The thymus is a soft organ comprising two asymmetric lobes joined
by an isthmus and invested by a fibrous capsule. The thymus gland
is embrylogically derived from the ventrolateral surfaces of the
third pharyngeal pouch in the form of paired primordia during the
sixth week of fetal life. The thymus occupies the anterior and superior
mediastinum, located behind the sternum in the midline. the upper
border of the thymus extends almost to the thyroid gland in the
neck and the lower margin overlaps the upper part of the heart,
occasionally extending as far as the diaphragm.
The thymus has its largest size relative to body weight at birth,
weighing on average 10-15 g.
A normal thymus is the most frequent cause of a widened mediastinum
in the newborn and infancy. The thymic lobes often differ significantly
in size and this can give the impression of a mass related to one
side of the mediastinum or the pulmonary hilum.
Thymic cysts are uncommon mediastinal lesions that account for only
1% to 2% of all tumors in the anterior mediastinum. It can be either
congenital or acquired.
the thymus can be imaged in most fetuses. Prenatal ultrasonography
is helpful in the discovery of a number of abnormalities in the
fetal thorax, including thymic cysts.
Thymus can be thought of as a soft pancake-like structure draped
over the mediastinal vessels.
Thymic cysts are rare lesions of the anterior mediastinum or neck
along the path of the thymopharyngeal ducts from the level of
the mandible to the chest.
Mediastinal thymic cysts account for about 1% of all mediastinal
masses. the cysts are unilocular or multilocular and range in
size from microscopic to as large diameter.
CT scan images can greatly assist in determining the exact location
of the mediastinal cyst and in determining its relationship to
Because the patient is not exposed to radiation, Magnetic resonance
imaging may be preferred to CT scan by some physicians for the
evaluation of a mediastinal cyst in a fetus.
resonance imaging :
MRI is useful in both the initial diagnosis of a mediastinal mass
and in follow-up evaluations. It provides superior vascular imaging
and can help better delineate the relationship of an identified
mediastinal mass to nearby intrathoracic structures.
Prenatal ultrasound can be a sensitive method
for the detection of pulmonary abnormalities, although thymic
cyst can be difficult to differentiate from other cystic chest
Congenital Diaphragmatic Hernia.
Congenital diaphragmatic hernias are unilateral in 97% and usually
on the left. This hernia most commonly results from failure of
the pleuroperitoneal canal to close completely. This should occur
by week 10 of gestation (12). Failure of closure results in a
herniation of abdominal viscera into the thorax, resulting in
a mass effect that can prevent normal lung development.
Ultrasonography may reveal poly-hydramnios and visualization of
abdominal viscera within the thorax. A unilateral pleural effusion
may be present. Characteristically, a fluid-filled mass is seen
behind the left atrium and ventricle, mediastinal shift is variable.
Real-time sonography may reveal bowel peristalsis within the thorax.
Ways of attempting to exclude a congenital diaphragmatic hernias
on prenatal ultrasound include visualization of an intact diaphragm
and an appropriately located stomach, liver and gallbladder. Fetal
breathing movements may assist in visualizing the bowel sliding
up and down in the chest through the diaphragmatic defect in some
cases of congenital diaphragmatic hernias.
Cystic Adenomatoid Malformation :
this is a rare congenital malformation, which appears to result
from an embryological anomaly occurring before the 7th week of
gestation (4). Ultrasonographically, the cystic adenomatoid malformation
may be multicystic with evidence of space occupation, or an echogenic
mass lesion. Macrocystic lesions contain single or multiple cysts
that are >5mm in diameter. Microcystic lesions are more solid,
appear echogenic on ultrasound, and contain cysts measuring <5
mm in diameter.
Bronchopulmonary Sequestration :
A portion of lung parenchyma does not communicate with the tracheobronchial
tree, and usually receives its arterial supply from a systemic
vessel. Cysts are occasionally seen with an intralobar sequestration,
which may make differentiation from other cystic intrathoracic
Teratoma is a rare cause of cystic lung lesions. Anterior mediastinal
teratomas are classified into mature and immature types. Mature
teratomas present as large fetal lung tumors with cystic and solid
components, polyhydramnios, hydrops fetalis, pleural effusions
and hyperechoic spots with of calcifications. Immature teratomas
form complex masses with multiloculate cystic components associated
with polyhydramnios but may not have calcification or fetal hydrops.
Dilated proximal pouch blind esophageal :
This is a large, purely cystic posterior mediastinal mass.
Mediastinal meningomyelocele, meningocele :
This appears cystic with variable thickness of its wall, which
may be associated with vertebral defect.
Bronchogenic Cyst :
Bronchogenic cysts are a rare bronchopulmonary foregut malformation
that result from an abnormal budding of the foregut. This may
be unilocular or multilocular, closely related to the trachea
or main bronchus and may communicate with the tracheobronchial
Simple lung cysts :
These appear as well defined cystic masses, unilocular, surrounded
by normal lung.
cysts Diagnosis is suggested by the position of the cyst adjacent
to the bowel and spine. They are often associated with vertebral
occasionally, these may be visualized as purely cystic mediastinal
Laryngeal Artesia :
Bronchial artesia results from a congenital obliteration of a
proximal segment of bronchus. This results in a collection of
mucus in the distal bronchi and the affected portion of lung appears
large and echogenic on prenatal ultrasound due to the accumulation
of fetal lung secretions.
Congenital left ventricular aneurysm :
Congenital left ventricular aneurysm is a Midline cystic intrathoracic
structure in close proximity to the heart, confirmed left ventrivle
aneurysm by color Doppler.
Pericardial cyst is usually attached to the pericardium overlying
the right heart border, or rarely presents at the right costophrenic
Heterotopic Brain Tissue :
There is an isolated case report of heterotopic brain tissue causing
an appearance consistent with macrocystic cyst postnatally. The
neonate had several other abnormalities present.
pleurisy appear anechoic and normally conform to the thoracic
- diaphragmatic contours
Cystic hygroma :
Usually nuchal, but may rarely extend into the thorax.
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