Delusions and hallucinations in Alzheimer's disease are quite common, affecting up to half of all Alzheimer's patients, usually in the middle stages of the disease. While they can occur in all five senses, seeing things (visual hallucinations) is the most common type.
While chances are reasonably good that if your loved one is seeing people in her home who are not there, it is a symptom of her dementia, it's important to realize that there are other possible causes as well. Hallucinations can occur for a variety of reasons in later life, with one large study in Sweden finding that almost 7% of all 85 year-old individuals without dementia were experiencing hallucinations.
In addition to dementia, hallucinations may occur in delirium, schizophrenia, major depression, and alcohol or drug intoxication or withdrawal.
Any sudden injury to the brain, including a stroke or a seizure, can cause auditory or visual hallucinations. Certain types of medications, including benzodiazepines (like Valium and Xanax), Parkinson's drugs (like Sinemet), and drugs for urinary retention (like Ditropan), can cause hallucinations.
Problems with vision or hearing may increase the chances of developing hallucinations. Charles Bonnet Syndrome is a condition that causes mentally healthy people with visual loss to have complex visual hallucinations. Images of complex colored patterns and of people, animals, and plants are common. In older people who lose their hearing, they may experience musical hallucinations, hearing music in their ears even when no music is being played near them.
As with many symptoms of dementia,the environment plays a role in hallucinations as well. Poorly lit rooms and loud, chaotic settings may lead to misperceptions and worsening of hallucinations. Nursing homes may have a PA system over which announcements are made -- residents with dementia may perceive this as hearing voices coming from the ceiling.
Blazer D., MD, Stefens D., MD, Busse E., MD, ed. Textbook of Geriatric Psychiatry, Third Edition, American Psychiatric Publishing, Inc., 2004, p.269, 273
Menon G., Rahman I. et al. Complex Visual Hallucinations in the Visually Impaired: The Charles Bonnet Syndrome. Survey of Ophthalmology January 2003 48,1; 58-72.
Rabins P., Lyketsos C., Steele C., Practical Dementia Care, Second Edition, Oxford University Press, 2006, pp. 159-60.