Multicystic Dysplastic Kidney Imaging

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Multicystic dysplastic kidney (MCDK) is a congenital maldevelopment in which the renal cortex is replaced by numerous cysts of multiple sizes. ^{[1, 2, 3, 4]} A dysplastic parenchyma anchors the cysts, the arrangement of which resembles a bunch of grapes. The calyceal drainage system is absent. Typically, MCDK is a unilateral disorder; the bilateral condition is incompatible with life. ^[5] Furthermore, MCDK with contralateral renal agenesis does not support life. ^[6] (See the image below.)

Several forms of MCDK have been described. The classic type and the less common hydronephrotic type have cysts of various sizes connected by loose, insubstantial fibrous tissue. No functional renal tissue can be identified. The classic type has a random configuration of cysts, whereas the hydronephrotic type presents with a discernible, dilated renal pelvis surrounded by cysts. Some sources identify a third type known as solid cystic dysplasia. Solid cystic dysplasia is composed of smaller cysts with a greater amount of nonfunctional parenchyma. ^[7]

MCDK should not be confused with polycystic kidney disease (PCKD) or other renal cystic diseases. ^[8] Spence recognized MCDK as a distinct entity in 1955. ^[9] In 1986, the Urology Section of the American Academy of Pediatrics established the National Multicystic Kidney Registry, which is a large, multicenter, longitudinal database that has helped clarify the appropriate management of MCDK.

Sonography is the preferred initial examination. In patients with a prenatal presentation, postnatal studies are required to differentiate MCDK from hydronephrosis. A sonogram should be obtained before a neonate is discharged from the nursery when MCDK is suspected. Reduced renal function and relative dehydration of the neonate should have no bearing on the detection of MCDK; therefore, there is no need to delay the initial sonogram. ^{[10, 11, 12]}

In a patient with a symptomatic presentation, such as a palpable abdominal mass, abdominal pain, incontinence, and recurrent urinary tract infections, sonography should be the initial study. The sonographic information provides clues of other urinary tract anomalies as well as intra-abdominal or retroperitoneal malignancies.

MCDK can be detected on other forms of imaging, but sonography is fast and accurate, and it does not require sedation, radiation, or other interventions. Radionuclide imaging can be used to further differentiate the hydronephrotic form of MCDK from an obstruction in a functioning kidney. Radionuclide imaging provides information about the function of the involved renal unit and is superior to intravenous pyelography (IVP) in children.

Technetium-99m (^99mTc) mercaptoacetyltriglycine (MAG-3) and ^99mTc dimercaptosuccinic acid (DMSA) studies can demonstrate lack of function in the affected kidney, but MAG-3 studies can also provide information regarding drainage in an obstructed hydronephrotic kidney.

Voiding cystourethrography (VCUG) is indicated in patients with MCDK to evaluate the urinary tract for vesicoureteral reflux (VUR) and other anomalies. Although a minority of patients will have this defect, VUR could lead to reflux nephropathy in the contralateral solitary kidney; therefore, it is important to perform a VCUG to detect a potentially damaging, but easily correctable, cause of renal damage.

The hydronephrotic form of MCDK can mimic ureteropelvic junction obstruction (UPJO), and radionuclide scanning is necessary following sonography to confirm the diagnosis. Renal function is relatively poor in the first month of life, and radionuclide imaging should be postponed until 1 month of age to avoid false-positive results.

Incidental findings on kidney, ureter, and bladder (KUB) images include displacement of the bowels when the affected kidney is enlarged (see the image below). Also, ringlike calcifications of the cyst walls may be seen on plain images.

Retrograde pyelography may demonstrate an atretic or absent ureter.

MCDK can be an incidental finding, but computed tomography (CT) scanning studies are not part of the diagnostic investigation. They show the typical multicystic appearance of MCDK with little or no parenchyma. Cyst wall calcification can be seen.

If a contrast-enhanced CT scan is performed, there is no excretion seen.

MCDK can be an incidental finding on magnetic resonance imaging (MRI) scans, but MRI is not part of the diagnostic investigation. MRIs show the typical multicystic appearance of MCDK with little or no parenchyma. (See the images below.)

Prenatal imaging findings (see the images below) include hypoechoic cysts of variable sizes and shapes, interfaces between cysts, a nonmedial location of large cysts, the absence of an identifiable renal sinus, a lack of communication between cysts on sonograms, and minimal surrounding parenchyma. Only 20% of MCDKs have an identifiable reniform shape (compared with 90% of hydronephrotic kidneys). Bilateral MCDK may occur with oligohydramnios as a result of poor urine production. The cysts of MCDK may become enlarged, may shrink, or may involute during fetal life. ^{[10, 11, 12]}

If MCDK is indicated on prenatal sonograms, postnatal sonography (shown in the images below) should be completed before the patient is discharged home from the hospital. The diagnostic criteria for postnatal sonography are the same as those for prenatal sonography.

