Aetiology

The two most important causes of RVH are Atherosclerotic renal artery stenosis (ARAS), which involves older population, and fibromuscular dysplasia (FMD) which more often affects children and young adults.

ARAS accounts for more than two-thirds cases of RVH. It tends to involve males above 45 years. It more often involves ostium and proximal one-third of renal artery, and bilateral affliction is seen in at least one-third of the cases. It is often associated with diabetes mellitus, dyslipidaemias and coexisting atherosclerosis of other arteries. The abdominal bruit in ARAS is of less diagnostic value.

FMD on the other hand tends involve female (F:M ratio of 8:1) <35 years of age. It is usually unilateral when diagnosed, but can involve both RA. It usually involves distal two-thirds and branches of RA. A high pitched, often intermittent bruit, best heard in mid epigastrium is audible in about 60% of patients. Three histological types of FMD exists – intimal, medial and adventitial. The medial FMD is most common (has classical string of beads appearance) and it shows progression in one-third of cases and complications like thrombosis and dissection are rare. The intimal and adventitial FMD are rarely encountered and commonly shows progression and aforementioned complications.

Other less common aetiologies of RAS are autoimmune diseases (Takayasu arteritis and polyarteritis nodosa), dissection, embolism, compression by extrinsic lesions, neurofibromatosis and hypercoagulable states.

Pathophysiology

It is well established that the principal mechanism underlying RVH is Renin angiotensin aldosterone system (RAAS).

Haemodynamically significant stenosis leads to decreased renal perfusion (ischemia) and as a compensatory response to this, affected kidney initiates hypersecretion of renin which in turn accelerates conversion of angiotensinogen to Angiotensin I, and further to Angiotensin II by angiotensin-converting enzyme (ACE).

Angiotensin II acts in multiple ways to restore glomerular filtration-vasoconstriction of efferent arteriole, increase aldosterone release from adrenal, increased sodium absorption from proximal tubule, sympathetic nerve system activation, intra-renal prostaglandin concentrations, and nitric oxide production; all contributing to HT.

Another important factor to consider in pathophysiology RAAS mediated RVH is presence or absence of contralateral kidney. In unilateral renal artery disease pressure diuresis and renin suppression by nonstenotic kidney mitigates the angiotensin II effects of ischemic kidney to some extent. But in a clinical scenario of solitary ischemic kidney or bilateral disease this salvage mechanism is absent, hence playing an additive role in the HT.

Long duration HT leads to reverse tachyphylaxis (reduction in renin level) thereby leading to increase in total fluid volume and cardiac output. It might be interesting to note that in prolonged RVH, the nephrosclerosis that happens in the opposite kidney (initially normal) becomes the culprit for HT and hence correction of stenosis in the affected kidney may not relieve the HT. However, nephrectomy of the contralateral kidney can lead to correction of HT.

When to clinically suspect RVH?

The clinical features of RVH include

Do all suspects need imaging evaluation? american college of radiology (ACR) appropriateness criteria

The evidence-based guidelines as listed in ACR Appropriateness Criteria are for specific clinical conditions that are reviewed annually by a multidisciplinary expert panel.

Although previously mentioned clinical scenarios points towards possibility of RVH, testing for RAS is not typically warranted for patients whose hypertension is well managed with medical therapy. This fact is supported by a randomized control trial of 947 patients from 113 centres ‘Cardiovascular Outcomes in Renal Atherosclerotic Lesions’ which showed no difference in multiple endpoints between medical therapy and renal stenting in patients with atherosclerotic RAS and hypertension or chronic kidney disease.

The testing for RAS is warranted in following clinical scenarios

The ACR appropriateness of imaging investigations in whom testing of RAS is warranted can be broadly categorized into two groups

Radiology investigations

Radiology imaging investigations may be categorized as (a) noninvasive and (b) minimally invasive, or it may be more logical group them as procedures defining (i) structural abnormalities and (ii) functional status of kidneys (Refer to Table 10.4.1.3).

Examination technique

CDUS requires great amount of skill, hence pre-scan patient preparation and knowledge of examination techniques are important from the perspective of performing radiologist/technologist.

From practical point of view obstacles encountered during scanning are obese habitus of patient, inability to hold breath and interposition of bowel gas, which limit optimal visualization of main RA in 20%–30% of the cases. First two are nonmodifiable factors and depends on skill of performing radiologist to some extent; however, problem to bowel gas can be mitigated by performing examination in early morning after overnight 12 hours fasting.

The procedure begins with 30 degrees reverse Trendelenburg supine position. Low-frequency probe (2.5–5 MHz) is required to visualize aorta and RA. Before starting dedicated CDUS examination, an overview of B mode appearance of bilateral kidneys is suggested to look for asymmetry in size and cortical echotexture, which may help localize the laterality, degree and duration of RAS and exclude other renal pathologies. The two main imaging techniques for examination are (a) Anterior approach (b) Flank, and other two are ‘banana peel view’ and rarely used posterior approach.

