Chronic, usually progressive deterioration of intellectual functions. Most common in the elderly, it usually begins with short-term-memory loss once thought a normal result of aging but now known to result from Alzheimer disease. Other common causes are Pick disease and vascular disease. Dementia also occurs in Huntington chorea, paresis (seeparalysis), and some types of encephalitis. Treatable causes include hypothyroidism (seethyroid gland), other metabolic diseases, and some malignant tumours. Treatment may arrest dementia's progress but usually does not reverse it.