Although delirium occurs at all ages, it is most frequently encountered in late life. This is because delirium is the result of an interaction between individual vulnerability factors (e.g. brain disease, sensory impairment) and external insults (e.g. physical illness, medication), the rates of which both increase with age. Our current concept of delirium derives principally from the florid clinical stereotype that has evolved from centuries of clinical observations on younger patients, and it may not be applicable to our historically unique ageing population. In younger adults, a major physical insult is usually necessary to precipitate delirium, which is often a dramatic disturbance. This is not the case in vulnerable elderly patients when relatively mild physical, psychological, or environmental upsets may be sufficient to bring about acute disturbances of mental functioning. These disturbances may be less obvious than in younger patients, particularly if they occur in the context of pre-existing cognitive impairment. Consequently, despite being common and problematic, delirium in elderly patients is frequently missed or misdiagnosed as dementia or depression by medical and nursing staff.⁽¹⁾ This is unfortunate, because delirium is an important non-specific sign of physical illness or intoxication, and if left untreated there may be costly consequences, both for the patient and for health services.

The clinical features of delirium are described in Chapter 4.1.1. Most delirium in elderly patients is of the quiet hypoactive variety, lacking the more florid disturbances in mood, perception, and behaviour that bring the disorder to clinical notice. Reversible cognitive impairment in elderly patients is associated with reduced conscious level, poor attention, poor contact with the patient, incoherent speech, reduced psychomotor activity, lack of awareness of surroundings,
poor orientation, and poor memory.⁽²⁾ Hyperactive delirium does occur in elderly patients, but it is less pronounced, with the overactivity usually confined to purposeless behaviour such as pulling at the bedclothes. Violent behaviour is uncommon; elderly patients are more likely to injure themselves than others.

The ICD-10 and DSM-IV diagnostic criteria for delirium are described in Chapter 4.1.1. They are not entirely concordant; ICD-10 is more restrictive, resulting in the diagnosis of fewer cases.⁽³⁾ However, the two systems agree on four essential features: disturbance of consciousness, disturbance of cognition, rapid onset/fluctuating course, and evidence of an external cause. Unfortunately, none of these features is specific for delirium as opposed to dementia, and the current diagnostic criteria are poor predictors of outcome, defined in terms of improvement in cognitive function. Reversibility of cognitive impairment may be the most discriminating feature of delirium,⁽²⁾ but is problematic as a diagnostic criterion since outcome is unknown at the outset.

Another shortcoming of the current classifications of delirium is that they do not recognize the partial and transitory disturbances that are commonly observed in elderly patients. Subsyndromal delirium is common, and is part of a continuum between normality and the full syndrome. Subsyndromal cases are clinically significant, since they have the same risk factors and the same increased mortality as syndromal cases.⁽⁴⁾

The diagnosis of delirium is a two-stage process: first, diagnose the delirium, and second, identify the underlying cause or causes. The diagnosis of delirium in elderly patients can be problematic, given the predominantly hypoactive clinical picture and the unreliability of ‘positive’ symptoms. However, it is important to consider the possibility if cognitive decline is rapid, and if any of the recognized signs and symptoms are present. A good informant history from relatives or ward staff is essential to establish the onset and course of the disorder. Routine screening procedures may be useful in identifying patients who develop delirium while in hospital. Brief instruments such as the Mini-Mental State Examination⁽⁵⁾ are not diagnostic, but will alert the clinician to any sudden decline in cognitive function. More extended diagnostic instruments are also available, such as the Delirium Rating Scale,⁽⁶⁾ the Confusion Assessment Method,⁽⁷⁾ and the Delirium Symptom Interview.⁽⁸⁾ Another approach to screening for delirium is to identify those at particular risk of developing the disorder. Predictive factors related to the patient include: visual impairment, severity of illness, cognitive impairment, and a blood urea nitrogen/creatinine ratio of 18 or more.⁽⁹⁾ Hospital- and treatment-related factors include: use of restraints, malnutrition, use of more than three medications, bladder catheterization, and the number of iatrogenic events.⁽¹⁰⁾ These factors are multiplicative in their effect.

