Megacystis

Diagnosis

A massively dilated and persistent fluid-filled structure arising from the fetal pelvis in the presence of oligohydramnios after 6 weeks is strong evidence for megacystis secondary to complete bladder obstruction. The bladder often assumes a fusiform or pear-shaped appearance with the neck of the pear representing the bladder neck and dilated upper portion of the urethra. Obstruction is not always complete or constant and associated features can be variable. Oligohydramnios is thought to occur in approximately 50% of fetuses with urethral level obstruction and 40% have ureterectasis or hydronephrosis. Detection of renal cortical cysts in the presence of urinary tract obstruction indicates renal dysplasia. Following aspiration, shunt placement, or spontaneous decompression of the urinary bladder, thickening of the bladder wall is seen secondary to hypertrophy of the detrusor muscle. Spontaneous decompression may cause extravasation of urine and result in urinary ascites or a paranephric urinoma. Decompression through the urachus may result in a dilated urachus or urachal cyst. With megaureter-megacystis syndrome or vesicoureteral reflux the degree of hydronephrosis is usually less severe and nonprogressive. Occasionally it is severe but in some cases it can resolve spontaneously. Since the mechanism of megacystis is not urethral obstruction but rather incomplete bladder emptying due to either poor musculature or reflux, the amniotic fluid volume may be normal or only slightly decreased. In megacystis-microcolon-hypoperistalsis syndrome the cardinal features include an enlarged bladder, dilated small bowel and polyhydramnios. Hydroureter and hydronephrosis are almost always present. This syndrome is frequently lethal so a precise diagnosis is extremely important.