DiGeorge Syndrome
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DiGeorge syndrome is an immunodeficiency that is caused by a 22q11.2 microdeletion. Since this is a microdeletion, DiGeorge patients cannot be diagnosed via karyotype. Instead, FISH (fluorescent in situ hybridization) can be used to make a diagnosis. The 22q11.2 microdeletion leads to a developmental failure of the 3rd and 4th pharyngeal pouches, which causes the various signs and symptoms of DiGeorge Syndrome. The thymus develops from these pouches, so DiGeorge patients are characterized by thymic aplasia and subsequent T cell deficiency. This causes an immunodeficiency in DiGeorge patients, and the syndrome is characterized by recurrent viral / fungal infections. Similarly, the abnormal pharyngeal pouches lead to parathyroid dysplasia and subsequent decreased PTH. The lowered PTH levels lead to a decrease in calcium levels and hypocalcemia. DiGeorge patients are also characterized by a number of clinical findings, such as abnormal facies and cardiac abnormalities. For the latter, conotruncal abnormalities such as truncus arteriosus and tetralogy of Fallot are common. Velocardiofacial syndrome is another 22q11.2 syndrome that is similar to DiGeorge syndrome. Velocardiofacial patients are characterized by cleft palate, abnormal facies, conotruncal abnormalities, and other similar findings. Since both syndromes have the same deletion, each syndrome is likely a different phenotypic presentations of the same disorder. The International 22q11.2 Foundation advocates for unifying the syndromes under a single name: 22q11.2 syndrome. That being said, it is important to recognize the names of both DiGeorge syndrome and velocardiofacial syndrome as either may appear on exams.