Thorax og lunger
EMBRYOLOGY
Many sonographically detectable pulmonary malformations can be linked to the
interruption of normal developmental sequences. During the first 5 weeks of
gestation, the lung buds grow out of the ventral aspect of the primitive foregut. If
these buds do not form, lung agenesis results. The trachea and esophagus become
separated by the fifth gestational week. Buds from the early trachea form and
penetrate the mesenchymal masses destined to become the lungs. An abnormal
budding of a segment of the tracheobronchial tree may result in formation of a
bronchogenic cyst. Between 5 and 16 weeks (the pseudoglandular period), the
bronchial tree is formed. Through a series of divisions and budding, bronchi give rise
to bronchioles, and each terminal bronchiole later gives rise to alveolar ducts and
alveoli. By 16 weeks, the formation of the bronchial tree is essentially complete.
Insults to the lung before then result in fewer than expected bronchi. Bronchial airway
generations are reduced in such conditions as congenital diaphragmatic hernia
(CDH), renal agenesis, absent phrenic nerve, rhesus isoimmunization, and idiopathic
lung hypoplasia.
Between 16 and 24 weeks (the canalicular period), one sees a dramatic increase in
the number and complexity of air spaces, large blood vessels, and capillaries. Insults
to the lungs during this phase result in smaller airways and a reduction in the number
and size of acini.
After 24 weeks, terminal sacs and alveoli continue to develop and mature (the
alveolar period), and the number and complexity of the airspaces are further
increased. The last 16 weeks of gestation are referred to as the terminal sac period.
Type II pneumocytes, the surfactant-producing cells, mature between 32 and 36
weeks. The number of alveoli continues to grow during childhood through alveolar
multiplication, after which alveolar expansion becomes the major means of lung
growth until adolescence. Investigators have speculated that, if an abnormality were
removed or interrupted before this stage (the alveolar period), the normal process of
air space maturation could potentially be renewed. Fetal surgical resections of lung
masses and repair of CDHs have been performed for this purpose.
The fetal thoracic cavity is bell-shaped and is bordered by the clavicles at the apex
and by the smooth, hypoechoic diaphragm inferiorly. The ribs, smoothly marginated
and regularly spaced, form the lateral boundaries and extend anteriorly more than
halfway around the thorax from their dorsal attachments.
The heart occupies approximately one third of the thoracic volume, and most of the
cardiac volume is located in the left anterior quadrant of the chest on an axial plane
at the level of the four-chamber. With real-time imaging, mediastinal and pulmonary
vessels can be easily appreciated; pulmonary veins can be followed to the left atrium,
to confirm normal venous connections to the heart. The fetal thymus is usually not
distinguishable unless large pleural effusions are present.
Normal fetal lungs are homogeneous in echotexture, and echogenicity may be
greater than, less than, or equal to the echogenicity of the liver. As a general
observation, the echogenicity of fetal lungs tends to increase progressively, to an
echogenicity greater than that of the liver during gestation.
Patologi
PULMONARY HYPOPLASIA
Pulmonary hypoplasia, pathologically defined as a low ratio of lung to body weight
and a low radial alveolar count, remains an important source of postnatal morbidity
and mortality. Four factors are most important for normal lung development: (1)
adequate gestational duration for lung maturation; (2) adequate amniotic fluid
volume; (3) intrathoracic space; and (4) fetal breathing movements.
Lung development can be compromised by a skeletal dysplasia (small thorax) or an
intrathoracic mass that compresses the developing lung and functionally reduces
thoracic space available for lung expansion and growth. Depressed fetal breathing
movements resulting from a severe neurologic defect. Although breathing
movements are important for normal lung development, they are not sufficient to
prevent pulmonary hypoplasia. Finally, severe prolonged oligohydramnios, whether
from chest compression, low amniotic fluid pressure, or reduction of fluid within the
fetal lung, is almost always associated with pulmonary hypoplasia.
