Cystic Diseases of the Kidney


US renal cyst classification

1. Simple

2. Complex or atypical

 Calcification

 Septa

 Intracystic lesion and wall thickness

3. Cysts of the renal sinus

 Parapelvic

 Peripelvic




Table 4.2
Cystic kidney diseases [1]


































Cystic kidney diseases

1. Renal cystic dysplasia – multilocular renal cysts (cystic nephroma)

2. Medullary cystic disease

 (a) Medullary sponge kidney

 (b) Uremic medullary cystic disease and nephronophthisis

3. Renal polycystic disease

 (a) Autosomal-dominant polycystic kidney disease

  Classic ADPKD

  Early-onset ADPKD in children

 (b) Autosomal-recessive polycystic kidney disease

  Classic ARPKD in neonates and infants

  Medullary duct ectasia in older children with hepatic fibrosis

4. Acquired renal cystic disease (associated with dialysis)

5. Renal cysts in hereditary malformative syndromes (tuberous sclerosis)


Ultrasound examination is the first imaging test, often with immediate and reliable diagnosis. Diagnostic accuracy depends on the operator experience, ultrasound device available, and ability to find valid acoustic windows due to patient’s habitus that are the same as those for the US examination of other abdominal organs. In selected cases, further imaging examinations by CT or MRI are preferred to obtain a better definition of cystic content and parietal aspects.

The diagnosis is almost always occasional, during US or CT examinations carried out in search of other pathologies. Rarely even large renal cysts cause symptoms or complications secondary to infection, bleeding, or urinary tract compression. The compression on the parenchyma only occurs for voluminous cystic formations that sometimes can stretch out on the renal pelvis, compressing it, with partial obstruction of the excretory duct.

Anatomical location of renal cysts is in the cortical, subcapsular, or renal hilum.

The role of the contrast-enhanced ultrasound (CEUS) in the identification and characterization of the complex renal cystic lesions is widely debated [3, 4].


4.1 Simple Renal Cyst


The vast majority of these are usually unilateral and solitary, sometimes multiple or bilateral (Fig. 4.1). It is believed that the development is due to a progressive dilation of the Bowman’s capsule or the contorted proximal tubule, so they have a cortical origin.

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Fig. 4.1
(a) Simple renal cyst with completely anechoic content. (b) Multiple simple renal cysts. (c) Intrarenal cystic lesion. (d) Exophytic cystic lesion

They can spread everywhere throughout the renal tissue. The causes of the onset are not certain, although it is possible to assume damages to the basement membrane of the distal tubules. Other causes can be medullary interstitial fibrosis and ischemic renal parenchymal damages. They are more frequent in males than in females, particularly in middle-aged people; the frequency increases in the elderly population, particularly in males [5, 6]. On the contrary, they are much less frequent in the pediatric population.

The classic simple renal cyst demonstrates well-defined features. They have a completely anechoic content, round/oval shape, and sharp, thin posterior walls (Fig. 4.2a); an imperceptibly thin wall with no enhancement, delimited by a thin layer of fibrous tissue covered by epithelium single layer, is undetectable when the cyst is parenchymal.

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Fig. 4.2
(a) Renal cyst without enhancement after infusion of contrast medium. (b) Roundish hypo-anechoic images in the renal cortex at the cortico-medullary passage: renal pyramids. Not to be confused with cortical cysts

It is important not to confuse the cysts with cortical medullary pyramids, taking hypoechoic appearance especially in the young (Fig. 4.2b).

The cysts generate the formation of artifacts due to the interaction of the ultrasound beam with the substrate analyzed; the ultrasonic beam crossing the cyst is neither attenuated nor absorbed and nor reflected, contrary to the surrounding tissues. The tissues located deeper than the fluid-filled structure emit signals that are reinforced, in comparison with the surrounding tissues. It follows the artifact of the increased through transmission (Fig. 4.3a). For this reason the back wall of the cyst appears thicker than how it really is. Another artifact is the lateral edge or acoustic shadows: thin acoustic shadow that appears behind edges of cystic structures (Fig. 4.3b). The speed of the ultrasounds in the fluid is faster than the propagation speed of the parenchymal tissues around the cystic formation. The matching between ultrasounds and the lateral profile of the cyst causes a refraction that makes the cystic lesion behave like an acoustic lens. This causes the formation of lateral acoustic shadows: the phenomenon of the refraction along the sidewalls. Anechoic renal cysts may show some artifact internal low-level echoes [7]. This may be improved by using harmonic imaging techniques.

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Fig. 4.3
(a) A large cyst with inhomogeneous intracystic material located in the peripheral parts and strong posterior increased through transmission. (b) Simple renal cyst with the artifact of lateral acoustic shadows

Complications are rare with a reported range of 2–4 %. The most common complications are hemorrhage, infections, or rupture. Generally, the increase of the inner content echogenicity relates with hemorrhagic complication (Fig. 4.4).

