Address correspondence to: Alicia Crum, RDMS, Baptist Hospital, Division of Maternal‑Fetal Medicine, 2010 Church Street, Suite 323, Nashville, TN 37203‑0338. Ph: 615‑329‑7833 Fax: 615‑340‑4637

Synonyms: Infantile or serous renal cyst.

Definition: Unilocular cyst with a single‑layer epithelial lining containing clear serous fluid. They are usually solitary but occasionally can be multiple. The walls are smooth, and there is no communication between the cyst cavity and the renal pelvis. The remainder of the kidney has a normal appearance^4,5,7.

Prevalence: Simple renal cysts in children are rare: less than 16:10,000 in children of less than one year^3‑7. They are equally distributed between males and females and the right and left kidneys^2,6. The upper pole is most frequently involved².

Etiology: The exact etiology of cyst formation is unknown. Theories presented have suggested they are acquired due to changes associated with aging⁵ or are formed during development⁴. Other possible causes are calyceal diverticula, abscess or hematoma⁵.

Associated anomalies: No anomalies are associated with a true simple cyst. However, renal cysts can be seen with cystic dysplasia, polycystic kidney disease, Meckel"s syndrome, chromosomal abnormalities and renal medullary cystic disorders¹.

Differential diagnosis: Duplication anomalies, hydronephrosis, calyceal diverticulum, multicystic kidney, polycystic kidney, abscess, necrotic tumor^3‑5.

Prognosis: Excellent; usually no complications, although long‑term outcome is not fully known⁶.

Recurrence risk: None reported².

Management: When no complications are present, occasional monitoring is sufficient^5,6.

MESH Kidney cyst ICD9 753.1 CDC 753.100

Introduction

Because renal cysts are infrequently seen in fetuses, the differential diagnosis of a simple renal cyst is often overlooked when considering other pathologies such as hydronephrosis, multicystic dysplastic kidneys, Wilms" tumor, etc. However, the physician and sonographer must be aware that with careful scanning and attention to detail, a simple renal cyst is a legitimate differential diagnosis.

Case report

An 18‑year‑old G₁P₀ patient was referred for routine sonography of a twin pregnancy. The ultrasound examination revealed a 27 week, diamniotic, twin pregnancy. Two small (approximately 4 and 5mm) cystic structures were observed on the lower pole of the left kidney of twin A. The remaining anatomy of twin A, as well as that of twin B, was unremarkable. A differential diagnosis of simple cysts versus lobar multicystic kidney was given. These cysts were not visualized on a previous scan done at 20 weeks. There was no change seen with a follow‑up exam done at 31 weeks.

Figure 1: Sagittal, oblique and axial views demonstrating the two small lower pole cysts.

Discussion

The pathogenesis of renal cyst is not entirely known. Because of the increasing frequency of renal cysts with age (they are found in over 50% of people over 50 years of age^2,5,7), it has been suggested that cyst formation is acquired— a result of the aging process^2,4,5,7. Vascular changes associated with age affect blood flow to the kidneys. This decreased blood flow causes areas of ischemia or infarct and obstruction of the renal tubules which leads to cyst formation^4,5,7. Another theory suggests that cysts are developmental in origin. During renal organogenesis, the second to fourth generations of uriniferous tubules do not coalesce or unite with later generations of collecting tubules, resulting in cyst formation⁴. Neither of these theories adequately explains the formation of all renal cysts. The first fails to explain the formation of pediatric and fetal cysts, while the second fails to explain cyst formation in adults. It is possible that the pathogenesis of cyst formation in adults and children is entirely different. Other possible causes of cyst formation in children and adults are liquefied hematomas, sterilized abscess, or calyceal diverticula that have lost their communication with the renal pelvis^2,7.

Prevalence

Simple renal cysts in children are rare. A study conducted at the Hospital for Sick Children in Toronto estimated the incidence rate at 0.22% for children 18 years and younger⁶. The rate for infants less than 1 year was 0.16%⁶. The distribution is equal between males and females and right and left kidneys². The upper pole is the most common site².

Ultrasound appearance

To be classified as a simple cyst, the following criteria must be met: unilocular, smooth walls with no communication to the collecting system^4,5. The contents should be free of internal echoes, and there should be posterior enhancement². Occasionally, internal echoes may be seen if there has been trauma or infection⁴, but this is seen in pediatric patients, not prenatally.

Size

The size may vary from millimeters to centimeters⁴. The Toronto study found a range of 3mm to 70mm, with a mean size of 10mm⁶.

Number

Usually the cysts are solitary but may be multiple, as was seen in one of our cases^2,4. There has been at least one case of bilateral cysts reported⁹. They can be medullary or cortical in origin⁴.

Differential diagnosis

Because cystic lesions can be seen with many types of renal and chromosomal abnormalities, meticulous scanning is a must. Thorough review of family history can also add valuable information. Differential diagnosis should include multicystic and polycystic kidney disease and structural anomalies such as duplication and calyceal diverticula. Tumor, abscess and hematoma must be considered, but they most likely will have internal echoes. Although renal cysts can be seen with chromosomal abnormalities, there are usually other anomalies present¹. When a cystic lesion is seen in the upper pole, an adrenal origin must also be considered. Finally, a cystic teratoma of the retroperitoneum can be considered.

Recurrence risk

There is no evidence of famillial tendency⁸.

Prognosis and management

The prognosis for children with simple renal cysts is excellent. Although data from long‑term studies is limited, it appears that most cysts do not change in size or cause problems with renal function². In most instances, if a firm diagnosis of simple cyst has been made, further investigation is not necessary^5,6,9. Barring complication, children with simple renal cysts can be managed with regular periodic monitoring^3,5,10.

References

1. Mir S, Rapola J, Koskimies O: Renal cysts in pediatric autopsy material. Nephron 33:189‑195, 1983.

2. Yamagishi F, Kitahara N, Mogi W, et al: Age related occurrence of simple renal cysts studied by ultrasonography. Klin Wochenschr 66:385‑387, 1988.

3. Steinhardt GF, Slovis TL, Perlmutter AD: Simple renal cysts in infants. Radiology 155:349‑350, 1985.

4. Siegel MJ, McAlister WH: Simple cysts of the kidney in children. J Urol 123:75‑78, 1980.

5. Gordon RI, Pollack HM, Popky GI, et al: Simple serous cysts of the kidney in children. Radiology 131:357‑361, 1991.

6. McHugh K, Stringer DA, Herbert D, et al: Simple renal cysts in children: Diagnosis and follow‑up with US. Radiology 178:383‑385, 1991.

7. Mosli H, MacDonald P, Schillinger J: Caliceal diverticula developing into simple renal cyst. J Urol 136:658‑661, 1986.

8. Grossman H, Rosenberg ER, Bowie JD, et al: Sonographic diagnosis of renal cyst diseases. AJR 140:81‑85, 1983.

9. Glassberg KI, Filmer RB: Renal dysplasia, hypoplasia and cystic disease of the kidney. In Kellalis PP, King LR, Belman (eds). Clinical Pediatric Urology, 2nd ed. Philadelphia: WB Saunders, 1985; 948‑971.

10. Chilcote QA: A simple renal cyst in a child. J Can Assoc Radiol 33:51‑52, 1982.