Longitudinal Clinical and Functional Outcomes in Adult-Onset Delusional Disorder and Schizophrenia: A 10-Year...

The distinction between delusional disorder (DD) and schizophrenia (SZ) remains debated, and onset age is an often-overlooked prognostic factor. We hypothesized that SZ would show greater diagnostic stability than DD, while functional and cognitive outcomes would be broadly comparable, with clinical differences emerging over time.

Diagnostic stability over a decade was examined in an age-matched first-episode DD and SZ cohort (n = 142). 10-year clinical, functional, and cognitive outcomes were then compared among participants who retained their original diagnosis throughout follow-up (n = 111). Missing data were addressed using multiple imputation, with sensitivity analyses conducted in complete cases (n = 82) and deceased-excluded cases (n = 100).

Over 10 years, 41% (29/71) of participants initially diagnosed with DD transitioned to SZ, compared with 3% (2/71) of those initially diagnosed with SZ who experienced a diagnostic shift. Among participants who retained their original diagnosis throughout follow-up (n = 111), SZ was associated with more relapses (B = 1.27, P = .012), hospitalizations (B = 0.47, P = .050), hallucinations (B = 3.35, P = .012), and perseverative errors on the modified Wisconsin Card Sorting Test (B = 4.31, P = .040), whereas DD showed poorer insight (Scale to Assess Unawareness of Mental Disorder: B = -1.38, P = .002). Sensitivity analyses yielded consistent results.

These findings support a psychosis-spectrum model in which stable DD and SZ show substantial longitudinal overlap in functioning and cognition, despite differences in diagnostic stability and selected clinical outcomes.