Disturbances of person identification in Alzheimer's disease. A retrospective study.

Person identification disturbances in Alzheimer's disease (AD) add to the suffering of both patients and caregivers. We assessed the prevalence of person identification disturbances in the records of 217 outpatients with AD. These disturbances occurred in 25.4% (N = 55) and included transient misidentifications of familiar persons (N = 34), the Capgras syndrome (N = 11), misidentification of themselves in mirrors (N = 5), prosopagnosia (N = 3), misidentification of unfamiliar persons as familiar (N = 1), and misidentification of another person as oneself (N = 1). Transient misidentifications were easily corrected misperceptions, and the Capgras syndrome and mirror difficulties were associated with suspiciousness/paranoia and delusions. In AD, these findings suggest that misidentifications of familiar persons result from misinterpretations due to cognitive impairments, and the Capgras syndrome and mirror difficulties ensue when these misinterpretations are elaborated by paranoid delusions.