Thymic hypoplasia is a disorder in which there is underdevelopment of the thymus gland. The thymus is located under the sternum, or breast bone, and it forms part of the immune system. Thymic hypoplasia can make people more prone to infections and it can occur as part of a congenital condition, which is present from birth, known as DiGeorge syndrome. DiGeorge syndrome may be associated with problems such as heart defects, facial abnormalities, seizures and learning difficulties. The degree of thymic hypoplasia can vary from moderate to severe and in some cases the thymus may be missing.
As part of the immune system, the thymus gland produces white blood cells known as T lymphocytes, which are involved in the body's response to infection. In most people, the thymus only develops until adolescence. From then on, the amount of cell growth decreases and the thymus begins to shrink, with the active cells slowly being replaced by scar tissue and fat. In spite of this shrinkage, the gland still produces lymphocytes and continues to do so into old age.
Fortunately, only a small proportion of people with DiGeorge syndrome have thymic aplasia, where there is no gland, or hypoplasia, which is severe enough to leave them prone to serious infections. For those with moderate thymic hypoplasia, recurrent infections may be a problem but these are more likely to be less serious illnesses such as colds. Many children with moderate thymic hypoplasia may even find that the function of their immune system improves as they get older.
Where the gland's function is severely impaired, the treatment of thymic hypoplasia could involve a bone marrow transplant or a transplant of thymus tissues from a donor. Sometimes, immunoglobulin therapy may be carried out. This involves giving immunoglobulins, or antibodies, extracted from donated blood plasma. Antibodies are special proteins which help to defend the body from infection, and this type of treatment is given into a vein.
The outlook for a person with thymic hypoplasia partly depends on the degree of loss of thymus function. It is also affected by the extent to which other organs, such as the heart, are defective, as part of DiGeorge syndrome. Problems of behavior, mental health and development may also occur as part of the condition, and may make it more difficult to manage. Usually a team of experts drawn from a number of disciplines is required to provide appropriate care.