MCI is "mild cognitive impairment." According to this WebMD story, it's prevalence is both understated and increasing.
This topic is one we have spent considerable time over the past several years researching, interviewing the top scientific experts, knowledgeable physicians (many times both categories embody the same individual), caregiving people, and researchers in aligned fields that you might say, are tangential.
The reason is that some have believed MCI was a precursor to Alzheimer's, some in fact that it was Alzheimer's - in early gestation, while others believed that MCI was a catchall category for undiagnosed, relatively asymptomatic memory loss attributable to a variety of factors ranging from cardiovascular disease to depression and sleeplessness.
However, it now appears that it is a definable condition and also that it occurs frequently. One thing is certain: people are going to live longer in the future, and therefore MCI is going to become a very big problem. Scientific consensus seems to be that cancer and cardio issues are being minimized and 'rolled back' a little more every few years, which contributes to the boom in longevity.
Once aging is slowed, which is already happening and has progressed at a steady pace since 1900 (Gompertz analysis) our biggest problem is going to be our perishable brains and mental faculties. The hard disk in our head gets corrupted, meaning that the software of life (consciousness, executive function, auto life-support aps like breathing) gets an ellipsis and cease to function.
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and that's it. If we cannot stop Alzheimer's, then it will become the terminal point of each person's life, given an extended lifespan.
The only other option (unfeasible at the moment) is to record consciousness and memories via artificial or bioorganic attahments, and then transfer this essence to a robot or an engineered replacement body. In this way, a single consciousness could span numerous lifetimes.