of predicting and following the results of surgery/angioplasty. [20] However, captopril renography is of poor utility in pre-existing renal dysfunction, bilateral RAS or solitary kidney with RAS. In addition, the renal function in such situations may deteriorate further, and hence should be used with caution. There are a few conditions that can produce false positive and false negative results on the scan, which the interpreter should be aware of, before RAS is implied or ruled out. [21]

Fig. 3

6. Angiography

Fig. 4	Fig. 5

Angiography remains the "gold standard" for evaluating RAS. Renal arteriography can be performed either by the conventional techniques or by using digital subtraction technology. Significant stenosis on either of these methods is indicated by reduction in calibre of the stenosed renal artery by at least 50% (Fig. 4). Ancillary signs such as reduction in renal size and demonstration of collaterals contribute to the diagnosis of RAS. There was a general resentment against the use of conventional angiography in screening for RAS and this was not surprising, considering the invasiveness of the procedure and the large volume of contrast required. Digital subtraction technology has to an extent alleviated this apprehension. In the 1980s, DSA images were obtained by intravenous injection of contrast through a central venous catheter, termed IV-DSA. However, the images lacked resolution with a large number of artefacts degrading the image quality. As many as 5-20% of the DSAs were not interpretable because of this. In addition, a high volume of contrast was usually needed. This technique was hence not largely favoured and was soon replaced by intra-arterial DSA. The procedural technique of IA-DSA is similar to conventional angiography, necessitatingarterial catheterisation. However image acquisition is done using computer technology. With the use of subtraction, confusing bony and soft tissue shadows that overlap the renal areas are eliminated (Fig. 5). Although requiring arterial puncture, the volume of contrast is markedly reduced and the smaller catheter size reduces the risk of angiography related complications. IA-DSA has been shown to be associated with a high diagnostic accuracy compared with conventional angiography. [22] With DSA, quantitative analysis of the stenosis is possible, by measuring the diameter of the involved artery and by measuring the pressure gradient across the stenosis (Fig. 6). Recently, carbon dioxide has emerged as an alternative angiographic contrast agent used in combination with DSA to avoid the risk of conventional nephrotoxic contrast agents in patients with severe renal insufficiency. However, the technique requires a dedicated CO2 injector which is currently unavailable in India.

Fig. 6

WHICH TEST TO CHOOSE?

Although there is a heated debate concerning which of the above-mentioned screening methods is best, selection of a particular test would depend on various aspects, such as patient compliance, cost-effectiveness, safety, availability and most importantly, accuracy of the test (Table 2).

Although the sensitivity and specificity of CTA and MRA are good, these methods are too expensive to be used as general screening tests. Furthermore, large amounts of contrast is required for CTA. MRA has a technical failure rate of about 10% due to claustrophobia or presence of a metallic implant. Captopril nuclear scan and colour Doppler are the least expensive methods. The high technical failure rate of Doppler coupled with high operator dependability has discredited this method. Captopril renogram can cause deterioration of renal function, though reversible. One advantage of this method is that it has prognostic value in identifying patients who are likely to benefit after successful intervention. However, this test does not perform equally well in patients with poor renal function and does not provide information on the anatomical severity of RAS. If a stenosis is identified on the non-invasive imaging modalities, angiography is invariably required to confirm the diagnosis. As regarding accuracy, IA-DSA is probably the best procedure amongst all the available tests in the current setting. It is highly accurate in determining the presence of RAS, can quantify the degree of stenosis and will be able to assess the causes of stenosis, with a fairly small volume of contrast required to carry out the test. A distinct advantage of angiography is that if the stenosis is found to be significant, it can be addressed to immediately by either angioplasty or stenting, precluding the necessity and discomfort of another arterial puncture at a later date.

WHEN TO TREAT RENAL ARTERY STENOSIS - CRITERIA FOR EVASCULARISATION

Once renal artery stenosis is established, the next dilemma would be when and how to treat it. Before discussing treatment strategies for renovascular hypertension, it must be emphasised that not all renal artery stenoses need to be treated. Renal artery lesions are often detected incidentally, especially after procedures such as coronary angiography and cardiac catheterisation, sometimes leading to a tendency to perform intervention in the absence of clear-cut medical indications. Avoiding this tendency will curb the incidence of rare but catastrophic complications, such as cholesterol embolisation, dissection, and other less serious complications and also lessen the financial burden faced by the patient in undergoing an unnecessary procedure. Secondly, it is not always justifiable to label hypertension in the presence of RAS as renovascular hypertension. The two conditions may simply coexist. Alternatively, it is even possible that the raised blood pressure is the cause of RAS and not the other way around i.e. hypertension increases the risk of atherosclerotic vascular disease which may involve the renal arteries. When this relation is suspected, physiological tests are invaluable in proving it.

