Severe combined immunodeficiency and DiGeorge syndrome

What it is

These two are the pair to hold together, because an absent thymic shadow on the chest x ray belongs to both.

DiGeorge syndrome, also called velocardiofacial syndrome, 22q11.2 deletion syndrome and Shprintzen syndrome, is a microdeletion in chromosome 22 with defective development of the pharyngeal pouches: failure to develop the 3rd and 4th pharyngeal pouches means the thymus and the parathyroids are absent. Mode of inheritance is autosomal dominant. It is the most common microdeletion syndrome and the most common T-cell disorder.

Severe combined immunodeficiency is a gene defect leading to failure of T cell development, with B cell dysfunction due to the absent T cells. Several types exist, including a defective IL-2R gamma chain (most common, X-linked recessive) and adenosine deaminase deficiency (autosomal recessive).

How it presents

DiGeorge syndrome is held in the mnemonic CATCH 22:

• Cardiac defect
• Abnormal face: hooded eyelids, ptosis, hypoplasia of auricles
• Thymic hypoplasia, with T cell abnormalities
• Cleft palate
• Parathyroid aplasia / hypoplasia
• Hypocalcemia

With it go hypernasal speech from velopharyngeal incompetence, hypotonia in infancy, and learning disabilities and behavioral problems. Hypocalcemia may show itself as tetany, or as a neonatal seizure.

Severe combined immunodeficiency presents with failure to thrive, chronic diarrhea and thrush, and with recurrent severe viral, bacterial, fungal and protozoal or opportunistic infections, for example Pneumocystis.