Djerba, Tunisia 4

Introduction: 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.

Case report:This is a 38-year-old woman, G4P3, referred at 18 weeks for a routine scan. The scan revelaled a 3-cm well-defined anechoic cyst on the right hemithorax. There was either a cyst on the left side of the chest, displacing the mediastinum and the heart. The stomach was visualized below the hemidiaphragm. The mass was believed to be a congenital cystic malformation.

CT imaging was performed and showed a bilocular thymic cyst. The cyst became progressively smaller during the pregnancy. After delivery, the thymic cyst disappeared. The baby has been developing without any abnormal findings.

Ultrasound findings: The thymus can be seen in most fetuses. Prenatal ultrasonography is helpful to diagnose a number of abnormalities in the fetal chest, including thymic cysts. Thymus can be seen 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 large diameter.

CT scan: CT scan images can help to determine the exact location of the mediastinal cyst and to determine its relationship to adjacent structures. Because the fetus 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.

Magnetic resonance imaging: MRI is useful  either to initial diagnosis of a mediastinal mass or in follow-up evaluations. It provides superior vascular imaging and it can help to better delineate the relationship of an identified mediastinal mass to nearby intrathoracic structures.

Differential diagnosis: Prenatal ultrasound has a good sensibility for the detection of pulmonary abnormalities, although thymic cyst can be difficult to differentiate from other cystic chest lesions:

• 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 masses difficult.
  
• Teratoma: 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 tree.
  
• Simple lung cysts: These appear as well defined cystic masses, unilocular, surrounded by normal lung.
  
• Enteric-neuroenteric cysts: Diagnosis is suggested by the position of the cyst adjacent to the bowel and spine. They are often associated with vertebral abnormalities.
  
• Neuroblastoma: occasionally, these may be visualized as purely cystic mediastinal masses
  
• Laryngeal Atresia: Bronchial atresia 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 ventricle aneurysm by color Doppler.
  
• Pericardial cyst: Pericardial cyst is usually attached to the pericardium overlying the right heart border, or rarely presents at the right costophrenic angle.
  
• 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.
  
• pleural effusion: 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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