Polyhydramnios

Synonym: hydramnios

What is polyhydramnios?

Polyhydramnios, or amniotic fluid disorder, is defined as an abnormally large volume of amniotic fluid. It is associated with adverse pregnancy outcomes. Greater deviations from the norm are more strongly associated with abnormality. However, up to 70% of women with polyhydramnios have an excellent prognosis.¹

Pathogenesis

Physiologically, the volume of amniotic fluid increases with gestation to a maximum at 36-37 weeks. It has a number of purposes, including protecting the foetus from trauma and infection, serving as a reservoir of fluid and nutrition, and allowing lung, muscle and digestive tract development.²

Mechanisms affecting amniotic fluid volume include:¹

• Impaired foetal swallowing. By term, the foetus swallows 210-760 ml of amniotic fluid per day. Polyhydramnios may result from any pathology that impairs foetal swallowing. These include:

  • Central nervous system lesions.

  • Neuromuscular dysfunction - for example, myotonic dystrophy.

  • Craniofacial abnormalities - for example, cleft palate.

  • Micrognathia.

  • Obstructive neck masses.

• Impaired gastrointestinal absorption. This is due to obstructions caused by:

  • Duodenal atresia.

  • Tracheoesophageal fistula.

  • Oesophageal atresia.

  • Thoracic mass.

  • Diaphragmatic hernia.

• Excessive foetal urine production. By term, foetal urinary output is around 700-900 ml per day. Increases in foetal urination can be caused by:

  • Maternal diabetes.

  • Foetal macrosomy.

  • High-output foetal cardiac failure due to:

    • Foetal anaemia, due to maternal alloimmunisation.

    • Parvovirus infection.

    • Thalassaemia or other or other haemolytic disorders.

    • Foetal tachyarrhythmias, including supraventricular tachycardia.

    • Arteriovenous shunting in conjunction with sacrococcygeal teratoma or chorioangioma.

    • Twin-twin transfusion syndrome. (One of the twins develops high-output cardiac failure with associated polyhydramnios whereas the other develops volume depletion and oligohydramnios.)

  • Bartter syndrome.³

Polyhydramnios epidemiology

Polyhydramnios occurs in between 1 and 2% of all pregnancies and this is generally similar across low- and middle-income countries.⁴ Rates are much higher in pregnancies of women with diabetes or gestational diabetes.

Approximately 65% to 70% of women with polyhydramnios have mild disease, whilst 20% have moderate disease, and less than 15% have severe disease.

Where polyhydramnios is mild, an underlying abnormality is found in only 17%, whereas in moderate-to-severe polyhydramnios, this figure rises to 91%. There is also an association with increasing maternal age and with foetal macrosomia.

Polyhydramnios causes ¹⁵

Polyhydramnios causes are numerous and include:

• Idiopathic - in between 60 and 65% of cases no underlying cause is found.

• Maternal causes account for 25-30% of cases:

  • Diabetes.

  • Maternal congestive cardiac failure.

• Foetal causes account for 10-20% of cases. These include:

  • Structural abnormalities:

    • Oesophageal or duodenal atresia.

    • Cardiovascular defects.

    • Microcephaly or anencephaly.

    • Neural tube defects.

    • Renal defects, including Bartter's syndrome.

  • Genetic disorders. 5.5% of patients with moderate or severe polyhydramnios had a foetus with a clinically significant genetic anomaly:

    • Trisomy 21, 18 and 13.

    • Pena-Shokeir syndrome (contractures of the joints (arthrogryposis), growth problems, underdeveloped lungs, facial deformities).

    • Beckwith-Wiedemann syndrome (overgrowth disorder).

  • Congenital infections:

    • Toxoplasmosis.

    • Parvovirus.

    • Rubella.

    • Cytomegalovirus.

  • Foetal anaemia.

• Maternal substance misuse has previously been postulated as a potential cause although there is little recent evidence.¹⁶

Polyhydramnios symptoms¹

Polyhydramnios may be suspected when antenatal examination reveals a uterus that is large for dates. Foetal parts may be difficult to palpate. A fundal height of greater than 3cm more than expected warrants an ultrasound scan to check for polyhydramnios or macrosomy.

Women with moderate to severe polyhydramnios may present with symptoms of dyspnoea due to increased splinting of the diaphragm. They may also have significant oedema of the legs and feet.

Differential diagnosis

When the uterus appears large for dates on clinical examination, ultrasound scans usually rule out the alternative diagnoses including:

• Multiple pregnancy.

• Foetal macrosomia.

