<p>There is considerable confusion regarding the diagnosis of patients presenting with non-affective psychosis in the absence of a dementia or secondary to a general medical condition in the fifth decade and beyond. A number of different terms, diagnostic criteria and age-cut-offs have been applied to this presentation posing a challenge to clinicians and researchers alike. Despite diagnostic inconsistencies and conceptual uncertainty, a remarkably consistent clinical picture has emerged. However, many questions still remain with regards to its underlying etiopathophysiological mechanisms, treatment and prognosis, including whether it is distinct from schizophrenia and whether it might be a prelude to cognitive deterioration. Currently there is no official diagnostic designation for patients who develop a primary psychosis in late life, with patients being typically diagnosed as either schizophrenia or delusional disorder, although the validity of such a distinction has been questioned.</p> <p>The following prospective longitudinal study sets out to characterize the largest known group of patients (n=102) with first-episode, late-onset (>age 40) psychotic disorder on demographic, clinical, treatment and prognostic variables. Given that one of the most contentious issues in the characterization of these patients has been that of diagnostic classification, we examined whether the currently nosological distinction of schizophrenia (SCZ) from delusional disorder (DD) has validity and/or utility. Patients were classified as either SCZ (n=47) or DD (n=55) according to DSM-IV criteria, and were then compared on a number of validators proposed as part of the DSM-V development process. As predicted, there were no significant differences between the two groups. In conclusion, our analysis did not find the current diagnostic distinction of SCZ from DD in the late-onset population to be valid and/or useful. We recommend the use of the more general diagnostic term, “Late-Onset Psychotic Disorder”, to refer to all patients who develop a primary psychosis in their forties or beyond.</p> / Master of Science (MSc)