**2.2 Duration of DM and cognitive impairment**

Several studies have studied about the linkage of DM with cognitive decline in elderly population. A perspective (over 20 years) cohort study in the USA with mid-age (mean age 58) diabetic patient reported that DM in the midlife was related to a significant increase in cognitive impairment. This study included 13,351 black and white adults aged 48–67 years old, and their cognitive function was examined using three cognitive tests, which are the delayed-word-recall test (DWRT), the digit substitution test (DSST) of Wechsler Adult Intelligence Scale-Revised (WAIS-R) and the word fluency test (WFT). The study also reported that a patient with poorly controlled DM might have bigger cognitive disorder than well-controlled ones and longer duration of DM will have increased chances of late-life cognitive disorder [26].

A cross-sectional study was conducted on 57 patients having type 2 DM. The result shows that patients with type 2 DM had low grades in the cognitive testing and poor performance in different cognitive function tasks which include the verbal relations, visual reasoning, short-term memory test and many more. Cognitive function is impaired more with the untreated DM patient than the treated group [25]. Cognitive dysfunction is nonlinearly related to the duration of diabetes. However, cognitive decline is more prominent when the duration of DM is more than 5 years and presence of hypertension which further increases the risk of cognitive impairment [21]. The patients having diabetes showed poor performance in the tests of recent memory, repetition and attention, as compared to the control group and DM, and the people with long history of DM are more at risk of cognitive decline [27]. It is concluded that cognitive function of diabetes type 2 patients should frequently be tested. This is because the duration of disease can be related with the decrease in cognitive function. As duration increases, impairment also increases [28].
