Diagnostic Yields Across Phenotypes

1. A 6-month-old with severe developmental delay, seizures, and no family history presents for genetic evaluation. Both parents are available for testing. Which testing strategy is best supported by current evidence?

1. A.Single-gene testing for the most common epilepsy genes, then a gene panel if those return negative
2. B.Chromosomal microarray alone — it is the only recommended first-tier test for neurodevelopmental disorders
3. C.Trio exome or genome sequencing (proband + both parents) — approximately doubles yield for de novo conditions✓
4. D.Sequential testing: CMA first, then gene panel, then exome — always proceeding stepwise to minimize cost

For severe early-onset neurodevelopmental phenotypes (DEE, GDD with seizures), trio sequencing is strongly supported by evidence. The key advantage is detecting de novo variants — spontaneous mutations present in the child but absent from both parents — which are the primary cause of many severe neurogenetic conditions. Meta-analysis shows trio sequencing approximately doubles yield compared to singleton testing (OR ~2.04). Trio analysis also enables phasing of compound heterozygous variants and parent-of-origin determination. While CMA remains valuable as a complementary test (detecting large CNVs, aneuploidy, UPD), it alone would miss the majority of diagnoses in this clinical context. Sequential testing delays diagnosis unnecessarily in severe early-onset presentations.

2. Two published studies report WES diagnostic yields for intellectual disability: Study A reports 45% yield, while Study B reports 20% yield. Both used trio WES with comparable bioinformatics. What is the most likely explanation for the difference?

1. A.Study A must have used a more sensitive sequencing machine with deeper coverage across the exome
2. B.Study A likely enrolled more severe and syndromic patients from a referral centre, while Study B enrolled a broader population✓
3. C.Study B must have had technical problems with sequencing quality or bioinformatic variant calling
4. D.The difference is statistically random and has no meaningful clinical or methodological explanation

Cohort selection is the single largest driver of yield variation across published genomic studies. Referral-centre or specialty-clinic cohorts are enriched for complex, treatment-resistant, or multi-system disease — these patients are more likely to have identifiable Mendelian conditions, driving yields of 40–60%+. Population-level or primary-care cohorts that include milder, less specific phenotypes yield 15–25% for the same test. Before citing any yield number, always ask: 'Who was in the denominator?' Other variables — severity, syndromic burden, younger age at onset, and comorbid epilepsy — all independently predict higher yield. Platform and bioinformatics differences exist but introduce smaller variations (5–10% relative).

3. A teenager with progressive ataxia and sensory neuropathy has whole exome sequencing that returns negative. Which critical next step should the clinician consider?

1. A.Accept the WES as final — if no variant was found, a genetic cause is excluded in this patient
2. B.Repeat the WES at a different laboratory with deeper coverage for a second opinion on variant calls
3. C.Order dedicated repeat expansion testing (Friedreich ataxia, SCA, CANVAS)✓
4. D.Proceed directly to whole genome sequencing

This is one of the most important 'testing blind spots' in clinical neurogenetics. The most common hereditary ataxias worldwide — Friedreich ataxia (FXN GAA repeat), spinocerebellar ataxias (SCA types with CAG repeats), and CANVAS (RFC1 AAGGG repeat) — are all caused by trinucleotide or pentanucleotide repeat expansions. Standard whole exome sequencing does NOT detect these expansions, and even standard short-read whole genome sequencing has variable sensitivity. Dedicated testing (repeat-primed PCR, Southern blot, or long-read sequencing) is required. This presentation — progressive ataxia with sensory neuropathy — is particularly suggestive of Friedreich ataxia or CANVAS. A negative WES in an ataxia patient should always prompt the question: 'Has dedicated repeat expansion testing been ordered?'

4. A colleague argues that chromosomal microarray (CMA) is obsolete now that whole exome sequencing is widely available. Which statement best counters this argument?

1. A.CMA is indeed obsolete — WES detects everything CMA can detect plus SNVs, making CMA redundant
2. B.CMA uniquely detects CNVs, aneuploidy, and (on SNP platforms) UPD and autozygosity — complementary to WES✓
3. C.CMA should only be used when exome sequencing is unavailable or unaffordable at the testing centre
4. D.CMA is only useful for prenatal testing and has no established role in postnatal neurogenetics

CMA is NOT obsolete — it provides information that WES does not. CMA detects copy number variants (CNVs) at higher resolution and sensitivity than WES-based CNV calling. Critically, SNP-array CMA detects uniparental disomy (UPD) and regions of autozygosity (AOH) — essential for diagnosing imprinting disorders (Angelman syndrome, Prader-Willi syndrome) and identifying consanguinity. CMA yields ~10% across broad NDD cohorts and is particularly productive in multiple congenital anomalies (15–25%), infantile spasms (14%), and ID. Many centres now run CMA + WES simultaneously rather than sequentially. While WES can detect some CNVs, its sensitivity for CNVs in segmental duplications and complex regions is lower than dedicated CMA.

5. Among the following pediatric neurogenetics phenotypes, which consistently shows the HIGHEST diagnostic yield from exome/genome sequencing?

1. A.Isolated autism spectrum disorder without intellectual disability or epilepsy
2. B.New-onset focal epilepsy in a child with otherwise normal development
3. C.Leukodystrophy with a characteristic MRI pattern guiding genetic testing✓
4. D.Mild global developmental delay presenting after age 3 without seizures

Leukodystrophy in MRI-selected cohorts achieves among the highest diagnostic yields in all of clinical neurogenetics: WES yields 50–72%, and dedicated WGS programmes report 72–90%+. The GWMD cohort (Neurology 2022, n=126) achieved 72% overall, 77% for onset <3 years, and 85% in the hypomyelination subgroup. The key reason: MRI pattern recognition dramatically narrows the differential diagnosis before sequencing begins. Categorizing the white matter abnormality (hypomyelination vs. demyelination vs. cystic vs. vacuolating) directs testing to the appropriate gene-disease context. By contrast, isolated ASD without ID has the lowest NDD yield (~10–15% WES), and mild GDD yields less than broad NDD cohorts. This illustrates a general principle: the more phenotypically specific and severe the presentation, the higher the diagnostic yield.

6. Rapid whole genome sequencing in NICU patients with unexplained encephalopathy has a diagnostic yield of 35–50%. Beyond identifying a diagnosis, what makes this one of the most impactful applications of genomic testing in pediatrics?

1. A.The diagnoses are primarily useful for genetic counseling and family planning but rarely change acute management
2. B.A molecular diagnosis changed clinical management in 38–50% of diagnosed NICU cases — the highest utility in pediatric genetics✓
3. C.NICU testing is valuable mainly because it is faster than standard WES, even though the clinical impact is comparable to other settings
4. D.Clinical utility is similar across all phenotypes — approximately 10% of diagnoses lead to management changes regardless of clinical setting

The NICU setting demonstrates the strongest evidence for clinical utility of genomic testing. Multiple studies, including the Maron et al. 2023 JAMA RCT, show that a molecular diagnosis changes clinical management in 38–50% of diagnosed neonatal cases. Management changes include: initiation of targeted therapy (e.g., ketogenic diet for GLUT1 deficiency, enzyme replacement for Pompe disease), avoidance of harmful medications, redirection of care when a devastating prognosis is confirmed, and surgical planning decisions. These are among the highest clinical utility figures in any medical context. Speed matters — rapid WGS (results in days rather than weeks) allows these management changes during the critical neonatal period. Approximately 18% of NICU admissions are estimated to carry a Mendelian disease, making this a high-impact target population.