Microdeletion 22q11.2 is the most common microdeletion with an incidence of at least 1:4,000 and with the highest variability of clinical symptoms. The severe presentation is defined by the complete clinical picture of DiGeorge syndrome with complex heart defects, thymic aplasia and resulting immunodeficiency, hypoparathyroidism due to aplasia of the parathyroid glands, developmental delay and short stature. The mild presentation shows for example only discrete signs of dysmorphia, possibly short stature and a hoarse voice. There is an overlap between the individual syndromes and even within a family, the degree of expression can vary significantly. The cause for this high variability even with the same deletion is not yet clear.
The microdeletion 22q11.2 causes a developmental field defect of the 3rd and 4th pharyngeal pockets as well as of the 4th gill arch and the structures deriving from it, such as vessels close to the heart, thymus, parathyroid glands. 75% of patients have heart defects, especially aortic arch anomalies and conotruncal heart defects. Up to 70% of patients have at least transient hypocalcemia. External features include a slightly wide interocular distance with a flat nasal root, hypoplastic nostrils, a narrow mouth with an arched upper lip overhanging the lower lip, and retrognathia. In adulthood, up to 20% are reported to have psychiatric abnormalities. In rare infantile psychosis, microdeletion 22q11.2 is part of the differential diagnosis. In 5-10% the microdeletion is already present in one parent, which leads to a 50% risk of recurrence for future children. Therefore, the parents of an affected child should also be examined, especially for future family planning purposes.
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