Oligohydramnios

Oligohydramnios is characterized by an abnormally low volume of amniotic fluid during pregnancy. It is diagnosed when the amniotic fluid index (AFI) measures less than 5 cm, or the single deepest vertical pocket is less than 2 cm.

It can occur at any stage of pregnancy, and its implications for maternal and fetal outcomes vary depending on the gestational age and severity.

Etiology and Risk Factors

Oligohydramnios can arise due to various factors, which can be categorized as fetal, maternal, placental, or idiopathic:

1. Fetal Causes

• Congenital Anomalies: Structural abnormalities of the fetal renal system (e.g., renal agenesis, multicystic dysplastic kidneys, obstructive uropathy) can reduce urine output, leading to decreased amniotic fluid.
• Fetal Growth Restriction (FGR): Intrauterine growth restriction can result in reduced fetal urine production due to placental insufficiency.
• Chromosomal Abnormalities: Genetic conditions like trisomy 18 and trisomy 13 may be associated with low amniotic fluid volume.
• Preterm Premature Rupture of Membranes (PPROM): Loss of amniotic fluid due to membrane rupture can lead to persistent oligohydramnios.

2. Maternal Causes

• Preeclampsia and Hypertensive Disorders: These conditions can compromise uteroplacental perfusion, leading to decreased amniotic fluid production.
• Dehydration: Severe maternal dehydration or hypovolemia can reduce fetal renal perfusion, leading to oligohydramnios.
• Use of Certain Medications: Medications such as angiotensin-converting enzyme (ACE) inhibitors and nonsteroidal anti-inflammatory drugs (NSAIDs) can reduce fetal urine production.

3. Placental Causes

• Placental Insufficiency: Conditions like placental abruption, infarction, or aging (post-term pregnancies) may impair the placental function, leading to oligohydramnios.

4. Idiopathic Oligohydramnios

• In many cases, no identifiable cause can be found, and these instances are considered idiopathic. These cases often occur in late pregnancy or near term.

Pathophysiology

Amniotic fluid volume is maintained through a balance between fetal urine production, fetal swallowing, respiratory tract absorption, and fluid exchange across the membranes. Oligohydramnios occurs when this balance is disrupted, leading to reduced production or increased loss of amniotic fluid.

Clinical Presentation

• Maternal Symptoms: Women may experience a reduction in fetal movements, or there may be no symptoms at all.
• Physical Examination: The uterus may appear smaller than expected for gestational age, and fetal parts may be more easily palpable due to the reduced fluid.

Diagnosis

• Ultrasound Assessment: The mainstay of diagnosis is through sonographic measurement:
  • Amniotic Fluid Index (AFI): Less than 5 cm indicates oligohydramnios.
  • Single Deepest Vertical Pocket: Less than 2 cm suggests oligohydramnios.
• Detailed Ultrasound Examination: Should include fetal anatomy screening to detect congenital anomalies and Doppler studies for fetal well-being assessment.
• Maternal Evaluation: Assess for risk factors such as hypertension, preeclampsia, or a history of PPROM.
• Non-Stress Test (NST) or Biophysical Profile (BPP): May be performed to evaluate fetal well-being.

Complications

• Maternal Risks:
  • May be associated with increased rates of labor induction and cesarean delivery due to fetal distress.
• Fetal Risks:
  • Pulmonary Hypoplasia: Particularly in cases of early-onset oligohydramnios (before 24 weeks), leading to underdeveloped lungs.
  • Cord Compression: Reduced amniotic fluid volume can increase the risk of cord compression, causing variable decelerations on fetal heart rate monitoring.
  • Fetal Growth Restriction (FGR): Oligohydramnios is often associated with placental insufficiency, leading to FGR.
  • Increased Perinatal Morbidity and Mortality: Higher rates of stillbirth, preterm delivery, and low Apgar scores.

Management

The management of oligohydramnios depends on the gestational age, severity, underlying cause, and fetal status:

1. Conservative Management

• Indicated for Mild or Idiopathic Oligohydramnios without immediate signs of fetal compromise.
• Serial Ultrasound Monitoring: Weekly or biweekly ultrasounds to monitor AFI or deepest pocket, and assess fetal growth.
• Non-Stress Tests (NST): Conduct twice-weekly NSTs or biophysical profiles from 32 weeks gestation or earlier if warranted.
• Maternal Hydration:
  • Oral Hydration: Encourage adequate fluid intake (at least 2-3 liters per day).
  • Intravenous Hydration: Administration of normal saline or Ringer’s lactate (1-2 liters over 2-4 hours) may temporarily improve amniotic fluid volume.

2. Pharmacological Management

• Discontinue Medications that May Reduce Amniotic Fluid: Stop ACE inhibitors or NSAIDs if they are contributing to oligohydramnios.
• Medications to Improve Uteroplacental Perfusion:
  • Aspirin (81 mg daily): Used in cases of preeclampsia to improve placental function.
  • Low Molecular Weight Heparin (LMWH): Considered in cases with a history of thrombophilia or uteroplacental insufficiency.

3. Amnioinfusion

• Indications: Useful during labor for cases with oligohydramnios where there are recurrent variable decelerations on fetal monitoring due to cord compression.
• Procedure:
  • Technique: Perform an intrauterine infusion of warmed sterile saline (500-1000 mL), administered slowly through an intrauterine catheter.
  • Monitoring: Fetal heart rate should be continuously monitored during and after the procedure to evaluate the resolution of variable decelerations.

4. Delivery Planning

• Timing of Delivery:
  • Term (≥37 weeks): Induce labor if oligohydramnios is present, even in the absence of other complications.
  • Preterm Oligohydramnios (24-36 weeks):
    • If fetal testing is reassuring and there are no signs of fetal compromise, conservative management with close monitoring is recommended.
    • Consider corticosteroid administration (betamethasone 12 mg IM, two doses 24 hours apart) for fetal lung maturity if delivery before 34 weeks is anticipated.
  • Severe Oligohydramnios (≤24 weeks): Poor prognosis with high risk for pulmonary hypoplasia. Consider the option of pregnancy termination based on patient counseling and fetal condition.

5. Management of Specific Causes

• Preterm Premature Rupture of Membranes (PPROM):
  • Antibiotic Prophylaxis: Administer antibiotics such as ampicillin (2 g IV every 6 hours for 48 hours) followed by amoxicillin (250 mg orally every 8 hours for 5 days) to prolong latency.
  • Antenatal Corticosteroids: Administer for fetal lung maturity if PPROM occurs before 34 weeks.
• Hypertensive Disorders:
  • Antihypertensive Therapy: Use labetalol (100-200 mg orally twice daily) or nifedipine (10-20 mg orally every 6-8 hours) to control blood pressure.
  • Delivery Planning: Early delivery may be indicated if there is severe oligohydramnios with deteriorating maternal or fetal conditions.

Prognosis

The prognosis of oligohydramnios depends on the underlying cause, severity, and gestational age at onset:

• Mild Idiopathic Cases: Generally have a good outcome, especially if occurring near term.
• Severe Cases before 24 Weeks: Associated with a poor prognosis due to the risk of pulmonary hypoplasia and preterm birth.

Emerging Research and Trends

• New imaging modalities, including 3D ultrasound and Doppler studies, are enhancing the evaluation of oligohydramnios.
• Amniotic Fluid Replenishment Therapies: Research into artificial amniotic fluid infusion and new drugs to stimulate fetal urine production is ongoing.
• Placental Transfusion Techniques: Being explored in cases of placental insufficiency to improve fetal outcomes.