Dementia is a syndrome characterized by loss of intellectual (cognitive) functions sufficient to interfere with social and occupational functioning. Cognition encompasses multiple functions including language, memory, perception, praxis, attentional mechanisms, and executive function (planning, reasoning). These elements may be affected selectively or globally: older definitions of dementia requiring global cognitive decline have now been superseded. Amnesia may or may not, depending on the classification system used, be a sine qua non for the diagnosis of dementia. Attentional mechanisms are largely preserved, certainly in comparison with delirium, a condition which precludes meaningful neuropsychological assessment because of profound attentional deficits. Although commoner in the elderly, dementia can also occur in the presenium and in children who may lose cognitive skills as a result of hereditary metabolic disorders. Failure to develop cognitive skills is termed learning disability. Multiple neuropsychological tests are available to test different areas of cognition. The heterogeneity of dementia is further exemplified by the fact that it may be acute or insidious in onset, and its course may be progressive, stable, or, in some instances, reversible ("dysmentia"). A distinction is drawn by some authors between cortical and subcortical dementia: in the former the pathology is predominantly cortical and neuropsychological findings are characterized by amnesia, agnosia, apraxia, and aphasia (e.g., Alzheimer’s disease); in the latter pathology is predominantly frontal-subcortical and neuropsychological deficits include psychomotor retardation, attentional deficits, with relative preservation of memory and language; movement disorders may also be apparent (e.g., Huntington’s disease, progressive supranuclear palsy). However, not all authors subscribe to this distinction, and considerable overlap may be observed clinically. Cognitive deficits also occur in affective disorders, such as depression, usually as a consequence of impaired attentional mechanisms. This syndrome is often labeled as "pseudodementia" since it is potentially reversible with treatment of the underlying affective disorder. It may be difficult to differentiate dementia originating from depressive or neurodegenerative disease, since depression may also be a feature of the latter. Impaired attentional mechanisms may account for the common complaint of not recalling conversations or instructions immediately after they happen (aprosexia). Behavioral abnormalities are common in dementias due to degenerative brain disease, and may require treatment in their own right.
Recognized causes of a dementia syndrome include:
- Neurodegenerative diseases:
- Alzheimer’s disease, frontotemporal lobar degenerations (frontotemporal dementia, encompassing Pick’s disease; semantic dementia; primary progressive aphasia), dementia with Lewy bodies, Huntington’s disease, progressive supranuclear palsy, corticobasal degeneration, prion disease, Down’s syndrome, dementia pugilistica.
- Cerebrovascular disease:
- focal strategic infarcts (e.g., paramedian thalamic infarction), multiple infarcts, subcortical vascular disease, Binswanger’s disease.
- Inflammatory disorders: multiple sclerosis, systemic lupus erythematosus.
- Structural disease: normal pressure hydrocephalus, tumors.
- Infection: HIV dementia, neurosyphilis, Whipple’s disease.
- Metabolic causes: Wernicke-Korsakoff syndrome, vitamin B12 deficiency, hypothyroidism, hyperparathyroidism/hypercalcemia, leukodystrophies, Wilson’s disease.
Cognitive dysfunction may be identified in many other neurological illnesses. Investigation of patients with dementia aims to identify its particular cause. Because of the possibility of progression, reversible causes are regularly sought though very rare. Specific treatments for dementia are few: cholinesterase inhibitors have been licensed for the treatment of mild to moderate Alzheimer’s disease and may find a role in other conditions, such as dementia with Lewy bodies and vascular dementia, for behavioral as well as mnestic features.
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