**3.1.2 Neurological disorders**

Dementia is a major predisposing factor for delirium, a meta-analysis suggesting a relative risk of 5.2 (Elie et al., 1998). Fick et al. reported that approximately 45% of patients with dementia develop delirium during hospitalization (2002). Elderly patients with dementia are at higher risk for developing delirium not only because they have the usual age-related decrease in acetylcholine described previously, but also have a focal loss of acetylcholine due to death of the cholinergic cells in the nucleus basalis of Meynert as a result of the disease process (Tune & Egeli, 1999).

In a study that included patients over the age of 65 years admitted to hospital with a fractured neck of femur, cognitive impairment which was measured by the Mini-Mental State Examination (MMSE), has been found to be the most significant predisposing factor for the development of delirium (Freter et al., 2005).

Other neurological causes are cerebrovascular diseases (thrombosis, embolism, arteritis, hemorrhage, hypertensive encephalopathy), degenerative disorders (multiple sclerosis), epilepsy, head trauma, space-occupying lesions (tumor, subdural hematoma, abscess, aneurysm) and encephalitis (Michaud et al., 2007; Fong et al., 2009).