• Abruptio placenta which may cause rapidly expanding uterine size due to the development of intrauterine haematoma. This is usually an easy differential diagnosis to make, as pain is a predominant feature.

• Chorioangioma - this is a benign lesion of the placenta due to excess capillary formation in the absence of villus differentiation. It may cause a 'large-for-dates' uterus per se, or be associated with polyhydramnios.

Investigations

Ultrasound examination can assess the fluid volume and make a diagnosis. At ultrasound, there are two methods used to quantify amniotic fluid volume:¹⁷

• Amniotic fluid index (AFI), or four-quadrant method: the uterine cavity is divided into four quadrants or pockets. The largest vertical pocket in each quadrant is measured in centimetres and the total volume is calculated by adding the four together. A total of more than 24 cm defines polyhydramnios.

• Single deepest pocket (SDP) method: the deepest pocket is measured vertically. A measurement under 2 cm defines oligohydramnios and where it is over 8 cm there is polyhydramnios.

Following the diagnosis of polyhydramnios, further screening for underlying causes may be advisable. In the UK these will always be carried out within the obstetric setting, but may include:

• Screening for alloimmunisation, aneuploidy, and congenital anomalies.

• Testing for gestational diabetes (a second test should be offered if it is more than a month since the last one).

• In a monochorionic twin pregnancy, screening for twin-twin transfusion is recommended every 2 weeks.

• Potential serum testing for infections including syphilis, rubella, parvovirus B19, HIV, hepatitis, cytomegalovirus, and toxoplasmosis.

• Genetic testing if there is evidence of possible congenital anomalies. A recent study found rates of genomic anomalies to be 4.5% even in apparently isolated polyhydramnios, suggesting that genetic testing may be of value even where no underlying abnormality is apparent.⁸

Polyhydramnios treatment and management¹

• The management of polyhydramnios depends on the underlying cause.

• Mild polyhydramnios can be simply monitored and treated conservatively.

• Foetal surveillance is recommended from 32 weeks in cases of moderate-severe polyhydramnios due to the increased risks of perinatal morbidity and mortality.

• There is no consensus around the frequency of repeat ultrasound scans in women with polyhydramnios. Macrosomic foetuses are found in up to 30% of women with polyhydramnios due to increased urine production. Polyhydramnios in the context of IUGR is more likely to be associated with pathology and therefore more frequent monitoring is likely to be required.

• Gestational diabetes needs active close management and monitoring. Amniotic fluid volumes are predictive of outcomes in this cohort.⁹

• Prostaglandin synthetase inhibitors, particularly indomethacin, may be used. This acts at least partly by reducing renal blood flow and reducing foetal urination. It is used usually for a maximum of 48 hours and regimes vary. After 32 weeks of pregnancy, indomethacin is contra-indicated and there is no evidence of benefit where indomethacin is used at any stage of pregnancy solely for polyhydramnios.

• Amnioreduction (drainage of amniotic fluid under ultrasound guidance) may be used in severe polyhydramnios, or in patients who are symptomatic. There is an increased risk of preterm labour, preterm prelabour rupture of membranes, intrauterine infection, and placental abruption in the first 48 hours after the procedure although these remain uncommon complications.

• Polyhydramnios associated with twin-to-twin syndrome may benefit from laser ablation of the connecting placental vessels.¹⁰

• Timing of delivery depends on the severity of the polyhydramnios. Women with mild polyhydramnios can usually deliver normally at term with spontaneous onset of labour. In women with moderate-severe polyhydramnios, induction of labour is not usually recommended before 39 weeks of gestation; women with severe polyhydramnios should usually deliver in a setting with facilities for both maternal and neonatal ICU. There is an increased risk of malpresentations, shoulder dystocia, postpartum haemorrhage and transient tachypnoea of the newborn.

Polyhydramnios prognosis¹⁷¹¹¹²

Mild polyhydramnios has an excellent prognosis.¹³

There is a higher incidence of preterm labour and delivery in moderate and severe polyhydramnios. This includes cases of idiopathic polyhydramnios. Polyhydramnios is also associated with a higher incidence of malpresentation, placental abruption, cord prolapse, Caesarean delivery (in part due to the higher risks of macrosomy and malpresentation), neonatal respiratory distress, low 5-minute APGAR scores, postpartum haemorrhage, and perinatal mortality.

Only 17% of pregnancies with mild polyhydramnios have a significant underlying cause. However, an underlying abnormality is associated with 91% of cases of moderate-to-severe polyhydramnios.

A large cohort study found rates of major congenital anomalies to be 8% in women with mild polyhydramnios, 12% in those with moderate polyhydramnios, and 31% in patients with severe polyhydramnios.