Does formal thought disorder differ among patients with schizophrenic, schizophreniform and manic schizoaffective disorders?

The aim of this study was to examine whether specific forms of formal thought disorder distinguish schizophrenia from schizophreniform and schizoaffective psychoses. The sample was composed of 82 consecutively admitted patients with schizophrenic symptoms. Of these, 28 had a diagnosis of schizophrenia by RDC and DSM-IIIR criteria, 28 a diagnosis of schizophrenia by RDC but not DSM-IIIR (consequently they were labeled schizophreniform), and 26 a diagnosis of manic schizoaffective disorder by RDC criteria. They were assessed by a semi-structured interview for schizophrenia, by scales for positive and negative symptoms (SAPS and SANS) and by prognostic indicators. The assessment of thought disorder was carried out by using the Thought, Language and Communication scale (TLC). The schizophrenic patients showed higher global scores on formal thought disorder, and some of its subtypes were 'most specific' to schizophrenia (poverty of speech, poverty of content of speech, illogicality, tangentiality and perseveration). Schizophrenics had more loose associations than schizophreniforms. Manic schizoaffectives had higher scores on positive versus negative formal thought disorder than schizophreniforms. We suggested that the assessment of disordered thinking by the TLC may facilitate the differential diagnosis of psychotic disorders during the acute phase of the illness.