Alzheimer’s disease (AD), the most common cause of dementia, is the fifth leading cause of death in the United States. An estimated 6.9 million Americans aged 65 years or older have AD. Comorbid conditions in AD may exacerbate the progression of dementia and negatively affect overall health.
Although the exact mechanisms remain unclear, systemic inflammation is thought to play a significant role in the development of many common comorbidities associated with AD. Among the most frequently observed comorbid conditions are hypertension, diabetes, and depression. The presence of these comorbidities affects the treatment and management of AD, underscoring the need to understand the mechanisms of their interrelationship and develop effective management strategies.
Hypertension
Hypertension is a well-established risk factor for numerous health conditions, including AD. A comprehensive review of five meta-analyses and 52 primary studies revealed that elevated systolic blood pressure (SBP) correlates with an 11 % increased risk of developing AD, raising the question of whether early intervention and control of blood pressure would mitigate the risk for AD later in life.
Findings from the Northern Manhattan Study suggest that although elevated SBP contributes to cognitive decline in older patients, the use of antihypertensive medications can neutralize the effects of high SBP on certain cognitive functions. Furthermore, a systematic review and meta-analysis comprising 12 trials (92,135 participants) demonstrated a significant reduction in the risk for dementia and cognitive impairment with antihypertensive treatment.
Notably, a retrospective cohort study involving 69,081 participants treated with beta-blockers for hypertension found that beta-blockers with high blood-brain barrier permeability were associated with a reduced risk for AD compared with those with low blood-brain barrier permeability. Additionally, a secondary analysis of the SPRINT trial found antihypertensive medications that stimulate vs inhibit type 2 and 4 angiotensin II receptors were associated with a lower incidence of cognitive impairment. Although further clinical trials are necessary to directly assess specific medications, these findings emphasize the potential of antihypertensive treatment as a strategic approach to reduce the risk for AD.
Type 2 Diabetes
The connection between AD and type 2 diabetes is such that AD is sometimes referred to as “type 3 diabetes.” Both diseases share some of the same underlying pathophysiologic mechanisms, particularly the development of insulin resistance and oxidative stress. A prospective cohort study of 10,095 participants showed that diabetes was significantly associated with a higher risk of developing dementia; this risk is even greater in patients who develop diabetes at an earlier age.
In an interview with this news organization, Alvaro Pascual-Leone, MD, PhD, a professor of neurology at Harvard Medical School, Boston, said, “In addition to being a comorbidity factor, diabetes appears to be a predisposing risk factor for AD.” This is supported by a comprehensive literature review showing an increased progression from mild cognitive impairment (MCI) to dementia in patients with diabetes, prediabetes, or metabolic syndrome, with a pooled odds ratio for dementia progression in individuals with diabetes of 1.53.
Owing to the overlapping pathophysiologic mechanisms in AD and diabetes, treating one condition may have beneficial effects on the other. A systematic umbrella review and meta-analysis that included 10 meta-analyses across nine classes of diabetes drugs found a protective effect against dementia with the use of metformin, thiazolidinediones (including pioglitazone), glucagon-like peptide 1 receptor agonists, and sodium-glucose cotransporter 2 inhibitors. Moreover, a cohort study of 12,220 patients who discontinued metformin early (ie, stopped using metformin without a prior history of abnormal kidney function) and 29,126 patients considered routine users found an increased risk for dementia in the early terminator group. Although further research is warranted, the concurrent treatment of AD and diabetes with antidiabetic agents holds considerable promise.