Patients with diabetes have a three-fold increased risk of progression from mild cognitive impairment to dementia, a primary care study has shown.

The longitudinal cohort study recruited 103 participants aged 65 years or above, from practices in South London, who had mild cognitive impairment. Patients were reassessed four years later. Mild cognitive impairment is a term used to describe people with memory impairment on formal testing who are otherwise functioning well and do not meet clinical criteria for dementia.¹

At follow-up, cognitive status was assessed by telephone interview, followed by a face-to-face evaluation where dementia was suspected.

Of the 61 participants who were successfully followed up, 19 were diagnosed with dementia. Of the ten participants with diabetes who completed follow-up, seven were diagnosed with dementia. Adjustment was made for sociodemographic factors, the APOE4 allele, premorbid IQ and other health conditions.

Patients with diabetes had a three-fold increased risk of progression from mild cognitive impairment to dementia, hazard ratio 2.9 (95% CI: 1.1-7.3).

Depression may cause a reversible impairment of cognitive function which can be easily mistaken for dementia. As the prevalence of depression is doubled in patients with diabetes, an important aspect of the study was discrimination of depression from dementia and adjustment for possible depression.

Diabetes is associated with a 47% increased risk for all dementia. It has been suggested that this is mainly due to increased vascular risk factors: whereas the risk of Alzheimer's dementia (AD) is increased by 39%, the risk of vascular dementia is more than doubled (risk ratio = 2.38).²

However, AD accounts for about two thirds of dementia cases (84% in this study), whereas vascular dementia accounts for only 15%.³

The majority of the additional cases in diabetes patients can therefore be expected to have a diagnosis of AD (six out of seven in this study). There are a number of potential mediating molecular mechanisms. For example, advanced glycation end products are found in amyloid plaques and neurofibrillary tangles, and raised insulin due to insulin resistance will inhibit the degradation of beta-amyloid.2

A priority objective of the National Dementia Strategy is ‘good quality early diagnosis and intervention for all'. ⁴ The difficulty which we face, however, is that, having made an early diagnosis of dementia, we have no treatment to offer which can alter the progression of the disease.

This study provides further evidence that diabetes is associated with faster cognitive decline but at the same time offers the hope that, for this group of patients, we may be able to prevent or slow progression of their condition by more rigorous control of their diabetes. Unfortunately, however, no methodologically sound randomised controlled trials have yet been carried out to investigate this possibility.^5,6