DiGeorge's or the III-IV pharyngeal pouch syndrome: pathology and a theory of pathogenesis.

Six new cases of the III-IV pharyngeal pouch syndrome were encountered at autopsy among 897 consecutive pediatric autoposies. All occurred in patients with conotruncal cardiac anomalies. The anatomic characteristics of the heart defect suggest a possible mechanism for the pathogenesis of this developmental anomaly. It is postulated that premature involution of the thyroidea ima artery, which is the principal embryonic blood source to the 3rd and 4th pharyngeal pouches and the ultimobranchial body, may be the critical event in the embryogenesis of the syndrome. Hemodynamic alterations in the course of anomalous morphogenesis of conotruncal anomalies may favor premature involution of certain vessels including the left 4th aortic arch and the thyroidea ima. Deficiencies of thymus, parathyroid glands and ultimobranchial tissue ("C" cells) may be the consequence of vascular deprivation during embryogenesis. These cases are discussed with regard to the proposed mechanism of pathogenesis and the influence of varying quantities of thymus upon peripheral lymphoid tissue. Both embryologic and clinical data support the possibility of a deficiency of ultimobranchial tissue. The range of minor and inconstant anomalies seen in patients with this syndrome is presented.