Polyhydramnios

Polyhydramnios refers to a situation where the amniotic fluid volume is more than expected for gestational age.

It is generally defined as:

• amniotic fluid index (AFI) > 25 cm
• largest fluid pocket depth (maximal vertical pocket (MVP)) greater than 8 cm ⁶: although some centres particularly in Australia, NZ and the UK use a cut off of 10 cm
• overall amniotic fluid volume larger than 1500-2000 cc³
• two diameter pocket (TDP) > 50 cm ²

Epidemiology

It can occur in approximately 1-1.5% of pregnancies.

Clinical presentation

The patient may clinically present as alarge for dates uterus. 

Pathology

Polyhydramnios occurs as a result of either increased production or decreased removal of amniotic fluid. The aetiology of polyhydramnios can be due to a vast variety of maternal and fetal disorders. 

It is usually detected after 20 weeks (often 3^rd trimester).

Causes

The potential causes of polyhydramnios are protean

• idiopathic: 60-65%: this is a diagnosis of exclusion despite accounting for a majority of cases, also termedidiopathic polyhydramnios
• maternal: 25-30%
  • diabetes: commonly gestational diabetes
  • maternal congestive heart failure ⁸
• fetal: 10-20%
  • CNS lesions (e.g. neural tube defects): fetal CNS abnormalities tend to be the commonest out of all fetal causative associations ³
  • proximal gastro-intestinal obstruction
    • gastrointestinal atresia(s) 
      • oesophageal atresia
      • duodenal atresia 
      • jejuno-ileal atresia
    • abdominal wall defects
      • gastroschisis
      • omphalocoele
    • fetal intestinal volvulus, e.g. from an intestinal malrotation
  • fetal cervico-thoracic abnormalities abnormalities 
    • fetal cervical masses
      • congenital cervical teratoma/epignathus
      • large fetal goitre
    • thoracic masses
    • congenital pulmonary airways malformation (CPAM)
    • congenital high airways obstruction syndrome (CHAOS)
    • congenital diaphragmatic herniation 
• fetal cardiovascular anomalies
  • sustained fetal tachycardia (e.g. supraventricular tachycardia (SVT), atrial flutter, ventricular tachycardia) ¹²
• twin pregnancy related complications
  • twin-twin transfusion: occurs in recipient
• hydrops fetalis: immune and non immune
• fetal sketetal abnormalities
  • reduced fetal movement

Associations

• fetal macrosomia: independant of maternal diabetes²,in idiopathic form
• mesoblastic nephroma
• Pena Shokeir syndrome
• maternal overhydration ^ref

Polyhydramios is associated with poor outcome if present in combination with intra uterine growth restriction (IUGR); usually seen in aneuploidies 18,13, and 21.

Classification

Some classify the severity of polyhydramnios as 

• mild: single deepest pocket at 8-11 cm
• moderate: single deepest pocket at 12-15 cm
• severe: single deepest pocket >16 cm

Treatment and prognosis

The prognosis is variable dependent on associated conditions. Usually minimal or no interventional required for idiopathic mild uncomplicated cases. Options include:

• improved maternal diabetes control
• caesarian section if there is profound macrosomia
• therapeutic amniocentesis / amnioreduction
• Indomethacin ^9-10

See also

• oligohydramnios