Mediastinal Widening

Alexander J. Towbin, MD

DIFFERENTIAL DIAGNOSIS

Common

Normal Thymus
Lymphoma
Reactive Lymphadenopathy

Less Common

Bronchogenic Cyst
Trauma
Enlargement of Ascending Aorta

Rare but Important

Germ Cell Tumor
Neurofibromatosis Type 1
Thyroid Carcinoma

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Normal pediatric mediastinum is wider than in adulthood due to thymus
Trauma is uncommon cause of mediastinal widening in children

Helpful Clues for Common Diagnoses

Normal Thymus
- Most common mediastinal “mass” in neonates and infants
- Most prominent in infancy
  - Has quadrilateral shape
- Thymus increases in weight until adolescence when it begins to involute
  - Gradually becomes triangular-shaped in childhood and teenage years
- Visible by radiograph until ˜ 5 years of age
- Appearance may change with respiration
- Thymus can vary in size depending on intercurrent illness and stress
  - May ↓ in size during illness/stress and → in size with recovery
- Often asymmetric across midline
  - Usually more prominent on right side
  - Mimic upper lobe consolidation
- Homogeneous appearance on CT and MR
  - Enhances homogeneously
- Multiple linear echoes and discrete echogenic foci on ultrasound
Lymphoma
- 3rd most common malignancy in children after leukemia and brain tumors
- Incidence increases with age
  - 25% of cancers in children 15-19 yo
- Most common mediastinal mass in teens
- Hodgkin lymphoma (HL) more common than non-Hodgkin lymphoma (NHL)
  - HL involves continuous nodal groups
  - NHL is more commonly extranodal
- NHL is more common than HL in children < 10 years of age
- Mediastinal disease is common in HL and NHL
  - Occurs in 2/3 with HL
- Mediastinal mass can cause tracheal compression
Reactive Lymphadenopathy
- Most common organism depends on geographic location
- Common causes: Tuberculosis (TB), histoplasmosis, coccidioidomycosis, and blastomycosis
  - TB and histoplasmosis may have lymph nodes with low-attenuation centers on contrast-enhanced CT in acute phase
  - Lymph node calcification in old disease
- Enlarged lymph nodes can compress superior vena cava (SVC)
  - May cause SVC syndrome or fibrosing mediastinitis

Helpful Clues for Less Common Diagnoses

Bronchogenic Cyst
- May have bronchial or esophageal origin
- Most common location: Paratracheal or subcarinal
- May lead to bronchial compression
  - May cause atelectasis or hyperinflation
- Round or oval with smooth contour
- Homogeneous appearance
  - Typically fluid attenuation on CT
  - Can have higher density with ↑ protein content, hemorrhage, or infection
  - Wall of cyst is thin and does not enhance unless complicated by infection
- Variable low T1 and homogeneously increased T2 signal on MR
  - Characteristically thin and nonenhancing wall unless infected
Trauma
- Most common cause of death in children
- Mediastinal hematoma is cause of mediastinal widening in trauma
- Traumatic aortic injuries are uncommon
  - Iatrogenic trauma is most common cause of aortic injury in children
- Aortic injury most common in teens
- Associated traumatic injuries are common
Enlargement of Ascending Aorta
- Aortic valve stenosis
  - Results in poststenotic dilatation
  - Can be valvular, subaortic, or supravalvular
  - Valvular aortic stenosis is most common
  - Can be seen in Turner syndrome and is associated with bicuspid aortic valve and coarctation of aorta
  - Supravalvar aortic stenosis is seen in Williams syndrome
- Dilated aorta often caused by aneurysm
  - Causes of aortic aneurysm: Connective tissue disorders, vasculitis, trauma, or infection
- Connective tissue disorders: Marfan syndrome, Ehlers-Danlos, Loeys-Dietz
  - Marfan and Ehlers-Danlos are disorders of collagen synthesis
  - Dilatation of sinus of Valsalva and ascending aorta
  - Other systemic manifestations
- Vasculitis: Takayasu arteritis
  - Large vessel vasculitis
  - Affects aorta, its main branches, and pulmonary arteries
- Mycotic aneurysm
  - Uncommon in children
  - Can occur in infants secondary to umbilical arterial line

Helpful Clues for Rare Diagnoses

Germ Cell Tumor
- Originate from germ cells that fail to complete migration from urogenital ridge
- Mediastinum is 4th most common site for teratoma (ovary, sacrococcygeal, testis)
- Often contain tissues that derive from germinal cell layers
- Can be cystic
Neurofibromatosis Type 1
- Autosomal dominant disorder
- Classical clinical findings of café au lait spots, axillary freckling, and dermal and plexiform neurofibromas
- Plexiform neurofibromas can occur anywhere
- Plexiform neurofibromas have targetoid appearance on MR
  - Loss of targetoid appearance should raise concern for degeneration into malignant peripheral nerve sheath tumor
Thyroid Carcinoma
- Uncommon in children
- Most common pediatric endocrine tumor
- Thyroid nodules seen in up to 1.5%
  - Nodules > 1 cm should be biopsied
- Radiation exposure is risk factor

Image Gallery

PA radiograph of the chest shows a sail-like appearance of the normal thymus

. The lateral border of the thymus typically has an undulating contour along the anterior lateral chest wall.

Lateral radiograph of the chest in the same patient shows the thymus in the anterior mediastinum filling in the retrosternal clear space

. This is a typical finding in young children.

(Left) Longitudinal ultrasound shows the normal thymus

just below the anterior chest wall. On ultrasound, the thymus has multiple linear echoes and discrete echogenic foci. Ultrasound is useful to distinguish a normal thymus from a mediastinal mass. (Right) Axial CECT shows the normal thymus