Reese H. Clark, M.D.
In general terms lung hypoplasia means under developed lungs. Hypo means small, plasia means formed. The lungs are a vital organ and without them we can not live. The lung is made up of small gas exchange units called alveoli. Alveoli are thin walled structures that are surrounded by small veins and arteries called capillaries. Gas in the alveoli is exchanged with gas in the blood allowing oxygen to be delivered to tissue as a key element for body function and carbon dioxide to be eliminated from the body.
When the lung is hypoplastic the number of alveoli that are available for gas exchange are decreased. If the lungs are very hypoplastic the number of gas exchange units reaches a critically low level and adequate gas exchange can not be maintained. Newborn babies with very hypoplastic lungs die of lung failure in the first few days of life if they cannot be supported long enough to grow more lung.
The lung begins forming very early in fetal development. Any thing that restricts growth of the chest can cause the lung to be under developed. It is important to distinguish lung hypoplasia from lung immaturity. They are not the same things though, functionally, they have the same effect. Both lead to inadequate gas exchange and lung failure. Babies born prematurely have immature lungs with a developmental normal number of alveoli. The goal in caring for these babies is to support them in a manner that prevents injury to the lung. If injury is avoided these babies can have normal lung development.
In contrast events that impact fetal lung growth may effect future lung growth and may prevent babies with lung hypoplasia from ever developing a normal complement of alveoli.
The most common causes of poor fetal lung growth are: inadequate amniotic fluid, congenital diaphragmatic hernia, hydrops fetalis, certain types of dwarfism, pulmonary agenesis, cystic adenomatous formation, and cystic hydroma. In each of these anomalies, the fetal lung does not grow to its normal size. Inadequate amniotic fluid is most commonly due to early leaking of amniotic fluid due to premature rupture of the membranes that surround the fetus. This is known as oligohydramnios (too little amniotic fluid). If amniotic fluid leaks out from around the baby, the chest wall movement that occurs with fetal breathing may be restricted. Fetal breathing and adequate fluid pressure are both believed vitally important for normal lung development.
The second most common cause for inadequate amniotic fluid is fetal renal anomalies. Amniotic fluid is produced by the amniotic membranes and by the fetal kidneys. Severe abnormalities of the kidneys (eg. polycystic kidneys, hydronephrosis, renal agenesis) can cause too little amniotic fluid to be formed (oligohydramnios) and are commonly associated with lung hypoplasia. Babies with kidney and lung problems have a particularly grave prognosis because they have two organ systems that have failed.
Structural problems in development may also impair lung growth. In babies with congenital diaphragmatic hernia, the diaphragm that separates the lung form the abdomen fails to develop. As a consequence, the intestines move into the chest cavity and restrict lung growth. Similarly, certain types of dwarfism or congenital anomalies of the lung restrict the area in which the lungs can grow normally. While most of these anomalies are rare (1:3000), they are commonly life threatening.
Small lungs fail to accomplish normal gas exchange (oxygen in, carbon dioxide out.)
The presentation is variable and dependent on the severity of the hypoplasia. Some babies may present with mild tachypnea (fast breathing) others may have signs of severe respiratory failure: fast breathing, labored breathing, blue color, and gasping.
The most important factors leading to a diagnosis are: history of fetal anomalies associated with lung hypoplasia, history of mom having too little amniotic fluid, and a chest radiograph showing small lungs.
Currently, treatment is primarily supportive. This means that there is currently no available medicine that makes babies grow lungs. So, until lung growth occurs to an extent that the lung can support normal gas exchange, the babies must be supported by artificial means. The main problem is that all modes of artificial respiratory support are associated with lung injury. The trick is to support normal gas exchange without causing injury and to support good nutrition so that the lung can grow. Babies have an incredible capacity to grow and develop. In time, if the lung is not too underdeveloped the baby can usually wean off artificial support and go home. Therapies used to support gas exchange in order of level of support are: oxygen, assisted ventilation, high frequency ventilation, and extracorporeal membrane oxygenation (ECMO).
There are no methods for preventing babies with certain anomalies from developing lung hypoplasia. Research is currently focused on maintaining normal amniotic fluid and pressure and prevention of restriction of lung growth. Investigators are also looking at factors that promote normal lung growth. The hope is that neonates with lung hypoplasia might be treated with lung growth factors that would promote growth of normal lung, reduce the need for artificial support and its attendant propensity to cause injury and allow for a healthier life.
About the Author
Dr. Clark, a leading clinical researcher for the care of critically ill newborns, is the Director of Research for Pediatrix Medical Group, Inc. and a Consulting Associate Professor at Duke University.
A native of North Carolina, Dr. Clark earned both his Bachelor of Arts and medical degrees from the University of North Carolina at Chapel Hill. He completed his pediatrics residency and his neonatal fellowship at Wilford Hall United States Air Forces Medical Center. He is a board-certified pediatrician and neonatologist.
Copyright 2012 Reese H. Clark, M.D., All Rights Reserved