Almost any physical illness can give rise to delirium in elderly patients. The most common physical causes are listed in Table..!. In many cases the underlying cause is not obvious, and the delirium may be the most prominent presenting feature. The aetiology is commonly multifactorial, 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.

Table 1 Common causes of delirium in elderly patients

Drugs are a particularly important cause of delirium in elderly patients, due primarily to age-associated changes in the distribution, metabolism, and excretion of drugs. These pharmacokinetic changes with age are very variable, with the result that toxicity at apparently therapeutic doses is not predictable. Certain drugs and drug groups are particularly prone to cause delirium in elderly patients, for example those agents having 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/1.3 Patients with Alzheimer's disease are particularly prone to develop delirium when given anticholinergic drugs, perhaps because their central cholinergic function is already impaired. Indeed, the pronounced deliriogenic effect of anticholinergic drugs supports the hypothesis that delirium is also the result of derangement of central cholinergic systems in the brain.

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 the stress response via the hypothalamic-pituitary-adrenal axis.

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