Investigating the neural mechanisms behind Charles Bonnet Syndrome

Research details

  • Type of funding: Fight for Sight / Esme's Umbrella Small Grant Award
  • Grant Holder: Professor Holly Bridge
  • Institute: University of Oxford
  • Region: South East
  • Start date: February 2021
  • End Date: July 2023
  • Priority: Understanding
  • Eye Category: AMD
Brief lay background

Charles Bonnet Syndrome (CBS) has been reported in people with visual impairment and is surprisingly common, with an estimated prevalence of 10 – 34% of patients. It consists of complex visual hallucinations such as seeing snakes rising from people’s heads or elephants inside a room.

Patients experience CBS hallucinations over many years, and although they retain knowledge that they are not real, the hallucinations can be frightening and cause considerable distress. Patients with CBS have a lower quality of life compared to patients with vision loss without CBS, and the condition is under-reported.

What problem/knowledge gap does it help address

There is currently no established treatment for CBS, with current management in low vision clinics revolving around reassurance. Many patients receive inappropriate care from other healthcare practitioners.

A clearer understanding of the mechanisms underlying CBS would help the medical community to adequately advise patients.

In addition, understanding the neural mechanisms of CBS can provide a model for investigating how the brain reorganises in response to sensory impairment and may eventually lead to improvement in the management and treatment of eye disease.

Visual processing becomes more complex between the retina, the primary visual cortex, and  the more specialised processing in higher visual areas. The complex nature of the hallucinations in CBS (faces, objects, animals), in spite of blurred vision, appears to implicate the higher visual areas. To measure neural changes across the visual brain during hallucinations is challenging given the transient nature of these events. For that reason, there have been relatively few imaging studies in CBS patients.

Aim of the project

To investigate CBS by providing an in-depth profile of the basic neurochemistry, structural and functional pathways of the visual brain in CBS.

Key procedures/objectives
  1. Evaluate vision-related quality of life and mental health in CBS using questionnaires.
  2. Determine the relationship of neurochemistry to visual function after vision loss.
  3. Explore structural and functional connectivity within visual cortex. Changes in connectivity are likely to underlie the symptoms of CBS.
  4. Increase awareness and understanding of meaningful CBS treatment.
  5. Collect pilot data for a larger study.
Potential impact on people with sight loss

The team predict that patients with CBS likely show 1) changes in the brain; 2) increased changes in higher visual areas and 3) abnormal visual activity. The goal is to characterise the visual cortex of CBS patients by using MR imaging techniques. Using state-of-the-art methods, the team will investigate visual activity in patients with CBS and visually impaired participants without CBS. In addition, the team will obtain clinical evaluation and qualitative assessments of hallucinations.