In ICD-10, the dementias are embedded in the section on organic and symptomatic mental disorders (codes F00-F09) which contains the following major rubrics:

• Dementia in Alzheimer’s disease.
  
• Vascular dementia.
  
• Dementia in diseases classified elsewhere.
  
• Unspecified dementia.
  
• Organic amnestic syndrome, other than induced by alcohol and drugs.
  
• Delirium, other than induced by alcohol and drugs.
  
• Other mental disorders due to brain damage and dysfunction and to physical disease.
  
• Personality and behavioural disorders due to brain disease, damage and dysfunction
  
• Unspecified organic or symptomatic mental disorder.

In contrast to ICD-9, the distinction between psychotic and non-psychotic illnesses is of no taxonomic consequence in ICD-10, where disorders of different psychopathological expression are grouped together on the basis of established or presumed common aetiology. In the particular instance of section F0, in which the dementing disorders are included, the underlying classificatory characteristic of “organic” is defined in the sense that “the syndrome so classified can be attributed to an independently diagnosable cerebral or systemic disease or disorder”. The subsidiary term “symptomatic” is not used in the titles of individual disorders but it is included in the overall title of the block F00 – F09. This is because it is widely used in many countries to indicate those organic mental disorders in which cerebral involvement is secondary to a systemic extra-cerebral disease or disorder. In other words, ‘symptomatic’ in this context is a subdivision of the wider term ‘organic’.

Another feature of ICD-10, as compared to earlier classifications, is the omission of any reference to age as a defining criterion of the disorders accompanied by a cognitive deficit. The terms “senile” and “presenile” are practically absent in the classification, and there is no provision for identifying any mental disorder as necessarily a result of aging. The classification does, however, allow the recording of an unusually early or late onset of the disorder. In other words, the mental disorders occurring in the elderly are no longer considered to belong in a separate category of morbidity. This is very much in line with research conducted in the past three decades which has demonstrated that the relatively high prevalence of mental morbidity in the elderly in Western cultures is related to a wide range of psychosocial factors (e.g. social isolation, cultural uprooting and institutionalization), as well as to physical comorbidity, but that the aging process itself does not produce nosologically specific forms of disorders.

If section F0 of ICD-10 is used as a diagnostic decision tree, there is a choice of five entry points at the level of clinical syndrome: (i) dementia; (ii) amnesic syndrome; (iii) delirium; (iv) organic quasifunctional disorder (affective, delusional, hallucinatory or other); and (v) personality or behavioural disorder. Once a disorder is identified at this general syndrome level, the next step is defined by the diagnostic guidelines which lead into more specific diagnostic categories. The diagnostic decision rules for dementia illustrate the point.

The syndrome of dementia is defined in ICD-10 by “evidence of a decline in both memory and thinking, which is of a degree sufficient to impair functioning in daily living”, in a setting of clear consciousness. For a confident diagnosis to be established, such disturbances should have been present for at least 6 months. Deterioration of emotional control, social behaviour and motivation represent significant additional features but the overriding criterion is the presence of memory, learning and reasoning decline. The ICD-10 DCR (research criteria) add anchor points for a grading of the deficits into mild, moderate and severe, separately for memory and intellectual capacity. The overall grading of the severity of dementia is made on the basis of the function which is more severely impaired.

Once the presence of the syndrome of dementia is established, the diagnostic process branches off into the different clinical varieties of dementia typical of Alzheimer’s disease, vascular dementia, and dementia in diseases classified elsewhere (including Pick’s disease, Creutzfeldt-Jakob disease, Huntington’s disease, Parkinson’s disease, HIV disease, and a range of systemic and infectious diseases such as hepatolenticular degeneration, lupus erythematosus, trypanosomiasis, and general paresis). Dementia in Alzheimer’s disease is subdivided into Type I (onset after the age of 65) and Type 2 (onset before the age of 65). Although the ICD-10-DCR criteria emphasize the ultimate criterion of the neuropathological examination and the supporting role of brain imaging, they nevertheless allow for a confident clinical diagnosis to be made, if clear evidence of a memory and intellectual performance deterioration has been present for 6 months or more. The ICD-10 criteria for vascular dementia are broader than the corresponding DSM-IV criteria: they include not only multi-infarct (predominantly cortical) vascular dementia, but also the subcortical dementias (Binswanger’s encephalopathy being an example), as well as the mixed cortical and subcortical forms.

As regards the diagnosis of delirium, ICD-10 has abandoned the distinction between acute and subacute deliria; the condition is defined as “a unitary syndrome of variable duration and degree of severity ranging from mild to very grave”, with an upper limit of 6 months’ duration and a subdivision into delirium superimposed on dementia and delirium not superimposed on dementia.