THE THYMIC "WAVE" SIGN.

Abstract

The purpose of this paper is to describe a new, common, and reliable roentgenologic sign for identification of the thymus and its differentiation from other mediastinal structures. Widening of the superior mediastinal shadow by a prominent thymus is commonly seen on the frontal film of infants. A large thymus is no longer accepted as a cause of respiratory distress or other symptoms, and its presence is probably indicative of normalcy rather than pathology (2). On occasion, however, a prominent thymus may be difficult to distinguish from significant abnormalities of the superior mediastinum (2, 8, 10). In healthy infants such widening of the superior mediastinum may be attributed to the thymus with a reasonable degree of assurance. In others, in whom the possibility of a pathologic process of the mediastinum exists, there may be a problem in differentiating a normal, prominent thymic shadow from neoplastic, inflammatory, or congenital abnormalities in that area. Certain known roentgen characteristics, if present, may permit identification of the thymus. The sail configuration (Fig. 1) is probably the most reliable roentgen sign of the thymus and is no longer felt to represent mediastinal pleurisy (4, 5, 7). Unfortunately, the sail sign is not always present; it was noted in 8.8 per cent of Kemp's series of 498 healthy children (7). It is usually seen on the right side, probably because it is obscured on the left, as the underside of the thymus lies against and blends with the heart to form the thymocardiac image (2). Occasionally, the sail sign will be identified on one film and not on another as a result of minor variations in the cardiac cycle, the phase of respiration, or the relative obliquity or angulation of the x-ray beam. Due to the vascularity and soft consistency of the thymus, its shadow may change strikingly in size from one film to another, with variations in the respiratory phase. Figures 2 and 6 are examples of such marked change in the apparent size of the thymus. But, while variability in size may be of help in distinguishing the thymus from pathologic processes of the mediastinum, it, like the sail sign, may not always be demonstrable. With stress, starvation, or debility, the thymus may regress in size; this atrophy is probably the result of the anabolic effect of adrenal steroids on lymphoid tissue. Caffey has noted the shrinking effect of exogenous steroids on thymic size and has pointed out that in problem cases this steroid effect may be useful in differentiating thymus from other causes of superior mediastinal widening. In one of the cases reported by him, mediastinal widening was not affected by steroid therapy in a patient later shown to have an anomalous venous return (1).