The three expressions of the clinical syndrome of dementia have been well documented: cognitive deficits—amnesia, aphasia, apraxia, and agnosia; neuropsychiatric features—a heterogeneous array of psychiatric symptoms and behavioural disturbances such as depression, delusions, hallucinations, misidentifications, aggression, agitation, wandering, collectively described as neuropsychiatric features, behavioural and psychological symptoms of dementia (BPSD),1 or non-cognitive features2; and problems withactivities of daily living. The history of interest in the neuropsychiatry of dementia is relatively short by comparison with research into cognitive dysfunction. Psychiatric symptomatology was only first described in detail in the 1980s and 1990s and has only recently been the subject of standardised and reliable methods of assessment (for example, the neuropsychiatric inventory3). In the paper by Holmes et al (this issue pp 777–779),4 the field takes a significant step forward in identifying some of the biological determinants of the expression of neuropsychiatric symptoms in Alzheimer's disease, the commonest cause of dementia.

Some aetiological factors have been implicated in the genesis of neuropsychiatric features. In Alzheimer's disease, associations have been described between the degree of neuronal loss and the histological changes of Alzheimer's disease with the presence of behaviours such as aggression and hypermetamorphosis. Changes in the aminergic brain stem nuclei are more pronounced in patients with Alzheimer's disease who have had depression. Increased sophistication of the measurement of neuropsychiatric features has emphasised that assessments of their phenomenology and occurrence are essentially drawn from the reports of caregivers. It is known that the environment in which a patient finds him or herself is a potent predictor of the presence of some behaviours (such as agitation) and often the interaction between a patient and carer (whether this be a paid or informal carer) can promote a reaction which can easily be interpreted and recorded as indicating the presence of a psychiatric symptom. Sensory deprivation such as poor vision and poor hearing can promote the presence of visual hallucinations and paranoid beliefs respectively.

The availability and ease of measurement of genetic markers in Alzheimer's disease has led to investigations examining the association between these biological markers and psychiatric symptoms.5

The importance of neuropsychiatric features in dementia are that they are very distressing to patients and carers, they are amenable to both environmental and pharmacological interventions, they may help in the differential diagnosis of the causes of dementia, and they may shed light on biological substrates of phenomenology in so called functional psychiatric disorders. They underscore the important role of the psychiatrist in the assessment and management of the dementias and, increasingly, in the understanding of the biological substrates of phenomenology.