A 5-month-old, full-term boy is brought to the office due to fever. He was doing well until this morning, when he felt warm to his mother. The patient has had a "runny nose" and intermittent coughing but otherwise has been breastfeeding well. His 3-year-old brother had an upper respiratory tract infection last week. The patient takes vitamin D, and vaccinations are up to date. Temperature is 38.9 C (102 F), blood pressure is 90/60 mm Hg, pulse is 120/min, and respirations are 32/min. Oxygen saturation is 96%. Examination shows crusted rhinorrhea at the nares. The patient's chest radiograph is shown in the image below:
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Which of the following is marked on this patient's chest radiograph?
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The thymus is normally visible on chest x-rays in children age <3. It is located in the anterior mediastinum behind the sternum and in front of the heart, aortic arch, and trachea. It appears prominent on infant x-rays due to its relatively large size compared to the small thorax. On frontal views, the thymus is most commonly recognized as the "sail sign" due to its triangular shape and uniform density. However, the shape can vary with the phases of respiration because the thymus is a soft gland. The thymus can also shrink during times of stress or illness and rebound after recovery (rebound hyperplasia).
In utero and during infancy and childhood, the thymus is an important organ for lymphocyte production and maturation. Its absence on neonatal x-ray may be suggestive of thymic hypoplasia or aplasia (eg, DiGeorge syndrome). The thymus normally atrophies and is replaced by fat after puberty, when it has completed the production of T lymphocytes. Therefore, adults with mediastinal opacities on x-ray should undergo further work-up for a pathologic cause (eg, lymphoma, germ cell tumor).
(Choice A) Hilar lymphadenopathy is never normal and raises concern for malignancy (eg, lymphoma, metastases), infection (eg, tuberculosis, histoplasmosis), or inflammation (eg, sarcoidosis). This infant has no symptoms or x-ray findings suggestive of an abnormal mediastinal mass.
(Choice B) Right atrial enlargement can give the appearance of cardiomegaly. Infants age <1 normally have a transverse cardiothoracic ratio of ≤60%. Children age ≥1 and adults should have a cardiothoracic ratio of ≤50%. This infant has a normal cardiac silhouette.
(Choices C and E) Right middle or upper lobe infiltrates are typical locations for aspiration pneumonia in infants or other patients who spend most of their time in the supine position. Infants with aspiration from swallowing dysfunction typically experience choking or gagging preceding respiratory distress. This infant has no infiltrates on x-ray.
(Choice D) Right upper lobe atelectasis appears as a linear density with associated shifting of the mediastinum toward the collapsed lung.
Educational objective:
A large thymic silhouette is a normal finding on frontal chest x-ray in children age <3 because it is relatively large compared to the small thorax. The thymus normally atrophies after puberty. Therefore, opacities in this location in adults raise concern for a pathologic cause (eg, lymphoma, germ cell tumor).