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Because of its relative prominence in infancy, a normal thymus gland can give the impression of cardiomegaly, a mediastinal mass or even lung pathology. Recognition of certain radiographic characteristics that pertain to this normal structure help to differentiate it from pathology.
The thymus is a soft organ comprising two asymmetric lobes joined by an isthmus and invested by a fibrous capsule. Embryologically it is derived from the third pharyngeal pouch. The thymus occupies the anterior and superior mediastinum, located behind the sternum in the midline. In infancy 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. It can be thought of as a soft pancake-like structure draped over the mediastinal vessels.
The thymus has a crucial role in the development of immunity, although its function has only been elucidated fairly recently. It serves as the site of differentiation of T lymphocytes in all mammals. These migrate to the thymus during fetal life and early infancy, emerging after a process of molecular modifications as specific types of immunocompetent cells involved in cell mediated immunity. Mature immunocompetent cells emerging from the bone marrow or thymus circulate and populate secondary lymphoid organs—the spleen and lymph nodes as well as aggregates of lymphatic tissue associated with mucosal surfaces such as the gut and bronchial system. The critical role of the thymus is completed in the first few days of life but it does have a continued role in immunity throughout life.
The thymus has its largest size relative to body weight at birth, weighing on average 10–15 g. It has an extremely variable size and appearance during early life but grows to a maximum weight of approx 35 g at puberty. Post-puberty the thymus involutes/atrophies, its glandular tissue gradually being replaced by fat.
Any type of stress such as an acute illness causes a rapid decrease (beginning within 24 hours) in thymic size due to the effects of endogenous steroids. On recovery there is often a “rebound” increase in the size of the thymus which later settles down to its regular size.
The characteristic features of DiGeorge syndrome (congenital thymic aplasia) are due to failure of normal development of the third and fourth branchial arches. In this syndrome, absence of the thymus and parathyroids, congenital heart disease, oesophageal atresia and facial abnormalities are seen and affected patients have a severe immunodeficiency. Absence of the thymus cannot be reliably diagnosed on chest radiograph (CXR), but the disorder is excluded if a thymic silhouette is seen.
A small thymus or structural thymic abnormalities are seen in severe combined immunodeficiency disease (SCID), which represents a group of rare, sometimes fatal, congenital disorders characterised by little or no immune response. The defining characteristic of this disorder is usually a severe defect in both the T- and B-lymphocyte systems.
The name thymus comes from the Latin derivation of the Greek thymos meaning warty excrescence due to its resemblance to the flowers of the thyme plant. Thymos also translates as soul or spirit. Throughout history the thymus has been hypothesised to have many roles, from being a purifier of the nervous system to no more than a protective cushion for the thorax. It was once designated the organ of mystery, probably the most apt description, since the true function of this organ remained unclear for almost 2000 years after its first discovery. However, in the late 1800s two alleged thymic conditions—thymic asthma and status thymicolymphaticus—were widely publicised. In thymic asthma the gland was thought to be responsible for mechanical compression of the trachea causing cough, stridor, and even sudden death. This was perpetuated by the finding that infants who died suddenly often had a large thymus at postmortem examination. In contrast, infants dying from an illness with even a relatively short duration and having a small, involuted thymus were thought to represent the normal state. Status thymicolymphaticus, a condition also associated with sudden death in infants but not coupled with respiratory symptoms, was thought to be caused by an underlying process that caused all lymphoid tissue to be enlarged and taken to represent an abnormal constitutional state which predisposed these infants to death from trivial causes. For those infants diagnosed before they succumbed to the condition, thymectomy was often performed.
Following the discovery of x rays in 1895, radiology played an important role not only in the diagnosis of these conditions but also in treatment. Thousands of children were irradiated prophylactically or therapeutically in the early 1900s, although it was later recognised that irradiation was responsible for increased rates of malignancy. When paediatric radiology developed as a specialty the normal appearance of the thymic shadow on CXR was finally recognised, although even as late as the 1940s, steroid treatment was given to shrink the thymus and improve visualisation of the heart and great vessels.
HOW TO RECOGNISE THE THYMUS ON CXR
The thymus has a very variable radiographic appearance. As it is soft it widens on expiration, narrowing and elongating on inspiration. The gland also changes in size and shape depending upon the age of an individual as well as their current state of health. It is usually seen on CXR as a prominent soft tissue density in the antero-superior mediastinum up to the age of 3 years, but its borders may be seen in many children up to 5 years of age. On lateral CXR it fills the retrosternal/precardiac space with a soft tissue density that cannot be separated from the heart border. In infancy a prominent thymus is almost always normal, reflecting good health and nutrition. A rebound increase in size seen following recovery from illness or stress-induced thymic atrophy is often responsible for the most striking variation in normal size. Occasionally the thymus extends into the middle or posterior mediastinum and may be perceived on the frontal CXR as a posterior mediastinal mass. In such cases computed tomography (CT) or magnetic resonance imaging (MRI) demonstrates that the abnormally sited tissue has identical characteristics to the normally located thymus and is usually in continuity. Similarly, ectopic thymic tissue that can be found in the neck or the trachea is identified as such by its characteristics on cross-sectional imaging. In these locations ectopic thymic tissue may cause problems such as airway compression or obstruction.
A normal thymus is the most frequent cause of a widened mediastinum on frontal CXR in infancy (fig 1). 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. One or more of the following characteristics are usually present and enable the thymus to be differentiated from other causes of a mediastinal mass or mediastinal widening such as lymphadenopathy, tumour or cardiomegaly.
The soft, normal thymus does not have any mass effect on adjacent structures such as the trachea, which should not be displaced or narrowed. The anterolateral margins of the gland are often indented by the anterior rib ends producing a scalloped appearance—the “wave” or “ripple” sign which may be seen on the left or right side (fig 2). Sharp, smooth, slightly convex lateral borders of one or both lobes are characteristic (fig 3) and a small notch often marks the inferior border between the thymus and the heart on either side (fig 4), although this can be quite subtle. The left thymic lobe overlaps the left pulmonary artery which can be seen through the thymus. The right lobe of the thymus often has a flattened inferior border near the horizontal fissure producing the so-called “sail sign” (fig 5); a left sided sail sign is less common although the appearance may even be bilateral.
CXR characteristics of the normal thymus: key points
Soft and moulds to the ribs, producing a scalloped border—“wave” sign
Straight inferior border—“sail” sign—usually right sided
No mass effect—therefore does not displace the trachea or vessels
Sharp, smooth borders which are slightly convex
Notches at its junction with the heart
Homogeneous, soft tissue density
Varies in size—particularly with different phases of respiration, and in related to general health
Most often seen on CXR < 3 years of age
The normal thymus may obscure anywhere from a third to the entire upper lobe of the lung on either side (fig 6) so may either simulate or obscure upper lobe pneumonia (fig 7). The absence of air bronchograms and a well defined strip of normally aerated lung peripherally helps to differentiate the thymus from consolidation; however, a lateral CXR may rarely be indicated to differentiate between upper lobe consolidation and an overlying thymic shadow. One or both of the thymic lobes may be prominent and simulate cardiomegaly (fig 8), however if there is doubt a lateral CXR will clarify. If the heart is enlarged, there is posterior displacement of the left ventricular margin so that it extends to or overlaps the spine.