A reader left a comment on a recent post pointing out that depression is also a relatively common cause of cognitive impairment. Because of childhood trauma, he'd also experienced anxiety and PTSD, and I suspect they belong on the list of potential causes, too. People sometimes ask me if depression makes a person more susceptible to Alzheimer's.

The question is important. There is certainly an association between the two, but the cause runs the other way: Alzheimer's (or any other cognitive impairment) can cause people to be depressed, but there is no evidence that depression causes Alzheimer's or even makes it more likely.

So depression must be on the list of the causes of mild cognitive impairment. If medical evaluation has ruled out the most obvious of the others, then depression moves higher in the list of possibilities. If a person is seriously depressed, one can see how the depression might affect his or her ability to think well.

Usually depression is obvious, but not always. There is (or, at least, was) a stigma around the disease and people don't want to acknowledge their inner pain. They need to cover it up and mask it with a smile. Even family members may not know how much the person suffers.

In some cases, the people themselves don't even know they're depressed.

I'm an example. I now know that I was depressed for the first half of my adult life, but at the time I didn't recognize it for what it was. I didn't have the usual symptoms: sleeping too much, being dysfunctional at work, thinking of suicide and so on. But I was almost always dissatisfied with my life, feeling I should be doing better, hyper-aware of my mistakes, despite having what might have appeared to be a perfect life as a successful small-town doctor.

I was mostly miserable. I thought my misery was caused by the difficulties of medical practice or my unreasonable expectations of life. The possibility of depression never entered my mind. It was more than a decade later, after I'd begun adequate anti-depressant medication and was beginning to experience how a non-depressed life felt, that I recognized I had depression.

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So, when faced with a person who has dementia of no discernible cause, a doctor must keep the possibility of depression in mind, even if the patient vigorously denies being depressed.

Having a doctor repeatedly return to the question of depression, however, can be frustrating for a patient who is convinced that they are not unusually depressed. It can raise doubts about the patient's own perceptions of their emotional state, especially if he or she does have minor bouts of mild depression. It may also seem as if the doctor is trying to dismiss the seriousness of the patient's complaints by pushing them off onto emotional causes. It's a frustrating experience for both patient and doctor, and can endanger mutual distrust.

Mild cognitive impairment of no obvious cause is difficult enough of an experience. The confusion around depression can make it worse.

Editor's note: David's journey with Mild Cognitive Impairment was chronicled in "Fade to Blank: Life Inside Alzheimer's," an in-depth look at the real lives of families impacted by the Alzheimer's epidemic. His story continues on his personal blog on AgingCare.com.