Definition of Alzheimer's Disease
With the development of drugs to treat AD, and a movement toward early identification and treatment, the definition of Alzheimer's disease is changing. AD has historically been defined as a syndrome of cognitive and functional decline with associated but variable, non-cognitive, behavioral features, and specific neuropathological and chemical changes. AD is still considered a cognitive syndrome affecting memory and at least one other area of cognition (i.e., language ability, attention and concentration, orientation, judgment, problem solving, reasoning, or the ability to perform already learned motor skills). However, it is no longer believed that everyone who develops AD will necessarily show functional decline (i.e., a diminishing ability to independently perform activities necessary for daily living, such as the ability to manage finances, cook, use a telephone, bathe, dress oneself, etc.). For those patients who are diagnosed with AD very late in life, but early in the course of their disease, the initiation of treatment may stabilize the condition before functional decline occurs, thereby allowing an individual to live the remainder of their natural life without significant change to their way of life.
The definition of AD is also changing in response to growing research on normal aging. By studying the brain tissue of normal adults, researchers have determined that Alzheimer's disease likely starts decades before the symptoms of the disease are recognized. In fact, it is hypothesized that the disease actually begins in the third, fourth, or fifth decade of life. What used to be considered a disease of advanced age is now viewed as a disease that develops across the lifespan.
There are many different conditions which cause “dementia”. Dementia is a broad term used to describe a decline in cognitive function which is usually severe enough to disrupt daily life and activities. Alzheimer's disease is the leading cause of dementia among those aged 65 and older. However, unlike a dementia due to stroke, where the onset of memory and thinking problems is sudden, AD results in a slow but ongoing loss of memory and thinking abilities. It is a progressive disorder which, left untreated, will continue to cause declines in cognition and functional abilities over time.
While AD always affects one of three primary domains (cognition), it may or may not impact the remaining two (functional ability and behavior). Depending on the age of the patient at the time of diagnosis, functional ability may be preserved in some cases. Most patients who live for a long time after an Alzheimer's disease diagnosis develop functional decline to variable degrees. The behavioral or "neuropsychiatric" features of AD may develop at any point in the course of the disease, or they may never develop. If behavioral symptoms of AD do occur, it does not necessarily mean that the disease is progressing. Examples of behavioral/neuropsychiatric features of AD include apathy (diminished interest, initiative, or feeling), anxiety, irritability, hallucinations (seeing or hearing things in the environment which aren't present) or delusions (holding false beliefs), disinhibition, agitation (restlessness, pacing, verbal or physical aggression), euphoria, or depression.
It is important to keep in mind that the behavioral or neuropsychiatric manifestations of AD vary widely from person to person. If behavioral symptoms occur, treatments (both non-pharmacological and pharmacological) are available, including the medications currently prescribed to treat the cognitive symptoms of AD. Originally thought to benefit cognition alone, research is demonstrating that cholinesterase inhibitors (e.g., Donepezil, Rivastigmine, Galantamine) and NMDA receptor antagonists (Memantine) may positively affect behavior and help maintain functioning for those with AD.