Some studies seem to show that some type 1 diabetic people who have suffered from repeated hypoglycemic episodes (low blood sugar episodes) can display temporary or perhaps some permanent loss of memory function. There is a good article on the following website about some studies on this issue, using both young and older type 1 patients. If you can skim through the technical language, it does offer some good information:
http://diabetes.diabetesjournals.org/cgi/content/full/52/1/149
As for how to help him... It really depends on what the cause of the memory loss is. You mention other drugs in addition to insulin, so the cause could have nothing to do with his diabetes treatment. And of course Alzheimer's is always a possibility. I would recommend that you or whoever is authorized to accompany him to his doctor have a discussion about this issue with his doctor. The doctor can then look at his health history and can better identify what might be the root of the problem. Yes, there are treatments that can help tremendously, depending on what is found to be the cause.
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