Real-world effectiveness of clozapine for intellectual disability: Results from a mirror-image and a...

Comorbid mental illness is common in patients with intellectual disability. Antipsychotics are widely used for these conditions, but the effect of clozapine remains largely unknown.

We aimed to investigate the effectiveness of clozapine on naturalistic outcomes in patients with intellectual disability.

By combining the national health registers, we identified all patients in Denmark with intellectual disability initiating clozapine treatment during the period 1996-2012. We used a mirror-image model to test whether initiation of clozapine treatment was associated with reduction in psychiatric admissions and inpatient days, reduction in the number of individuals performing intentional self-harm or overdose, and less frequent use of concomitant psychopharmacological treatment. Similar outcome measures were used in a reverse mirror-image model to investigate the effects of clozapine termination.

A total of 405 patients with intellectual disability redeemed clozapine. After initiation of clozapine the number of psychiatric admissions were reduced by 0.65 admissions (95% CI: 0.31-1.00) and the inpatient days were reduced by 67.2 days (95% CI: 51.2-83.3), with a similar decrease for patients with intellectual disability without psychiatric comorbidity. Clozapine treatment was not found to reduce the number of individuals with intentional self-harm, incidents of overdose, or the use of concomitant psychotropics. In cases where clozapine treatment was terminated, the number of psychiatric admissions increased by 0.57 admissions (95% CI: 0.01-1.12).

This nationwide study, which is the largest to date, suggests that treatment with clozapine is associated with a reduction in psychiatric admissions and inpatient days in patients with intellectual disability. Further studies evaluating the effects of clozapine in patients with intellectual disability are warranted.