Maldescent of the thymus: 34 necropsy and 10 surgical cases, including 7 thymuses medial to the mandible.

Among 3236 pediatric necropsies over 23 years, abnormal position of thymic tissue was recorded in 34 cases. Cardiac anomalies, predominantly those seen in DiGeorge syndrome, were present in 24; 3 had noncardiac anomalies only, 4 had other diseases, and 3 were sudden infant deaths. Mediastinal thymic tissue was absent in 22 cases, small or unilateral in 7, and normal in 5. The maldescended thymic tissue was unilateral in 18, bilateral in 11, and multiple on one or both sides in 5. It was situated near the thyroid gland in 19, lower in the neck in 6, and higher in 9, including 7 medial to the submandibular salivary gland and 1 at the base of the skull. The maldescended tissue approached the size of a normal lobe of thymus in eight and was 2.4 to 0.1 cm in the remainder. Over the same period, in only two cases was no thymic tissue found. Among 68,000 surgical specimens over 40 years, there were 10 cases of ectopic thymus in the neck, including 1 cutaneous and 4 cystic. This frequency of thymic tissue in the neck may explain why lymphoid tissues are sometimes relatively normal in cases of absent mediastinal thymus and behooves a search in the submandibular salivary gland region and higher before diagnosing thymic agenesis.