Intellectual Developmental Disorder and Specific Learning Disorders

Risk factors

• Genetic syndromes: Down syndrome (trisomy 21), Fragile X (most common heritable cause of IDD), Prader-Willi, Williams, velocardiofacial (22q11.2 deletion), Rett syndrome, tuberous sclerosis
• Prenatal: fetal alcohol spectrum disorder (leading preventable cause), maternal infection (CMV, rubella, Zika), maternal phenylketonuria, malnutrition
• Perinatal: hypoxic-ischemic encephalopathy, kernicterus, prematurity
• Postnatal: lead poisoning, traumatic brain injury, meningitis, encephalitis, severe malnutrition
• SLD: family history of SLD, prematurity, prenatal nicotine exposure, low socioeconomic status (interacts with biology)

Pathophysiology

IDD is a heterogeneous endpoint of any insult disrupting brain development. SLD: convergent evidence implicates left-hemisphere temporo-parietal and occipito-temporal circuitry in dyslexia (reduced activation during phonologic processing); intraparietal sulcus in dyscalculia. Strong heritability for both.

Differential diagnosis

• Global developmental delay (<5 yo) — Used when child <5 cannot complete standardized testing; reclassify when older
• Autism spectrum disorder — Social-communication deficits and restricted/repetitive behaviors; IQ can be normal or low; SLD or IDD may coexist
• ADHD — Inattention impairs academic performance but underlying ability intact; SLD may coexist
• Hearing or vision impairment — Sensory deficit accounts for difficulty; rule out at any new learning concern
• Inadequate instruction or English language learner — Difficulty resolves with appropriate teaching; SLD by definition is not explained by these
• Anxiety, depression, trauma — Acquired performance drop; emotional features predominate
• Acquired neurologic injury (TBI, seizure) — History clarifies onset

Diagnostic workup

Diagnostic algorithm

Complications

• IDD: increased risk of seizures, sensory impairment, mental health comorbidity (~30-40%), behavioral disturbance, abuse and exploitation
• SLD: school dropout, employment limitations, secondary depression and anxiety, low self-esteem
• Healthcare disparities: diagnostic overshadowing (attributing new symptoms to IDD), reduced cancer screening, reduced preventive care
• Polypharmacy and metabolic syndrome in patients receiving atypical antipsychotics for behavior

PANCE pearls

• DSM-5-TR uses 'intellectual developmental disorder'; IDEA and many systems still use 'intellectual disability' — the entities are equivalent.
• Severity in IDD is determined by adaptive functioning, NOT IQ.
• Fragile X is the most common heritable cause of IDD; fetal alcohol spectrum disorder is the most common preventable cause.
• Chromosomal microarray is first-line genetic testing for unexplained IDD.
• Dyslexia is not a disorder of vision — it is a phonologic processing deficit; eye exercises do not treat it.
• Early reading intervention (kindergarten-2nd grade) is dramatically more effective than late intervention.
• Patients with IDD have shortened life expectancy and underrecognized medical issues — avoid diagnostic overshadowing.

References

• DSM-5-TR — American Psychiatric Association. DSM-5-TR. 2022.
• AAP 2014 — Moeschler JB, Shevell M; AAP Committee on Genetics. Comprehensive evaluation of the child with intellectual disability or global developmental delays. Pediatrics 2014;134:e903-18.
• IDEA — Individuals with Disabilities Education Act, 20 U.S.C. § 1400 (2004) — federal special education law providing IEP and 504 frameworks.
• AACAP 2011 — American Academy of Child and Adolescent Psychiatry. Practice Parameter for the Assessment and Treatment of Children and Adolescents With Intellectual Disability. J Am Acad Child Adolesc Psychiatry 2011;50:801.