In the ICD-10, the diagnosis of schizophrenia depends upon the presence of characteristic symptoms, a minimum duration of those symptoms, and a differentiation from affective, other psychotic, organic or substance-induced disorders. The ICD-10 diagnostic criteria for research of schizophrenia are shown in Table 1.1.

Table 1.1 ICD-10 research diagnostic criteria for schizophrenia

G1. Either at least one of the syndromes, symptoms, and signs listed under (1) below, or at least two of the symptoms and signs listed under (2) should be present for most of the time during an episode of psychotic illness lasting for at least 1 month (or at some time during most of the days).

(1) At least one of the following must be present:

(a) thought echo, thought insertion or withdrawal, or thought broadcasting;

(b) delusions of control, influence, or passivity, clearly referred to body or limb movements or specific thoughts, actions, or sensations; delusional perception;

(c) hallucinatory voices giving a running commentary on the patient's behaviour, or discussing the patient among themselves, or other types of hallucinatory voices coming from some part of the body;

(d) persistent delusions of other kinds that are culturally inappropriate and completely impossible (e.g. being able to control the weather, or being in communication with aliens from another world).

(2) Or at least two of the following:

(a) persistent hallucinations in any modality, when occurring every day for at least 1 month, when accompanied by delusions (which may be fleeting or half-formed) without clear affective content, or when accompanied by persistent over-valued ideas;

(b) neologisms, breaks, or interpolations in the train of thought, resulting in incoherence or irrelevant speech;

(c) catatonic behaviour, such as excitement, posturing or waxy flexibility, negativism, mutism, and stupor;

(d) ''negative'' symptoms, such as marked apathy, paucity of speech, and blunting or incongruity of emotional responses (it must be clear that these are not due to depression or to neuroleptic medication).

G2. Most commonly used exclusion clauses

(1) If the patient also meets criteria for manic episode (F30.—) or depressive episode (F32.—), the criteria listed under G1 (1) and G1 (2) above must have been met before the disturbance of mood developed;

(2) The disorder is not attributable to organic brain disease (in the sense of F00-F09), or to alcohol- or drug-related intoxication (F1x.0), dependence (F1x.2), or withdrawal (F1x.3 and F1x.4).

The ICD-10 recognizes that no strictly pathognomonic symptoms can be identified. Characteristic symptoms are divided into eight groups that have special importance for the diagnosis and often occur together. The first four groups are comprised of typical delusions and hallucinations; the last four groups are comprised of less typical delusions and hallucinations, as well as neologisms, breaks or interpolations in the train of thought, catatonic behaviour, and negative symptoms such as marked apathy, paucity of speech, and blunting or incongruity of emotional responses.

The delusions and hallucinations described in the first three groups are all Schneiderian first-rank symptoms, while the delusions listed in the fourth group are defined as culturally inappropriate and completely impossible (they probably correspond to the definition of bizarre delusions in other classifications).

The requirement for a diagnosis of schizophrenia is that a minimum of one symptom belonging to any one of the first four groups, or symptoms from at least two of the last four groups, should have been clearly present for most of the time during a period of one month or more.

While adopting a one-month duration of typical psychotic symptoms, the ICD-10 rejects the assumption that schizophrenia must be of comparatively long duration. According to the authors of the ICD-10, a substantial proportion of patients who have clear and typical schizophrenic symptoms lasting for more than one month but less than 6 months, have been shown to make good, if not complete, recoveries from the disorder. According to the authors of the ICD-10, in the present state of ignorance, there appear to be no advantages in restricting the diagnosis of schizophrenia by specifying a longer overall duration of the disorder and it seems best to avoid any assumptions about necessary chronicity.

The authors of the ICD-10 recognize that before the appearance of typical schizophrenic symptoms, there may be a period of weeks or months characterized by the appearance of a prodrome of non-specific symptoms. If a prodrome typical of and specific to schizophrenia could be identified, described reliably, and shown to be uncommon in patients with other psychiatric disorders and in subjects with no disorder at all, it would be justifiable to include such a prodrome among the optional criteria for schizophrenia. It was considered that insufficient information was available on this issue at present to justify the inclusion of a prodromal state as a contributor to the diagnosis in ICD-10.

As a general rule, interference with the performance of social roles has not been used as a diagnostic criterion in ICD-10. This decision has been retained for the diagnosis of schizophrenia, with the exception of simple schizophrenia, in which marked decline in social, scholastic or occupational performance is part of the concept.

According to ICD-10, the course of schizophrenia is variable. The course may be specified after at least one year has elapsed since onset of the disorder. The following patterns are listed: continuous; episodic with progressive deficit; episodic with stable deficit; episodic remittent; incomplete remission; complete remission; other.

Concerning differential diagnosis, the ICD-10 provides clear guidelines on how to differentiate schizophrenia from affective and schizoaffective disorders, other transient or persistent psychotic disorders, as well as any organic or substance-induced mental disorder. The diagnosis of schizophrenia should not be made in the presence of extensive depressive or manic symptoms unless it is clear that schizophrenic symptoms antedated the affective disturbance. If both schizophrenic and affective symptoms develop together and are evenly balanced, the diagnosis of schizoaffective disorder should be made, even if the schizophrenic symptoms by themselves would have justified the diagnosis of schizophrenia.

The separation of acute and transient psychotic disorders and schizophrenia rests upon the presence and duration of symptoms of the schizophrenic type. If such symptoms are present and persist for more than one month, the diagnosis should be changed to schizophrenia.

In persistent delusional disorders, delusions must be present for at least 3 months and the general criteria for schizophrenia are not fulfilled.

Finally, schizophrenia should not be diagnosed in the presence of overt brain disease or during states of drug intoxication or withdrawal.

The ICD-10 describes seven subtypes of schizophrenia: paranoid, hebe-phrenic, catatonic, post-schizophrenic depression, undifferentiated, residual and simple.

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