The differential diagnosis of delirium includes most other psychiatric disorders in this age group. These disorders are themselves risk factors for delirium, so the possibility of co-morbidity must always be considered. When in doubt, investigate and manage as delirium until the situation is clear.

The community prevalence of delirium increases with age, rising to 14 per cent in those aged 85 years and older. In medical and surgical inpatients, the rates of delirium vary considerably (prevalence, 10-30 per cent; incidence, 4-53 per cent), because of methodological and population differences. Similar rates are also found in studies of acute psychogeriatric admissions.⁽¹¹⁾ Some patient groups, such as those with hip fractures, have consistently higher rates. Other at-risk populations, such as nursing home residents, have received less systematic investigation, but the available evidence
suggests that they also have rates of delirium comparable to those found in elderly inpatients.

Almost any physical illness can give rise to delirium in elderly patients. The most common physical causes are listed in Table 8.5.1.1. In many cases the underlying cause is not obvious, and the delirium may be the most prominent presenting feature. The aetiology is commonly multi-factorial, and all contributory factors need to be identified and treated. As a rule, hyperactive delirium is more commonly due to infection and toxic/withdrawal states, whereas hypoactive delirium is more commonly due to metabolic abnormalities.

Drugs are an important cause of delirium in elderly patients, due to age-associated changes in their distribution, metabolism, and excretion. These pharmacokinetic changes are very variable, with the result that toxicity at apparently therapeutic doses is unpredictable. Certain drugs are particularly prone to cause delirium in elderly patients, for example those with anticholinergic activity. Tricyclic antidepressants, thioridazine, and benzhexol are particularly toxic in this respect, but many of the drugs commonly prescribed to elderly patients have some degree of anticholinergic activity, for example, digoxin, prednisolone, cimetidine, ampicillin, and warfarin. Individually, this activity may be small, but the cumulative effect can be significant if patients are on multiple medications.⁽¹²⁾ Patients with Alzheimer’s disease are particularly prone to develop delirium when given anticholinergic drugs, perhaps because their central cholinergic function is already impaired. In a minority of particularly vulnerable elderly patients, purely environmental and psychological insults are sufficient to cause delirium. The mechanisms of action in these cases are not known, but may involve factors such as sensory deprivation and stress responses via the hypothalamic-pituitary-adrenal axis.

Traditionally, delirium has been regarded as a transient condition that proceeds to either recovery or death. In the majority of cases, the delirium is brief, but about one-third of patients have prolonged or recurrent episodes.⁽¹³⁾ Delirium is associated with increased short-term mortality in elderly patients, mainly because of the severity of the underlying illness. Delirium interferes with the processes of diagnosis, treatment, and rehabilitation, and as a result patients have longer hospital stays and higher rates of functional decline and discharge to nursing homes.⁽¹⁴⁾ Increased length of stay and mortality are particularly associated with hypoactive delirium. In general, patients with hyperactive delirium appear to be less severely ill than those with hypoactive delirium; this may be due to differences in the cause of the delirium, or to the fact that hyperactive delirium is more likely to be identified and the causes treated.

Prospective studies have shown that the prognosis, in terms of persistent or recurrent symptoms, is relatively poor in elderly patients.⁽¹⁵⁾ This is probably because those who experience delirium are a vulnerable group who are likely to develop the condition provided there is sufficient external insult. A proportion will also be suffering from a form of dementia, which will increase the vulnerability to delirium as it progresses. There is evidence that delirium is followed by persistent cognitive decline,⁽¹⁶⁾ which raises the possibility that it (or the underlying cause) is a risk factor for the development or exacerbation of dementia.

Evidence regarding the efficacy of treatments for delirium is sparse (Chapter 4.1.1). The cholinergic hypothesis of delirium raises the possibility that cholinergic agonists, such as the cholinesterase inhibitors licensed for the treatment of Alzheimer’s disease, may be of value in the prevention and treatment of delirium.

There are four important steps in the management of delirium⁽¹⁷⁾: