7 Unless they have a genetic syndrome that causes other congenital anomalies, infants with severe combined immunodeficiency appear healthy at birth, which contributes to the challenge of early recognition. 10, 11 A Canadian national surveillance study found a high incidence of the disorder in First Nations, Métis and Inuit Canadian children of 1:23 000 live births, and an incidence of 1:71 000 live births in non–First Nations, Métis and Inuit Canadian children. Improved recognition through newborn screening has enabled a more accurate estimate of 1:58 000. 2 The incidence of severe combined immunodeficiency was previously thought to be 1:100 000. Severe combined immunodeficiency is caused by a variety of genetic defects that severely impair the development and function of T cells. Because the T cell activates other cells of the immune system (B cells, monocytes), a defect in T cells confers an associated defect in B cells - hence the term “combined” immune deficiency. The T lymphocyte, or T cell, is an essential part of the immune response: it protects against infectious pathogens, contributes to immunity against cancer and aids in preventing self-reactive processes such as autoimmunity. What is severe combined immunodeficiency? We describe the epidemiology, diagnosis and screening of this disorder and provide a brief summary of available treatments ( Box 1). This review informs physicians who may treat newborns in their practice (e.g., family physicians, obstetricians, pediatricians) on how to approach patients and counsel families who are faced with an abnormal screen for severe combined immunodeficiency. The screening program has since expanded to the Maritime provinces and is being established in several other provinces and territories. 9 In 2013, a newborn screening program for severe combined immunodeficiency was introduced in Ontario, following an application made by Immunodeficiency Canada, a national nonprofit advocacy organization with a mission to study and cure inherited immunodeficiency. 8 This assay was established in 2005 and is carried out on dried blood spots (also known as the Guthrie card), which are already taken as part of the routine newborn screen. 7 The clear need for early diagnosis and treatment led to the development of a newborn screen for severe combined immunodeficiency, known as the T-cell receptor excision circle assay. In Canada, severe combined immunodeficiency is diagnosed at a mean age of 4.2 months and carries a mortality rate of 30%, with nearly 60% of deaths caused by infection before patients are able to receive a transplant. 3, 4 Early diagnosis is essential, as patients who receive a transplant before 3.5 months of age have the best outcomes. Infants with severe combined immunodeficiency commonly appear healthy at birth consequently, the condition often presents only when the child has already had many infections and secondary organ damage. 1, 2 The condition is fatal early in life if affected infants do not receive therapy to restore immune function in the form of hematopoietic stem cell transplantation, enzyme replacement therapy or gene therapy. Severe combined immunodeficiency is caused by genetic defects that profoundly impair development of the immune system. Infants with a positive newborn screen for severe combined immunodeficiency require referral to a clinical immunologist, as well as precautionary measures against infection while they undergo evaluation. The newborn screen for severe combined immunodeficiency evaluates for T-cell lymphopenia using a T-cell receptor excision circle assay, which can be performed on the dried newborn blood spot. Outcomes for severe combined immunodeficiency are greatly improved by early diagnosis and treatment. Severe combined immunodeficiency is a medical emergency that can potentially be cured by hematopoietic stem cell transplantation or gene therapy (in specific diseases).
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