Visual Hallucinations in Alzheimer's Disease: Possible Involvement of Low Visual Acuity and Dementia With Lewy Bodies

SIR: I read with interest the article by Holroyd et al.¹ where they showed that Alzheimer's disease (AD) patients with visual hallucinations had a significantly smaller occipital/whole brain ratio than AD patients without visual hallucinations. Although they mentioned limitations to their study such as small sample size, there are another two limitations as follows.

First, low visual acuity may be associated with visual hallucinations in AD patients. For example, Chapman et al.² have reported 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 in AD patients. Accordingly, it is necessary to compare visual acuity between AD patients with visual hallucinations and those without them in the patients of Holroyd et al.¹ If the two groups had comparable visual acuity, their visual hallucinations might have derived from neuropathology of the occipital lobe. However, if AD patients with visual hallucinations had significantly lower visual acuity, it is uncertain whether their visual hallucinations derived from low visual acuity or from neuropathology of the occipital lobe or both. Actually, even psychologically normal old people can suffer from visual hallucinations, which is called Charles Bonnet syndrome.^3,4

Second, although Holroyd et al.¹ diagnosed probable AD according to NINCDS-ADRDA criteria, there is a possibility that their patients included patients suffering from dementia with Lewy bodies (DLB) because AD+DLB patients are not easily distinguished clinically from pure AD cases.⁵ Moreover, visual hallucinations occur at significantly higher rates in DLB patients than in AD patients.^5,6 If Holroyd and colleagues'¹ group of probable AD patients with visual hallucinations actually included more DLB patients than their AD group without visual hallucinations, DLB rather than occipital atrophy might have been associated with visual hallucinations.