Oligohydramnios

What is Oligohydramnios?

Oligohydramnios is an abnormal reduction in the amount of amniotic fluid surrounding the fetus. At term, there should be approximately 800 mL of amniotic fluid present. Often defined as a single deepest pocket of amniotic fluid of 2 cm or less or an amniotic fluid index (sum of maximum vertical fluid pocket in each quadrant not containing umbilical cord or fetal extremities) of 5 cm or less on ultrasonography.

Prevalence: Rare in early pregnancy, common in post-term pregnancies (12%-25% at 41 weeks) and during labor following rupture of the fetal membranes.

Predominant Age: Reproductive age.

Genetics: No genetic pattern.

Causes and Pathogenesis of  Oligohydramnios

Causes: Unknown. Generally associated with a reduction in fetal urine production ( renal agenesis, urinary tract obstruction, and fetal death), chronic amniotic leak or preterm rupture of the membranes (35%), maternal disease (hypertension, diabetes, uteroplacental insufficiency, preeclampsia).

Risk Factors Oligohydramnios: Fetal chromosomal or congenital abnormalities, Fetal growth restriction or demise, post-term pregnancy, maternal hypertension, diabetes, preeclampsia, and prostaglandin synthetase inhibitors.

Chorioamnionitis: Causes, Symptoms, Risk Factors and Treatment

Signs and Symptoms of  Oligohydramnios

1. Uterine size smaller than normal for stage of pregnancy.
2. Reduced amniotic fluid measured by ultrasonography. 

Abnormality (Anomalies) Associated with Oligohydramnios

1. Amniotic band syndrome
2. Cardiac anomalies: tetralogy of Fallot, septal defects.
3. Central nervous system: holoprosencephaly, meningocele, encephalocele, microcephaly.
4. Chromosomal: triploidy, trisomy 18, Turner syndrome.
5. Cloacal dysgenesis
6. Cystic hygroma.
7. Diaphragmatic hernia.
8. Genitourinary tract: renal agenesis, renal dysplasia, urethral obstruction, bladder exstrophy, Meckel-Gruber syndrome, prune belly syndrome.
9. Hypothyroidism.
10. Multiple gestation: twin-twin transfusion syndrome, twin reverse arterial perfusion sequences (TRAP).
11. Musculoskeletal: sirenomelia, sacral agenesis, absent radius, facial clefting.
12. VACTERL ( vertebral, anal, cardiac, tracheoesophageal, renal, limb) association.

Diagnostic Approach

1. Inaccurate gestational age.
2. Intrauterine growth restriction.
3. Fetal anomalies.
4. Premature rupture of the membranes.

Associated Conditions: Fetal-renal and urinary tract anomalies, intrauterine fetal growth restriction, pulmonary hypoplasia, musculoskeletal defects, meconium-stained amniotic fluid. Fetal anomalies are present in 15%-25% of cases. Maternal-chronic disease ( diabetes, hypertension)

Ectopic pregnancy: Causes, Symptoms, Diagnosis, and Complications

Workup and Evaluation

Laboratory: No evaluation indicated.

Imaging: Amniotic fluid index calculated by adding the vertical depths of the largest pockets of amniotic fluid in each quadrant of the uterus ( average at term = 12.5 cm, 95th percentile = 21.4). Borderline values should always be rechecked before any intervention is undertaken. Fetal anomalies may also be documented.

Special Test: Nonstress or contraction stress testing to evaluate fetal health.

Diagnostic procedures: Physical examination, ultrasonography.

Pathologic Findings

Reduced amniotic fluid (other findings based on the cause).

Management and Therapy

Non-pharmacologic

General measures: Evaluation. Mild degree may be managed expectantly. Maternal oral hydration may improve amniotic fluid volume.

Specific Measures: Amnioinfusion (the introduction of normal saline via an intrauterine catheter placed through the partially dilated cervix during labor) has been used to reduce the incidence of umbilical cord compression during labor. This does not reduce the risk for meconium aspiration.

Diet: No specific dietary changes indicated.

Activity: No restriction.

Drug of Choice: None 

Follow-up 

Patient Monitoring: Intensive fetal surveillance is required 

Prevention/Avoidance: None 

Uterine prolapse: Management, Complications

Possible Complications of Oligohydramnios

1. Amniotic band syndrome
2. Pulmonary hypoplasia.
3. Premature labor.
4. Club foot.
5. Meconium-stained amniotic fluid.
6. Umbilical cord compression.
7. Fetal death.

The prognosis is inversely related to gestational age: the early the Oligohydramnios occurs, the worse the outcome.

Expected Outcome: When Oligohydramnios occurs in term or post-term pregnancies, it is associated with fetuses that do not tolerate labor well ( five-fold to seven-fold increase in the rate of cesarean delivery).

Reference: Netter Obstetrics and Gynecology