2. Abdominal ultrasound
The use of abdominal ultrasound in the evaluation of hypertension is twofold.
In the detection of a secondary forms of hypertension.
In the evaluation of subclinical organ damage induced by hypertension.
In the current European Society of Cardiology/European Society of Hypertension (ESC/ESH ) guidelines for hypertension the use of abdominal ultrasound is recommended as a part of the evaluation of hypertensive individuals. The abdominal ultrasound supplies information about the etiology of hypertension as well as possible end organ damages.
The main interest is the morphology of the kidneys, the adrenal glands and of the aorta. Due to their retroperitoneal position, kidneys are completely and easily detectable. A 3.5-5 MHz probe is typically used to scan the kidney. The examination from dorsolateral allows the evasion of the intestinal loops and thus allows for a non-overlapping imaging in the supine position. The formerly widely spread examination in the lateral recumbent position is nowadays used only in exceptional cases. Renal ultrasound has now almost completely replaced intravenous urography in the anatomical exploration of the kidney. While the latter requires the injection of potentially nephrotoxic contrast medium, ultrasound is non-invasive and provides the necessary anatomic data about kidney size and shape, cortical thickness, urinary tract obstruction and renal masses [1]. Renal parenchymal disease is one of the most common causes of secondary hypertension which leads to a wide spectrum of morphologic alterations. The finding of bilateral upper abdominal masses at physical examination is consistent with polycystic kidney disease and should warrant an abdominal ultrasound examination. Acute parenchymal inflammatory processes like crescentic glomerulonephritis or acute interstitial nephritis sometimes predisposes individuals to measurable organ swelling. The cortical and medullary pyramids have in this case an anechoic profile. However, the morphological alterations seen via ultrasound in the acute kidney processes are less prominent than those seen in chronic kidney damage. Thus, the diagnostic performance in acute inflammatory renal disease is less effective. Chronic parenchymal diseases, such as chronic interstitial nephritis, glomerulonephritis or nephrosclerosis contribute to a progressive decrease in organ size. A kidney size below 90 mm should be interpreted as pathological. Over the course of the organ decrease, small scarring cortical retractions develop, which give the kidney surface a humped aspect. The renal parenchyma develops a hyperechoic pattern through progressive scarring. The border between parenchyma and pyelon becomes progressively nondescript. A variety of chronic parenchymal diseases can lead to the morphological end stage of a shrunken kidney. Sonographically, it is not possible to differentiate whether small kidneys are the cause or the result of hypertension. A unilateral small kidney as a possible indicator for a hemodynamic relevant renal artery stenosis should always lead to a further evaluation of the renal arteries. Renal cell carcinomas as a rare cause of hypertension are depicted as a well delimitable structure from the surrounding tissue. Usually they can be depicted via ultrasound when they exceed 1 cm. With increasingly size there is an increase in their inhomogeneity, so that it is possible to detect areas of liquefied necrosis for example.
In the screening of secondary forms of hypertension abdominal ultrasound plays also a role in the depiction of the adrenal glands. For this purpose a detailed knowledge of the local anatomy is required. The adrenal glands are located within the retroperitoneum. The left adrenal gland, lacking the acoustic window of the liver and being obscured by air in the stomach, is inherently more difficult to scan than the right adrenal gland. On the right side, the right kidney and the inferior vena cava are landmarks for the examination of adrenal glands, whereas on the left side the aorta, the lower pole of the spleen and the upper pole of the kidney are points of orientation. The right adrenal gland is usually scanned with a right transcostal scan or a subcostal flank scan or oblique subcostal scan. On the left side it is better to use an intercostal flank scan through the spleen. The normal sized adrenal glands are only visible with trained examination techniques and by using high resolution technology, whereas enlarged adrenal glands are detectable in a high percentage of cases. Thirty percent of cases of primary aldosteronism are caused by adrenal adenomas. Seventy percent of cases are caused by adrenal hyperplasia. There are rare cases of adrenal carcinoma and the autosomal dominant condition of glucocorticoid remediable aldosteronism [2]. The micronodular hyperplasia is not possible to be detected via sonography. Adrenal adenomas have a round to oval shape and are uniformly hypoechoic with smooth margins, although some lesions have scalloped borders (polycyclic). Adenomas occasionally have an inhomogeneous appearance. Autopsy statistics indicate that they are quite common (10–20%), but most adenomas (90%) produce no endocrine symptoms, they are silent and too small to be detected by ultrasound. In one study the average size of adenomas was reported to be 1.5 cm, although they may exceed 5 cm in diameter. In a small percentage of patients adenomas are bilateral. Functioning and nonfunctioning adenomas are indistinguishable by their sonographic features [3]. Thus, ultrasound is not a sufficient test in the morphologic diagnosis of Conn syndrome. Upon the detection of a high aldosterone-to-renin ratio and after a confirmation test (e.g. suppression after administration of sodium chloride) the use of a CT or MRT scan is indicated. On the other hand, the detection of a unilateral adrenal mass seen in the ultrasound should be followed by a laboratory evaluation for the evaluation of Conn-Syndrome.
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Phaeochromocytoma, a tumor of the adrenal medulla, is a rare secondary cause of hypertension (0.2 – 0.4% of all cases of elevated blood pressure) with an estimated annual incidence of 2 – 8 per million population.[4]. It can be inherited or acquired. Hypertension occurs in about 70% of all cases of phaeochromocytoma, being stable or paroxysmal in approximately equal proportions. The diagnosis is based on establishing an increase in plasma or urinary catecholamines or their metabolites (e.g. (nor-) metanephrines). Following the appearance of clinical symptoms (hypertension and tachycardia caused by increased catecholamine secretion), pheochromocytoma can be detected in 80-90% of cases via abdominal ultrasound. Most pheochromocytomas are already several centimeters in diameter when diagnosed. They have smooth margins, a round shape, and an inhomogeneous or complex echo structure. Hypoechoic liquid components may also be observed. A spectrum of appearances is possible. Pheochromocytomas are bilateral in approximately 10% of cases and extra-adrenal in 10–20%. The organ of Zuckerkandl should be looked for at the level of the origin of the inferior mesenteric artery, anterior to the aorta. Other extra-adrenal sites are the renal hilum, bladder wall, and thorax. Pheochromocytoma is occasionally seen posterior to the renal vein in transverse scans. Rarely, pheochromocytoma is diagnosed in the setting of multiple endocrine neoplasia (MEN). About 2% to 5% of pheochromocytomas are malignant. In recent years endosopic sonography is being used to obtain an adrenal gland biopsy [5-7].
