When Does Mild Cognitive Impairment Develop into Alzheimer's Disease?

People are always asking when mild cognitive impairment turns into Alzheimer's disease. I commonly encounter this question in the press, I hear it discussed at conferences, and physicians wrestle with it regularly. Despite the centrality of this question in so many forums of cognitive health, it is a nonsensical inquiry with inherent flaws. Additionally, it propagates confusion that I want to clarify today.

There are many medical conditions that can cause a subtle cognitive deficit. Pondering whether or not mild cognitive impairment will "convert" to Alzheimer's disease obscures the fact that mild cognitive impairment is a symptom of an underlying medical problem, not the problem itself. In fact, some mild cognitive impairment is actually caused by Alzheimer's disease; therefore, the prospect of conversion does not belong in a logical, informed discussion.

The Correct Question

When mild cognitive impairment is present, we should not ask if or when it will convert to Alzheimer's disease. The correct question is: what is the cause of the impairment?

If the answer is Alzheimer's disease, then the folly of a conversion outcome is clear: the disease precedes the impairment and not vice versa. If the mild cognitive impairment is caused by some other medical condition (e.g., depression, vascular disease, thyroid disease, anxiety, etc.) then it is equally futile to consider whether or not it will convert to Alzheimer's disease, as these medical problems are separate and distinct.

All of this is not to say that a person with mild cognitive impairment caused by untreated depression will never get Alzheimer's disease because they may. In fact, some medical conditions that can cause mild cognitive impairment confer a greater risk for Alzheimer's disease. Nonetheless, the notion of "converting" from mild cognitive impairment to Alzheimer's disease is illogical.