Dementia (taken from Latin, originally meaning madness, from de- (without) + ment, the root of mens (mind) is a serious loss of global cognitive ability in a previously unimpaired person, beyond what might be expected from normal ageing. It may be static, the result of a unique global brain injury, or progressive, resulting in long-term decline due to damage or disease in the body. Although dementia is far more common in the geriatric population, it can occur before the age of 65, in which case it is termed "early onset dementia".
Dementia is not a single disease, but a non-specific illness syndrome (i.e., set of signs and symptoms). Affected cognitive areas can be memory, attention, language, and problem solving. Normally, symptoms must be present for at least six months to support a diagnosis. Cognitive dysfunction of shorter duration is called delirium. In all types of general cognitive dysfunction, higher mental functions are affected first in the process.
Especially in later stages of the condition, subjects may be disoriented in time (not knowing the day, week, or even year), in place (not knowing where they are), and in person (not knowing who they, or others around them, are). Dementia, though often treatable to some degree, is usually due to causes that are progressive and incurable, as observed in primary progressive aphasia (PPA).
Symptoms of dementia can be classified as either reversible or irreversible, depending upon the etiology of the disease. Fewer than 10% of cases of dementia are due to causes that may presently be reversed with treatment. Causes include many different specific disease processes, in the same way that symptoms of organ dysfunction such as shortness of breath, jaundice, or pain are attributable to many etiologies.
Delirium can be easily confused with dementia due to similar symptoms. Delirium is characterized by a sudden onset, fluctuating course, a short duration (often lasting from hours to weeks), and is primarily related to a somatic (or medical) disturbance. In comparison, dementia has typically an insidious onset (except in the cases of a stroke or trauma), slow decline of mental functioning, as well as a longer duration (from months to years). Some mental illnesses, including depression and psychosis, may produce symptoms that must be differentiated from both delirium and dementia.
There are many specific types (causes) of dementia, often showing slightly different symptoms. However, the symptom overlap is such that it is impossible to diagnose the type of dementia by symptomatology alone, and in only a few cases are symptoms enough to give a high probability of some specific cause. Diagnosis is therefore aided by nuclear medicine brain scanning techniques. Certainty cannot be attained except with brain biopsy during life, or at autopsy in death.
Some of the most common forms of dementia are: Alzheimer's disease, vascular dementia, frontotemporal dementia, semantic dementia and dementia with Lewy bodies. It is possible for a patient to exhibit two or more dementing processes at the same time, as none of the known types of dementia protects against the others. Indeed, about ten per cent of people with dementia have what is known as mixed dementia, which may be a combination of Alzheimer's disease and multi-infarct dementia.