The inhaled meconium can partially or completely block the baby's airways. Although air can flow past the meconium trapped in the baby's airways as the baby breathes in, the meconium becomes trapped in the airways when the baby breathes out. And so, the inhaled meconium irritates the baby's airways and makes it difficult to breathe.
MAS can affect the baby's breathing in a number of ways, including chemical irritation to the lung tissue, airway obstruction by a meconium plug, infection, and the inactivation of surfactant by the meconium (surfactant is a natural substance that helps the lungs expand properly).
The severity of MAS depends on the amount of meconium the baby inhales as well as underlying conditions, such as infections within the uterus or postmaturity (when a baby is overdue, or more than 40 weeks' gestational age). Generally, the more meconium a baby inhales, the more serious the condition.
Normally, fluid is moved in and out of only the trachea (the upper portion of the airway) when there's breathing activity in the fetus. Meconium can be inhaled into the lungs when the baby gasps while still in the womb or during the initial gasping breaths after delivery. This gasping typically happens when there has been a problem (i.e., an infection or compression of the umbilical cord) that causes the baby to have difficulty getting enough oxygen in the womb.
Although 6% to 25% of babies delivered have meconium-stained amniotic fluid, not all infants who pass meconium during labor and delivery develop MAS. Of the babies who either pass meconium during birth or are delivered having meconium-stained fluid, 2% to 36% either inhale the meconium in utero (while still in the uterus) or with the first breath. Of the infants born with meconium-stained amniotic fluid, 11% of them experience some degree of MAS.
MAS is often related to fetal stress. Fetal stress can be caused by problems in the womb, such as infections, or by difficulties during the labor process. A distressed baby may experience hypoxia (decreased oxygen), which may make the baby's intestinal activity increase and may cause relaxation of the anal sphincter (the muscular valve that controls the passage of feces out of the anus). This relaxation then moves meconium into the amniotic fluid that envelops the baby.
But meconium passage during labor and delivery isn't always associated with fetal distress. Occasionally, babies who aren't distressed during the birth process pass meconium before birth. In either case, a baby that gasps or inhales meconium can develop MAS.
Additional risk factors for MAS include:
- a difficult delivery
- advanced gestational age (or postmaturity)
- a mother who smokes cigarettes heavily
- infants of diabetic mothers,
- a mother with high blood pressure, or chronic respiratory or cardiovascular disease
- umbilical cord complications
- poor intrauterine growth (poor growth of the baby while in the uterus)
Prematurity is not a risk factor. In fact, MAS is rare in babies born before 34 weeks.
Signs and Symptoms
At birth, the doctor will likely notice one or more symptoms of MAS, including:
- meconium or dark green streaks or stains in the amniotic fluid
- discoloration of the baby's skin — either blue (cyanosis) or green (from being stained by the meconium)
- problems with breathing — including rapid breathing (tachypnea), labored (difficulty) breathing, or suspension of breathing (apnea)
- low heart rate in the baby before birth
- low apgar score (the Apgar test is given to newborns just after birth to quickly evaluate color, heartbeat, reflexes, muscle tone, and breathing)
- limpness in the baby
- postmaturity (signs that a baby is overdue such as long nails)
If a baby is thought to have inhaled meconium, treatment will begin during delivery. If the baby has any depression in breathing, the doctor taking care of the baby will insert a laryngoscope into the baby’s trachea to remove any meconium that might be present. The doctor will also probably listen to the baby's chest with a stethoscope for sounds in the lungs that are common in infants with MAS.
The doctor may also order tests — a blood test (called a blood gas analysis) that helps determine if the baby is getting enough oxygen and a chest X-ray that can show patches or streaks on the lungs that are found in babies with MAS.
Current recommendations say that if an infant has inhaled meconium but looks active, appears well, and has a strong heartbeat (>100 bpm), the delivery team can watch the baby for MAS symptoms, which typically appear within the first 24 hours. So the baby is observed for such signs as increased respiratory rate, grunting, or cyanosis.
For an infant that has inhaled meconium and shows signs of poor activity level, has a lower heart rate (<100 bpm), is limp, and has poor muscle tone, the goal is to clear the airway as much as possible to decrease the amount of meconium that's aspirated. This is done by putting in an endotracheal tube (a plastic tube that's placed into the baby's windpipe through the mouth or nose) and applying suction as the tube is slowly removed. This allows the infant to receive suctioning of both the upper and lower airways. The neonatal caregivers will continue trying to clear the airway until there's no meconium in the suctioned fluids.
Most babies with MAS improve within a few days or weeks, depending on the severity of the aspiration. Although a baby's rapid breathing may continue for days after birth, there's usually no severe permanent lung damage. There are studies, however, indicating that those born with MAS are at a higher risk of having reactive airway disease (lungs that are more sensitive and can possibly lead to an asthmatic condition).
Babies with MAS may be sent to a intensive care nursery (NICU) to be closely monitored for the next few days.
Treatments may include:
oxygen therapy by oxygen hood or ventilation
use of surfactant
obtaining blood routinely to determine if the baby is receiving enough oxygen
Babies who have severe aspiration and require mechanical ventilation are at increased risk for bronchopulmonary dysplasia, a lung condition that can be treated with medication or oxygen. Another complication associated with MAS is a collapsed lung. Also known as pneumothorax, a collapsed lung is treated by reinflating the lung (inserting a tube between the ribs, allowing the lung to gradually re-expand).
Although rare, a small percentage of babies with severe MAS develop aspiration pneumonia. If this occurs, the doctor may recommend advanced lung rescue therapy.
Three therapies are currently used to treat aspiration pneumonia and severe forms of MAS:
Surfactant therapy — An artificial surfactant is instilled into the baby's lungs, which helps to keep the air sacs open.
High-frequency oscillation — This special ventilator vibrates air enriched with extra oxygen into the baby's lungs.
Rescue therapy — Nitric oxide is added to the oxygen in the ventilator. It dilates the blood vessels and allows more blood flow and oxygen to reach the baby's lungs.
If one of these therapies (or a combination of them) doesn't work, there is another alternative. Extra corporeal membrane oxygenation (ECMO) is a form of cardiopulmonary bypass, meaning that an artificial heart and lung will temporarily take over to supply bloodflow to the baby's body. ECMO reduces the fatality rate for these severely distressed infants from 80% to 10%. Not all hospitals are ECMO centers, so babies that require ECMO may need to be moved to another hospital.
Babies with severe cases of MAS may come home from the hospital on oxygen. They may be more likely to have wheezing and lung infections during their first year, but lungs can regenerate new air sacs, so the long-term prognosis for their lungs is excellent.
Severely affected babies are at risk for developing chronic lung disease and may also have developmental abnormalities and hearing loss. Babies diagnosed with MAS will be screened at the hospital for hearing problems or neurological damage. Although very rare, severe cases of MAS may be fatal. Studies have indicated that deaths from MAS have decreased significantly through interventions such as suctioning and reducing the number of post-term births.
It's important for a pregnant woman to tell her doctor immediately if meconium is present in the amniotic fluid when her water breaks, or if the fluid has dark green stains or streaks. Doctors also use a fetal monitor during labor to observe the baby's heart rate for any signs of fetal distress. In some cases doctors may recommend amnioinfusion, the dilution of the amniotic fluid with saline, to wash meconium out of the amniotic sac before the baby has a chance to inhale it at birth.
Although MAS is a frightening complication for parents to face during the birth of their child, the majority of cases are not severe. Most infants are monitored for fetal distress during labor, and doctors pay careful attention to any signs that would indicate meconium aspiration. If it does occur, treatment will begin immediately and can prevent further complications.