Intellectual disability (formerly mental retardation, terminology updated in DSM-5) is characterized by significant limitations in both intellectual functioning AND adaptive functioning, with onset during the developmental period. DSM-5 explicitly bases severity on adaptive functioning rather than IQ score alone — recognizing that real-world function matters more than test performance.
The three domains: conceptual (language, reading, writing, math, reasoning, knowledge, memory), social (empathy, communication, friendship, social judgment), practical (self-care, employment, recreation, managing money, transportation, organizing tasks). Severity ratings (mild, moderate, severe, profound) reflect support needs across these domains.
Major etiology categories:
Genetic — Down syndrome (trisomy 21), fragile X syndrome (most common inherited cause), Williams syndrome, Prader-Willi syndrome, many other genetic conditions. Genetic causes increasingly identifiable with modern sequencing.
Prenatal — fetal alcohol spectrum disorders, congenital infections (TORCH), placental insufficiency, intrauterine exposures, maternal phenylketonuria, severe maternal malnutrition.
Perinatal — birth complications, severe prematurity with intraventricular hemorrhage, kernicterus.
Postnatal — CNS infections (meningitis, encephalitis), traumatic brain injury, severe neglect, lead poisoning, metabolic disorders.
Idiopathic — substantial proportion remain without identified cause despite workup.
Co-occurring psychiatric illness is the rule. Rates of psychiatric comorbidity in ID populations are 3-4 times higher than general population: mood disorders, anxiety, ADHD, autism, psychotic disorders. The presentation often differs — communication limitations affect symptom reporting, and behavioral changes may be the presenting feature. Patients with ID can have any psychiatric disorder, and they deserve specific treatment.
Diagnostic overshadowing is the common clinical error — attributing new symptoms to the underlying ID rather than recognizing a treatable psychiatric or medical condition. The patient with ID who develops sleep disturbance, agitation, or behavioral change deserves the same workup as any other patient: pain (often overlooked in non-verbal patients), constipation, urinary retention, infection, medication side effects, environmental changes, depression, anxiety, psychosis.
Approach to medication in ID: start low, go slow. Use evidence-based agents for specific diagnoses (SSRIs for depression/anxiety, antipsychotics for psychosis, stimulants for ADHD, mood stabilizers for bipolar disorder). Address sleep and behavioral issues with behavioral interventions before medication. Polypharmacy substantially increases adverse effects in this population.
Family and caregiver support is essential. Resources, respite, advocacy through national and local organizations. The patient's social support network often determines functional outcomes more than the underlying ID severity.
When you encounter a patient with intellectual disability, the psychiatric care is real care. The patient deserves specific evaluation and treatment of any psychiatric symptoms — not generic behavioral management. Recognition matters; treatment works.