Is there a link between multiple sclerosis and Alzheimer disease? A critical note

Multiple sclerosis (MS) and Alzheimer disease (AD) are two neurological disorders that represent a major health problem, as they pose an enormous burden to the patients' quality of life. Although no cure is available, due to recent therapeutic advancements that significantly extend the lifespan, increased prevalence of AD is seen in elderly MS patients. Although cognitive dysfunction affects 40-60% of people with MS, little is known about the co-occurrence of these two conditions and their pathogenetic relationship. About 125 cases of comorbid MS and AD necessitate an evaluation of the literature on the overlaps between AD and MS. Although the etiology of both disorders is unknown, with the emerging recognition that demyelination plays a role in the risk and progression of AD, it has been proposed that both MS and AD share genetic and environmental factors inducing immune cell dysfunctions, inflammation, and neurodegeneration. These mechanisms may be triggered by an early viral infection, Epstein-Barr virus infection being a major risk factor. Both MS and AD show signs of amyloid pathology in the brain and impaired Aβ metabolism in CSF, suggesting overlapping pathobiology. Recent studies revealed similar polygenicity of AD and MS, providing molecular genetic insight into the immune mechanisms involved in both diseases, with similar patterns of microglial activation promoting neuroinflammation. The accumulating damage leads to demyelination, amyloid dysfunction, and damaged blood-brain barrier, together with mitochondrial impairment and other molecular mechanisms leading to neurodegeneration and development of dementia later in life. This short review highlights the overlapping pathophysiological processes promoting the development of AD pathology in progressed MS.