Meconium-stained amniotic fluid

Definition

• Meconium-stained amniotic fluid results when the fetus passes meconium, the newborn's first stool, before or during birth.
• Meconium is a black-green, sterile, viscous material consisting of water, desquamated skin and intestinal cells, gastrointestinal secretions, bile, mucus, lanugo, vernix and amniotic fluid.
• Meconium aspiration syndrome is respiratory distress in a newborn with meconium-stained amniotic fluid when the respiratory symptoms cannot be attributed to another cause.

Occurrence & epidemiology

Epidemiology

• Meconium-stained amniotic fluid is found in 4 to 22 per cent of all births and in up to 23 to 52 per cent beyond 42 weeks.
• Only 3 to 12 per cent of infants born through meconium-stained fluid develop meconium aspiration syndrome.

Aetiopathogenesis

Aetiology and risk factors

• Before 34 weeks meconium in the amniotic fluid is uncommon because peristalsis and motilin levels are low and anal sphincter tone is good; meconium aspiration syndrome therefore mainly affects term and post-term newborns.
• In a preterm infant, meconium passage is never considered normal.
• Fetal stress from hypoxia and acidosis, for example with umbilical cord compression or placental insufficiency, or infection leads via a vagal response with increased peristalsis and anal sphincter relaxation to meconium passage.
• Risk factors are placental insufficiency, maternal hypertension, pre-eclampsia, oligohydramnios with cord compression, infections and maternal nicotine and cocaine use.

Clinical features

Clinical features and complications

• Complete bronchial obstruction causes atelectasis, partial obstruction via a ball-valve effect causes air trapping with hyperinflation and air leak such as pneumothorax or pneumomediastinum.
• Persistent pulmonary hypertension of the newborn can ultimately be the more life-threatening complication.
• Clinical signs are tachypnoea, nasal flaring, retractions, cyanosis or desaturation, rales and rhonchi.

Diagnosis

• It is suspected with respiratory distress after birth through meconium-stained fluid; greenish-yellow staining of the cord, nail beds or skin indicates exposure for several hours but is not diagnostic.
• The diagnosis is confirmed by chest radiograph showing hyperinflation, variable areas of atelectasis, patchy or streaky opacities and diaphragmatic flattening.

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