Cognitive Ability and Diabetes

Summarized by Robert W. Griffith, MD
June 14, 2007

Summary

A study of 1,440 type 1 diabetics showed that those on an intensive antidiabetic regime as well as those on a less-intensive, more conventional, treatment plan had no deterioration in their cognitive ability over an 18-year follow-up period.

Introduction

In April 2007 an article was published in the Archives of Neurology reporting that type 2 diabetes is associated with the development of mild cognitive impairment (MCI). Over 900 healthy New Yorkers aged 65 and above were followed for an average of 6 years; every 18 months they had a full medical assessment.¹ There were 24% diabetics among the participants. After adjustments for known risk factors for MCI it was found that diabetes was related to a significantly higher risk of MCI; the risk was greater in African-Americans and Hispanics than in non-Hispanic white people.

This somewhat alarming finding was countered, to a large extent, by the report of the long-term findings from the Diabetes Control and Complications Trial and Epidemiology of Diabetes Interventions and Complications (DCCT/EDIC) Study. The results, which came 12 to 18 years after the start of the study, were reported in the New England Journal of Medicine, and are summarized here.

What was done

There were 1,441 type 1 diabetics enrolled in the DCCT/EDIC study, who were examined at enrollment and again approximately 18 years later. Their ages ranged from 13 through 39. There were two classes of patients - those without any evidence of eye or kidney complications (retinopathy or nephropathy), and those with mild to moderate retinopathy and protein in the urine at entry.

Roughly half (711) of the patients were randomly assigned to have intensive treatment to control their blood sugar levels as closely as possible. This meant 3 or more insulin injections daily or the use of an insulin pump, guided by frequent self-determinations of blood glucose levels. The targeted fasting glucose level was between 70 and 120 mg/dL, and the target HbA1c (measured monthly) was below 6%. Blood sugar levels below 50 mg/dL or symptoms that could be readily reversed by oral or intravenous glucose were diagnostic of hypoglycemia.

The remaining 730 patients had 'conventional' anti-diabetic therapy; one or two insulin shots daily, without target blood levels, but aiming to avoid diabetic or 'hypo' (hypoglycemic) attacks.

It was known that the intensive treatment would inevitably be associated with occasional hypoglycemic incidents, and there was fear that these might impact the cognitive function of the patients. For this reason, a cognitive-test battery lasting 4-5 hours was done at entry to the study, at the end of the follow-up period, and at intervals in between. The battery covered 8 areas: problem solving, learning, immediate memory, delayed recall, spatial information, attention, psychomotor efficiency, and speed of movement. The results of each person's test were adjusted for age at baseline, gender, years of education, length of follow-up, and symptoms of eye or nerve complications (retinopathy, neuropathy).

What was found

At the end of the study, the average age of the participants was 46 (ranging from 29 to 62). Women constituted 47% of the population, and 96% of them were white. As many as 97% of the conventional treatment patients had developed retinopathy, compared to 89% of the intensive treatment group. Vision was impaired in 4% of the intensive treatment and in 5% of the conventional treatment subjects. In both groups, 30% of the subjects had peripheral neuropathy, and there was kidney insufficiency in 7 intensive- and 13 conventionally-treated patients.

There were 1,355 episodes of coma or seizure related to hypoglycemia over the 18 years; 896 episodes in 262 patients in the intensive treatment group, and 459 in 191 patients in the conventional group. Not surprisingly, the frequency of these episodes was higher with the intensive attempts at blood sugar control.

The scores for the cognitive tests were, on average, all within the normal values established for healthy people without diabetes. They were uninfluenced by the subjects' assignment to intensive- or conventional-treatment groups, or the number of hypoglycemic events. The only associations found were higher values for HbA1c levels in those subjects with moderate declines in psychomotor efficiency and speed of movement.

What these findings mean

The original purpose of this study was to determine if the increased episodes of hypoglycemia seen with tight control of diabetes were responsible for a decrease in cognitive ability. Fortunately, this is not the case, as the results show.

However, it would be nice to be able to say that the results - no cognitive decline in diabetics who have intensive treatment to maintain tight control of blood sugars - counteract the previous report cited above.¹ Regrettably, the findings were obtained in subjects with type 1 diabetes, whereas the previous report was based on findings in type 2 diabetes. So the possibility still exists that uncontrolled blood sugar levels in type 2 diabetics ed be linked to mild cognitive impairment. After all, vascular disorders - commonly seen in type 2 diabetes - are associated with an increased risk of dementia (albeit not Alzheimer's disease, but multi-infarct dementia).

To resolve the problem, a similar study to this one should be conducted in type 2 diabetics. Doubtless, something along these lines is planned or being done. In the meantime, all diabetics - type 1 and type 2 - should take care to monitor their disease carefully and treat adequately to maintain their blood sugar levels in the recommended range. This should minimize the likelihood of cognitive impairment - something that's worth serious consideration.

Source

• The Diabetes Control and Complications Trial/Epidemiology of Diabetes Interventions and Complications (DCCT/EDIC) Study Research Group. Long-term effects of diabetes and its treatment on cognitive function. N Engl J Med 2007;Vol. 356: pp. 1842-1852

Footnotes
1. Relation of diabetes to mild cognitive impairment. JA. Luchsinger, C. Reitz, B. Patel,  et al. , Arch Neurol, 2007, vol. 64, pp. 570--575

Related Links
Tight Blood Sugar Control Wards Off Diabetes Complications
Diabetes, high blood pressure and mild mental decline
Diagnosing Mild Cognitive Impairment

Please take a moment to give us your comments. For questions about Health matters you may check our "Questions & Answers" Portal and Service.