What causes HMD?
HMD occurs when there is not enough of a substance in the lungs called surfactant. Surfactant is made by the cells in the airways and consists of phospholipids and protein. It begins to be produced in the fetus at about 24 to 28 weeks of pregnancy, and is found in amniotic fluid between 28 and 32 weeks. By about 35 weeks gestation, most babies have developed adequate amounts of surfactant.
What does surfactant do?
In healthy lungs, surfactant is released into the lung tissues where it helps lower surface tension in the airways, which helps keep the lung alveoli (air sacs) open. When there is not enough surfactant, the tiny alveoli collapse with each breath. As the alveoli collapse, damaged cells collect in the airways, which makes it even harder to breath. These cells are called hyaline membranes. Your baby works harder and harder at breathing, trying to re-inflate the collapsed airways.
As your baby's lung function decreases, less oxygen is taken in and more carbon dioxide builds up in the blood. This can lead to acidosis (increased acid in the blood), a condition that can affect other body organs. Without treatment, your baby becomes exhausted trying to breathe and eventually gives up. A mechanical ventilator (breathing machine) must do the work of breathing instead.
What factors determine how HMD progresses?
The course of illness with HMD depends on the size and gestational age of your baby, the severity of the disease, the presence of infection, whether or not your baby has a patent ductus arteriosus (a heart condition) and whether or not she needs mechanical help to breathe.
Who is affected by HMD?
HMD occurs in about 60 to 80 percent of babies born before 28 weeks gestation, but only in 15 to 30 percent of those born between 32 and 36 weeks. About 25 percent of babies born at 30 weeks develop HMD severe enough to need a mechanical ventilator (breathing machine).
Although most babies with HMD are premature, other factors can influence the chances of developing the disease. These include the following:
- Caucasian or male babies
- previous birth of baby with HMD
- Cesarean delivery
- perinatal asphyxia (lack of air immediately before, during or after birth)
- cold stress (a condition that suppresses surfactant production)
- perinatal infection
- multiple births (multiple birth babies are often premature)
- infants of diabetic mothers (too much insulin in a baby's system due to maternal diabetes can delay surfactant production)
- babies with patent ductus arteriosus
What are the symptoms of HMD?
While each baby may experience symptoms differently, some of the most common symptoms of HMD include:
- difficulty breathing at birth that gets progressively worse
- cyanosis (blue coloring)
- flaring of the nostrils
- tachypnea (rapid breathing)
- grunting sounds with breathing
- chest retractions (pulling in at the ribs and sternum during
Symptoms of HMD usually peak by the third day and may resolve quickly when your baby begins to diurese (excrete excess water in urine) and needs less oxygen and mechanical help to breathe.
What complications are associated with HMD?
Your baby may develop complications of the disease or problems as side effects of treatment. As with any disease, more severe cases often have greater risks for complications. Some complications associated with HMD include the following:
- air leaks of the lung tissues such as:
- pneumomediastinum - air leaks into the mediastinum (the space between the two pleural sacs containing the lungs).
- pneumothorax - air leaks into the space between the chest wall and the outer tissues of the lungs
- pneumopericardium - air leaks into the sac surrounding the heart
- pulmonary interstitial emphysema (PIE) - air leaks and becomes trapped between the alveoli, the tiny air sacs of the lungs
- chronic lung disease, sometimes called bronchopulmonary dysplasia
Can HMD be prevented?
The best way of preventing HMD is by preventing a preterm birth. When a preterm birth cannot be prevented, giving the mother medications called corticosteroids before delivery has been shown to dramatically lower the risk and severity of HMD in the baby. These steroids are often given to women between 24 and 34 weeks gestation who are at risk of early delivery.