Hallucinations in Alzheimer’s Disease and Charles Bonnet Syndrome

To the Editor: I read with interest the article by Fiona M. Chapman, F.R.C.Ophth., et al. (1) in which an association between visual hallucinations and impaired visual acuity was demonstrated in patients with probable Alzheimer’s disease. It seems reasonable and useful for them to have concluded that glasses and cataract surgery need evaluation as prophylactic or adjunctive treatments for visual hallucinations in patients with probable Alzheimer’s disease.

The association between visual hallucinations and impaired visual acuity in psychologically normal people is known as Charles Bonnet syndrome. For example, Teunisse et al. (2) found that 60 of 505 (12%) of visually handicapped patients suffered from Charles Bonnet syndrome. This suggests that visual hallucinations may occur in visually handicapped patients even if they do not suffer from Alzheimer’s disease. Thus, there is a possibility that Dr. Chapman and colleagues" patients with visual hallucinations included those suffering from visual hallucinations due not to Alzheimer’s disease but to impaired visual acuity. This possibility is supported by the fact that only impaired visual acuity was entered into the equation when logistic regression analysis examined cognition, visual acuity, and gender as associates of visual hallucinations. As the authors pointed out, however, it is necessary to do controlled trials to compare the prevalence of visual hallucinations in psychologically normal people with visual impairment and Alzheimer’s patients with visual impairment under age- and gender-matched conditions.

If that possibility is the case, pharmacotherapy for Charles Bonnet syndrome can be applied to some patients with probable Alzheimer’s disease when glasses and cataract surgery turn out to be useless, because some patients with Charles Bonnet syndrome do not respond to glasses and cataract surgery and need pharmacotherapy. At present there are several candidate treatments for Charles Bonnet syndrome, such as carbamazepine (3, 4), carbamazepine plus clonazepam (5), and valproate (unpublished data). These drugs are anticonvulsants, but they may be effective for the treatment of Charles Bonnet syndrome. These may be more effective and be associated with fewer side effects than neuroleptics for visual hallucinations in Alzheimer’s disease patients. Since these treatments are not established, it is necessary to perform randomized placebo- or neuroleptic-controlled, double-blind studies to investigate the side effects of these drugs and their effects on visual hallucinations in Alzheimer’s disease as well as in Charles Bonnet syndrome.