Charles Bonnet Syndrome (CBS) is the rather strange phenomenon of complex visual hallucinations in people with vision loss with no neurological or psychiatric problem. These experiences can be alarming for patients and their families - ‘Mum is seeing monkeys in trees’ will get our attention.

A study into the prevalence of CBS was published in the Canadian Journal of Ophthalmology (1). 2,565 new clients aged 40 years and older at a national low vision service in Canada were carefully questioned; 18.8 % of clients reported that they experienced hallucinations, forming visual images that they knew were not there. Perhaps surprisingly, about 15% of clients with only mild vision loss (BCVA between 6/6 and 6/17 in the better eye) also experienced hallucinations.

Similar rates of CBS were reported by people with AMD, glaucoma, diabetic retinopathy and other eye diseases, suggesting that it is vision loss rather than a specific disease.

There was also no difference in the likelihood of reporting hallucinations in people aged older than 80 compared with those aged 40 - 80 years. More CBS was reported in females. The author suggested that this may have been due to a greater reluctance of males to report the condition.

Given the high proportion of CBS in people with even mild vision loss, it is a reminder for us to be aware that many low vision patients may be experiencing CBS. These people will benefit from reassurance that their experiences are not atypical and that they are not suffering from a mental health disorder.

(1) Gordon K. Can J Ophthalmol 2016; 51: 3