Multicystic kidney disease
Multicystic kidney disease has an incidence of approximately 1/10,000 and presents a variable picture with large or small kidneys, bilateral, unilateral or segmental involvement, and cysts of varying size and number. When bilateral, it is always lethal. Multicystic kidney is the most common type of renal defect in newborn infants with Potterís syndrome. The pathogenesis is obscure. In some cases it may result from inhibition of the normal induction of nephron development by the ampulla which affects normal branching of the ureteric bud so that the terminal ampullary portions become cystic. Alternatively a lack of condensed metanephrogenic mesenchyme apposed to the ureteric ampullae may result in the primitive ducts of renal dysplasia. In addition, since urethral obstruction ultimately results in cystic dysplasia, it is difficult to distinguish between a primary developmental error of the ureteric bud or metanephrogenic mesenchyme and secondary pathogeneses related to urinary tract obstruction. Early fetal ultrasound examinations may suggest which mechanism has been implicated.
Multicystic kidneys are frequently enlarged, but may be small and atrophic. Typically the cysts are multiple, peripheral, round and of varying size. They may be bilateral, unilateral or segmental (unilocular cysts). If bilateral, the fetal bladder may not be visualised; oligohydramnios and signs of Potterís syndrome may be present. If hydronephrosis (cysts radiating from the renal pelvis) or a dilated ureter are seen, this is not consistent with MKD and may indicate a potentially correctable obstructive lesion. A search should be made for other major organ system anomalies including cleft palate, diaphragmatic hernia, microphthalmia, duodenal stenosis, imperforate anus, tracheoesophageal fistula, and absence of radii. This is especially important for lethal conditions associated with MKD such as Meckel-Gruber (occipital cephalocele, polydactyly) and the short-rib polydactyly skeletal dysplasias. Unilateral cases may be only segmental (multilocular); there is an increased incidence of congenital malformations associated with unilateral MKD.
Bilaterally enlarged kidneys must be distinguished from those found in infantile polycystic kidney disease (IPKD) in which the cysts are diffuse, microscopic, and uniform in size. Consequently the enlarged kidneys have a solid appearance and occupy greater than 1/3 of the fetal abdominal area in a transverse plane; the fetal bladder may not be seen and oligohydramnios may be present, usually by the third trimester. Lower tract obstructive lesions can usually be easily distinguished by the presence of dilated ureters and/or a distended fetal bladder. Ureteropelvic junction (UPJ) obstruction can be more difficult to exclude. It is characterised by cystic structures communicating with or from the renal pelvis which itself may be markedly dilated. Usually more renal parenchyma is present in UPJ obstruction except for the most severe end-stages, in which case findings may become indistinguishable from MKD. Occasionally a large single cyst may replace the renal pelvis with little or no parenchyma remaining. Unilateral MKD must also be distinguished from obstructive lesions as above. Segmental cysts must be distinguished from Wilmsí tumours, hamartomas, adrenal tumours and haemorrhage.
Unilateral or bilateral cystic kidneys
Multiple round cysts of variable size
Kidneys usually enlarged but may be small and atrophic
When bilateral – bladder may not be visualized and oligohydramnios may be present
When unilateral – hypoplasia or hydronephrosis may be evident in the contralateral kidney
- Dandy-Walker malformation
- Renal dysplasia-aplasia
- Trisomy 9
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