Schizophrenia in adults with developmental disability

Adults with intellectual or developmental disabilities (IDD) — a group that includes intellectual disability, autism spectrum disorder, and several genetic syndromes — develop schizophrenia at rates several times higher than the general population. The exact figures vary by definition and population, but most reviews put the lifetime prevalence of psychotic disorders in adults with IDD at roughly 3–5%, compared with about 1% in the general population. Despite the elevated rate, schizophrenia in this population is often missed, mistreated, or mislabelled — frequently because of diagnostic overshadowing, the well-documented tendency for clinicians to attribute new symptoms to the pre-existing disability rather than to a separate condition.

Why the rate is higher

Several reasons converge:

• Genetic overlap. Some genetic syndromes — most notably 22q11.2 deletion syndrome — carry both intellectual disability and a substantially elevated risk of schizophrenia (see our 22q11 article).
• Neurodevelopmental vulnerability. Brain regions implicated in schizophrenia overlap with those affected in many IDD conditions.
• Adverse experiences. Adults with IDD experience high rates of trauma, abuse, and bullying, which contribute to risk.
• Substance use. Although less common, substance use in this population is often unrecognised.

Diagnostic overshadowing

This is the central clinical problem. New auditory hallucinations may be dismissed as "self-talk." Paranoia may be attributed to "rigidity." Disorganised speech may be ascribed to baseline communication style. The result is that a treatable psychotic illness goes unrecognised for years.

Clinicians who treat this population well do several things:

• Establish a careful baseline of communication and behaviour, ideally from caregivers who have known the person for years
• Look for change from baseline — new behaviours, lost skills, new fears, new content of speech
• Use validated assessment tools adapted for IDD, such as the Psychiatric Assessment Schedule for Adults with Developmental Disability or the DM-ID-3
• Take adequate time — assessment often requires multiple visits
• Triangulate with multiple informants (family, day program, residential staff)

Symptom presentation

Schizophrenia in adults with IDD can look slightly different:

• Positive symptoms may be less elaborate or less verbalised. A non-verbal person may begin staring at empty corners or covering ears.
• Negative symptoms can be especially hard to distinguish from baseline.
• Disorganisation may show up as new agitation, sleep disruption, or loss of previously mastered self-care tasks.
• Behavioural change — new aggression or self-injury — is sometimes the first sign of an underlying psychotic episode.

Treatment principles

Start low, go slow

Adults with IDD are often more sensitive to side effects and tolerate dose changes less well. Lower starting doses and slower titration reduce the risk of adverse effects driving discontinuation.

Avoid polypharmacy when possible

Many adults with IDD arrive in psychiatric care already on multiple medications, including layered antipsychotics, mood stabilisers, and benzodiazepines. Each medication adds side effects and complicates assessment. Periodic, structured medication reviews — sometimes called "deprescribing" reviews — are recommended by groups including the CDC and AAIDD.

Watch for tardive dyskinesia and metabolic effects

Adults with IDD are at higher risk of long-term side effects, in part because antipsychotics are sometimes continued indefinitely without re-evaluation. Annual AIMS exams and metabolic monitoring are standard of care.

Avoid using antipsychotics for behavioural control alone

Behavioural symptoms in IDD often have non-psychiatric causes — pain, constipation, dental problems, sensory overload, environmental change. The FDA boxed warning against use of antipsychotics for dementia-related agitation has parallels in IDD: psychotropic use without a clear psychiatric indication is widely overused.

Communication-adapted therapy

Adapted CBT for psychosis, simplified language psychoeducation, and visual supports work better than standard talk therapy. Therapists experienced with IDD know how to scale the work.

Coordination across systems

Care for adults with IDD often involves multiple systems — Medicaid waiver services, day programs, residential providers, primary care, and psychiatry. A coordinated care plan with one shared document and one identified care coordinator makes a measurable difference. The Arc and state Developmental Disabilities Councils can help families navigate.

Family and supporter perspective

Families who have parented an adult with IDD into a new diagnosis of schizophrenia often describe a particular grief — not the same as a single new diagnosis, but a layered one. They also describe the relief, sometimes years later, of finally having a name for what they had been observing. Both can be true at once.

Resources

• American Association on Intellectual and Developmental Disabilities
• The Arc
• National Association for the Dually Diagnosed (NADD)
• State Developmental Disabilities agencies

See related articles on 22q11 deletion syndrome, schizophrenia and autism, and childhood-onset schizophrenia.

The bottom line

Adults with intellectual and developmental disability deserve the same careful diagnostic work and the same evidence-based treatment as anyone else. The combination of overshadowing and undertreatment has historically meant they get neither. The fix is mostly procedural — longer visits, multiple informants, validated tools, careful prescribing — and within reach of any clinic that takes the time.

This article is for educational purposes only and is not medical advice, diagnosis, or treatment. Always consult a qualified mental health professional. If you or someone you know is in crisis, call or text 988 in the US, or your local emergency number.