We read with interest the paper by Tan et al¹ on Charles Bonnet syndrome (CBS) in Asian patients. Their finding of a lower CBS prevalence than European or North American surveys demands further investigation, although this may reflect the stringent criteria of hallucination complexity they used in making the diagnosis (thus excluding the commonest CBS hallucinations of coloured blobs and grid-like “tesselloptic” patterns^2,3) and, as pointed out in the accompanying editorial comment, the relatively low prevalence of macular disease in their cohort. However, it is not this aspect of the report we found most intriguing—it was the observation that CBS occurred with good acuity. In fact, three of the four CBS patients described had a degree of impairment which placed them at risk for CBS (best eye acuity 0.3 or worse⁴). It is the remaining patient (patient three, a 72 year old man) who is of particular importance as his relative preservation of acuity bilaterally (20/30 RE, 20/40 LE) challenges the view that significant acuity loss is a prerequisite for “ophthalmological” visual hallucinations. This case mirrors four patients we have recently studied with CBS secondary to glaucoma and bilaterally good acuity. We describe the cases below and offer a pathophysiological mechanism for the association.

In one sense, the finding that CBS occurs with preserved acuity is hardly novel. As cited by Tan et al,¹ several previous reports have found such an association. However, all is not as it seems, the term CBS being used in different ways by different authors. Some use the term to describe visual hallucinations with insight, irrespective of the presence of eye disease, age or clinical context.^5,6 Others use the term to describe the association of visual hallucinations with age and intact cognition, without reference to eye disease …