Although it would be ideal for obstetric management and parental counseling to be
able to predict the severity of pulmonary underdevelopment based on the observed
sonographic abnormality, this is not always possible. Concern about potentially
significant pulmonary hypoplasia is warranted in any pregnancy affected by
prolonged oligohydramnios (especially if it occurs before 26 weeks and lasts more
than 5 weeks) or by a large space-occupying fetal chest lesion.
The rate of growth of the fetal TC is linear between 16 and 40 weeks, similar to other
biometric parameters. Thoracic size has been correlated with normal pulmonary
development. A correlation between pulmonary hypoplasia and a small thoracic
circumference (TC) has been demonstrated. Therefore, the ratio of TC to AC, HC, or
FL is constant, and these ratios may function as useful gestational age-independent
parameters . TC:AC seems to be the ratio with the smallest variability in normal
fetuses, and it is more than 0.80 (mean, 0.89- 0.85) in nearly all normal pregnancies
beyond 20 weeks.
FETAL THORACIC MALFORMATIONS
A few clinically important thoracic malformations have been observed prenatally.
Both extrinsic (ie, abnormalities of the bony thorax) and intrathoracic abnormalities
can interfere with normal lung growth and development.
Bony thoracic abnormalities usually present as part of a multisystemic or
generalized fetal abnormality. The fetal thorax may be abnormally small or
misshapen, with fractured or shortened ribs. The latter should raise the suspicion of a
primary skeletal dysplasia . A small but otherwise normal-appearing thorax usually
has another cause, such as severe and prolonged oligohydramnios or severe fetal
neurologic disorder.
Marked mediastinal shift probably contributes to the development of hydrops by
impeding cardiac venous return and elevating central venous pressure.
Three lesions constitute most unilateral fetal chest masses: CDH, CAM, and BPS.
Bronchogenic cyst, unilateral bronchial atresia, and bronchial stenosis are rarer but
also have been detected in the fetus
CONGENITAL CYSTIC ADENOMATOID MALFORMATION
CAM is described pathologically as a hamartoma or focal dysplasia of the lung, and
it accounts for approximately 25% of congenital lung lesions. Investigators have
speculated that this malformation results from an early embryonic insult (earlier than
8 weeks), because the bronchial system within the lesion is poorly developed. CAM
is thought to occur from a failure of the endodermal bronchiolar epithelium to induce
surrounding mesenchyme to form normal bronchopulmonary segments. Stocker and
associates described three pathologic categories of CAM: type I, macrocystic with
cysts 2 to 10 cm, type II, medium-sized cysts; and type III, microcystic with cysts 0.3
to 0.5 cm. Reports of associated renal and chromosomal anomalies (3% to 8%),
CAMs may also cause polyhydramnios, speculated to result from compression of the
fetal esophagus, impairing fetal swallowing, or possibly from excess lung fluid
produced by the lesion. Prognosis is worse if polyhydramnios is present, as it is if the
lesion is large and associated with severe shift of the cardiomediastinal structures.
Bromley and coworkers reported a 76% survival rate among fetuses with absent or
mild mediastinal shift, compared with a 37% survival rate among those with moderate
or severe shift, and they also reported 100% survival of fetuses with normal amniotic
fluid volume, compared with only 50% survival among fetuses with polyhydramnios.
BRONCHOPULMONARY SEQUESTRATIONS
A BPS is a mass of pulmonary tissue separated from its normal bronchial and
vascular connections and supplied by systemic arteries.
Two forms of sequestrations have been described, intralobar and extralobar. Both
are supplied by a systemic artery from the thoracic or abdominal aorta.
Sonographically, a BPS appears as a homogeneous, echogenic lung mass. The
hyperechogenicity is thought to result from multiple reflecting interfaces of tiny dilated
bronchioles and air spaces..
BRONCHOGENIC CYSTS
Bronchogenic cysts are bronchopulmonary foregut malformations thought to occur
as a result of abnormal budding of the ventral diverticulum of the foregut. Therefore,
most such cysts probably develop between the 26th and 40th day of fetal life.
Prenatal detection with ultrasound is rare, but it has been accomplished by observing
(1) a single unilocular pulmonary cyst or (2) an echogenic, distended lung obstructed
by a small bronchogenic cyst.