Anterior approach demonstrates bilateral main renal arteries

Right RA originates from anterolateral aspect of aorta and courses behind IVC. The problem frequently encountered in this approach is that Doppler angle is perpendicular to renal artery flow and interposition of IVC. These technical difficulties are alleviated by flank approach in which angle of insonation becomes parallel to blood flow and renal artery is visualized separately from IVC.

Left RA originates from posterolateral surface of aorta and imaging clue to visualize it is identification left renal vein which is large and easy to locate, behind which lies artery. A pitfall that should be avoided is mistaking of inferior mesenteric artery (IMA) as left RA; however, this can be dealt with knowledge that IMA shows high resistance waveform and RA low resistance and typical IMA arises much lower in aorta. A second approach to visualize left RA is modified flank approach in which patient is turned to right lateral decubitus position resulting in mild midline shift of kidney, which then acts as its own window.

Flank Approach demonstrates intra-renal/parenchymal arteries, and bilateral main RA as described above.

Banana peel view is particularly useful to identify the ostium of RA, where most stenosis occurs in elderly. In this approach also, the patient is turned to the opposite decubitus position from the vessel being examined. The transducer is placed longitudinally, aorta is identified, and then probe is moved in an AP direction until the RA is identified arising from it.

Knobology

Correct use and knowledge of controls of USG machine (Knobology) is essential, and important ones required in CDUS are Doppler gain, pulse repetition frequency/Velocity scale, Doppler gate and Doppler angle.

Doppler gain should be optimized to detect flow by increasing the gain to a level just below colour artefact. The pulse repetition frequency is the frequency of sampling; under sampling may underestimate peak velocities. To get the largest spectral images possible, one should decrease the velocity scale until the waveform fills almost the entire field and increase sweep speed. The Doppler gate should be set to include the entire artery lumen and Doppler angle/angle of insonation should be ≤60 degrees.

Normal doppler findings

The spectral waveform of renal artery is a typically low resistance waveform (just like any other parenchymal organ) with rapid early upstroke.

The normal peak systolic velocity in main renal artery without any stenosis is <100 cm/s and it decreases distally in intrarenal branches. The resistive index (RI) value of 0.7 is used to discriminate normal and pathologic resistance to flow. The normal acceleration time (AT) is <0.07 s and acceleration index (AI) is >3.5 m/s². The acceleration indices should be measured along the initial portion of systolic upstroke, avoiding compliance peak. The strongest and reproducible signals come from segmental and interlobar arteries as they course directly towards transducer.

Doppler criteria for diagnosis of RAS

The ideal situation is when CDUS demonstrates bilateral RA along its entire length from ostium to its divisions into segmental and interlobar arteries. The Doppler criteria for RAS can broadly grouped as (a) Proximal/Direct and (b) Distal/Indirect.

Diagnostic criteria

Consensus panels have recommended that the test be interpreted as low, intermediate (indeterminate) and high probability for RVH.

A low-probability test shows a normal ACE inhibition renogram or a grade II renogram that is unchanged or improves after ACE inhibition.

An intermediate probably test shows a kidney that is small and poorly functioning or kidneys with abnormal baseline renograms (grades III–V) that are unchanged after ACE inhibition.

A high probability test or findings suggestive of haemodynamically significant stenosis are worsening of the scintigraphic curve (exaggeration of baseline asymmetry), change in the scintigraphic grade, decrease in the relative uptake, parenchymal retention, increase in the 20-min/peak uptake ratio or renal cortical activity at 20 min (RCA₂₀), and prolongation of Tp (time to peak).

The ACE inhibitor scintigraphy can be very accurate in selected high-risk cases. Acosta GMJ et al. reported a specificity of 91%, but the sensitivity was 60%. The shortcomings of this test are low sensitivity in presence of bilateral RAS, impaired renal function (Sr Creatinine > 2 mg/dL), chronic intake of ACE inhibitors and obstructive uropathy.

Clinical presentation

The most common presenting complaints of urinary calculi include pain, haematuria or urinary tract infection. Small nonobstructive stones are usually silent. The passage of stones into the ureter can cause obstructive symptoms of colicky pain and vomiting.

Large staghorn calculi are often relatively asymptomatic. It refers to the presence of a branched kidney stone occupying the renal pelvis and at least one calyceal system. They manifest as either haematuria or UTI rather than acute pain.

