Loneliness might be a preclinical sign for Alzheimer's. Earlier this year, a study published in the International Journal of Geriatric Psychiatry looked at the reciprocal relationship between loneliness and cognitive function in older adults. It reported that loneliness and depressive symptoms were linked to worsening cognitive function over a 12-year period, whereas initially poor cognitive function did not lead to increased loneliness.(1)
(1) Int J Geriatr Psychiatry. 2017 May;32(5):564-573. doi: 10.1002/gps.4495. Epub 2016 May 9. Loneliness, depression and cognitive function in older U.S. adults.Donovan NJ, Wu Q, Rentz DM, Sperling RA, Marshall GA, Glymour MM.