Except in the cases of stroke or traumatic brain injury, loss of cognitive function is not something that happens all at once. Cognitive impairment that comes with age may be thought of as belonging on a continuum, with one end being no cognitive impairment and the other end being dementia, of which Alzheimer's is the most common type.
Most older adults are actually at the "no impairment" end of the continuum. A further 30-40% of adults over 65 will have what is called "age-related memory impairment", which is the type of cognitive loss we regard as a normal consequence of age -- a measurable (but slight) decline on memory tests; a feeling that you're not quite as sharp or as good at remembering, as you used to be.
Only about 1% of these people will develop Alzheimer's.
But around 10% of adults over 65 develop "mild cognitive impairment", and this is a precursor of Alzheimer's. This doesn't mean someone with MCI will inevitably get Alzheimer's in their lifetime, but their likelihood of doing so is substantially increased.
Whether you are one of those 10% depends in part on your age and your level of education. A study2 of nearly 4000 people from the general population of a Minnesota county, run by the Mayo Clinic, indicates 9% of those aged 70 to 79 and nearly 18% of those 80 to 89 have MCI. The prevalence decreased with years of education: it was 25% in those with up to eight years of education, 14% in those with nine to 12 years, 9% in those with 13 to 16 years, and 8.5% in those with greater than 16 years.
Whether or not this will develop into Alzheimer’s can be predicted with a reasonably high level of accuracy (75%) by the rate at which brain tissue is being lost, and in particular the rate at which it is being lost in the hippocampus (arguably the most important region for memory in the brain). Whether actions known to build brain tissue (physical exercise, mental stimulation) can counteract that in this population is not yet known — but it certainly can’t hurt!
Mild cognitive impairment doesn’t necessarily mean memory problems. There are two types of MCI: those with the amnesic subtype (MCI-A) have memory impairments only, while those with the multiple cognitive domain subtype (MCI-MCD) have other types of mild impairments, such as in judgment or language, and mild or no memory loss. Both sub-types progress to Alzheimer's disease at the same rate, but they do have different pathologies in the brain.
Mild cognitive impairment is not necessarily obvious to outside observers. A person with it can function perfectly well, and although they may feel their impairment is obvious to all around them, it's not likely to be obvious to anyone not living with them.
A person suffering from mild cognitive impairment may find that they have problems with:
- finding the right words
- making decisions
- remembering recent events
- placing things in space (for example, getting the proportions right when drawing a simple object such as a box).
Essentially, age-related cognitive impairment might be thought of as slight, non-important, cognitive impairment, while mild cognitive impairment is a condition where significant cognitive impairment exists which nevertheless doesn't affect daily functioning. Dementia is significant cognitive impairment that does interfere with daily life.