The big question: 'How much vision loss?'

 

Despite being first clinically identified in the mid-18th century, the world of Charles Bonnet syndrome (CBS) remains shrouded in misconceptions. One of these is the idea that significant vision impairment is necessary in order to be at risk of developing CBS.

Some ophthalmologists fuel this misunderstanding by scoffing at patients who ask whether they may have CBS by responding:

• "You can't possibly have CBS because your eye sight is too good" or
• "You read too far down the eye chart to be a CBS candidate."

 

In fact, there have been various reports over the decades that have indicated people can experience CBS when their vision loss is either mild or moderate. In line with this view, a new study from the UK has found that vision loss in just one eye can be sufficient to trigger the syndrome.¹ Historically, most studies have only explored those people with significant vision loss in both eyes and this has skewed perceptions of the syndrome. 

It is also known that some people with CBS have nothing wrong with their eyes as such but rather there has been some interruption or damage somewhere along the visual pathways of the brain. This could be due to, for example, a brain aneurysm, tumour or stroke. 

Medically speaking, there remains no agreed upon visual threshold that applies to CBS. The syndrome occurs across a wide range of degrees of sight loss: from mild and moderate forms through to significant and even in cases of no perception of light (blindness).

 

¹   Forte, G et al. (2025). Charles Bonnet Syndrome associated with unilateral vision loss: A new diagnostic perspective. Ophthalmic and Physiological Optics Volume 45(3): 681-688.