Until recently, delirium was classified as part of a group of 'organic brain disorders' in both the main international classifications. However, differences between them have emerged with the advent of ICD-10 and DSM-IV. DSM-IV dropped the section on 'Organic mental syndromes and disorders', which had appeared in previous editions, because 'the term organic mental disorders ... incorrectly implies that 'non-organic' mental disorders do not have a biological basis'. DSM-IV introduced a new general category of 'Delirium, dementia, amnestic and other cognitive disorders' grouped into three sections.

1. Dementia, delirium and amnestic and other cognitive disorders.

2. Mental disorders due to a general medical condition.

3. Substance-related disorders.

Relevant DSM-IV codings of subcategories include:

• 291.0 delirium due to alcohol (intoxication or withdrawal)

• 292.0 delirium due to other substances (intoxication or withdrawal)

• 293.0 delirium due to a general metabolic condition, delirium due to multiple aetiologies

• 780.09: delirium not otherwise specified.

The category 'delirium due to multiple aetiologies' is useful, as it reflects clinical experience that delirium often results from several factors (e.g. dehydration, fever, and polypharmacy).

In contrast, ICD-10 retains the general term 'Organic mental disorders'. ICD diagnostic criteria for the presence of delirium are somewhat more restrictive than those of DSM. Symptoms have to be present in all the following areas:

• impairment of consciousness

• global disturbance of cognition

• psychomotor disturbances

• disturbance of sleep-wake cycle

• emotional disturbance.

Like DSM, ICD-10 also has subcategories for the various causes and types of delirium. Delirium due to substance use is coded F1 x.0 (intoxication) or F1 x.3 (withdrawal), where x denotes a numerical code for the particular substance concerned. There are further subcodings to denote the presence or absence of convulsions and of dependence. ICD-10's other main category of delirium is defined by exclusion: 'F05, Delirium, not induced by alcohol and other psychoactive substances'. There is also a subcategory 'F05.1 Delirium, superimposed on dementia'. Other categories are described very generally only ('F05.0 Delirium, not superimposed on dementia'; 'F05.8 Other delirium', 'F05.9 Delirium, unspecified').

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