In a 3‑year‑old female undergoing repair of tetralogy of Fallot, what is the condition called when the thymus is absent and what associated anomalies are typical?

DiGeorge Syndrome (Complete DiGeorge Syndrome)

The absence of thymus gland discovered during tetralogy of Fallot repair in a 3-year-old is most consistent with DiGeorge syndrome (DGS), specifically complete DiGeorge syndrome when thymic aplasia is present. This represents a critical finding with life-threatening implications for immune function and requires immediate immunologic evaluation and consideration for thymus transplantation 1.

What This Condition Is Called

The clinical diagnosis is DiGeorge syndrome (DGS), defined by the classic triad of:

• Thymic hypoplasia/aplasia (as found in your patient)
• Hypoparathyroidism
• Congenital heart disease (tetralogy of Fallot in this case)

When complete thymic aplasia is present (as discovered at surgery), this was historically termed "complete DiGeorge syndrome" and occurs in <1% of 22q11.2 deletion syndrome cases 1. This represents the most severe end of the spectrum with profound T-cell immunodeficiency.

Associated Anomalies

Cardiac Anomalies (Already Present)

• Conotruncal defects are most common, including tetralogy of Fallot (as in your patient), truncus arteriosus, interrupted aortic arch
• Right aortic arch occurs in approximately 42% of cases 2
• The presence of conotruncal defects with complete DGS carries higher mortality risk (45% die before transplantation) 3

Parathyroid Dysfunction

• Hypoparathyroidism with hypocalcemia is a hallmark feature
• Critical to monitor calcium levels perioperatively and postoperatively 4
• Can cause seizures if untreated

Craniofacial Abnormalities

• Hypertelorism, low-set ears, small mouth, short philtrum, micrognathia 2
• Palatal defects, especially velopharyngeal deficiency 5
• These features result from abnormal neural crest cell migration affecting pharyngeal arch development 1

Immunologic Features (Critical in Complete DGS)

• Complete absence of T cells (T-B+NK+ immunophenotype) due to thymic aplasia 1
• Severe combined immunodeficiency-like presentation
• Fatal without thymus or bone marrow transplantation 3
• Recurrent, severe infections are the primary cause of death 3

Genitourinary Abnormalities (15% of cases)

• Renal anomalies: hydronephrosis, unilateral renal agenesis, multicystic dysplastic kidney 4
• Genital anomalies more common in males (cryptorchidism, hypospadias) 4

Hematologic Issues

• Thrombocytopenia with increased platelet volume 4
• Increased bleeding risk perioperatively
• Risk of autoimmune cytopenias (20% develop autoimmunity) 4

Developmental and Neuropsychiatric

• Developmental delays are nearly universal 6
• Increased risk of psychiatric disorders later in life 6, 5

Most Common Genetic Cause

22q11.2 deletion syndrome (22q11.2DS) is the most frequent etiology, occurring in 1:4000 live births 1. The deletion involves the TBX1 gene, which regulates pharyngeal arch development. However, the second most common cause is CHARGE syndrome (CHD7 gene mutations), which includes coloboma, heart defects, choanal atresia, growth/developmental retardation, genitourinary anomalies, and ear anomalies 1.

Critical Management Implications

Immediate Perioperative Concerns

• Monitor calcium levels closely - hypocalcemia can cause seizures 4
• Check platelet counts - bleeding risk 4
• Anticipate difficult intubation - smaller equipment may be needed 4
• Blood products must be irradiated if severe T-cell deficiency confirmed 4

Urgent Immunologic Evaluation Required

This child needs immediate T-cell evaluation to determine severity:

• Absolute T-cell counts (CD3+, CD4+, CD8+)
• T-cell proliferation studies
• TREC levels (will be absent/severely reduced in complete athymia) 1
• Immunoglobulin levels

Life-Saving Treatment Consideration

Thymus transplantation is the standard of care for complete DiGeorge syndrome and should be pursued urgently 1. Survival with thymus transplantation is approximately 75%, which is superior to hematopoietic cell transplantation (which has poorer outcomes and severe graft-versus-host disease risk) 1. The main cause of death is infection before immune reconstitution is established 1.

Common Pitfall

Do not delay genetic testing and immunologic workup. The mortality rate for complete DGS with congenital heart disease is 67% overall, with infection being the leading cause of death both before and after transplantation 3. Conotruncal defects (like TOF) are associated with higher pre-transplant mortality (45%) compared to other cardiac defects 3.

The combination of absent thymus at cardiac surgery plus tetralogy of Fallot creates a high-risk scenario requiring multidisciplinary coordination between cardiology, immunology, genetics, and potentially transplant services.