TABLE 2 Diagnostic accuracy of different methods for detection of RAS?
	Technical failure	Sensitivity	Specificity
Colour Doppler sonography	15%	92%	95%
Captopril nuclear scan	0%	86%	93%
Magnetic resonance imaging	10%	96%	74%
Spiral computed tomography	0%	98%	94%
Intra-venous digital subtraction angiography	5%	85%	83%
Intra-arterial digital subtraction angiography	0%	100%	100%

In general, patients are considered for revascularisation after all four important criteria are met:

i. A significant clinical problem has been clearly established : Poorly controlled hypertension is the most common indication for revascularisation. This includes patients who are intolerant to the current medications, patients whose blood pressure cannot be controlled on medications and patients with continued lability of their blood pressure. Patients who show a deterioration of renal function after ACE inhibitors also need to be revascularised. Patients with renal dysfunction are likely candidates for revascularisation to improve or prevent further deterioration of renal function. Other less common indications include episodic flash pulmonary oedema, congestive cardiac failure and unstable angina. Asymptomatic severe stenotic lesions are considered pre-occlusive and are recommended for revascularisation if it is bilateral or if it occurs in a solitary functioning kidney.

ii. The stenosis is likely to be haemodynamically significant : Haemodynamic significance of the lesion is indicated by at least 50% reduction in diameter and a significant pressure gradient of at least 5 mm Hg across the lesion.

iii. The involved kidney is believed to be salvageable : Renal salvageability can be based on several factors. Renal size on USG is a very practical and commonly used indicator. Revascularisation is usually not attempted if the renal length is less than 8 cm, unless the indication is preservation of renal function. Some physicians prefer to obtain a renogram before deciding on revascularisation, as this is of physiological importance. Kidneys that are contributing less than 15% of total renal function are usually not considered suitable candidates.

iv. No significant contraindications such as severe bleeding diathesis are present.

WHAT TREATMENT OPTION TO CHOOSE : ANGIOPLASTY OR SURGERY?

Revascularisation treatment involves restoring blood flow to the ischaemic kidney by either a surgical bypass or by endovascular techniques such as angioplasty and stenting. Majority of renal artery stenotic lesions are amenable to percutaneous therapy and this has gained acceptance in most of the centres that offer it, becoming the procedure of choice. Surgical treatment is usually reserved for the following indications, although certain institutions with experienced interventional radiologists may elect to treat some of these percutaneously.

1. Failure of angioplasty/stenting
2. Completely occluded renal arteries
3. Long segment renal arterial disease
4. Patients undergoing vascular surgery for other problems (e.g. treatment of abdominal aortic aneurysm)
5. Solitary kidney
6. Very severe diffuse atherosclerotic disease in the aorta and other access vessels that may present a high risk of embolism during the procedure.

With surgery, the restoration of vascular patency is usually permanent, but surgery is more costly, mainly due to the long hospitalisation required and more procedure-related complications, including death (mortality rate 2.2-7.8%). [23-26] Although the cost of surgery is almost three times the cost of angioplasty, it may be argued that in the long run, angioplasty may be more expensive, as it is not as long lasting as surgery and would require repeated interventions. This is not true. In the long run, the overall cost is more or less the same even after a secondary procedure. Also, the patency rates after this second intervention is the same inboth sub-groups. [26] There is no change in the prognostic outcome with either of the two techniques, with about 10% patients progressing to end-stage renal disease in 15 years. [27] Hence surgical correction is not superior to angioplasty. The less invasive nature of angioplasty alongwith its lower morbidity and mortality, even in critically ill patients, makes angioplasty the primary method when technically possible.

Angioplasty

Percutaneous transluminal angioplasty involves dilatation of a balloon within the lumen of a stenosed artery. This "controlled" injury breaks the intimal plaque and/or stretches the walls of the artery, enlarging the effective lumen of the artery. The aetiology and anatomical location of the lesion are important determinants of technically successful angioplasty and subsequent clinical benefit.

