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Although there are many types of dementia, they have a number of features in common. This section contains a discussion of dementia in general and brief descriptions of some of its more common types.

Definition of dementia
The essential features of a dementia are multiple cognitive deficits that include memory impairment and at least one of the following: aphasia, apraxia, agnosia, or a disturbance in executive functioning (the ability to think abstractly and to plan, initiate, sequence, monitor, and stop complex behavior). The order of onset and relative prominence of the cognitive disturbances and associated symptoms vary with the specific type of dementia, as is discussed below:

Memory impairment is generally a prominent early symptom. Individuals with dementia have difficulty learning new material and may lose valuables, such as wallets and keys, or forget food cooking on the stove. In more severe dementia, individuals also forget previously learned material, including the names of loved ones. Individuals with dementia may have difficulty with spatial tasks, such as navigating around the house or in the immediate neighborhood (where difficulties with memory are unlikely to play a role). Poor judgment and poor insight are common as well. Individuals may exhibit little or no awareness of memory loss or other cognitive abnormalities. They may make unrealistic assessments of their abilities and make plans that are not congruent with their deficits and prognosis (eg.planning to start a new business). They may underestimate the risks involved in activities (eg.driving). In order to make a diagnosis of dementia, the cognitive deficits must be sufficiently severe to cause impairment in occupational or social functioning and must represent a decline from a previous level of functioning. The nature and degree of impairment are variable and often depend on the particular social setting of the individual. For example, mild cognitive impairment may significantly impair an individual’s ability to perform a complex job but not a less demanding one.

Associated features
Some individuals with dementia show disinhibited behavior, including making inappropriate jokes, neglecting personal hygiene, exhibiting undue familiarity with strangers, or disregarding conventional rules of social conduct. Occasionally, they may harm others by striking out. Suicidal behavior may occur, especially in mildly impaired individuals, who are more likely to have insight into their deficits and to be capable of formulating (and carrying out) a plan of action. Anxiety is fairly common, and some patients manifest “Catastrophic reactions”, overwhelming emotional responses to relatively minor stressors, such as changes in routine or environment. Depressed mood, with or without neurovegetative changes, is quite common, as are sleep disturbances independent of depression. Delusions can occur, especially those involving themes of persecution (eg., the belief that misplaced possessions have been stolen). Misidentifications of familiar people as unfamiliar (or vice versa) frequently occur. Hallucinations can occur in all sensory modalities, but visual hallucinations are most common. Some patients exhibit a peak period of agitation (or other behavioral disturbances) during the evening hours, which is sometimes referred to as “Sundowning”.

Delirium is frequently superimposed on dementia because the underlying brain disease increases susceptibility to the effects of medications or concurrent general medical conditions. Individuals with dementia may also be especially vulnerable to psychosocial stressors (eg. going to the hospital, bereavement, etc.), which may exacerbate their intellectual deficits and associated problems.

Dementia is sometimes accompanied by motor disturbances, which may include gait difficulties, slurred speech, and a variety of abnormal movements. Other neurological symptoms, such as myoclonus and seizures, may also occur.

Dementia due to other causes
In addition to the preceding categories, a number of general medical conditions can cause dementia. These conditions include structural lesions (primary or secondary brain tumor, subdural hematoma, slowly progressive or normal-pressure hydrocephalus), head trauma, endocrine conditions (hypothyroidism, hypercalcemia, hypoglycemia), nutritional conditions (deficiency of thiamin, niacin, or vitamin B12), other infectious conditions (HIV, neurosyphilis, cryptococcus), derangements of renal and hepatic function, neurological conditions (eg., multiple sclerosis), effects of medications (eg. benzodiazepines, ß blockers, diphenhydramine), and the toxic effect of long–standing substance abuse, especially alcohol abuse. It is critical that psychiatrists caring for demented individuals be familiar with the general medical and neurological causes of dementia in order to assure that the diagnosis is accurate and, in particular, that potentially treatable conditions are not missed.