Sophisticated epidemiological data are now available for most psychiatric disorders. Accurate data on incidence, prevalence, and mortality in delirium, however, are difficult to come by. The comparison of estimates across studies is hampered by methodological differences. Methods of case finding and diagnosis will influence rates obtained, as will the patient population and the setting (community, general medical, surgical, or geriatric inpatient) in which the disorder is diagnosed.17 Most incidence and prevalence studies of delirium have been conducted in inpatient settings. One notable exception is the Eastern Baltimore Mental Survey, a large community based survey forming part of the Epidemiologic Catchment Area (ECA) programme.18 This study aimed to look at the prevalence of delirium in the general adult population aged between 18 and 64 years. A total of 810 individuals were subjected to psychiatric evaluation and a point prevalence rate of 04% was calculated, rising to 11% for those aged 55 years and over.

Whereas prevalence rates in the community are low, in hospital settings they are high. It has been estimated that 10% of all medical and surgical inpatients meet criteria for delirium at some point during their stay.4 Delirium may therefore represent the mental disorder with the single highest incidence.19

As with community subjects, advanced age is a potent risk factor for delirium in inpatient surveys. Among elderly general medical inpatients, rates of 30%20 or even 50%21 have been reported, although a more conservative estimate of 17%22 is probably more realistic. Rates of delirium in elderly surgical patients are broadly similar, although following operation for hip fracture the rate may be as high as 50%.23

A pre-existing dementia seems also to be a risk factor, independent of age. A large Finnish study24 found that in 2000 patients aged 55 and over admitted to medical wards, delirium was present in over 40% of those patients with evidence for an established dementia, compared with 15% on admission for the group as a whole.

The diagnosis of delirium carries a significant mortality rate.6 It is significantly higher than non-delirious controls during hospital admission and long term follow up. In most studies, hospital mortality in the elderly varies from 8% to 16% compared to 1-5% in controls.25-28 A few studies have an even higher mortality, up to 65%.29,30 One explanation for this is that they tend to include younger patients, who usually require a severe underlying illness to produce delirium. In the elderly the higher mortality persists even up to two years later with a mortality of 39% compared to 23% in controls.31 Some studies have found delirium to be an independent predictor of both hospital28 and long term mortality.30 Other studies, however, have found that delirium is not associated with increased mortality when confounding factors such as severity of acute illness, cognitive impairment, and co-morbid disease were taken into account.25,27 It is important to stress that the successful treatment of delirium eliminates much of the excess hospital mortality. This underlines the importance of early detection and urgent treatment.

The course of the symptoms has been found in some studies to be much less transient than previously acknowledged. In one study only 18% of patients had resolved all new symptoms six months after discharge.14 Delirium has also been found to be associated with prolonged hospital stay, increased risk of hospital acquired complications, and increased risk of admission to institutional care.27

In the elderly, delirium is best thought of as a multifactorial condition similar to other old age conditions, such as falls or incontinence. Risk factors can be divided into vulnerability factors (present on admission) and precipitating factors (acquired during hospital stay). Most studies have examined both factors simultaneously; they are certainly highly interrelated. Those patients with a high number of vulnerability factors only require a small precipitating insult to produce delirium, whereas those with a low vulnerability require a much greater insult to produce the same effect.32

Despite using different populations and definitions, there is consensus between studies about three vulnerability factors: pre-existing cognitive impairment, severe chronic illnesses, and functional impairment.33 The precipitating factors commonly mentioned in the literature include medications (particularly those with psychoactive and/or anticholinergic properties), infections, metabolic disturbances, alcohol withdrawal, dehydration, and malnutrition. Factors that reduce mobility, such as intravenous lines, indwelling catheters, and use of physical restraints (in the United States), have also been suggested as having a precipitant role.

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