Oligohydramnios & Polyhydramnios Flashcards
Define oligohydramnios and polyhydramnios.
Oligohydramnios - reduced amniotic fluid,usually defined as:
- amniotic fluid of <500ml at 32-36 weeks
- maximum vertical pocket (MVP) of <2 cm from late mid-trimester
- amniotic fluid index (AFI) <5cm or <5th percentile from late mid-trimester
What is the peak amniotic fluid volume in pregnancy and when is it reached?
Peak of ~1 L at 34-36 weeks of gestation.
Amniotic fluid volume (AFV) then decreases towards term, with a mean AFV of 800 ml at 40 weeks
Describe the normal mechanisms affecting volume of amniotic fluid. How can each be affected?
Exchange rate can be as high as 3600ml/hr
Sources:
-
Fetal urination - should be 700-900ml/day by term
- Affected by renal agenesis or fetal urinary tract obstruction
- Respiratory tract, oral and nasal cavity fluids
Cleared by:
-
Fetal swallowing - by term swallows 210-760ml per day
- Affected by atresia of the upper GI tract, fetal hypoxia, neuromuscular disorders, brain abnormalities
- Skin absorption (before keratinisation at 22-25 weeks)
Both: absorption via intramembranous and transmembranous pathways inlc fluid between fetal blood and placenta
What are the functions of amniotic fluid?
- Protect fetus from trauma and infection
- Allow lung development
- Facilitates development and movement of limb and other skeletal parts
List 5 causes of oligohydramnios.
Fetal causes:
- premature rupture of membranes (PROM)
- fetal renal problems e.g. renal agenesis
- IUGR
- fetal death
Placental causes:
- Twin-to-twin transfusion syndrome (MC)
- abruption
Maternal causes:
- maternal dehydration
- post-term gestation - may be due to decreased efficiency of placenta
- pre-eclampsia, HTN
- diabetes
- chronic hypoxia
Idiopathic
Why might you test for SLE in oligohydramnios?
SLE can cause immune-mediated infarcts in the placenta and placental insufficiency
How is oligohydramnios diagnosed?
USS - should rule out renal problems by examining fetal kidneys and bladder.
What further investigations are done to check for possible causes of oligohydramnios?
USS - fetal kidneys, bladder to exclude pathology and check fetal growth
Sterile speculum examination +/- nitrazine stick test (turns blue as AF pH is ~6.5 compared to vaginal pH 4.5) - check for ROM
What is the management of oligohydramnios at different stages of pregnancy? What simple treatment may reverse oligohydramios?
Pre-term: expectant management, serial fetal growth and AFV USS, continuous FH rate monitoring if in labour
Term: delivery or expectant management if reassuring fetal testing
Post-term: expectant management, no evidence for IOL.
NB: maternal rehydration with oral or IV fluid has been shown to increase the AFV by 30%
Describe these 2 treatments for oligohydramnios:
- Amnioinfusion
- Vesico-amniotic shunts
Amnioinfusion - NaCl or Ringer’s lactate is infused under US guidance through uterine wall or transcervically via intrauterine catheter. Not recommended for PROM by RCOG.
Vesico-amniotic shunts - divert fetal urine to the amniotic fluid cavity where there is fetal obstructive uropathy; only done in specialist centres
What are the complications of oligohydramnios?
- Potter syndrome - death at time of birth or shortly after
- Pulmonary hypoplasia
- Amniotic band syndrome
- Increased risk of fetal infection (in prolonged ROM)
What is Potter syndrome?
Rare condition characterized by the physical characteristics of a fetus that develop when there is too little amniotic fluid in the uterus (in utero) during pregnancy.
What is amniotic band syndrome?
AKA constriction ring syndrome
Happens when fibrous bands of the amniotic sac (the lining inside the uterus that contains a fetus) get tangled around a developing fetus. In rare cases, the bands wrap around the fetus’ head or umbilical cord.
Define polyhydramnios.
= an abnormally large volume of amniotic fluid that surrounding the fetus
- AFI total >24 cm
- AFI >95th centile for gestation on ultrasound estimation
- SDP measurement >8 cm
List 5 causes of polyhydramnios.
Idiopathic (most common)
Fetal:
- Congenital disorders causing e.g. oesophageal/duodenal atresia, cardiovascular defects, microcephaly/anencephaly, neural tube defects, renal defects (Batter’s syndrome)
- Genetic disorders e.g. trisomy 13, 18, 21;
- Multiple pregnancy
- Fetal anaemia
- Hyrops fetalis
- Congenital infections e.g. toxo, parvovirus, rubella, CMV
Maternal
- Diabetes
- Substance misuse
- Metabolic abnormalities e.g. hypercalcaemia
