Thought Disorder Index: a longitudinal study of severity levels and schizophrenia factors.

BACKGROUND Thought disturbances are commonly associated with psychiatric disorders, especially schizophrenia. Our aim was to clarify whether thought disorders are only stable at certain severity levels and in the presence of certain schizophrenia factors of the Thought Disorder Index (TDI) scale. Furthermore, we also examined the significance of genetic status and the psychiatric disorder for the persistence of TDI severity levels and factors. METHODS The thought disorders of 158 adoptees genetically at high-risk or low-risk for schizophrenia participating the Finnish Adoptive Family Study of Schizophrenia were evaluated twice at a mean interval of 11 years. Thought Disorder Index (TDI) was used to assess the severity levels and schizophrenia factors. TDI identifies 23 different items of thinking disturbances, which are weighted along a continuum of severity. RESULTS Thought disorders at the 0.50 and 0.75 severity levels and idiosyncratic verbalization indicative of the schizophrenia factors turned out to be stable phenomena throughout the follow-up period. The adoptees' genetic or psychiatric status was not associated with the results. CONCLUSIONS The study shows that the stability of TDI seems to be related to the most severe categories of thought disorders. However, of the specific schizophrenia factors, idiosyncratic verbalization, but not confusion and fluid thinking, showed stability over time. Although idiosyncratic verbalization does not necessarily represent the most severe type of thought disturbance, it turned out relatively stable and we can speculate that idiosyncratic verbalization have some predictive value, too.