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Fig. 4.4
(a) Exophytic cyst with inhomogeneous and corpuscular content in a patient with fever and shaking chills: infected cyst. (b) Partly exophytic cystic formation with endocystic iso-anechoic level: intracystic bleeding with declive layers of echoes. (c) Cyst containing blood. US (multiple low-level echoes) and CT axial view (hyperdense oval lesion without intravenous administration of iodinated contrast medium)

A sudden intracystic hemorrhage can clinically occur with pain, and sometimes it is possible to document a fluid-fluid level of the content, with layers of echoes because of the concentration of the protein components deriving from intracystic bleeding.

Hemorrhage generally affects only one cyst, but frequently it is possible to notice multiple cysts with hemorrhagic content in the same kidney. Those with a diameter less than 3 cm, if homogeneous and without calcifications, can be regarded as lacking in particular clinical implications. Sometimes they develop residual calcification in a central pattern or within the cystic wall that becomes thickened and develops septa and the cyst becoming multilocular or multilobular, acquiring the features of a complex cyst (Fig. 4.5) [1, 8].

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Fig. 4.5
Cyst with a fluid-fluid level due to an intracystic bleeding with parietal calcifications


4.2 Complex or Atypical Cysts


Complex or atypical cysts differ in shape and content from simple cysts. Sonographically, the criteria for a simple cyst are (1) an anechoic lesion, (2) sharply defined walls, and (3) acoustic enhancement behind the lesion. Complex or atypical cyst is defined as any deviations from at least one of these standards. A complex cyst includes irregular walls or non-anechoic content or calcifications or diffuse or focal low-level echoes [3, 9].

Oval, polygonal, or elongated shape is suspicious for a complex cyst (Fig. 4.6). As hemorrhagic cysts resolve, they develop residual calcification in a central pattern or within the cystic wall that becomes thickened and develops septa (Fig. 4.7) with the cyst becoming multilocular or multilobular; also the wall of infected cysts is often thickened markedly and calcified occasionally and has parietal irregularities such as endocystic vegetations [110] (Fig. 4.8). They have intrinsic echo content and sometimes clots that are indicative of bleeding.

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Fig. 4.6
Elongated cystic lesion


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Fig. 4.7
(a) Cyst with a thin internal septum. (b) Two adjacent cystic lesions: the walls of the cysts come into contact appearing as just only one cyst, not to be confused with an intracystic septum


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Fig. 4.8
(a) Cyst with a thin focal parietal thickening in the posterior wall. (b) Cyst with a little endocystic vegetation emerging from the anterior wall. (c) Cystic formation with internal low-level echoes: hemorrhagic cyst

The definition of complex cyst is made by the presence of one or more atypical aspects. Furthermore, the septum vascularization on color Doppler is suspicious for cancer. Also CEUS may be useful to show vascularity of septa or nodular protuberances in a renal cyst and can help differentiate a benign cyst from an indeterminate cyst or a malignant-appearing cyst [11]. During US examination, underlining this kind of aspects can raise suspicion of the presence of a malignant lesion. Therefore, a correct evaluation of the cystic lesion is essential in order to recommend an in-depth diagnosis by means of CT or MRI exams.


4.2.1 Cyst with Calcifications


Calcification can occur in the wall or the septa of benign as well as malignant lesions. The presence of calcification may influence the decision to surgically explore or follow up a lesion [12, 13]. CT allows higher spatial resolution than US in the evaluation of cystic parietal and endoluminal calcifications. The posterior acoustic shadowing and the reverberation effect of the echoes in the presence of cystic calcifications during US examination can constitute a limit on the detection of the other components of the cyst itself. There are three categories of calcifications:


  1. 1.


    Small and regular calcification: it is a small quantity of calcium which linearly settles on the wall (Fig. 4.9a, b) or on a septum or, appearing like “milk of calcium,” on the declive part of the cystic lumen.

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    Fig. 4.9
    (a) A cystic lesion with parietal and inner calcifications. This determines the formation of a wide posterior acoustic shadowing. (b) CT axial view. (c) Echoic annular image with posterior positive through transmission due to a cyst with parietal lamellar calcifications. (d) Echoic image with a posterior positive through transmission. (e) CT sagittal view

     

  2. 2.


    Multiple and irregular calcification: it is a calcification associated with parietal thickening of the cyst, with nodules or with aspects of cystic vascularization.

     

  3. 3.


    Single or few isolated calcifications with thickness (Fig. 4.9c) or nodular appearance (Fig. 4.9d, e), without signs of vascularization and without parietal nodules, worthy of follow-up in time.

     

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Jul 10, 2017 | Posted by in UROLOGY | Comments Off on Cystic Diseases of the Kidney
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