Retinopathy and cataracts each affect about 15 per cent of patients and may result in a significant loss of independence for these individuals. Maculopathy is an especially common form of retinopathy in type 2 and may threaten vision; 31 it may be very difficult to diagnose at routine fundoscopy. By contrast, proliferative retinopathy, the most common cause of blindness in type 1 diabetes, is rare. Nephropathy is probably as likely to develop in type 2 as it is in type 1, although its prevalence is lower because most type 2 patients, being substantially older, have a shorter exposure to hyperglycemia and therefore less opportunity to progress to end-stage nephropathy with renal failure. In addition, because type 2 diabetes is now so common, these patients constitute the majority of those requiring renal replacement therapy in many countries. Finally, neuropathy is a common complication and causes serious morbidity in a substantial proportion of type 2 patients, about eight per cent of whom have painful, rather than asymptomatic, neuropathy.7 At least one-third of male type 2 patients, when directly questioned, have some degree of erectile dysfunction.32....
Research on cognitive function in older people with type 2 diabetes has been performed primarily by small, cross-sectional studies and, to a lesser extent, by epidemiological studies involving much larger samples. Within each type of study, however, the evidence generated has been inconsistent. There are two main reasons for this lack of consensus. First, older people with diabetes usually also have other medical conditions that impair cognitive functioning, such as hypertension and cardiovascular disease. Therefore, identifying the independent contributions of diabetes and of other conditions, or indeed controlling for the influence of the latter, presents a challenge for researchers.33,34 Second, there is a lack of consensus over the cognitive functions that ought to be tested, as well as the assessment instruments that should be used. As a result, no two studies have used the identical test battery.
Factors that may further complicate the relationship between cognitive function and diabetes are age, duration of illness, diabetes complications and glycemic control and these have been considered in research studies in the field. However, separating the unique contribution of these factors is a challenge that has yet to be met.
Notwithstanding these difficulties, simple and more complex cognitive processes have been studied. In general, performance on complex cognitive tasks such as those assessing abstract reasoning, verbal memory and mental flexibility has often been shown to be impaired in people with type 2 diabetes.35-37 Learning and memory in particular have further been proposed to be affected by older age.38 More complicated diabetes 39 and a longer illness duration40 are also related to greater cognitive decline in patients with the illness.
Finally, a recent review of prospective studies41 has also concluded that type 2 diabetes may well be associated with a higher probability of cognitive decline or indeed accelerated dementia. The strength of prospective studies lies in the fact that they have examined large samples, and hence are powerful enough to detect small changes in cognitive performance; however, at the same time, they suffer from methodological difficulties such as relying on patient self-report for diagnosis of the illness,40 failing to control for pre-morbid intelligence42 or using crude measures of dementia (the Mini Mental State Examination, MMSE).43
We conclude that older people with type 2 diabetes are more likely to have some cognitive impairment in comparison to people without the illness. However, such impairment is probably limited to relatively complex cognitive processes and may be attributable to other conditions that often accompany diabetes and that are likely to interfere with cognitive function. The precise extent and severity of cognitive impairment is unlikely to be established until a consensus is achieved on the cognitive functions that should be routinely assessed and on the precise battery of cognitive tests that is likely to be sensitive enough to detect these cognitive changes.
So what are the implications of these findings for the management of older adults with diabetes? Although much research has been conducted to investigate the extent and magnitude of diabetes-related deterioration in cognitive performance, few studies have attempted to relate these findings to diabetes self-management. In other words, although most researchers in the field warn of potential cognitive decline in patients with the illness, only very few44,22 have explored the extent to which cognitive function predicts diabetes self-management in patients with type 2 diabetes. From the limited amount of work in this area it would appear that cognitive performance is not a major predictor of diabetes self-care activities in older adults with the illness. In our work22 we found only moderate relationships between performance on a battery of cognitive tests and diabetes self-care as assessed by self-report, suggesting that the relatively small changes in cognitive function that we observed may not be a severe threat to self-management of the illness. Interestingly, we also found that patients' subjective perceptions of their everyday memory performance, rather than objective neuropsychological test performance, is associated with diabetes-related problem solving activities in older people with the illness. These findings would suggest that clinicians should be aware that some of their type 2 patients may well be suffering from cognitive difficulties, over and above those that would be expected as a result of old age and other comorbidities. This cognitive decline, however, is unlikely to heavily compromise self-care efforts; instead, patients' beliefs about their cognitive function may well be more useful in predicting diabetes self-management.
Depression and anxiety
Older people are well known to be less depressed and less anxious than their younger counterparts.45 Chronically ill older patients are also less likely than their younger chronically ill counterparts to be depressed or anxious.46 Diabetes, however, has been extensively reported to be associated with both depressive illness and anxiety disorder. Although depression is not particularly more prevalent in diabetes than the rates reported in other chronic illnesses, 47 it is more prevalent among people with diabetes than the general population, affecting around 15–20 per cent of patients. Women with type 2 diabetes are twice as likely as men to be depressed, 48 with body weight a significant predictor of depressive illness. Diabetes complications such as neuropathy, retinopathy, macrovascular complications and sexual dysfunction have also been associated with depressive illness; 49 however, the presence of uncomplicated diabetes in the absence of another chronic illness does not seem to predispose to depression.50
The aetiology of depression in older adults with diabetes is not yet clear, although both physiological and psychological factors have been explored.51 Recent work52 has indicated that depression increases diabetes symptoms by complicating patients' self-care efforts. In particular, it has been proposed53 that depression affects diabetes self-management in three distinctive ways: first by affecting older patients' overall quality of life,54 second by reducing physical activity levels (which have been shown to be restored with depression treatment55) and third by impairing patients' ability to communicate effectively with their heath care teams.
Depression in diabetes remains underdiagnosed and undertreated. The few studies that have been conducted to investigate treatment effectiveness have produced encouraging results. Lustman et al.56 showed that cognitive behavioral therapy (CBT) was effective in remitting depression, although these results were compromised in the presence of diabetes complications and lower frequency of blood glucose testing. In similar work,56 CBT was found to be effective in reducing glycosylated hemoglobin 6 months after treatment. In more recent work, Williams and his team55 showed that a combined care approach for depression improved diabetes patients' depressive symptoms and their overall functioning as well as their weekly frequency of physical activity. HbA1c levels were unaffected.
Overall, and as Snoek and Skinner51 (p. 265) have argued, '. . . evidence to support the effect of psychological treatment in problematic diabetes is still scarce. . .'. This is not just true for depression, but also for anxiety disorder. A review of anxiety prevalence rates in diabetes patients57 showed that around 14 per cent of patients have clinical characteristics of generalized anxiety disorder (GAD), whilst around 40 per cent present with elevated symptoms of anxiety, with the presence of elevated symptoms being significantly higher in women (around 55 per cent) than men.
At the moment, routine depression and anxiety screening and treatment are not processes that are a routine part of diabetes care in general. Given the high prevalence of both disorders however, and their potentially devastating effect on diabetes self-care, routine screening of the older patient with diabetes is recommended.
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