This disrupts the work of the brain cells affected and triggers a toxic cascade, ultimately leading to cell death and later brain shrinkage.Alzheimers disease is believed to occur when abnormal amounts of amyloid beta (Aβ), accumulating extracellularly as amyloid plaques and tau proteins, or intracellularly as neurofibrillary tangles, form in the brain, affecting neuronal functioning and connectivity, resulting in a progressive loss of brain function. This altered protein clearance ability is age-related, regulated by brain cholesterol, and associated with other neurodegenerative diseases.Advances in brain imaging techniques allow researchers to see the development and spread of abnormal amyloid and tau proteins in the living brain, as well as changes in brain structure and function. Beta-amyloid is a fragment of a larger protein. When these fragments cluster together, a toxic effect appears on neurons  and disrupt cell-to-cell communication. Larger deposits called amyloid plaques are thus further formed.Tau proteins are responsible in neurons internal support and transport system to carry nutrients and other essential materials. In Alzheimers disease, the shape of tau proteins is altered and thus organize themselves into structures called neurofibrillary tangles. The tangles disrupt the transport system and are toxic to cells. The cause for most Alzheimers cases is still mostly unknown, except for 1–2% of cases where deterministic genetic differences have been identified.
Eventually, the term Alzheimers disease was formally adopted in medical nomenclature to describe individuals of all ages with a characteristic common symptom pattern, disease course, and neuropathology.The National Institute of Neurological and Communicative Disorders and Stroke (NINCDS) and the Alzheimers Disease and Related Disorders Association (ADRDA, now known as the Alzheimers Association) established the most commonly used NINCDS-ADRDA Alzheimers Criteria for diagnosis in 1984, extensively updated in 2007. These criteria require that the presence of cognitive impairment, and a suspected dementia syndrome, be confirmed by neuropsychological testing for a clinical diagnosis of possible or probable Alzheimers disease. A histopathologic confirmation including a microscopic examination of brain tissue is required for a definitive diagnosis. Good statistical reliability and validity have been shown between the diagnostic criteria and definitive histopathological confirmation. Society and culture
Social costs
Dementia, and specifically Alzheimers disease, may be among the most costly diseases for societies worldwide. As populations age, these costs will probably increase and become an important social problem and economic burden. Costs associated with AD include direct and indirect medical costs, which vary between countries depending on social care for a person with AD. Direct costs include doctor visits, hospital care, medical treatments, nursing home care, specialized equipment, and household expenses.