Noncontrast CT or CT-KUB

In patients with nephrolithiasis, noncontrast CT or CT-KUB radiography is most often used. The ACR estimates the specificity of CT to be 97% when a patient presents with acute flank pain suspicious of an obstructing stone. The sensitivity of CT for detecting kidney stones is the highest of all the available modalities and reasonable estimates suggest it is ∼95%. All types of renal stones are usually seen on CT, rare exceptions include stones that are caused by the precipitation of protease-inhibitor medications in the urine. CT generates a 3D image of the stone and the surrounding anatomy, which can be reconstructed into multiple viewing planes. Few large stones are missed using CT but small stones (<3 mm) might slip between the imaged tissue planes and not be detected.

The accuracy of CT is significantly better than that of ultrasonography. Standard CT is the imaging modality of choice for patients with a BMI >30 according to the ACR, AUA and EAU.

A standard CT scan exposes patient to an effective dose of ionizing radiation of ∼10 mSv. Low-dose CT is used to reduce radiation exposure by lowering the tube current to the radiation source.

Overall, CT is a highly sensitive and specific technique for imaging stones in patients presenting with renal colic in emergency, which is useful in assisting surgical decision making, owing to the superior anatomical detail obtained.

The ACR and AUA both recommend CT as the first-line investigation for adult patients with symptoms suggestive of obstructive urolithiasis. When considering low-dose protocols, one should keep in mind patient’s age, BMI and degree of clinical suspicion of stones in order to choose the optimal imaging option.

CT urography

It requires additional images after administration of IV contrast. First scan is done after 90–100 s and second scan after 5–15 min of contrast administration.

Unenhanced CT KUB has many advantages over Ct urography (CTU) for detection of urinary calculi with sensitivity ranging from 98% to 100% and specificity of 92%–100%.

Due to administration of IV contrast, there is always risk of nephrotoxicity associated with CTU which is eliminated by the use of unenhanced CT without any compromise in the detection rate of calculi.

Dual-energy CT

Dual-energy CT can be used to differentiate between uric acid and nonuric acid stones. Further characterization of nonuric acid stones into cystine, struvite, calcium oxalate and hydroxyapatite stones can be done by applying additional tin filtration to the high-energy spectrum. Various colour coding systems are provided by different vendors for different calculi (Fig. 10.4.2.1). Accuracy of stone characterization is the highest for stones larger than 3–5 mm in diameter. Small stones have tendency to pass spontaneously, so this limitation may not be clinically significant. Another advantage of dual-energy CT in CTU is virtual nonenhanced images allowing detection of urinary calculi that would otherwise be obscured.

Ultrasonography

The ACR and American Urological Association recommend CT evaluation as a first-line investigation of patients with suspected kidney stones, whereas the European Association of Urology recommends ultrasonography. Ultrasound has a low sensitivity of 45% for detection of ureteric and renal calculi and a specificity of 94% and 88% for the same.

Ultrasonography can also be used to detect surrogate markers of obstructing stones such as hydronephrosis, and a lack of a ureteral jet using Doppler ultrasonography.

Ultrasonography is very limited in its ability to detect small stones (<5 mm), due to partial volume effect or the absence of posterior shadowing. Stones in the mid ureters are also not detectable, due to masking by overlying intestinal loops and gas, especially in obese patients.

The accuracy of diagnosing renal obstruction and stones can be improved by combining Doppler US and Colour Doppler for identifying secondary signs.

An absent, asymmetric and/or reduced ureteric jet from the ureteric orifices usually suggests presence of ureteric obstruction. However, the contrary may not be true always, that is, presence of a positive ureteral jet does not rule out the presence of ureteral stones 14 since ureteral stones quite often only cause partial obstruction.

A renal resistivity index (RI) >0.70 and/or a 10% difference between the kidneys is considered as diagnostic of obstructive uropathy.

To summarize ultrasonography can reliably detect hydronephrosis but is less sensitive and specific than CT imaging for detecting and sizing of stones. It is recommended as the first-line imaging modality for pregnant and paediatric patients.

MRI

The sensitivity of MRI for detection of renal or ureteric calculi is variable, can be augmented by hydro nephrosis. Using standard MRI sequences, stones appear as a nonspecific signal void;

The sensitivity of MRI is 82% more than that of ultrasonography and X-ray but less than that of CT.

A major advantage of MRI is that it provides 3D imaging without radiation. Unfortunately, the drawbacks of MRI like high cost, lower accuracy and longer image acquisition times prevent it from widespread use in stone imaging.

The ACR, AUA and EAU guidelines suggest that MRI be used as a second-line modality when ultrasonography is nondiagnostic in pregnant and paediatric patients.

X-RAY: KUB

This technique uses the same fundamental concepts as CT but in a single plane. Generally, 75% of stones are calcium-based and should be visible on plain film, only about 60% are found to be visible on plain films.

Intravenous pyelography

IVP is less sensitive than noncontrast helical CT. CT scanning with delayed contrast series and thin slices has reduced the need for IVP in most cases.

The advantage of IVP:

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