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Key Features

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Essentials of Diagnosis
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  • Onset in first year of life

  • Recurrent infections caused by bacteria, viruses, fungi, and opportunistic pathogens

  • Chronic diarrhea and failure to thrive

  • Absent lymphoid tissue

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General Considerations
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  • Defining characteristic is severe deficiency of T-cell function and number

  • Due to the centrality of T cells in the immune system, the severity of T-cell deficit results in widespread immunologic dysfunction and broad susceptibility to infection

  • Suspected SCID is a medical emergency; confirming diagnosis and initiating treatment must be done rapidly

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Clinical Findings

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  • Frequently presents with opportunistic, unusual, and persistent infection

  • Common organisms include but are not limited to Pneumocystis jirovecii, candida, and cytomegalovirus

  • In the absence of an identified microorganism, SCID may present with any combination of the following: failure to thrive, chronic diarrhea, or unexplained chronic respiratory illness

  • Physical examination is notable for a lack of lymphoid tissue, including tonsils and lymph nodes

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Diagnosis

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Laboratory Findings
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  • Characteristic feature is the deficiency in production of T cells

  • Associated laboratory findings can include decreases in numbers of NK cells and B cells, poor lymphocyte proliferative response to mitogens, and low immunoglobulin levels

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Imaging
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  • Chest radiograph usually demonstrates an absent thymic shadow

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Diagnostic Procedures
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  • Genetic testing should be pursued to confirm the diagnosis; treatment should not be delayed while awaiting results

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Treatment
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  • A variety of therapies are used, including

    • Hematopoietic stem cell transplantation (HSCT)

    • Gene therapy

    • Thymus transplant

    • Enzyme replacement

  • Choice of therapy depends on the specific genetic defect, age at diagnosis, access to a suitable HSCT donor, and comorbidities

  • Replacement Ig therapy should be initiated

  • Patients in whom SCID is suspected should not receive any live vaccines

  • If transfusion is needed, patients should only receive CMV-negative, irradiated blood products

  • If the patient has received BCG vaccination, specific therapy should be considered

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Outcome
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Prevention
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  • Antimicrobial prophylaxis should be initiated with the aim of preventing pulmonary infection with Pneumocystis as well as other fungal pathogens

  • Antiviral prophylaxis can be considered as well

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Prognosis
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  • Left untreated, SCID uniformly results in death before the first year of life

  • Transplant outcomes are favorable if performed within the first 3 months of life, or if performed prior to the onset of SCID-associated chronic infections

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References

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Pai  SY  et al: Transplantation outcomes for severe combined immunodeficiency, 2000-2009. N Engl J Med 2014;371:434–446
[PubMed: 25075835]
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Rivers  L, Gaspar  HB: Severe combined immunodeficiency: recent developments and guidance on clinical management. Arch Dis Child 2015;100:667–672
[PubMed: 25564533]

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