Ultrasonography is an excellent diagnostic test for MCDK, with a high degree of confidence. An obstructive uropathy with little renal parenchyma can mimic MCDK, but radionuclide studies can provide confirmation of the diagnosis. Autosomal recessive PCKD is not usually mistaken for MCDK, as the cysts in PCKD are too small to be visualized on sonograms, and the parenchyma is generally homogeneously hyperechoic. Other cystic diseases typically appear with some functional parenchyma.

The greatest source of false-positive errors occurs in the setting of hydronephrosis or UPJO. A reniform shape and/or a large cystic structure in the medial portion of the kidney are more indicative of hydronephrosis than MCDK. Unlike in hydronephrosis, where there is communication with the central renal pelvis, the cysts of the classic form of MCDK do not communicate.

Regarding false-negative results, renal agenesis with non-renal cystic structures in the retroperitoneum (eg, an adrenal cystic mass) could potentially be mistaken for MCDK.

Nuclear renograms are used to evaluate the perfusion, function, and drainage of the kidneys, as there is uptake of the radiotracer into a functioning kidney and excretion into the renal pelvis, ureter, and bladder. In the area of the MCDK, a photopenic region is present that represents displaced tissue with background activity only. ^[10] (See the images below.)

Nuclear medicine studies are the best imaging studies for differentiating between MCDK and hydronephrosis; however, severe hydronephrosis with poor renal function may still be difficult to distinguish from MCDK. The similarity may not be clinically relevant because reconstructive surgery is usually not indicated in the setting of poor (ie, < 10-15%) differential function.

MAG-3, diethylenetriamine-pentaacetic acid (DTPA), and dimercaptosuccinic acid (DMSA) are the preferred agents. Tests with these agents are sensitive for detecting renal function, but the anatomic resolution is relatively poor. Other forms of imaging are required to visualize the specific anatomy of the kidney.

On a nuclear renogram, anything that impairs the renal circulation, such as a renal artery stenosis or a renal vein thrombosis, appears as a nonfunctioning kidney; nevertheless, the renal parenchyma should appear relatively normal with ultrasonography. In addition, renal agenesis may mimic the nonfunctioning kidney of MCDK, but the tissue is absent on sonograms.

False-negative results may arise in the setting of severe hydronephrosis. In a poorly functioning kidney, the intervention is the same as in MCDK; therefore, discriminating between these 2 disorders may not be imperative.

Angiography is not indicated for the evaluation of MCDK; however, if catheterization, CT scanning, or magnetic resonance angiography (MRA) is performed, the ipsilateral renal artery is atretic or absent.

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John S Wiener, MD, FACS, FAAP Associate Professor of Surgery and Associate Residency Program Director, Division of Urologic Surgery, Associate Professor of Pediatrics, Duke University School of Medicine

John S Wiener, MD, FACS, FAAP is a member of the following medical societies: Alpha Omega Alpha, American Academy of Pediatrics, Society for Pediatric Urology, Society of University Urologists, Society for Fetal Urology, American College of Surgeons, American Medical Association, American Urological Association

Disclosure: Nothing to disclose.

Ana Maria Gaca, MD Assistant Professor, Division of Pediatric Radiology, Duke University Medical Center

Disclosure: Nothing to disclose.

Bernard D Coombs, MB, ChB, PhD Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand

Disclosure: Nothing to disclose.

Kieran McHugh, MB, BCh Honorary Lecturer, The Institute of Child Health; Consultant Pediatric Radiologist, Department of Radiology, Great Ormond Street Hospital for Children, London, UK

Kieran McHugh, MB, BCh is a member of the following medical societies: American Roentgen Ray Society, Royal College of Radiologists

Disclosure: Nothing to disclose.

Eugene C Lin, MD Attending Radiologist, Teaching Coordinator for Cardiac Imaging, Radiology Residency Program, Virginia Mason Medical Center; Clinical Assistant Professor of Radiology, University of Washington School of Medicine

Eugene C Lin, MD is a member of the following medical societies: American College of Nuclear Medicine, American College of Radiology, Radiological Society of North America, Society of Nuclear Medicine and Molecular Imaging

Disclosure: Nothing to disclose.

Lori Lee Barr, MD, FACR, FAIUM Clinical Assistant Professor of Radiology, University of Texas Medical Branch at Galveston School of Medicine; Member, Board of Directors, Austin Radiological Association; Consulting Staff, Seton Health Network, Columbia/St David’s Healthcare System, Healthsouth Rehabilitation Hospital of Austin, Georgetown Hospital, St Mark’s Medical Center, Cedar Park Regional Medical Center

Lori Lee Barr, MD, FACR, FAIUM is a member of the following medical societies: American Roentgen Ray Society, Association of University Radiologists, Southern Medical Association, Undersea and Hyperbaric Medical Society, American Society of Pediatric Neuroradiology, Society of Radiologists in Ultrasound, Texas Radiological Society, American Association for Women Radiologists, American College of Radiology, American Institute of Ultrasound in Medicine, Radiological Society of North America, Society for Pediatric Radiology

Disclosure: Nothing to disclose.

Michaella E Maloney Duke University School of Medicine

Disclosure: Nothing to disclose.

Multicystic Dysplastic Kidney Imaging

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