What are the causes of polyhydramnios?

Causes of Polyhydramnios

Polyhydramnios is primarily caused by fetal conditions that increase amniotic fluid production or decrease fetal swallowing, with Bartter syndrome being the most common genetic cause of severe prenatal polyhydramnios due to excessive fetal polyuria. 1

Definition and Diagnosis

Polyhydramnios is defined as an abnormal increase in amniotic fluid volume, diagnosed by either:

• Deepest vertical pocket ≥8 cm
• Amniotic fluid index ≥24 cm

Major Etiologies

Fetal Causes

1. Genetic and Renal Disorders

   • Bartter syndrome - Various types (BS1-5) with different severity of polyhydramnios:
     • BS1, BS2: Severe polyhydramnios
     • BS4a, BS4b: Severe to very severe polyhydramnios
     • BS5: Transient polyhydramnios with preterm delivery
     • BS3: Usually absent or mild polyhydramnios 1

2. Gastrointestinal Anomalies (impaired swallowing)

   • Esophageal atresia
   • Duodenal atresia
   • Jejunal atresia
   • Diaphragmatic hernia
   • Midgut volvulus
   • Gastrointestinal obstruction
   • Malrotation of intestines
   • Meconium peritonitis 1

3. Neurological Disorders (impaired swallowing)

   • Anencephaly
   • Hydrocephaly
   • Microcephaly
   • Myotonic dystrophy

4. Cardiovascular Anomalies

   • Cardiac defects (most common fetal anomalies associated with polyhydramnios, ~33%) 2
   • Cardiac arrhythmias
   • Cardiac rhabdomyomas (associated with tuberous sclerosis) 1

5. Other Fetal Conditions

   • Congenital infections (TORCH infections) - 2.9% of cases 2
   • Fetal anemia
   • Fetal hydrops
   • Twin-twin transfusion syndrome
   • Sacrococcygeal teratoma and other tumors 1

Maternal Causes

1. Diabetes mellitus - 19.8% of polyhydramnios cases 2

   • Associated with fetal macrosomia
   • Increased fetal urination due to fetal hyperglycemia

2. Alloimmunization

   • Rh incompatibility
   • Other blood group incompatibilities

Idiopathic

• Approximately 60-70% of cases have no identifiable cause (68.8% in one large study) 2
• More common in mild polyhydramnios

Pathophysiologic Mechanisms

Polyhydramnios develops through three main mechanisms:

1. Increased production of amniotic fluid

   • Fetal polyuria (as in Bartter syndrome)
   • Maternal diabetes (fetal hyperglycemia leading to polyuria)

2. Decreased fetal swallowing

   • Gastrointestinal obstruction
   • Neurological disorders affecting swallowing
   • Facial anomalies preventing normal swallowing

3. Increased fluid transfer across fetal membranes

   • Twin-twin transfusion syndrome

Clinical Significance

Polyhydramnios is associated with:

• Maternal dyspnea and discomfort
• Preterm labor
• Premature rupture of membranes
• Abnormal fetal presentation
• Cord prolapse
• Postpartum hemorrhage 3
• Increased risk of cesarean delivery 2
• Higher rates of fetal distress in labor 4
• Increased perinatal mortality 5

Diagnostic Approach

When polyhydramnios is identified, the following evaluations should be performed:

1. Detailed fetal anatomical survey with special attention to:

   • Cardiac structures (echocardiography)
   • Gastrointestinal tract
   • Central nervous system
   • Face and neck

2. Maternal evaluation:

   • Glucose tolerance testing (even in absence of known diabetes)
   • TORCH serology

3. Genetic evaluation when indicated:

   • Consider genetic testing, particularly if Bartter syndrome is suspected
   • Especially important with severe polyhydramnios or when associated with other anomalies 1

Management Considerations

• Mild idiopathic polyhydramnios generally doesn't require antenatal fetal surveillance
• Severe polyhydramnios warrants delivery at a tertiary care center due to high likelihood of fetal anomalies 6
• Amnioreduction should only be considered for severe maternal discomfort or dyspnea 6

Key Pitfalls to Avoid

1. Failing to perform glucose tolerance testing - diabetes is a common cause of polyhydramnios
2. Missing cardiac defects - always perform fetal echocardiography
3. Overlooking Bartter syndrome - consider in cases of severe polyhydramnios, especially with preterm delivery
4. Assuming idiopathic cause prematurely - thorough evaluation is necessary before labeling as idiopathic
5. Unnecessary intervention - mild idiopathic polyhydramnios doesn't require early delivery or special surveillance

Remember that polyhydramnios severity often correlates with the likelihood of finding an underlying cause, with severe cases more frequently associated with fetal anomalies.