Genetic Testing in Intellectual Disability

Definition And Clinical Context

Intellectual disability (ID) is a neurodevelopmental disorder defined by limitations in intellectual functioning (IQ <70-75) and adaptive behavior, with onset before 18 years of age.
Global developmental delay (GDD) describes children under 5 years presenting with delays in two or more domains (motor, language, cognition, social).
Genetic etiology accounts for 30-50% of all ID cases, escalating to 50-80% in severe or syndromic presentations.

Comprehensive clinical evaluation is mandatory prior to testing to maximize diagnostic utility and minimize the burden of Variants of Uncertain Significance (VUS).
Detailed developmental history assessing static versus regressive trajectories and specific behavioral phenotypes.
Three-generation pedigree formulation to evaluate consanguinity, miscarriages, and affected relatives.
Systematic physical examination documenting dysmorphology using standardized Human Phenotype Ontology terms, growth parameters, and neurocutaneous stigmata.
Execution of ancillary assessments including vision/hearing screening, electroencephalogram (EEG), and brain magnetic resonance imaging (MRI) based on focal signs or regression.
Exclusion of non-genetic etiologies such as perinatal asphyxia, congenital infections (TORCH), and toxin exposure.

Modern diagnostic pathways emphasize a tiered approach, utilizing advanced genomic interrogation over traditional cytogenetics.

Testing Tier	Modality	Clinical Indications	Diagnostic Yield
Tier 1: First-Line Agnostic	Chromosomal Microarray (CMA)	Unexplained GDD/ID, autism spectrum disorder, congenital anomalies.	15-20%.
Tier 1: First-Line Agnostic	Exome Sequencing (ES) / Genome Sequencing (GS)	Unexplained ID/GDD; Trio testing (proband and parents) preferred.	38-63%.
Tier 1: Targeted	Fragile X Testing (FMR1 PCR)	All males with nonsyndromic ID; females with suggestive family history.	~1-2%.
Tier 2: Phenotype-Driven	Metabolic Screening	Regression, multisystem involvement, abnormal MRI findings.	1-5%.
Tier 2: Phenotype-Driven	Targeted Gene Panels	Specific clinical clues (e.g., epilepsy, skeletal dysplasia).	Variable.
Tier 3: Advanced/Residual	Iterative Reanalysis	Negative Tier 1/2 results; reanalyze ES/GS data every 1-2 years.	Increases yield by 10-15%.
Tier 3: Advanced/Residual	Methylation / MLPA	Suspected imprinting disorders (e.g., Prader-Willi/Angelman syndromes).	Syndrome specific.

Chromosomal Microarray (CMA): Single Nucleotide Polymorphism (SNP) array is the preferred platform. It detects Copy Number Variants (CNVs) and Regions of Homozygosity (ROH). Identifying ROH is critical in consanguineous families to pinpoint autosomal recessive conditions.
Exome And Genome Sequencing (ES/GS): Captures protein-coding regions (ES) or the entire genome (GS). Utilizing a trio analysis rapidly filters benign familial variants and efficiently identifies de novo and compound heterozygous mutations.
Fragile X Testing: Evaluates CGG repeat expansions. It is essential because standard ES poorly covers repeat expansion disorders.

Clinical management is directly altered in 50-60% of diagnosed cases.
Enables precision interventions, including enzyme replacement therapy, dietary modification, and gene therapy.
Directs specific surveillance protocols for known comorbidities, such as tumor screening or cardiac monitoring.
Defines exact recurrence risks (e.g., 1% for de novo, 25% for autosomal recessive) and opens avenues for preimplantation genetic testing and prenatal diagnosis.

Variant classification relies strictly on the American College of Medical Genetics and Genomics / Association for Molecular Pathology (ACMG/AMP) guidelines.
Pre-test counseling must address the 81 ACMG actionable genes regarding secondary or incidental findings.
A major challenge is the high rate of VUS, requiring expert interpretation and periodic clinical re-review.
Negative genetic testing does not exclude a genetic etiology; standard of care dictates continuous reanalysis of sequencing data as genomic knowledge evolves.