|Diabetic complications, such as retinopathy and peripheral neuropathy, have been studied extensively and are known to impair balance and increase fall risk. However, the effect of diabetes on the vestibular system is not clear. The vestibular system plays an important role in maintaining static and dynamic balance, by providing spatially orienting information. Hence, damage to the vestibular system could potentially increase imbalance and risk of falls. The primary purpose of this work was to examine the effect of diabetes on the peripheral vestibular system, and the resulting impact on symptoms, mobility and balance. In Chapter 1, we have presented a review of the literature on the pathophysiology of diabetes-related complications and their influence on balance and falls, with specific attention to emerging evidence of vestibular dysfunction due to diabetes. We have provided a perspective on the impact of vestibular complications, the need for a thorough vestibular evaluation, and recommendations for vestibular rehabilitation techniques, for people with diabetes. Chapter 2 describes the results of our pilot investigation, where we focused our attention to identify if a specific vestibular disorder may be present in higher frequency in individuals with type 2 diabetes (T2D). In this retrospective analysis of electronic health records, we examined data from 3933 individuals with nine common vestibular disorders. Of the nine vestibular disorders commonly seen in the clinic setting, the prevalence of one condition, benign paroxysmal positional vertigo (BPPV), was significantly higher in individuals with diabetes. Factors that were predictive of BPPV included age, female sex, race and presence of hypertension. Hypertension was the mediating factor that increased the prevalence of BPPV in people with diabetes. Building upon this study, we identified benign paroxysmal positional vertigo as our clinical condition of interest. We developed our specific aims to examine the direct effect of diabetes on the vestibular system using evoked potential studies, as well as the indirect effects on symptoms, mobility and balance, in people diagnosed with BPPV. Chapter 2 showed that the prevalence of BPPV was higher in people with diabetes. BPPV results when otoconia fragments dislodge from the otolith organs of the inner ear, and cause symptoms of vertigo. The effect of diabetes on the saccule and utricle of the vestibular system is not clear, and the combined effect of BPPV and diabetes on the otolith organs, has not been studied. In Chapter 3, our main purpose was to analyze otolith function using vestibular evoked myogenic potential (VEMP) tests in people with diabetes and concurrent BPPV, and to examine the relationships between VEMP variables and diabetes-related variables. For this study, we recruited people who were 40 to 65 years of age, into four groups; 20 controls, 19 individuals with T2D without vestibular dysfunction, 18 individuals with BPPV, and 14 individuals with BPPV and diabetes (BPPV+DM). Results of this study showed that the frequency of delayed and absent saccule responses was significantly higher in people with T2D, BPPV, and BPPV+DM compared to healthy controls. Delayed latency of the saccule responses were associated with higher HbA1c levels. Utricle function did not appear to be as affected by diabetes as the saccule. Although, BPPV and diabetes independently affected utricle and saccule function, they did not appear to have a distinct cumulative effect. The true impact of a disease is the resulting impairment to the affected individual, and the effect on daily activities. Chapter 3 revealed otolith dysfunction in people with T2D, BPPV and BPPV+DM. Because otolith dysfunction is associated with balance deficits and increased postural sway, in chapter 4, we examined postural sway. In this study, we compared the postural sway of people with BPPV and BPPV+DM who were between 40 to 65 years of age. We expanded the scope of our data collection to include healthy controls and people with diabetes (without vestibular dysfunction) for comparison with BPPV and BPPV+DM, as normative values for postural sway using the specific accelerometry procedures and conditions we tested is not available. Not all participants from chapter 3 were included in this study due to equipment malfunction and other issues. Ultimately, we recruited 14 controls, 14 individuals with T2D only, 13 individuals with BPPV only and 11 individuals with BPPV+DM for this study. We measured postural sway using an accelerometer positioned over the L3 spine level, in five conditions that progressively challenged the vestibular system. Results of this study showed that participants with BPPV+DM had higher postural sway measures compared to controls and the T2D group, which was particularly evident in the anteroposterior direction. Standing on foam with eyes closed and tandem stance conditions were the most challenging conditions for people with BPPV+DM. Besides causing symptoms of vertigo, BPPV is known to affect balance and functional mobility. However, BPPV can be effectively treated with canalith repositioning maneuvers. The effect of diabetes on symptom severity, mobility and balance and the response to treatment maneuvers in people with BPPV is unknown. In Chapter 5, we examined handicap due to dizziness, mobility, and balance in people between 40 to 80 years of age, in two groups, 34 individuals with BPPV and 16 with BPPV+DM, before and after treatment with the canalith repositioning maneuver. To identify these deficits, all individuals completed the Dizziness Handicap Inventory (DHI), the Functional Gait Assessment (FGA) and postural sway tests, in quiet stance with altered visual and proprioceptive feedback. We found no differences between people with BPPV and those with BPPV+DM, on the DHI or FGA scores, at baseline and after symptom resolution. Significant differences were seen in postural sway between the groups at baseline. However, after resolution of dizziness, there were no differences between the two groups on any postural sway measures. The number of treatment maneuvers required for resolution of vertigo did not differ between groups. People with BPPV with or without diabetes, made significant improvements in symptoms, mobility, and balance after their vertigo had resolved. In summary, this body of work makes significant contributions to the existing literature examining the vestibular complications due to diabetes. Our central hypothesis was that diabetes would affect the otolith organs of the vestibular system, thereby increasing symptom severity, functional deficits, and balance sway, compared to people without T2D. Our findings were that although the prevalence of BPPV was higher in people with diabetes, hypertension was the complete mediator in the relationship between BPPV and T2D. People with BPPV+DM did not have increased otolith dysfunction, compared to those with BPPV or T2D only; however, postural sway was higher in people with BPPV+DM. The higher postural sway in people with BPPV+DM may be due to the presence of diabetic peripheral neuropathy; however, we did not analyze the effect of neuropathy on mobility and balance. There was no difference in the severity of symptoms and mobility deficits of people with BPPV + DM compared to people with BPPV only. Of importance, we found that people with BPPV+DM do respond well to treatment maneuvers with significant improvements in symptoms, mobility, and balance, and do not require additional treatment maneuvers, when compared to people with BPPV. Although our results did not fully support our central hypothesis, it provides valuable information. This body of work emphasizes the need for early diagnosis and prompt treatment of vestibular dysfunction in individuals with BPPV+DM. Future studies examining falls in people with BPPV+DM considering the influence of other diabetic complications, will help elucidate the relationship between diabetic complications and fall risk.