Amongst the various conditions associated with RAS, angioplasty works best in fibromuscular dysplasia. Its low recurrence rates and success in controlling blood pressure and preserving renal function is well established and hence has largely replaced surgery for this disease (Figs. 7,8). [28]

With atherosclerotic RAS, the success of angioplasty would depend mainly on the location of the stenotic lesion. A stenosis away from the ostium of the renal artery is more likely to respond to angioplasty than a lesion at the ostium (Figs. 9,10). This is probably due to elastic recoil from the aortic plaques encroaching on the renal artery ostium.

RAS due to aortoarteritis does not respond well to angioplasty due to the tight nature of the stenosis and severe involvement of the adjacent aorta. The lesion is often difficult to cross, is resistant to dilatation, and associated with a high complication and recurrence rate. [29] Most of these lesions ultimately have to undergo either repeated dilatations or surgical bypass (Fig. 11).

The results obtained with angioplasty in all these differing scenarios are conceptualised in Table 3.

Stenosis can also occur at surgical anastomoses of the renal artery in renal transplantation or at the aorto-renal bypass graft. A second surgery in such a situation is usually difficult due to excessive fibrosis encountered by the surgeon at the operative site, hence endovascular treatment is the best option (Figs. 12,13). Angioplasty is successful in almost 90% of cases and results in a good control of hypertension in majority of cases. More importantly, it preserves the transplanted kidney for a much longer time. [30] However, a quarter of these patients tend to have a recurrence, which would require a repeat dilatation or may be even stent placement.

Fig. 7a	Fig. 7b

TABLE 3 Results of angioplasty in RAS
Disease	Technical success	Cured/Improved hypertension	Improved/Stabilised renal function
Fibromuscular disease	100%	98%	100%
Aortoarteritis	85%	72%	NA
Atherosclerosis	20-29% (ostial) 75-95% (non-ostial)	66%	83%

Fig. 8a	Fig. 8b
Fig. 9a	Fig. 9b

Stent placement

The use of stents in endovascular treatment of RAS is rapidly gaining acceptance, for overcomingthe problems of angioplasty. Available data suggests that renal artery stent placement is particularly appropriate in the following situations (Table 4).

The results that have been accumulated so far have been promising. The 99% overall technical success rate of stents is markedly superior to angioplasty alone. Reported clinical results for hypertension and renal failure in atherosclerotic RAS are not markedly different when compared with angioplasty. About 61% of patients appear to benefit when stent are inserted for control of hypertension and 70% show improvement or stabilisation of renal function. [31]

Fig. 10a	Fig.10b
Fig.11a	Fig. 11b

TABLE 4 Indications for renal stents

Technically unsatisfactory angioplasty (residual stenosis, dissection) (Fig. 14) Stenosis at the ostium of the renal artery (Fig. 15) Restenosis following angioplasty Revascularisation of an occluded renal artery

A major drawback of stents is that they may induce intimal hyperplasia, which in turn could cause restenosis in about one-fourth of the cases. This would necessitate another endovascular procedure or possibly a surgical bypass to regain clinical benefit. Since the long-term implications of renal stents are unclear, it is necessary that they are used judiciously in younger patients. Theauthor hence does not recommend their primary use in fibromuscular disease and aortoarteritis at present.

Stents are quite expensive, each costing a minimum of Rs. 35,000. But when one considers the progression to renal failure and entering a dialysis programme, it certainly becomes cost-effective to undergo the endovascular procedure and prolonging the renal function.

Fig. 12a	Fig.12b
Fig.13a	Fig. 13b
Fig.13c	Fig.14a
Fig.14b	Fig.14c

CONCLUSION

RAS impairs blood flow to the kidney and consequently causes renovascular hypertension and renal failure. Although the prevalence of this condition among patients with hypertension is low, improved therapeutic options for relieving RAS, such as angioplasty and stenting, makes the searchfor RAS worthwhile. Renal angiography is the gold standard for diagnosing RAS, but should be used selectively, based on clinical suspicion. Endovascular treatment by angioplasty with or without stent placement is now established as the procedure of choice in treating a majority of the renovascular lesions. In order to ensure optimum results, this form of treatment should be undertaken in a well-equipped vascular and interventional radiology department, having sufficient expertise in handling these procedures.

Fig.15a	Fig.15b

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