Oligohydromnios, Pulmonary hypoplasia, Intrauterine vesicoamniotic shunting

Pulmonary Hypoplasia

• Nearly always secondary to another process
  • Restriction in fetal thoracic volume
  • Oligohydramnios
  • Decreased fetal breathing
  • Decreased pulmonary blood flow
  • Trisomy 13, 18, 21
• Degree of severity depends on timing of insult
  • Conducting airways formed by 16 weeks gestation
  • Bronchioles continue to develop until 26 weeks
  • 90% of alveoli develop after birth
• Late insults may be asymptomatic
• Early damage may result in
  • pulmonary hypertension
  • hypoventilation
  • death

Amniotic fluid volume is a good indicator of fetal renal function and of the potential success of vesicoamniotic shunting. Failure of urine to fill the amniotic space leads to progressive oligohydramnios, which results in poor pulmonary growth. The primary cause of death in prenatally diagnosed obstructive uropathy is pulmonary hypoplasia. The degree of renal dysplasia and pulmonary hypoplasia depends on the gestational age at onset and the duration of obstruction. Intrauterine relief of obstruction and restoration of amniotic fluid volume leads to normal lung growth and development, which is the basis for vesicoamniotic shunting as a means of preventing pulmonary hypoplasia. 

The shape of the urinary bladder, rather than its volume, determines the outcome following vesicoamniotic shunting. Complete obstruction of the urethra, as in urethral atresia, leads to a round, thick-walled bladder. In contrast, partial obstruction of the urethra, as in posterior urethral valve, results in a tubular shape with symmetric wall thickening. Functional obstruction of the urethra, as in prune belly syndrome, leads to a segmented bladder appearance, with thinning of the dome  and thickening of the bladder neck. Urethral atresia carries a poor prognosis.

In female fetuses, lower urinary tract obstruction often is associated with developmental anomalies of the cloaca and other syndromic abnormalities. In contrast, male fetuses usually have an isolated urethral obstruction. Female gender, therefore, carries a poor prognosis for intrauterine intervention.

Analysis of fetal urine for electrolytes has been proposed as a test for determining the outcome of vesicoamniotic shunting. Hypertonicity of urine from salt wasting is believed to indicate poor renal function and, hence, a poor outcome. However, this interpretation remains unconfirmed. Moreover, the measurement of urine electrolytes from a single bladder drainage may  not be predictive of fetal renal function.

Ultrasonographic measurement of the renal pelvis diameter has been proposed as another test for determining the outcome of vesicoamniotic shunting. However, the usefulness of this test is controversial.Continued surveillance of renal pelvis diameter at various gestations and correlation with postnatal diagnosis and outcome are needed.

References:
Jona JZ. Advances in fetal surgery. Pediatr Clin North Am. 1998;45:599-604
Walsh DS, Johnson MP. Fetal interventions for obstructive uropathy.
Semin Perinatol. 1999;23:484-495