A 2-month-old infant born to non-consanguineous parents presents with severe cyanosis (SpO₂ 65%) and poor feeding. Echocardiography confirms tetralogy of Fallot with severe right ventricular outflow tract obstruction. The parents inquire about the presence of associated genetic syndromes. Which investigation is most appropriate to screen for chromosomal and genetic abnormalities in this neonate?

Explanation

## Genetic Screening in Tetralogy of Fallot **Key Point:** Karyotyping with chromosomal microarray analysis is the most appropriate initial investigation to screen for genetic and chromosomal abnormalities associated with TOF, particularly 22q11 deletion syndrome. ### Association of TOF with Genetic Syndromes **High-Yield:** TOF is associated with multiple genetic conditions: - **22q11 deletion syndrome (DiGeorge syndrome)**: Present in 15–30% of TOF cases; most common genetic association - **Trisomy 21 (Down syndrome)**: Present in 10–15% of TOF cases - **Trisomy 18 (Edwards syndrome)** and **Trisomy 13 (Patau syndrome)**: Less common but significant - **CHARGE syndrome, Williams syndrome, Alagille syndrome**: Other syndromic associations ### Why Karyotyping with Microarray is Optimal **Clinical Pearl:** Karyotyping with chromosomal microarray provides: - Detection of numerical chromosomal abnormalities (trisomies, monosomies) - Detection of submicroscopic deletions and duplications (including 22q11 deletion) - Comprehensive screening in a single test - Cost-effective approach for initial genetic evaluation - High sensitivity (>99%) for clinically significant abnormalities - Standard-of-care first-tier investigation for congenital heart disease with suspected genetic etiology ### Comparison of Genetic Investigations | Investigation | Detects | Utility in TOF | Limitation | |---|---|---|---| | **Karyotyping + Microarray** | Numerical abnormalities, submicroscopic deletions/duplications | First-tier; detects 22q11 deletion, trisomies | Requires 1–2 weeks for results | | **FISH for 22q11** | Only 22q11 microdeletion | Targeted; faster results (24–48 hrs) | Misses other genetic abnormalities; not comprehensive | | **Whole exome sequencing** | Single-gene mutations, rare variants | Useful if microarray negative and syndromic features present | Expensive; time-consuming; requires interpretation; not first-line | | **Targeted cardiac gene panel** | Mutations in cardiac transcription factors (TBX1, NKX2-5, etc.) | Useful for familial TOF or syndromic cases | Misses chromosomal abnormalities; not comprehensive | **Mnemonic:** **MICRO-ARRAY** = **M**ultiple abnormalities, **I**nitial test, **C**ost-effective, **R**apid (relatively), **O**verall best, **A**bnormalities detected (numerical + submicroscopic), **R**ecommended first-tier, **R**obust sensitivity, **A**ssesses all chromosomes, **Y**ields comprehensive data. ### Clinical Approach ```mermaid flowchart TD A[Infant with TOF diagnosed]:::outcome --> B{Syndromic features or family history?}:::decision B -->|Yes or Suspected| C[Karyotyping + Chromosomal Microarray]:::action B -->|No syndromic features| D[Consider karyotyping + microarray anyway]:::action C --> E{Abnormality detected?}:::decision E -->|Yes: 22q11 deletion| F[Confirm with FISH if needed; Genetic counseling]:::action E -->|Yes: Trisomy| G[Manage accordingly; Genetic counseling]:::action E -->|No abnormality| H[Consider WES or targeted panel if syndromic]:::action ``` **Tip:** Even in the absence of obvious syndromic features, karyotyping with microarray should be offered to all infants with TOF because genetic abnormalities (especially 22q11 deletion) have implications for management, prognosis, and family counseling.