Visualization of the thymus in myasthenia gravis

We investigated whether CT had any advantage over pneumomediastinography (PMG) for the visualization of thymus in twenty-one patients with myasthenia gravis (MG). In two cases of thymoma which had been suspected with chest radiography, CT demonstrated an anterior mediastinal tumor distinguishable from other mediastinal organs, and for these cases PMG was not performed. Excluding three patients who were submitted to PMG only, CT of the anterior mediastinum was carried out in sixteen patients of MG, revealing thymic shadows in seven (44%). PMG followed by conventional tomography was done subsequently in three of these seven cases, for whom thymectomy was indicated because of uncontrollable myasthenic symptoms, and in all patients finger-like thymic shadows were disclosed. The vertical extension of thymus was more easily demonstrable by PMG than CT. PMG was carried out in six of nine patients in whom CT was negative, and in all cases thymic shadows were obvious with subsequent conventional tomography. Consequently, false negative rate of CT was at least 38% (6/16) with regard to the visualization of the nontumorous thymus. Although CT of the anterior mediastinum is useful as a screening method because of its non-invasiveness, its negative result does not rule out an absence of the pathologic thymus in view of its high false negative rate. In this regard, PMG is still necessary for the final determination of the thymic configuration in the MG patients. (J.P.N.)