BRONCHIAL ATRESIA
Atresia of the segmental, lobar, or main stem bronchus is thought to occur
secondary to a vascular accident during development. The bronchi distal to the
atresia are usually normal in number. The insult is thought to occur after the 16th
week of gestation. This abnormality has been detected in the fetus sonographically
as a large echogenic fetal lung mass. The abnormality may not be visible before 24
weeks.
CONGENITAL DIAPHRAGMATIC HERNIA (RIGHT-SIDED AND LEFT-SIDED)
CDH describes a defect in the diaphragm thought to result from incomplete fusion
of the pleuroperitoneal membrane at 6 to 10 weeks. The developing lungs are
severely compressed by the herniated viscera, resulting in nearly universal
pulmonary hypoplasia, which is often lethal. The incidence in live births is estimated
to be 1 in 3000 to 5000, but, owing to unrecognized CDH in fetal and neonatal
deaths, the overall incidence is thought to be as high as 1 in 2,200 births. CDH
occurs more commonly in females (3:2). The most common location of CDH is
posterolateral and on the left (85% to 90%). The size of the defect in the diaphragm
varies, and in 1% to 2%, the diaphragm is completely absent. Associated
malformations are common (25% to 57%), most notably cardiac defects (9% to 23%),
neural tube defects (28%), spinal defects, trisomies (trisomy 21 and 18, in 10%
diagnosed prenatally), intestinal atresias, hydronephrosis and renal agenesis,
anencephaly, spina bifida, and certain well-defined syndromes. Most cases occur
sporadically, with anecdotal reports of familial CDH. Sonographically, mediastinal
shift is one of the first abnormalities observed . If a fluid-filled stomach cannot be
detected below the diaphragm, CDH should be strongly considered in the differential
diagnosis of a cystic chest mass. The herniated viscera most commonly include the
stomach and bowel and, in large defects, the spleen, the left kidney, and the left lobe
of the liver may herniate as well.
Prognosis of fetuses with CDH is guarded. Overall mortality in prenatally diagnosed
cases is variable, but it is often reported to be more than 70% or approximately 60%
if the CDH is isolated and extracorporeal membrane oxygenation (ECMO) is
available. Poor prognostic indicators include preterm birth, additional malformations,
and large hernias. Fetuses in whom the initial diagnosis is made after birth have
much better outcomes than those in whom the CDH is detected antenatally.
Prognosis of fetuses with CDH has also been linked to the size of the contralateral
lung observed on the sonogram.
Some degree of pulmonary hypoplasia is usually present in fetuses with CDHs, and
it is the leading cause of perinatal mortality. As with CAMs and BPSs, most
investigators believe that compression of the developing lung by the herniated
viscera interrupts normal pulmonary development. Morphometric analyses of
neonates with CDH confirm an arrest in pulmonary development (reduced number of
bronchial branches and alveoli), and infants dying of a large CDH usually have lung
weights 15% to 40% of those expected for gestational age. The pulmonary vascular
bed is also abnormal, with a reduced number of vessels, and increased medial
muscular hyperplasia in small peripheral arteries.
Right-sided posterolateral hernias are much less common than those on the left. The
right lobe of the liver most commonly herniates, and the small bowel and kidney are
thus affected less commonly.
Pleural Effusions
Fetal hydrothorax may be an isolated primary abnormality (fetal chylothorax).
Pleural fluid in the fetus is always abnormal. In most cases, fetal pleural effusion is a
secondary manifestation of one of many possible disorders, including immune and
nonimmune fetal anemia, chromosomal (especially trisomy 21 and 45 XO), cardiac,
and metabolic abnormalities, pulmonary masses, and abnormalities of the placenta
and umbilical cord.
Causes of Pleural Effusion
Idiopathic primary chylothorax
Hydrops fetalis (multiple causes)
Abnormal karyotype, mostly 45 XO, trisomy 21
Lung mass: sequestration, congenital diaphragmatic hernia
The mortality rate among fetuses with hydrothoraces is considerably higher (50%)
than that in newborns with chylothorax (15%). Fetal pleural effusions may resolve
spontaneously in utero in as many as 10%.