Diagnostic ultrasound ( PDFDrive )

CHAPTER 39 The Fetal Urogenital Tract 1361
A
B
FIG. 39.33 Posterior Urethral Valves Causing Urethral Level Obstruction. (A) Dilated urinary bladder (B) and proximal urethra (*) give the
appearance of a keyhole, characteristic of urethral obstruction in a 21-week fetus. (B) Coronal scan shows dilated tortuous ureters (arrows).
Posterior urethral valves are the most common cause of
lower urinary tract obstruction (LUTO), followed by urethral
atresia or stricture. Posterior urethral valves are seen exclusively
in males and may cause total, intermittent, or partial obstruction,
with variable prognosis. Most cases are sporadic, occurring in
1 : 5000 male births; and the recurrence risk is small. 195 Intravesical
pressure generated secondary to the obstruction results in a
persistently dilated urinary bladder, with a dilated proximal
urethra (keyhole sign) (Fig. 39.33). However, the keyhole sign
is not a reliable indicator of posterior urethral valves because it
can also be seen in fetuses with other pathologies, including
VUR and primary megaureter. 196 here may be thickening of
the bladder wall (>3 mm), bilateral tortuous hydroureters, and
hydronephrosis. If the obstruction is severe and long-standing,
progressive renal parenchymal ibrosis and dysplasia develop,
resulting in severe oligohydramnios, pulmonary hypoplasia, and
compression deformities (Potter syndrome). here may be
spontaneous bladder rupture with urinary ascites (Fig. 39.34)
or a calyceal rupture with perirenal urinoma. 197 If spontaneous
decompression occurs, this “safety valve” may protect the kidneys
from further prenatal damage and may diminish the degree of
hydronephrosis.
Urethral atresia causes the most severe, and earliest detected,
form of obstructive uropathy. he sonographic features include
a greatly distended bladder that may ill the whole abdomen,
and anhydramnios ater the irst trimester (Fig. 39.35). Urethral
atresia is almost always fatal, because of associated renal dysplasia
and pulmonary hypoplasia, and only case reports of survivors
following antenatal treatment have been described. 195,198
Prune belly syndrome is a rare congenital disorder characterized
by megacystis, deiciency of abdominal musculature, and
cryptorchidism. It is rarely reported in females. Several mechanisms
for the pathogenesis of prune belly syndrome have been
suggested. Some authors believe that it results from a primary
mesodermal defect, but the more prevailing theory is that in
utero urethral obstruction leads to a distended urinary system
with secondary defective abdominal musculature. 199 he abdominal
wall has a laccid, wrinkled appearance; hence the name
FIG. 39.34 Urinary Ascites. Longitudinal scan of a 22-week fetus
shows a thick-walled bladder (B) and urinary ascites (*) caused by
spontaneous rupture of severe megacystis.
FIG. 39.35 Urethral Atresia. Coronal scan of a 17-week fetus shows
a greatly distended bladder (B) that occupies the entire abdomen. The
thorax (arrows) is compressed and bell shaped because of pulmonary
hypoplasia. There is anhydramnios. P, Placenta.