Alzheimer’s disease is a type of brain disease, just as coronary artery disease is a type of heart disease. It is also a progressive disease, meaning that it becomes worse with time. Alzheimer’s disease is thought to begin 20 years or more before symptoms arise.1-8 It starts with changes in the brain that are unnoticeable to the person affected. Only after years of brain changes do individuals experience noticeable symptoms such as memory loss and language problems. Symptoms occur because nerve cells (neurons) in parts of the brain involved in thinking, learning and memory (cognitive function) have been damaged or destroyed. As the disease progresses, neurons in other parts of the brain are damaged or destroyed as well. Eventually, neurons in parts of the brain that enable a person to carry out basic bodily functions, such as walking and swallowing, are affected. Individuals become bed-bound and require around-the-clock care. Alzheimer’s disease is ultimately fatal. Brain Changes of Alzheimer’s Disease A healthy adult brain has about 100 billion neurons, each with long, branching extensions. These extensions enable individual neurons to form connections with other neurons. At such connections, called synapses, information flows in tiny bursts of chemicals that are released by one neuron and detected by another neuron. The brain contains about 100 trillion synapses. They allow signals to travel rapidly through the brain, and the information they carry creates the cellular basis of memories, thoughts, sensations, emotions, movements and skills. The accumulation of the protein fragment beta-amyloid into clumps (called beta-amyloid plaques) outside neurons and the accumulation of an abnormal form of the protein tau (called tau tangles) inside neurons are two of several brain changes associated with Alzheimer’s. Plaques and smaller accumulations of beta-amyloid called oligomers may contribute to the damage and death of neurons (neurodegeneration) by interfering with neuron-to-neuron communication at synapses. Inside neurons, tau tangles block the transport of nutrients and other molecules essential for normal function and neurons’ survival. Although the complete sequence of events is unclear, beta-amyloid may begin accumulating before abnormal tau, and increasing beta-amyloid accumulation is associated with subsequent increases in tau.9-10 Other brain changes associated with Alzheimer’s include inflammation and atrophy (decreased brain volume). The presence of toxic beta-amyloid and tau proteins are believed to activate immune system cells in the brain called microglia. Microglia try to clear the toxic proteins as well as widespread debris from dead and dying cells. Chronic inflammation may set in when the microglia can't keep up with all that needs to be cleared. Atrophy occurs because of cell loss. Normal brain function is further compromised in Alzheimer’s disease by decreases in the brain's ability to metabolize glucose, its main fuel. These brain changes are considered biomarkers of Alzheimer's. Biomarkers are biological changes that can be measured to indicate the presence or absence of a disease or the risk of developing a disease. For example, the level of glucose in blood is a biomarker of diabetes, and cholesterol level is a biomarker of one's risk of cardiovascular disease. A study7 of people with rare genetic mutations that cause Alzheimer’s, called dominantly inherited Alzheimer’s disease (DIAD), found that levels of beta-amyloid in the brain were significantly increased starting 22 years before symptoms were expected to develop (individuals with Overview Alzheimer’s Disease or Dementia? Many people wonder what the difference is between Alzheimer’s disease and dementia. Dementia is an overall term for a particular group of symptoms. The characteristic symptoms of dementia are difficulties with memory, language, problem-solving and other thinking skills that affect a person's ability to perform everyday activities. Dementia has many causes (see Table 1, page 6). Alzheimer’s disease is the most common cause of dementia. In this report, Alzheimer’s dementia refers to dementia that is caused by, or believed to be caused by, the brain changes of Alzheimer’s disease. 6 Alzheimer’s Association. 2021 Alzheimer’s Disease Facts and Figures. Alzheimers Dement 2021;17(3). Common Causes of Dementia and Associated Characteristics Alzheimer’s disease Alzheimer's disease is the most common cause of dementia, accounting for an estimated 60% to 80% of cases. Recent large autopsy studies show that more than half of individuals with Alzheimer's dementia have Alzheimer's disease brain changes (pathology) as well as the brain changes of one or more other causes of dementia, such as cerebrovascular disease or Lewy body disease.11-12 This is called mixed pathologies, and if recognized during life is called mixed dementia.