Thymic aplasia

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Thymic aplasia


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USMLE® Step 1 questions

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Thymic aplasia

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A 10-day-old boy is brought to the emergency department for evaluation of generalized tonic-clonic seizures. The parents note recurrent upper and lower extremity muscle spasms. He has had no fever, cough, or runny nose. The patient was born full term via an uncomplicated pregnancy and normal vaginal delivery. Family history is unremarkable. Physical examination demonstrates a neonate with rhythmic jerking movements of the upper and lower of extremities bilaterally. The anterior fontanelle is open, pupils are reactive to light, and fundi are normal. Facial examination reveals a small lower jaw, low set ears, and a submucosal cleft palate. Lungs are clear to auscultation bilaterally. Cardiac auscultation reveals a harsh holosystolic murmur at the left lower sternal border. Chest x-ray demonstrates decreased soft tissue attenuation in the right anterior mediastinum. Which of the following immunologic derrangements is most likely to be observed in this patient?

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In thymic hypoplasia, thymic refers to the thymus which is an immune organ that sits between the lungs, hypo- refers to under, and -plasia refers to development.

So, thymic hypoplasia is a condition where the thymus is underdeveloped and has a reduced number of cells.

By week 4 of development, the embryo takes on a more recognizably “human” form—but to be honest, it still looks more like a shrimp than a baby.

At the head end of this little shrimp-like creature, a set of structures called the pharyngeal apparatus begins to develop, consisting of pharyngeal arches, clefts, and pouches.

The components of the pharyngeal apparatus develop into various head and neck structures, and sometimes multiple arches join together to give rise to a single structure.

Now, the epithelial tissue of the embryo’s third and fourth pouch turns into the inferior parathyroid glands and superior parathyroid glands, while the epithelial tissue that lines the ventral region of the third pouch forms the thymus.

Both glands then go on to break off from the pharyngeal wall and eventually attach to the posterior side of the thyroid.

The thymus now free, migrates down the middle of the pharynx, until it ends up in its final position in the front of the thorax where it fuses with its counterpart from the opposite side.

During childhood, the thymus occupies considerable room behind the sternum, in a part of the chest known as the mediastinum, a space in the chest between the lungs that also contains the heart.

But when people become older, it atrophies and is replaced by fatty tissue.

It's here in the thymus, where certain immune cells from the bone marrow mature into T lymphocytes or T cells, where the T stands for Thymus.


Thymic hypoplasia is a condition in which the thymus is underdeveloped or involuted, leading to a reduced number of T cells. There are two main causes of thymic hypoplasia, DiGeorge syndrome, and Ataxia-telangiectasia syndrome, both cause a reduction of thymic cells. In DiGeorge syndrome the parathyroid glands are underdeveloped, this results in less parathyroid hormones which results in hypocalcemia. On the other hand, in Ataxia-telangiectasia syndrome, the parathyroid glands develop normally, this results in normal levels of parathyroid hormones which results in normal levels of calcium in the blood.

Symptoms of thymic hypoplasia include repeated infections because of a weakened immune system due to a T cell deficiency. In DiGeorge syndrome, there are also symptoms caused by hypocalcemia such as osteoporosis and tetany, or involuntary contraction of muscles. Other symptoms associated with this disorder are congenital cardiac defects like tetralogy of Fallot, cleft palate, and learning disabilities. In ataxia telangiectasia, there can also be symptoms like ataxia which is a problem with muscle coordination, telangiectasia, which are dilated blood vessels in the skin and eye, and an increased risk of cancer.

Treatment is focused on managing symptoms and preventing complications, and can include antibiotics and antiviral medications to prevent infections, nutritional support to promote growth, and surgical interventions for cardiac abnormalities or cleft palate.


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