12 Intellectual disability12.2 Epidemiology
12.2.2 Occurrence of coexisting disorders and impairments
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Written by Resident Doctor Committee · Published
New in the 2027 syllabus
The College added this topic for exams from January 2027.
Telling the outcomes apart
Coexisting disorders include mental illness, behaviour that challenges, epilepsy, sensory loss and physical impairment. Each needs its own definition and its own method of assessment.
Behaviour that challenges describes an interaction between a person and their circumstances. It does not by itself show psychiatric illness.
Point prevalence counts episodes that are already going on as well as new ones.
Incidence counts only new episodes that start during follow-up.
The two measures answer different questions, so neither can stand in for the other.
Evidence from a follow-up study
A population-based cohort of Scottish adults reported these two-year incidence figures in 2007. They added to the cohort's earlier prevalence findings.
Outcome | Two-year incidence |
|---|---|
Mental ill health, including problem behaviours | 16.3% |
Mental ill health, not including problem behaviours | 12.6% |
Problem behaviours | 4.6% |
The categories overlap, so adding the percentages would count some people twice.
Follow-up retention was 70% (651 participants). Loss to follow-up and the historical setting limit how far the results apply to other populations.
A two-year percentage is not a yearly rate. Do not report it as an annual incidence.
Worked example: incidence and prevalence
These invented figures concern new depressive episodes in adults with intellectual disability. Assume complete one-year follow-up, no deaths and no repeat episodes counted.
Item | Number |
|---|---|
Adults assessed at the start | 100 |
Adults already in a depressive episode at the start | 10 |
New episodes among the other adults | 9 |
Find who is at risk of a new episode. People already depressed are not, so 100 - 10 = 90 adults.
Work out the one-year incidence proportion: 9 / 90 = 0.10, or 10%.
Keep the starting prevalence separate: 10 / 100 = 10%. The percentage is the same, but it answers a different question.
Other impairments
Hearing and visual impairment can limit communication and participation. Assessment should separate an unrecognised sensory problem from psychiatric withdrawal or a loss of understanding.
Epilepsy, dysphagia (difficulty swallowing) and mobility problems can coexist with mental illness and change the support the person needs. They are not diagnostic groups that rule each other out.
Physical health monitoring includes direct assessment and accessible annual health checks, because routine records can miss problems the person cannot easily describe.
Applying study figures: before using a study estimate for an individual or a service, consider age, severity of disability, any syndrome, the setting and how the diagnosis was made.