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) > 20 - 25
• 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 cc ³
• 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 a large for dates uterus. 

Pathophysiology

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) due to the fetus not starting swallowing in early pregnancy. Fetal swallowing is a major route of removal of amniotic fluid.

Causes

The potential causes of polyhydramnios are protean

• maternal  : 25 - 30 %
  • diabetes : commonly gestational diabetes
  • hypertension / pre-eclampsia
  • 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 maltoration
  • fetal cervico-thoracic 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
  • twin pregnancy related complications
    • twin-twin transfusion : occurs in recipient
  • hydrops fetalis : immune and non immune
  • fetal sketetal abnormalities
  • reduced fetal movement
• idiopathic : 60 - 65 % : this is a diagnosis of exclusion despite accounting for a majority of cases : also termed idiopathic polyhydramnios

Associations

• fetal macrosomia : independant of maternal diabetes ²
• mesoblastic nephroma
• Pena Shokeir syndrome
• maternal overhydration ^ref

Polyhydramios is associated with poor outcome if present in combination with intra uterine growth restriction (IUGR).

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

Management options

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