Polyhydramnios

Overview

Definition

Polyhydramnios is an abnormally high level of amniotic fluid in the amniotic sac.

Epidemiology

• Incidence range: 0.2%–1.6%
• Identification is often incidental during the 3rd trimester.
• Polyhydramnios is idiopathic in 60%–70% of cases.
• 20% of cases are due to congenital anomalies.

Etiology

• Idiopathic
• Fetal anomalies:
  • Gastrointestinal:
    • Esophageal atresia
    • Duodenal atresia
    • Intestinal obstruction
  • CNS:
    • Anencephaly
    • Dandy-Walker malformation
  • Pulmonary:
    • Diaphragmatic hernia
    • Congenital pulmonary airway obstruction
  • Neuromuscular:
    • Fetal akinesia deformation sequence
    • Skeletal dysplasia
• Fetal chromosomal abnormalities:
  • Trisomy 18
  • Trisomy 21
• Fetal anemia:
  • Alloimmunization
  • Congenital infection (e.g., parvovirus)
  • Fetomaternal hemorrhage
• Hydrops fetalis
• Twin-to-twin transfusion syndrome
• Gestational diabetes
• Maternal uremia
• Maternal hypercalcemia
• Fetal and placental tumors
• Intrapartum infections (e.g., TORCH infections)
• Maternal intake of lithium

Pathophysiology

Normal physiologic conditions:

• Amniotic fluid is derived from fetal urination.
• Fluid absorption occurs through fetal swallowing.
• Equilibrium develops between the production and excretion of amniotic fluid.

Two major causes of polyhydramnios:

1. Increased fetal urination:
   • High cardiac output (fetal anemia)
   • Volume overload (twin-to-twin transfusion syndrome)
   • Osmotic diuresis (maternal diabetes, maternal uremia)
2. Decreased fetal swallowing

Clinical Presentation

• Most patients are asymptomatic with polyhydramnios an incidental finding on ultrasound.
• Symptomatic patients may have:
  • Dyspnea
  • Extremity edema
  • Constipation
  • Abdominal pain
  • Tightness of the abdominal wall
  • Rapidly enlarging abdomen
  • Decreased fetal movement
• The baby is often in breech presentation.

Diagnosis

History

• Often unhelpful
• Focus on the presence of risk factors (e.g., diabetes, genetic diseases).
• Ask about the maternal perception of fetal movement (often decreased).

Physical exam

• Assessments:
  • Abdominal size and tightness
  • Lower extremity edema
• Measurement of fundal height (> 3 cm above gestational age is suspicious):
  • Use a flexible tape measure.
  • Measure from the pubic symphysis to the top of the fundus.
  • Measure in centimeters.
  • The age of gestation is associated with the measurement between 16–36 weeks (e.g., 25 weeks is 25 cm).

Diagnostic testing

• Amniotic fluid assessment with ultrasound:
  • Divide the uterus into 4 quadrants.
  • Measure the amniotic fluid vertically.
  • Amniotic fluid index (AFI):
    • A sum of the deepest amniotic pocket in all 4 quadrants
    • Values between 8–18 cm are normal.
  • Single deep pocket (SDP):
    • Amniotic fluid volume in the deepest pocket
    • Values between 2–8 cm are normal.
• Fetal organ screening
• Middle cerebral artery peak systolic velocity measurement: 
  • Measured by ultrasound
  • ↑ Value indicates fetal anemia
• Amniocentesis:
  • Fetal karyotyping for trisomy 21, 13, and 18
  • PCR for detection of congenital infections
  • Dye dilution technique:
    • The gold standard to test the amniotic fluid measurement
    • Rarely done
• Other:
  • Glucose tolerance test
  • Screening for congenital infections:
    • TORCH
    • HIV
    • Hepatitis
  • Maternal blood type
  • Rhesus factor
  • Screening for antibodies (Kell, Duffy, D, and C)
  • Lithium levels
  • Kleihauer-Betke test: evaluates fetal-maternal hemorrhage
  • Hemoglobin Barts (significant in Asian descent): may be heterozygous for α-thalassemia

Management

Prenatal care

• Treatment is dependent on the severity of polyhydramnios.
• No treatment is required for mild forms of polyhydramnios.
• Maternal-fetal counseling is recommended.
• Serial ultrasound is performed to assess fetal growth and AFI.
• Bed rest to ↓ the likelihood of preterm labor
• Severe polyhydramnios is treated with:
  • Amnioreduction: Serial AFI monitoring is recommended every 1–3 weeks after the procedure.
  • Prostaglandin synthetase inhibitors: stimulation of fetal ADH secretion → ↓ of renal blood flow → ↓ diuresis
  • Sulindac
• Genetic counseling
• Intrauterine blood transfusion (fetal anemia)
• Laser photocoagulation (twin-to-twin transfusion syndrome)

Delivery

• Labor induction is not clearly indicated.
• Continuous fetal monitoring is recommended.
• Use steroids to enhance fetal lung maturity if preterm delivery is anticipated.
• Controlled induction can be performed at 38 weeks of gestation for:
  • Severe polyhydramnios
  • Fetal abnormalities

Complications

• Preterm labor
• Premature rupture of membranes
• Postpartum hemorrhage
• Fetal malposition
• Umbilical cord prolapse
• Fetal death

Differential Diagnosis

• Preeclampsia: a condition characterized by new-onset hypertension after 20 weeks of gestation, and proteinuria or signs of end-organ damage. Patients present with visual abnormalities, headache, shortness of breath, and epigastric pain. Diagnosis is based on new-onset hypertension and the presence of proteinuria or end-organ damage. Management includes the administration of antihypertensives and possibly labor induction.
• Placental abruption: the placenta prematurely separates from the inner lining of the uterus. Placental abruption is a dangerous complication of pregnancy. Patients often present with painful vaginal bleeding, uterine contractions, abdominal or back pain, and premature birth. Management depends on gestational age and hemodynamic status of the mother and fetus. Placental abruption includes inpatient admission of the patient and possibly delivery. 
• Congestive heart failure: the heart is unable to pump enough blood to meet the metabolic requirements of the body. Patients often present with exertional dyspnea, chest pain, paroxysmal nocturnal dyspnea, and abdominal distention due to ascites and/or hepatomegaly. Echocardiography confirms the diagnosis. Management includes sodium restriction, administration of diuretics, inotropic agents, and vasodilators.