ADHD and schizophrenia: Mere prodromal variant or homogeneous subgroup?

Introduction: Attention deficit hyperactivity disorder (ADHD) diagnosed in childhood is associated with a relative
risk of 4.74 (95 % CI, 4.11–5.46) for developing schizophrenia spectrum disorder (SSD) later in life; if other
comorbidities exist the risk is 2.1-fold higher. There is no guideline on treating ADHD in SSD and no research on
the effect of this combination on length of inpatient treatment, type of pharmacotherapy and employment status.
This study aims to further explore the role of ADHD in SSD.
Methods: Latent Class Analysis (LCA) uses no a priori assumptions in testing for homogeneous subgroups within a
data sample of 2871 inpatient treatment cases of SSD from three psychiatric hospitals. Data was extracted from
case files and statistical reports to the federal statistical office.
Results: Two subgroups are identified. One primarily consists of individuals with SSD and ADHD (estimated
population size of 3 %). In comparison to the other subgroup with SSD and no ADHD (97 %), these individuals
more frequently have other mental comorbidities, especially substance use disorders, are unemployed and about
half are administered stimulants. All studied individuals were administered antipsychotics and length of inpatient
stay was similar in both subgroups.
Conclusion: ADHD and SSD define a subgroup of individuals with specific treatment needs and additional burden
of disease. ADHD is more than an initial misdiagnosis or random precursor disease of SSD. Treating psychiatrists
seem to frequently administer stimulants.