This chapter has reviewed the evidence regarding the diagnosis of schizophrenia. There are several areas where evidence is consolidated, others where evidence is uncertain, and quite a few that are completely open to research.

Consistent Evidence

It is now well established that schizophrenia is a heterogeneous group of syndromes which differ in symptomatology, course and outcome. Although some of the symptoms are more characteristic of the disorder than others, there are no specific or pathognomonic symptoms of schizophrenia. Characteristic symptoms may be subdivided into positive and negative symptoms. Prominent among positive symptoms are hallucinations, delusions and major disturbances in the form of thought. Prominent among negative symptoms are affective flattening, avolition and alogia.

Standardized and structured instruments are available for the assessment of characteristic signs and symptoms and for making a diagnosis of schizophrenia.

There is convincing evidence that schizophrenia is a disorder of all ages. Although onset of the disorder is usually in adolescence or early adulthood, schizophrenia may start in childhood and in late life. It has also been clearly established that there usually have been non-psychotic symptoms, often for several years, before the appearance of characteristic psychotic symptoms. Prominent pre-psychotic manifestations include symptoms related to impairment of cognitive functioning, in particular impairment of attention.

Outcome studies have consistently shown that the course of schizophrenia is poor in at least 50% of cases, but that a substantial proportion of patients make good, if not complete, recoveries from the disorder.

Incomplete Evidence

The current definitions of schizophrenia, as described in the explicit diagnostic criteria provided in ICD-10 and DSM-IV, are, at best, validated in part only. Owing to the polythetic nature of the symptom criteria, both the ICD-10 and the DSM-IV criteria define a group of disorders which is possibly extremely heterogeneous.

Although there is some evidence to support the distinction between the paranoid subtype and the non-paranoid subtypes, there is little evidence to support the differentiation of the individual classical subtypes of schizophrenia. Alternatives to the classical subtyping of schizophrenia, such as the three-factor dimensional descriptors that are proposed in DSM-IV to describe current and lifetime symptomatology, have not been fully investigated up to now.

While there is substantial evidence supporting the importance of a differentiation between positive and negative symptoms, there is considerably less evidence supporting the type I-type II or positive-negative dichotomy of schizophrenia. In particular, further studies are needed to determine whether the development of prominent negative symptoms justifies a diagnosis of schizophrenia (simple schizophrenia) in a patient who has never shown any positive symptoms.

Inclusion of prodromal or residual signs and symptoms in the definition of the disorder remains controversial, since no prodromal or residual manifestations have as yet been identified that are typical and specific of schizophrenia.

Attempts to differentiate schizophrenia more clearly from non-schizophrenic psychotic disorders, either acute or chronic, have not been conclusive. There also remain unsolved questions with regard to the distinction between schizophrenia and mood disorders, whenever schizophrenic and mood symptoms are present, either simultaneously or consecutively, in the same patient.

There is, up to now, only limited information on schizotypal disorder, and additional studies are needed to clarify the relationship between schizophrenia, schizotypal disorder and the other schizophrenia spectrum personality disorders.

Areas Still Open to Research

Very promising results have been obtained on biological indicators, such as abnormalities in eye movement, electrodermal activity, event-related brain potentials, disturbances in attention and information processing, and brain imaging. However, the importance of these abnormalities for diagnostic purposes remains, at best, controversial. The validity of the same abnormalities as trait markers for schizophrenia also needs to be confirmed by further studies.

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